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Development of a high-concentration hydrogen sensor and a gas sensor test bench Fan, Lilian Lai Yee 2007

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DEVELOPMENT OF A HIGH-CONCENTRATION HYDROGEN SENSOR AND A GAS SENSOR TEST BENCH by Lilian Lai Yee Fan B.A.Sc, The University of British Columbia, 2002 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES (Engineering Physics) THE UNIVERSITY OF BRITISH COLUMBIA April 2007 © Lilian Lai Yee Fan, 2007 Abstract Resistive hydrogen gas sensors based on palladium-hydrogen (Pd-H2) interactions were fabricated and tested. This thesis presents expected resistance characteristics for these sensors, and describes their fabrication process. Test results and analysis identified the H 2 sensing mechanisms in these sensors. In particular, linearity between sensor resistance and the approximate atomic hydrogen content in the Pd was established, and the estimated proportionality constant obtained is 2.7± 0.9 (a.u.). A low flow rate sensor test bench featuring a line-switching mechanism was designed and built also. A description of the test bench and its operation principles are discussed. Response times obtained from initial tests of the un-optimized system is 16s ± 9s, comparable to the system response time of a commercial gas sensor test station presently available. Recommendations for future work to optimize this test bench are discussed, as are recommendations for future sensor design and development. ii J ( Table of Contents Abstract Table of Contents :.. iii List of Tables vi List of Figures vii Acknowledgements -. xii Dedication x ' i ' Chapter 1 Introduction 1 1.1 Motivation and objectives 1 1.2 Applications 1 1.2.a Manufacturing 2 1.2.b Medicine 2 1.2.c Environmental 3 1.2. d Fuel regulation and monitoring 3 1.3 Recent research 3 1.3. a Resistive sensors.... ; 3 1.3.b Semiconductor sensors 5 1.3.c Other sensor structures 7 1.3.C.1 Thermoelectric sensors 7 1.3.C.2 Optical sensors 8 1.3.C.3 Cantilever sensors 8 1.3.C.4 Pyroelectric sensors 9 »1.3.C.5 Surface acoustic wave sensors 9 1.4 Sensor development 9 Chapter 2 Theory and Simulation of Pd Mesowire Resistors 11 2.1 Resistive mesowire sensors 11 2.2 Basic sensor structure 11 2.3 Electrical resistance of Pd hydrides 12 2.3.a Pressure-concentration (p-C) isotherms for the Pd-H2 system 12 2.3.b Resistivity vs. [H]/[Pd] 14 2.3.c Supersaturation ; : 14 2.3.d Magnitude of changes in resistance 15 2.4 Percolation mechanism 15 2.5 Temperature effects 16 iii 2.5.a Temperature coefficient of resistance in Pd hydrides 16 2.5.b Pressure-concentration changes with temperature 16 2.5.c Other temperature effects IV 2.5.d Temperature modelling 17 2.6 Anticipated observations • 22 Chapter 3 Fabrication of Pd Mesowire Resistors 23 3.1 Fabricated sensors 23 3.2 Process description '. 26 3.2.a Substrate preparation 26 3.2.b Sensor patterning 29 3.2.c Developing the resist 31 3.2.d Sputtering and liftoff. : 39 3.2.e Packaging •• -45 3.3 Sensor testing 45 Chapter 4 Development of a High Concentration H 2 Test Bench ...46 4.1 Application requirements 46 4.2 Description of the facility 46 4.2.a Gas manifold 46 4.2.a. 1 Supply to MFC controllers 48 4.2.a.2 MFCs to LINEOUT: 49 4.2.a.3 Purge line and safety alarms 50 4.2.b Testchamber 50 4.2.b. 1 Vacuum system '. ( • 51 4.2.b.2 Sensor mount 51 4.2.b.3 Operation of the gas manifold and test chamber 54 4.2. c Data acquisition electronics 54 4.2.c.l Sensor excitation and signal detection circuitry.... 55 4.2.C.2 LabVIEW data logging program 55 4.3 Commissioning and testing with the test bench 57 4.3. a Test bench commissioning 57 4.3.b Testing with the test bench 57 Chapter 5 Results and Analysis • 61 5.1 Test results 61 r- 5.1.a Early sensors • 61 5.1.b Failure mechanisms 66 iv 5.1. c Sample response curves 68 5.2 Response analysis 73 5.2. a Small signal step response 73 5.2.b Background response 78 5.2.b. 1 Lattice and grain boundary effects 79 5.2.b.2 Percolation effects 79 5.2.b.3 Auxiliary effects 81 Chapter 6 Conclusion -82 6.1 Test bench development summary 82 6.2 Sensor development summary 83 6.3 Significance of results and comparison with literature 83 6.4 Additional recommendations 85 References • 87 Appendices ..r. .....95 Appendix A. LabVIEW control panels and block diagrams 95 Appendix B. Calibration charts for gas sensor test bench 100 v List of Tables Table 1.1 Some resistive sensor characteristics 5 Table 1.2 Some semiconductor sensor characteristics 6 Table 3.1 JEOL 840 SEM Settings 29 Table 3.2 NPGS e-beam lithography program settings 30 Table 3.3 Equivalent line and area dose from NPGS program 30 Table 3.4 Gaussian best-fit parameters for Pd wire cross-section 43 Table 4.1 Mass flow controller characteristics 49 Table 4.2 Operation of line-selecting solenoid valves 49 Table 4.3 Test chamber physical data 51 Table 4.4 Average response times for commercial test station and test bench 59 Table 5.1 Reported (p-C) isotherm values of fi Pd hydrides from literature .•. 73 Table 5.2 Summary of pressure interpolation constants for determining [H]/[Pd] ratios 73 Table 5.3 Average values for fit coefficients 77 Table B. 1 Conversion factors for MFCs 100 vi List of Figures Figure 2.1 Top and cross-section view of basic sensor structure. Sensors developed in this study are variations of this geometry 11 Figure 2.2 Shape of the pressure-concentration (p-C) isotherm of hydrogen in Pd. The first rise corresponds to the a-hydride phase and follows a linear form whereas the second sharp rise is due to the P-hydride phase and follows a logarithmic form. The plateau corresponds to the ct-P transition region and shows the effect of hysteresis 13 Figure 2.3 R/Ro behaviour on temperature as reported by Sakamoto et al. Changes in plateau resistance are not monotonic with temperature 17 Figure 2.4 Heat Transport model of Pd mesowires with inset (not to scale) showing model structure. The Pd wire is mirrored across the boundary to the right, and is half the length of the simulation by symmetry 18 Figure 2.5 Estimated temperature along Pd wire at various voltages. Modelled properties include conduction, convection and thermal effects on resistance. Thermal coefficient of 3.3 x 10" 3 /K is used 19 Figure 2.6 Estimated temperature along Pd wire at voltages varying from 20mVDC to 120mVDC 20 Figure 2.7 Model structure of Pd mesowire with a delaminated section (not to scale) for heat transport simulations 20 Figure 2.8 Temperature simulation of a delaminated Pd wire (over a 12.5|rm air gap). The structure of this simulation was illustrated in Figure 2.7. This figure clearly demonstrates the deleterious effect of delamination on sensor performance 21 Figure 2.9 Temperature simulation of a delaminated Pd wire (over a 12.5um air gap) for voltages between 0.02VDC and 0.12VDC 21 Figure 3.1 Lithography and liftoff process: A) Coating with resist and patterning with electron beam; B) developing exposed resist; C) depositing the desired material for the pattern and D) removing the resist layer and leaving behind the patterned layer 23 Figure 3.2 Typical early design sensor showing many-wire Pd array across Au/Cr contacts ^ approximately 65pm apart 24 Figure 3.3 Later design sensor with five-wire Pd array. These sensors have improved liftoff yield and higher overall resistance than many-wire arrays 24 Figure 3.4 Sensor with U-shaped symmetric structure. Instead of patterning gaps along a wire by turning off the e-beam, these samples are patterned by having the e-beam travel half of the gap and then returning to the same side. A U shape was used on each side (symmetrically) vii to lessen overexposure of the resist at the turning points. Attempts at generating gaps in this manner were unsuccessful 25 Figure 3.5 Five-wire device sensor with crosslinked polymer platform. The presence of imperfections along these wires have less effects on the overall resistance than in other sensors developed 25 Figure 3.6 Au contact pads of a sample, shown with bonded Au wires. Crosses are alignment marks to ease navigation of electron beam during sample exposure 26 Figure 3.7 Mask for photolithography of Au/Cr contact pads for fabricated sensors. Pad sizes are ~lmm x 1mm. The patterns are rotated to align with the cleave planes of Si(l 11) wafers. ...27 Figure 3.8 Bilayer resist cross-section on SiC>2. The upper layer is PMMA, the middle layer isPMGI and the bottom is Si02. Thicknesses of the resist layers are about 200nm each 28 Figure 3.9 Partial Liftoff of Pd from e-beam exposure of PMMA. This shows PMMA working as a positive resist at moderate electron-beam dose 32 Figure,3.10 Effects of overdeveloping the PMGI undercut layer. The PMMA layer becomes entirely delaminated from the substrate 33 Figure 3.11 PMMA development: IminOOsec, 1:3 MIBK:IPA. Very little development is observed at this point 33 Figure3.12 PMMA development: 2min00sec, 1:3 MD3K:IPA. Faint traces corresponding to developed lines begin to appear 34 Figure 3.13 PMMA development: 3min00sec, 1:3 MIBK:IPA 34 Figure 3.14 PMMA development: 4min00sec, 1:3 MIBK:IPA. Patterns in PMMA are now easily distinguishable ......34 Figure3.15 PMMA development: 5min06sec, 1:3 MIBK:IPA. End of the MIBK development process. 3 5 Figure 3.16 PMGI development: 0min20sec 2.2% TMAH. Patterns begin to broaden 35 Figure 3.17 PMGI development: 0min40sec 2.2% TMAH. Cavities under the PMMA layer (dissolved PMGI) begin to appear 35 Figure 3.18 PMGI development: 1 minOOsec 2.2% TMAH. Mild delamination (circled) of the PMMA begins to appear 36 Figure 3.19 PMGI development: lmin20sec 2.2% TMAH. Further widening of PMGI cavity 36 Figure 3.20 Micrograph showing top view of sample cleaved in the center of the device. Delamination and debris from sample cross-sectioning are clearly visible 37 Figure 3.21 PMMA Resist peeling off is clearly visible from sample cross-sectioned area 37 Figure 3.22 Cross-section of developed PMMA/PMGI bilayer. The layer to the left is PMMA, which can be seen with a large flap/extension left behind from the cleaving process. The layer with the cavity is the PMGI layer, sandwiched between the PMMA and the Si0 2 38 viii Figure 3.23 Chart showing the width of the developed PMGI cavities as a function of electron dosage. Only a few data points are available because of difficulties in obtaining intact cross- -sections of polymer layers 38 Figure 3.24 Pd wires after liftoff from single layer resist 39 Figure 3.25 Pd wire after liftoff from bilayer resist. Pd wires fabricated from the bilayer process are thinner than with the single layer PMMA process 40 Figure 3.26 Secondary Electrons image of a thin Pd wire sputtered through bilayer resist 41 Figure 3.27 Electron counts associated with Si obtained from EDX measurements. Slightly lower Si count density is observed across the lower half of the image corresponding to the location ofthePdwire '. 41 Figure 3.28 Electron counts associated with Pd obtained from EDX measurements. Pd is present as a very faint bar of higher count density across the lower half of the image! These measurements prompted further characterization of the bilayer samples with a field emission scanning electron microscope (FESEM) and an atomic force microscope (AFM), which determined unambiguously the presence of the Pd wires as a continuous thin film 41 Figure 3.29 FESEM micrograph of a Pd wire on oxide fabricated with single-layer resist 42 Figure 3.30 FESEM micrograph of a Pd wire on oxide fabricated with bilayer resist 42 Figure 3.31 Cross-section of Pd mesowire sputtered through bilayer resist. Fitted line follows the Gaussian form given by Wo+W^expHCx-W^/Ws)2) : 43 Figure 3.32 Top view of Pd wire deposited through bilayer resist. Sample was exposed with l.OnC/cm electron beam dose, and subsequently developed in 1:3 MIBKTPA and MF-319 for 5m06s and lm20s respectively 44 Figure 3.33 Sputtered Pd wire widths with increasing e-beam dosage. Widths are normalized with the measured width of wires exposed with 4.0nC/cm electron dose 44 Figure 3.34 Packaged, bonded chips for sensor testing 45 Figure 4.1 Gas manifold and test chamber schematic for the test bench 47 Figure 4.2 Test chamber showing t est sensors, pressure transducers and vacuum pumps mounted on numerous ports 50 Figure 4.3 Diagram of slit nozzle. Gas entering through the port at one side of the nozzle is distributed across a slit orifice close to the sample 52 Figure 4.4 Simulation model structure for slit nozzle. Gas enters through the circular inlet on the left and exits to the right through a slit. Samples would be located at the bottom surface (extrusion) of the model 53 Figure 4.5 Vertical section of simulated gas flow through a slit nozzle. Flow velocity is uniform across much of the sensor cavity. Sensors are attached at the centre of the bottom surface. ..53 ix Figure 4.6 Horizontal section of simulated pressure in a slit nozzle. Pressure appears to be uniform across the sensor cavity. Sensors are attached at the centre of the bottom surface 53 Figure 4.7 Excitation and DAQ interface circuit to power resistive sensors and measure changes in resistance 55 Figure 4.8 Sample measurement obtained from LabVIEW-controlled DAQ setup 56 Figure 4.9 Sample response of a wire sensor fabricated with bilayer resist at very high concentrations ofH 2 58 Figure 4.10 Sample response of a capacitor sensor tested with the test bench. Response of these sensors are compared with response of the same sensors tested with a commercial test station ; 59 Figure 4.11 Sample response of the same sample as in Figure 4.10, tested in the Greenlight commercial sensor test station 59 Figure 5.1 Resistance response from an intact array of many Pd wires. A very slow and gradual response was observed, in addition to a large hysteresis 62 Figure 5.2 Voltage across defect-ridden Pd wire array. Response is indicative of percolation effects. Wires that are open circuits in air become conductive at 100% H 2 62 Figure 5.3 Intact wire array sample corresponding to test results shown in Figure 5.1 63 Figure 5.4 Wire array with multiple defects corresponding to test results shown in Figure 5.2 63 Figure 5.5 Unstable response of the defect-ridden many-wire array of Figure 5.4. 0-100% H 2 Step response of this sensor was shown in Figure 5.2 64 Figure 5.6 Failure response of the defect-ridden sensor of Figure 5.4. This test was performed immediately after the sensor test shown in Figure 5.5 65 Figure 5.7 Small signal dependence of resistance on H 2 concentration: very small changes 65 Figure 5.8 Sample with imperfections on the underlying oxide layer, as fabricated. The compromised ^ region is the darker patch of oxide between the contact pads, to the right 67 Figure 5.9 Failure of the sample of Figure 5.8. Portions of wires spanning oxide with imperfections can be seen to have melted. The adhesion between the wires and the oxide layer at these areas was compromised, which led to wire delamination and overheating 67 Figure 5.10 I-V curve for Pd wires with good adhesion to the oxide substrate. Trend line is a linear regression of I=0.008*V based on data up to 6.25VDC 68 Figure 5.11 Response from bilayer PMMA/PMGI resist sample. This sample is one with a platform structure. See text for circled property 69 Figure 5.12 Step response portion of Figure 5.11 in detail. See text for circled properties '. 69 Figure 5.13 Response from sample fabricated with the single PMMA resist process, 5 wire structure. See text for circled property 70 > Figure 5.14 Step response portion of Figure 5.13 in detail 70 Figure 5.15 Repeated testing of a platform sensor 71 Figure 5.16 Step response portion of Figure 5.15 in detail 71 Figure 5.17 Logarithmic fit of small signal resistance measured as a function of H 2 concentration 74 Figure 5.18 Sample response along with small signal component for H 2 concentrations over 10% 74 Figure 5.19 Sample background response left over after small signal component is removed 75 Figure 5.20 Reference resistances for samples analysed with respect to their initial resistance. Samples marked with t before their labels did not liftoff properly. Sample marked with * was an older, many-wire sensor with compromised signal-to-noise. These sensors (the three on the right) were not used to calculate averages 76 Figure 5.21 Coefficients (arbitrary normalized units) extracted from small signal response fitting. Again, samples marked with t before their labels did not liftoff properly. Sample marked with * was an older, many-wire sensor with compromised signal-to-nbise. These sensors (the three on the right) were not used to calculate averages. .77 Figure 5.22 Percolation-like effects in resistance response. Change in resistance due to (a) expansion of Pd wire segments, forming an additional conductive path and (b) contraction of Pd segments 80 Figure A.l Main control program front panel (MultiChannelDAQ.vi) 95 Figure A.2 Block diagram of main control program (MultiChannelDAQ.vi) : 96 Figure A.3 Time reference case in main control program (MultiChannelDAQ.vi) 97 Figure A.4 Case for no plot data in main control program (MultiChannelDAQ.vi) 97 Figure A.5 Front Panel of module for parsing and extracting data to be logged (Array2DExtract01.vi)..97 Figure A.6 Block Diagram of module for parsing and extracting data to be logged (Array2DExtractO 1 .vi) 98 Figure A.7 Front Panel for plotting module. Plots scrolling data (MultiPlot2DArray.vi) 98 Figure A.8 Block Diagram for plotting module (MultiPlot2DArray.vi) 99 Figure A.9 Case for no data to plot in plotting module (MultiPlot2DArray.vi) 99 Figure A.lOCase for only one point to plot in plotting module (MultiPlot2DArray.vi) 99 Figure B.l Cross reference chart for N 2 MFCs, received new from manufacturer (ASGE) 101 Figure B.2 Real flow conversion chart for an older H 2 MFC 101 Figure B.3 Actual flow of older H 2 MFC as the console setpoints vary 102 Figure B.4 Cross Reference curve for Cole Parmer MFC with ASGE MFC. Discrepancies of about 2slpm were observed between the two MFCs 102 xi Acknowledgements I would like to express my thanks to the people who have made this thesis work possible. In particular, I would like to thank Dr. Jonathan Wu, now at the University of Windsor, for giving me the opportunity, support and encouragement to pursue this Master's degree. I thank Dr. Kevin Stanley, Dr. Cheng Hu, Dr. Eva Czyzewska, Tom Vanderhoek, and Dr. Yanghua Tang for their friendship, advice, and invaluable contributions to the thesis. Many thanks to Weimin Qian and Shuozhi Yang, who helped carry out much of the testing for this work. I offer my gratitude to Dr. Andras Pattantyus-Abraham, Dr. Mario Beaudoin, John Yuen and my family for their unfailing support to solve all kinds of issues I have, whether in the cleanroom, laboratory, cubicle or at home. • To my supervisory committee and reviewers also I owe many thanks, for their willingness to read and provide useful suggestions on my thesis within such a tight schedule. Thank you Dr. Tiedje, Dr. Wu, Mario, Jun and Cheng! Finally, I would like to thank the National Research Council of Canada - Institute for Fuel Cell Innovation for their support. It has indeed been a privilege to have the opportunity to work with so many admirable researchers at such an open and creative organization. xii To my parents. xiii Chapter 1 Introduction ' With the commercialization of fuel cell and hydrogen technologies, there is a pressing need for small, fast, economical and efficient hydrogen (H2) monitoring devices. Interest in H 2 sensors arises not only from safety concerns, but also from their potential for monitoring and optimizing the operation of fuel system components. Numerous technology gaps exist, however, and development of sensors suitable for commercial applications such as embedded fuel cell and balance-of-plant* monitors remains a challenge that a number of research groups around the world are trying to address. 1.1 Motivation and objectives The goal of the present work is to develop a micro-sensor capable of detecting H 2 concentration variations between 90% and 100%) in nitrogen (N2) for on-line monitoring of fuel cell systems. The sensor should be fast, with response and recovery times preferably below one second. The sensor's response should also be relatively stable and reliable. Its fabrication process should require little or no manual assembly to facilitate transfer of developed technologies to industry. To this end, resistive palladium (Pd) mesowire sensors that operate between 10% and 100%#H2 have been successfully fabricated and tested. A test bench was also designed and assembled for H 2 sensor testing with the objective of determining detailed time response of the developed sensors. The time response characterization is not yet accomplished, but initial test bench calibration results have been obtained and are promising. While the direct motivation for this work is fuelled by the emerging fuel cell industry, it is important to recognize that H 2 sensors are also used in numerous other industries. The present chapter describes some applications of H 2 sensors and provides an overview of recent research reported in literature. Emphasis will be placed on resistive devices based on Pd-H2 interactions, as these are most similar in nature to the sensors developed in this work. 1.2 Applications Hydrogen is the lightest element known on earth. It is a flammable gas above 20.35K for concentrations between 4% and 75% in air, and has an auto-ignition temperature of 773K [1]. Atomic hydrogen (H) is a * Balance-of-plant refers auxiliary systems needed for fuel cell operation, such as storage tanks, compressors and control electronics. constituent of many common substances, and hydrogen gas (H2) is frequently used in a wide array of industries and research. 1.2.a Manufacturing Many industrial processes use H 2 as a raw material, while numerous others produce H 2 as end products. Of these, only a small portion is sold commercially, as most H 2 requirements for industry are typically produced and consumed on-site. H 2 sensors are thus present in many facilities to monitor the safety and optimize the efficiency of these industrial processes. H 2 is primarily produced by reforming natural gas. The two largest chemical industries at present that use H 2 are the ammonia and petroleum refining (plastics) industries [2]. Other industrial users of H 2 include chemical plants that produce methanol, hydrogen peroxide, acids and metals. A different niche manufacturing application for H 2 sensors is its use as a tracer gas for locating leaks and cracks in objects ranging from automotive exhaust pipes to food packaging. Average background levels of H 2 are typically on the order of 0.5ppm. As such, tracer gas mixtures of H 2 in nitrogen (N2) may be easily distinguished and located [3]. The food industries, in particular, use H 2 sensors in various ways. Many food processing facilities modify hydrogenation levels in fats for attaining desired baking/cooking results. H 2 sensors are reportedly used for monitoring cooking progress, and are used in studies for rapid identification of prior irradiation in certain foods [4, 5]. J 1.2.b Medicine H 2 sensors have also found their way into medical laboratories, where "Hydrogen breath tests" are used for diagnosis of gastrointestinal symptoms. Sugars and carbohydrates are usually digested and absorbed in the stomach and small intestines, before the remaining foodstuff reaches the colon. When these sugars are not absorbed properly, large amounts of these sugars may become accessible^ to certain anaerobic bacteria in the colon. These bacteria will digest them, releasing H 2 as one of the by-products. Some of the H 2 is absorbed into the blood stream and taken to the lungs where the gas is exhaled. Identification of specific medical disorders is done through analyzing the timing, frequency and amount of H 2 exhaled. Examples include tests to identify patient intolerance to particular sugars (such as lactose) [6]. At 2 present, breath samples are generally trapped and analyzed with gas chromatography where H2-concentration changes down to ppm levels are monitored [7]. 1.2.c Environmental H 2 sensors are also used in atmospheric and environmental studies. Mass spectrometry is used to determine hydrogen and deuterium concentrations in the earth's lower atmospheres, shedding light on studies of natural H 2 uptake in soil [8]. Other uses of H 2 sensors in environmental studies include their use in space missions to provide key information on the presence and volume of water or ice on planetary bodies and are aiding scientists around the world study climate changes on the planet [9, 10, 11] 1.2. d Fuel regulation and monitoring More in line with the objectives of this thesis are H 2 sensors in fuel monitoring. H 2 detectors play a crucial role in monitoring the safety, operation and fuel quality of H 2 powered technologies such as space shuttles and fuel cells. Environmental concerns over the use of fossil fuels and advancements in fuel cell technologies in recent years lead to ever increasing use of H 2 as an alternative, environmentally conscious means of energy delivery. Specifically, mounting demands for device miniaturization in the fuel cell industries [12] is driving the development of tiny, novel sensors capable of fast and accurate monitoring of H 2 environments. The majority of H 2 sensor research in literature are interested in developing sensors for fuel cell applications. Leak sensors optimized for low concentration detection are needed for safety monitoring, while embedded sensors that distinguish a wide range of concentrations are needed for performance monitoring. Successful commercialization of H 2 fuel cells depends on the appropriate use of sensing devices in related areas such as fuel storage, fuel,processing (reforming) and transportation technologies. The performance of commercially available H 2 sensors follows developments in H 2 sensor research closely. The following section surveys of some recent experimental studies in the field. 1.3 Recent research 1.3. a Resistive sensors Resistive measurements of thick and thin-film semiconducting materials is a common method to determine gas concentrations. Longstanding research on semiconducting materials such as Sn02 [13, 14, 15, 16] 3 continue to bring progress in this area, as does recent work with materials such as Ti0 2 [17, 18] and W0 3 [19]. Emphasis of these research endeavours is placed (for the most part) on detecting minute concentrations of H 2 . Oxide resistive sensors operate on the basis of an increase in excess electrons from absorbed H 2 molecules, thus lowering the overall resistivity of the material. These sensors typically have simple structures, but suffer from poor selectivity and high operating temperature requirements. Documented resistance changes in Pd-H2 systems predate semiconducting oxide sensors research by almost a century, with an excellent reference summarizing studies on H 2 interactions with bulk Pd written back in 1967 by F.A. Lewis [20]. By far the majority of work on resistive H 2 sensors to this day use Pd-based materials as the active sensing element. Pd as a sensor material is used as catalyst powders (thermoelectric sensors), thin films, gate films (in semiconductor sensors), nanowires and nanoclusters, among others. Some of these sensors will be described briefly in the following paragraphs. A description of the sensing mechanisms present in Pd-based resistive sensors will be presented in the following chapter. Stability and degradation has always been a prime concern for Pd-based H 2 sensors. Pd-Si devices are known to degrade with the formation of palladium silicides [21]. Blister formation and delamination routinely occur in Pd thin film devices exposed to high concentrations of H 2 [22]. Methods of stabilizing Pd films include using an adhesion layer between the Pd and the substrate, or alloying Pd with impurities [23, 24, 41]. In general, the introduction of alloy impurities in Pd increases in the upper concentration-sensitivity limit of the device, and improves the mechanical stability of the Pd. These changes are typically at the expense of reduced sensitivity at lower concentrations and, more importantly, slower time response [40]. In the past decade, a few researchers have investigated Pd-based resistive sensors operating at concentrations up to 100% H 2 . Examples include sensors featuring Pd-Ni alloy films evaporated on Si 3N 4 substrates [25]. These Pd-Ni sensors feature good mechanical stability at Ni contents over 8%, and exhibit reasonable dynamic response (film resistance) at high H 2 concentrations. Comparable results from Pd-Ni alloy films were obtained on A1203 substrates in a different study [26]. Thick-film resistive pastes were also studied as H 2 sensing materials. This class of sensors include thick film sintered glass binders with Pd or Pd-Ag alloy [27, 28, 29]. The response behaviour of these and the Pd-Ni film sensors mentioned earlier are similar, showing an increase in resistance upon exposure to H 2 . Response time however for these thick-film sensors are longer, with reports typically on the order of one to several minutes. 4 Interesting, novel sensing mechanisms are emerging as the use of micro and nanofabrication technologies become increasingly common. One such example is a sensor structure that operates based on the swelling and contraction of Pd clusters or grains. Palladium Mesowire Arrays (PMA) [30, 31] are electrodeposited Pd wires between 50-600nm in width. 'Gaps' along these wires close due to Pd lattice expansion in the presence of H 2 , leading to a large change in the overall resistance of the wires. Sensors based on this activation mechanism were tested with H 2 concentrations between 0% and 10% in air. While signal saturation occurred as the H 2 concentration approached 5%, the sensor enjoys excellent response and recovery times from tens of milliseconds up to several seconds. Similar sensing mechanism was observed in other nanoscale Pd sensors, including Pd nanowire bundles electrodeposited into a sacrificial porous alumina host [32], discontinuous films of Pd clusters close to their percolation limit [33], and Pd nanowires electrodeposited onto patterned Si microchannels [34, 35]. Other resistive sensors in recent developments include Pd-coated rare-earth thin films supported on a micro-hotplate [36] and Pd-coated polysilicon mesoswires [37]. A number of resistive sensor structures are listed in Table 1.1 with representative characteristics. Table 1.1 Some resistive sensor characteristics Sensor Type Material/Structure H 2 Concentration range Response/ Recovery time(s) Operation Temperature (K) Ref. Oxide Sn02 0.31% 100s 473-873K [-14] Oxide Ti0 2 thermal 0.001%- 1% 2-10s 423-573K [18] Oxide W03, Pt-W03, Au-W03 0.125%-1.25% 200s 300-573K [19] Metal Pd/Ni-Ti-Si3N4 < 100%, increasing R 20s/50s 383K [25] Metal Pd mesowires 0.5%-10% 20ms-5s 300K [30] Metal Pd/rare earth/hotplate 0.02% - 30% < ls-200s N/S [36] 1.3.b Semiconductor sensors Semiconductor sensors encompass a large portion of H 2 sensor research. Included within this class of gas sensors are Schottky metal-semiconductor (MS) sensors, metal-insulator-semiconductor (MIS) and metal-insulator-metal (MIM) tunneling devices [23]. Pd-gate MOS devices were first reported by Lundstrom et al. 5 [38, 39]. In general, adsorption of H 2 changes the work function of the gas-sensitive metal and causes shifts in the barrier height and flat-band voltages for semiconductor devices. In the case where Pd is used as the active metal, further changes are typically observed from atomic hydrogen migration to the Pd/dielectric interface, which causes charge redistribution [40]. The majority of state-of-the-art research into semiconductor sensors focus on lowering the H 2 detection limit and minimizing the response and recovery times at low concentrations. As such, only a handful of recent research in this category of H 2 sensors provided experimental results for H 2 concentrations over 1%, among which even less reported results close to 100% [41]. A list of some semiconductor sensors are given in Table 1.2 to illustrate general operating conditions and properties for these sensors. Table 1.2 Some semiconductor sensor characteristics Sensor Type Material/Structure H 2 Concentrations range Response/ Recovery time (s) Operation Temperature (K) Ref. MOS Pd/Ni capacitors 0.01 torrT-700 torr Not Specified 300K [41] MOS HEMT Pd-InGaAs-AlGaAs 0.0014%-0.997% 22-400s 300-460K [42] FET Pt-commercial Ta205 gate FET 0.001%-10% 2s-1500s 305K-393K [43] Schottky Pd-Nb205 0.001%-0.8% 5s-500s 300-473K [44] Schottky Ru-SiC <2% 12s/ 150s 673K [45] pHEMT1 Pd-AlGaAs-InGaAs-GaAs < 0.4% 25-600s 300K [46] MIS Pt-TaSix-Si02-SiC 1% <10ms/ <150ms 673-1073K [47] P-N p ZnO-n ZnO 0.05%, 0.1% 146s/86s 573-673K [55] Semiconductor H 2 sensors are very sensitive at low concentrations, though this sensitivity typically drops off with signal saturation at H 2 concentrations greater than several percent. Response and recovery times of these sensors range from several seconds to hours, and typically improve with higher operating temperatures [47, Atmospheric pressure is 760 torr acronym for pseudorhorphic High Electron Mobi l i ty transistor 6 48]. Some microsensors described in literature are now designed and built with integrated heaters [65]. It is useful to note however that response times on the order of several hundred milliseconds were reported from the early work of Poteat et al. at 760Torr H 2 at room temperature [49, 50]. As such, long response and recovery times attributed to semiconductor-based sensors may be characteristic only to low concentration detection. A number of groups developed semiconductor sensors using Ru, Pt, and Ir as the active metal [45, 47, 51,52, 53, 54, 47, 48]. Pt especially is commonly used in recent semiconductor H 2 sensor work. While devices based on these metals are much more stable mechanically than Pd-based sensors, they exhibit less sensitivity to H 2 [40], and are typically responsive to 0 2, CO, N 2 as well as numerous other hydrocarbon-based gases. Other novel materials in recent semiconductor H 2 sensor research include doped ZnO p-n junctions [55] and porous silicon [56, 57], Results from these novel sensors are promising, albeit less refined than results from conventional semiconductor sensors. 1.3.c Other sensor structures 1.3.C.1 Thermoelectric sensors Thermoelectricity, also known as the Seebeck effect, is induced when a temperature gradient between two points in a material generates a potential difference between the two points. Early work using thermoelectric voltages as the basis for H 2 sensing was done with Pt or Pd coated Sn02 pellets [58]. Changes in temperature across Sn02 are produced by catalytic H 2 oxidation on the metals' surface. In recent years, thermoelectric Hydrogen Sensors (THS) exhibiting good linear response from 0.5ppm to 10% H 2 in synthetic air was reported in the literature [59, 60, 61, 62, 63, 64, 65]. These sensors consist of Pt catalysts deposited on a portion of the surface of thermoelectric Li- and Na- doped NiQ and SiGe films. The exothermic reaction of H 2 dissociation on the Pt surface generates a temperature gradient across a thermoelectric film, which in turn produces a voltage across the sensor. Recent research into miniaturizing the device using MEMS techniques resulted in the integration of the sensor (as a catalyst-loaded membrane) with a micro-hotplate [65]. The rise time and recovery time of these sensors are reported to be on the order of 50s or less. Cross sensitivity exists to a significant extent as the Pt catalyst is sensitive to numerous other gases [66]. 7 1.3.C.2 Optical sensors The majority of optical sensors are Pd-based and measures changes in the metal's optical or mechanical characteristics. Optical sensors feature "remote" detection capability that minimizes the potential for generating electrical sparks in an H2-rich environment. For the same reason, however, these sensors typically have complicated measurement setups (both electrical and optical), and require significant operating power to drive both the excitation and detection circuitry. Recent research in optical sensors include studies on the transmittance changes of a Pd/PVDF polymer bilayer structure in the presence of H 2 [67]. Photodiode measurements of laser excitation through thin Pd/PVDF films are compared with a reference beam, and transmittance is shown to increase when such films are exposed to H 2 . Measurements were made between 0.2% and 100%) H 2 in N 2 and excellent Pd adhesion and mechanical stability was reported. Sensor sensitivity, however, tends to drop off as H 2 concentration increases beyond 10% H 2 . Sensor response times are on the order of 50s at 3% H 2 , and studies were done to integrate these sensors in a dual photopyroelectric/optical-transmittance H 2 sensor [68]. Transmittance measurements to detect H 2 were also done with Pd/ V 2 O s films on glass substrates between 0 and 10% H 2 [69]. Signal saturation occurred at around 6% H 2 and response and recovery times were on the order of 20s were at 4% H 2 in N 2 . Other research on optical sensors include work studying surface plasmon resonances in thin film structures deposited at the end of a fibre optic cable [70, 71], plasmon-induced losses along Pd-coated fibres [72, 73, 74, 75], transmittance studies of fibres of various cladding materials [76, 77, 78], reflectivity and transmission measurements of Pd mirrors at the end of optical fibres [79, 80, 81]. Among these studies, a few were tested up to 100%H2[80, 76]. 1.3.C.3 Cantilever sensors Some groups have developed micro-cantilever sensors for H 2 detection. These cantilevers are predominantly driven by the physical expansion of Pd metal in the presence of H 2 , and deflections are measured with optics [82, 83, 84, 85] or capacitance [86] typically. In recent reports, H2-sensitive sensors were fabricated using arrays of cantilevers coated with Pd-Ni alloys [86, 87, 88]. These cantilevers operate by detecting variations in the deflection-sensitive capacitance between the cantilever and a stationary baseplate. Test results for these sensors exhibit good sensitivity range for H 2 concentrations up to 90% in N 2 and in air. Typical response times ranged from 60s for higher concentrations to hours for smaller concentrations. 8 Optical cantilever sensors include thin glass substrates coated with Pd or Pd-Ag alloy [83]. Changes in the curvature of these sensors were observed in a vacuum chamber at H 2 pressures up to 600Torr between 20°C ^ and 50°C. The response time for the Pd-coated cantilevers at H 2 pressures under 30Torr (4% partial pressure) is approximately 1 minute, while recovery times are typically at least 10 minutes. 1.3.C.4 Pyroelectric sensors -Pyroelectricity is the potential difference generated by certain anisotropic solids that become spontaneously and temporarily polarized when a temperature change is induced within the material [40]. Thermal waves and interference patterns are induced within PVDF fdms by ac laser excitation. Signal differences are observed and compared between Pd coated fdms and reference electrodes [89, 90]. Standing wave patterns could also be modified / matched by varying the relative intensity of the excitation laser. Variations in the pyroelectric signal intensity due to difference in the optical absorptances of the electrodes were measured upon the sensor's exposure to H 2 . Typical operating results from these sensors saturate at H 2 concentrations greater than 1.5% in N 2 , though good signal-to-noise is still discernible down to approximately 200ppm in N 2 . 1.3.C.5 Surface acoustic wave sensors Recent research on Surface Acoustic Wave (SAW) H 2 sensors are typically also Pd-based. SAW sensors measure perturbations of a propagating acoustic wave along the surface of a piezoelectric crystal [91, 92]. Perturbations could be in the form of a change in amplitude, phase or frequency. In the presence of H 2 , SAW sensor elements (again, typically Pd) change in density and elasticity, modifying wave propagation velocities along the sensor when compared to reference electrodes. Some SAW sensor structures studied include thin palladium / metal-phthalocyannine (Pd/CuPc and Pd/NiPc) bilayers on a LiNbOs piezoelectric substrate. Changes in the wave frequency at H 2 concentrations between 0.5%. and 4% were reported, with response and recovery times on the order of several hundred seconds. 1.4 Sensor development Despite the great variety of sensors in current state-of-the-art research, it is difficult to identify one single sensor that is superior across all concentrations of H 2 in terms of its selectivity, sensitivity, speed, stability (chemical and physical), complexity, and efficiency. In the present study, resistive Pd structures are fabricated and their response to concentrations of H 2 between 10% and 100% will be discussed. We choose to study these sensors on the basis of the following observations from literature: 9 1. Pd is known to be selective to H 2 gas. Pd is mildly cross-sensitive to a number of other chemical species such as 0 2, N 2 , H2S and CO. Its response to H 2 , however, is by far the dominant response. 2. Although the pre-saturation operating range for the majority of H 2 sensors described in the literature are limited to concentrations less than several percent, it is clear that resistive changes in Pd can be detected across all concentrations of H 2 . 3. The response and recovery speed of sensors reviewed range from several seconds to hours. Thick-film Pd resistors are characteristically slow, as the sensing mechanism is rate-limited by the diffusion of hydrogen atoms into the bulk material. In this respect, Pd-based semiconductor sensors excel as their operation are more dependent on changes in surface.charges. In recent years, however, use of micro and nanofabrication technologies has resulted in faster resistive Pd devices, as device dimensions and diffusion lengths diminishes. 4. There is simplicity in using resistive sensors. In a monitoring and diagnosis system, it is likely that other sensors and electronics are used to provide key information on the system. It is ideal to use simple detection mechanisms that do not require complex electronics and optics to power and extract sensor signals. Complex excitation and detection circuitry are typically expensive in terms of the spatial requirements, power consumption, and manufacturing costs. Optical and pyroelectric sensors, for example, require externally powered devices (lasers, photodiodes) in addition to the usual signal processing circuitry. Many semiconductor sensors require elevated operating temperatures. For devices that will ultimately be embedded in and integrated with portable fuel cell components, it is not practical to consider devices that consume a lot of power. 5. There are numerous mechanisms in the interaction of Pd and hydrogen atoms that translate to changes in the Pd's electrical resistance. These mechanisms have the potential to be integrated together to compensate for deficiencies in one another, optimizing the overall response. Given the above considerations, a study of resistive H 2 sensors was performed-. Details of the study are described in the following chapters. A description of the theoretical characteristics of Pd in the presence of H 2 will be presented first (Chapter 1), followed by an account of the processes involved in fabricating Pd mesowire resistors (Chapter 1). A digression from this sensor work follows with a description of a test bench developed for testing fast transients of H 2 gas sensors (Chapter 1). This is followed by test results of sensors developed in this work (Chapter 1), obtained from a commercial gas sensor test station. The results chapter describes observations of changes and failure modes in Pd mesowire resistors upon exposure to H 2 . An analysis of changes in the electrical resistance of these sensors will also be presented. Finally, the significance and relevance of these results, along with recommendations for future work, will be discussed in the concluding chapter (Chapter 6). 10 Chapter 2 Theory and Simulation of Pd Mesowire Resistors 2.1 Resistive mesowire sensors Sensors are devices that respond to environmental variations with changes in their physical properties. As discussed in the introductory chapter, there is a wide variety of materials and structures that are sensitive to the presence of H 2 gas. The present study investigates the theoretical background relevant to the operation of resistive Pd sensors. 2.2 Basic sensor structure The basic structure of sensors we developed consists of thin Pd wires deposited across two contact pads on oxidized Si substrates, as shown diagrammatically in Figure 2.1. The resistance of these Pd wires can be measured by applying a potential difference across the contacts and measuring the corresponding current. A detailed description of the sensors developed in this work will be given at a later chapter devoted to sensor fabrication (Chapter 1). Top View Au/Cr Au/Cr t Pd wires Side View Pd Au/Cr contacts •SiO, Figure 2.1 Top and cross-section view of basic sensor structure. Sensors developed in this study are variations of this geometry. 1 1 2.3 Electrical resistance of Pd hydrides There is a large quantity of literature on the Pd-H2 system. It is well known that atomic hydrogen solubility in Pd is very high, and that spontaneous dissociation of H 2 molecules and chemisorption of hydrogen atoms at Pd surfaces occur readily. The absorption of hydrogen atoms into Pd causes physical expansion in the Pd lattice, causing changes .its electronic states and physical characteristics. Two phases of hydrides exists in the Pd-H2 system. The a-hydride corresponds to low concentrations of hydrogen atoms in Pd, while the P-hydride corresponds to high atomic hydrogen content and is characterized by large dislocation densities in the Pd host. Resistance measurements of bulk Pd in the presence of H 2 have been measured since the mid-nineteenth century. Several conventions were used to represent the hydrogen content in Pd, a traditional one being [H]/[Pd] atomic ratios, the concentration of hydrogen atoms in solid (Pd) solution. The electrical resistance of bulk Pd exposed to H 2 is traditionally described as a roughly linear function of atomic content. The slopes corresponding to different phases of the Pd hydrides differ from one study to another however [20]. ; Concentrations of H 2 gas at atmospheric pressure between 10% and 100% correspond to [H]/[Pd] ratios of 0.68 to 0.73, well into the (3-hydrides region. Reported slopes of normalized resistance (R/Ro) vs. [H]/[Pd] in the P region, on H 2 absorption, vary from 0.87 to 8.3 depending on the study [20, 95]. 2.3.a Pressure-concentration (p-C) isotherms for the Pd-H 2 system It is generally easier, however, to measure the H 2 concentration in the vicinity of the sensor rather than measuring the atomic content of the material. Empirical equations describing pressure-concentration (p-C) isotherms can be used to convert H 2 concentration in the sensor's surroundings to an estimate of the atomic [H]/[Pd] ratio. A number of studies have been reported in the literature that measure these isotherms, and exact numeric results vary depending on the method and the form of the material used (eg. Pd black, Pd wire) [20, 93, 94]. Despite some differences in reported plateaux pressures and a-P phase boundaries, several generalizations can be made. A It is generally agreed that H 2 absorption in Pd for small [H]/[Pd] ratios at room temperature follows a Sievert's Law relationship: [H]/[Pd] = S-p[H2]U2 (2.1) where [H]/[Pd] is the atomic concentration, S is an empirical constant and p[H2] is the partial pressure of H 2 . The region over which this relation is valid corresponds to the a-region of the Pd hydride. 12 As [H]/[Pd] concentrations enter the two-phase region, the (p-C) isotherm enters a plateau, with atomic concentration of hydrogen in solution increasing quickly over small increases in the ambient H 2 pressure. As hydrogen content increases beyond this o>P transition into the p region, [H]/[Pd] content increases much more slowly with H 2 pressure. The P-hydride isotherm can be approximated with the following equation: [H]/[Pd] = A + B\np[H2] (2.2) N where A and B are constants. The pressure at which the phase transformation occurs is a function of temperature and absorption history. It is generally agreed that hysteresis occurs in the absorption/desorption isotherms. A diagram showing the relation between the (p-C) isotherm and hydrogen content in Pd is shown in Figure 2.2. The presence of the hysteresis poses a limitation for hydride resistance measurements to identify H 2 gas concentrations corresponding to the phase transition. From the introductory chapter, one may observe that sensors reported in literature using pure Pd generally operate at low H 2 concentrations. Sensor studies at higher concentrations typically feature Pd alloys such as Pd-Ni, in which the P-hydride is suppressed [25]. 100 P r e s s u r e - C o n c e n t r a t i o n Isotherm (Not to scale) a. CM I a. o 0.1 Desorption -Absorption 0.1 0.2 0.3 0.4 0.5 H/Pd Atomic Ratio 0.6 0.7 0.8 Figure 2.2 Shape of the pressure-concentration (p-C) isotherm of hydrogen in Pd. The first rise corresponds to the a-hydride phase and follows a linear form whereas the second sharp rise is due to the P-hydride phase and follows a logarithmic form. The plateau corresponds to the a-p transition region and shows the effect of hysteresis. 13 Our study of Pd resistive sensors is interested in looking at the behaviour of H 2 in Pd at atmospheric pressure. This corresponds to a solubility limit of approximately [H]/[Pd]=0.73 [93]. The ratio of H 2 partial pressure to atmospheric pressure can be considered directly as the H 2 gas concentration in a H 2 /N 2 gas mixture. 2.3.b Resistivity vs. [H]/[Pd] Electrical resistivity measurements of Pd hydrides are typically normalized with the initial resistance (R0) of the Pd metal. Early studies of changes in resistance as a function of atomic content show ah almost linear relationship, piecewise for each phase. The slope of resistance over atomic content (R/Ro vs. [H]/[Pd]) rises less steeply in the phase transition than in the a region, and depending on the study, either levels out or continues rising in the P-hydride region [20]. More recently, Sakamoto et ah [95, 96] reported Pd resistance data on H 2 absorption with R/R0 vs. [H]/[Pd] slopes that change drastically between different phases of the hydrides, to the extent where it follows the form of the (p-C) isotherms (Figure 2.2) very closely. In their results, resistance increases sharply in the a region, hardly changes over the phase transition and increases sharply again in the p region. At 323K, the slope of R/Ro vs. [H]/[Pd] relation drops off again within the p region (between 40% and 100% H2), albeit not as level as over the o>P transition. On desorption, the flatter slope observed during absorption from 40% to 100% H 2 persists, causing a large hysteresis with subsequent desorption and re-absorption within the P and o>P transition region. The authors attributed this behaviour to P-induced lattice strains, as similar effects are observed with heavily cold-worked Pd [97]. The presence of a substantial'hysteresis will challenge direct application of resistive Pd sensors over the o>p transition. N Another recent study in the a-hydride region reports a correlation between changes in resistance and the surrounding H 2 pressure [98]. The data was approximated with a linear fit to the square root of the H 2 pressure, which corresponds roughly to a linear relationship with the atomic [H]/[Pd] ratio according to Sievert's Law (Equation (2.1)). Deviations from the trend were consistently observed however at both ends of the fit for all three sets of data given. 2.3.c Supersaturation Another effect observed in some Pd-H2 studies is the presence of supersaturated a-hydrides at the initial onset of the o>P transition. It was observed in some studies that a local maxima in pressure exists in the (p-C) isotherm, immediately before the a-P plateau [20]. This effect is found to be attributed to freshly annealed Pd samples, and is non-existent in cold-worked Pd samples or samples which have undergone previous a-P transformations [97]. Moreover, this supersaturation effect was found to account for variations in the 14 [H]/[Pd] ratios corresponding to the onset of the o>p transition, and changes the slope of the (p-C) isotherm in the a region. 2.3.d Magnitude of changes in resistance R/Pvo values for effects discussed in the above section varies from 1.0 to about 1.8 over gas concentrations between 0% and 100% H 2 , regardless of the variety of R/Ro vs. [H]/[Pd] slopes reported. Observed differences in various studies are best described with their reported slopes of the phase transition region. A number of groups claim increases in R/Ro of about 0.5 (>60% of the R/Ro range) [20], while another reports increases of less than 0.1 (-13% of the range) [95]. 2.4 Percolation mechanism A different mechanism affecting the electrical characteristics of Pd in the presence of H 2 is observed for semi-continuous Pd fdms and wire segments near their percolation threshold. Percolation threshold refers to the point where the first continuous path appears across a film or line of growing Pd clusters. The overall electrical resistance of a metallic material changes over this non-conductive/conductive transition. In the case of Pd-H2 systems, studies have successfully shown Pd mesowires [30] and films [33, 99] that detect the presence of H 2 based on this mechanism. Pd clusters formed via Joule heating of thin film Pd [33], and as-deposited Pd clusters on siloxane self-assembled monolayers [99], showed increased conductivity in the presence of H 2 . Pd clusters swell in size due to lattice expansion, directly generating conductive paths in some cases and narrowing inter-cluster gaps to lower the electron tunnelling barrier in others. The former effect is most evident around 1-2% H 2 as the a-P hydride transition occurs. The latter effect is dominant for lower H 2 concentrations, and is useful for reflecting small lattice changes within the a-region. Pd mesowires about 150nm in diameter were also shown to exhibit percolation effects [30, 31]. These wires were electrodeposited on highly oriented pyrolytic graphite and subsequently transferred onto cyanoacrylate for H 2 testing. In this case, aggregates and wire segments are formed after the wires' initial H 2 exposure, as hydrogen atoms desorbs from the Pd and the lattice contracts. Electrical conductivity is restored when the wire segments re-connect upon subsequent H 2 exposures. The effective resistance for an electron tunnelling across two points through a barrier is the inverse of the electron's transmission coefficient, and grows exponentially as the distance between these two points 15 increases for a constant electron potential. A number of groups have derived related formalisms and approximations to the conduction of electrons in semi-continuous media [100, 101]. It is sufficient for the purposes of this work to refer to physical observations of tunneling effects in the context of H 2 absorption into Pd metal. R/Ro values of 0.95 were reported for discontinuous Pd films about 1.6nm thick, exposed to 0.26% H 2 at room temperature [101]. Similar work on (smaller) monolayer-promoted Pd clusters showed comparable R/Ro values at approximately 0.025% H 2 [99]. j 1 The overall changes in resistance from percolation effects are much larger as H 2 concentrations increase, with reported film resistance down to half of their original values in one study for H 2 pressures up to 90% [33]. Examples reported in the mesowire studies showed R/Ro values down to 0.25 upon exposure to 10% H 2 [30]. 2.5 Temperature effects 2.5.a Temperature coefficient of resistance in Pd hydrides Temperature plays a significant role in the electrical characteristics of Pd. The melting point of Pd metal is 1828K [102], and the temperature coefficient of resistance for bulk palladium is approximately 3.8 x 10"3/K [20]. This coefficient is related to changes in resistance according to Equation (2.3). —L = arAT (2.3) Where ART is the change in resistance due to temperature effects, R0 is the initial resistance, aT is the temperature coefficient of resistance and AT is the change in temperature. v The temperature coefficient of Pd hydrides is a function of atomic content of hydrogen, increasing to ~ 4.2 x 10-3/Katthe a region and dropping off as hydrogen concentration increases, down to about 1.8 x 10"3/K at [H]/[Pd]=0.7 [20], or 100% H 2 . 2.5.b Pressure-concentration changes with temperature The increase in hydride resistance with temperature is offset by changes in the overall hydride content however. (p-C) studies generally agree on an upward shift and narrowing of the a-P transition plateau as temperature increases. Plateau pressure is reported to increase beyond 1 atmosphere by 433K [94]. This 16 translates to significant changes in the concentration and phase of the Pd hydrides present at a given H 2 pressure. 2.5.C Other temperature effects In the case of Sakamoto's study [96], which presented R/Ro vs. [H]/[Pd] relationships similar in shape to the (p-C) isotherm shown in Figure 2.2, changes in temperature between 323K and 433K during H 2 absorption causes changes in the height of the R/R0 vs. [H]/[Pd] plateau by up to 0.6 (75% of the range of R/R0 at atmospheric pressure). Little change in the width of the plateau was observed however. Plateau height jumps from about R/R0 = 1.15 to 1.75 between 323K and 348K, then slowly decreases as temperature continues to increase up to 433K. A diagram illustrating this behaviour is shown in Figure 2.3. Reported R/Ro on Hz absorption (Not to scale) Plateau height decreases from 323K to 433K i I 1 /1 / 1 r l i ji r 1 Plateau height increases from 323K to 348K 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 H/Pd Atomic Ratio Figure 2.3 R/R0 behaviour on temperature as reported by Sakamoto et al. Changes in plateau resistance are not monotonic with temperature. The thickness of the Pd metal plays a role in its temperature coefficient, dropping off as film thicknesses decrease to less than lOnm [103]. Temperature also affects percolation films, where resistances (again) vary non-monotonically with temperature [101]. These temperature effects are exemplified in the variety of temperature dependencies observed from a number of Pd resistive sensors [36, 29]. 2.5.d Temperature modelling Since the Pd structures studiedin this work behave as resistors across an applied voltage, it is instructive ,to estimate the extent to which their temperature increase during operation. To this end, a two-dimensional 17 COMSOL Multiphysics simulation was done to model the heat profile along a Pd mesowire 200nm wide, lOOnm thick and 60um long resting on thick Si02. Resistivity is set up as a function of resistance in the models. The initial wire resistance used in the simulation is 325Q, and is estimated from where p is the resistivity, approximately 10.8 x 10" (Q, m) for bulk Pd at 300K [102], L is the length and A is the cross-section area of the modeled wire. This simulation was constructed and set up by Kevin Stanley at the National Research Council of Canada - Institute for Fuel Cell Innovation (NRC-IFCI). The model is shown in Figure 2.4 with labelled regions in the inset (not to scale). 171x10 4 COMSOL Multiphysics model 5 4 250urn ^ Air ^ 1 ^ 30um 30um X 3 Air Pd JOOnrX i Oxide 600nrrY 2 Si 7 1 0 250um 001 i 1 2 3 4 s m*io4 Figure 2.4 Heat Transport model of Pd mesowires with inset (not to scale) showing model structure. The Pd wire is mirrored across the boundary to the right, and is half the length of the simulation by symmetry. Hydrogen content is not taken into account in this modelling study. Parameters modelled include: 18 1. Temperature changes from Joule heating, where the heat source is the applied voltages across the wire, 2. Resistance changes from temperature differences (using a thermal coefficient of 3.3 x 10"3/K); 3. Radiative, convective and conductive heat dissipation. Plots of the temperature profile along the wire with voltages stepping from 0.5VDC to 7.0VDC are shown in Figure 2.5. From the simulation, the estimated voltage that brings the wire to its melting point is about 6.8VDC at a maximum temperature of 1825K. Temperature along Pd wire 0.5V — 3 — 1.0V 1.5V 2.0V - * - 2.5V * - 3.0V 3.5V 4.0V — 3 — 4.5V 5.0V <•>•• 5.5V —X— 6.0V 6.5V 7.0V 0.5 1 1.5 2 2.5 3 Arc-length (m, edge to mid-span) x i o " 5 Figure 2.5 Estimated temperature along Pd wire at various voltages. Modelled properties include conduction, convection and thermal effects on resistance. Thermal coefficient of 3.3 x 10"3/K is used. At excitation voltages below 0.1 VDC, temperature variations within the Pd wire is less than 0.34K. The temperature of the wire at mid-span increases from 300K to 301.7K as the applied voltage approaches 0.1 VDC. This is shown in Figure 2.6. 19 302.5 302 & ^301.5 3 E E 301 Temperature along Pd wire 300.5 300 --0- «-g _j r# —:— ^ i i i i ; ^0.02V - a - 0.04V -•-0.06V 0.08V -*-Q.10V * 0.12V 0 0.5 1 1.5 2 2.5 3 Arc-length (m, edge to mid-span) X 10 5 Figure 2.6 Estimated temperature along Pd wire at voltages varying from 20mVDC to 120mVDC Effects of a changing temperature coefficient on temperature was investigated and a decrease of 0.006K was noted at an applied voltage of 0. IV over the coefficient increase from 1.8x10"3/K to 4.2 x 10"3/K. By far the dominant course of heat dissipation in our Pd wires is through conduction to the oxide layer. It is likely, therefore, that localized heating will occur in the event that portions of the wire delaminates from the oxide layer. A similar simulation was done for a structure in which the layer beneath the wire is sectioned into oxide and air partitions to simulate the presence of a delaminated wire section. A diagram of the model is shown in Figure 2.7, and simulated temperatures from a voltage sweep of 0.2VDC to 1.0VDC are shown in Figure 2.8. 30pm A ' r 30pm Air Pd Oxide Air Oxide 40pm M 1*4 • gj 12.5pm 7.5pm 250pm 100nm 600nm 250pm Figure 2.7 Model structure of Pd mesowire with a delaminated section (not to scale) for heat transport simulations. 20 Temperature along delaminated Pd wire Figure 2.8 Temperature simulation of a delaminated Pd wire (over a 12.5um air gap). The structure of this simulation was illustrated in Figure 2.7. This figure clearly demonstrates the deleterious effect of delamination on sensor performance. The simulations above consist of a 12.5um air gap beneath the Pd wire, located lOum from the edge of the wire. Given that Pd is prone to delaminate to some extent in the presence of H 2 , even moderate voltages applied to probe the wire sensors may cause a substantial temperature effect in the sensors' response. Figure 2.9 shows plots of the same delaminated wire model for a range of smaller voltages. Applying 0.1V across the wire will result in an estimated temperature change of about 1 IK for a delaminated length of 12.5(rm. Temperature along delaminated Pd wire 0.02V 0.04V 0.06V 0.08V ^O.IOV ..... 0.12V 0 0.5 1 1.5 2 2.5 3 Arc-length (m, edge to mid-span) x i o " 5 Figure 2.9 Temperature simulation of a delaminated Pd wire (over a 12.5um air gap) for voltages between 0.02VDC and 0.12VDC. 21 As discussed earlier, temperature increases will generally cause an upward shift in Pd and Pd hydride resistance. For a temperature increase of 1 IK, an estimate of /\RT/RQ — 0.04 at 0% H 2 can be obtained (Equation (2.3)). Changes in resistance due to variations in the hydrogen solubility, however, are less predictable. These modelling results are important for selecting a suitable excitation voltage that provides good signal-to-noise while maintaining reasonable uniformity in sensor temperature. 2.6 Anticipated observations Given the multitude of mechanisms occurring in Pd-H2 systems that directly affect their electrical characteristics, it is useful to summarize some anticipated changes in resistance we expect to observe in the sensors developed for this thesis. Relative resistance increase (R/Ro) of up to 1.8 is expected when sensor wires are exposed to H 2 concentrations between 0% and 100% due to hydride resistance changes. This change in resistance is expected to be non-linear with H 2 gas concentration, and the shape of the relationship is expected to be different for H 2 concentrations corresponding to different phases of the Pd hydrides. A hysteresis is expected to be present over the a-P hydride transition, and some irreversible changes in resistance may be observed due to supersaturation and cold-work-like effects. For sensors in which a large number of defects are present, resistance changes in the form of large step decreases may also occur due to percolation effects. Temperature effects onthe Pd resistance are expected to be negligible, as sensors will be probed with voltages less than lOOmV to ensure reasonable temperature uniformity. Test results obtained from sensors developed in this thesis will be evaluated with the above theoretical considerations in Chapter 1. 22 Chapter 3 Fabrication of Pd Mesowire Resistors A description of the sensors developed in this thesis is presented in this chapter. Several sensor designs were fabricated, all of which consist of Pd wire structures spanning across two metallized contact pads on an insulator. A diagram showing the top view and cross-section of the general structure of these sensors was shown earlier in Figure 2.1. Actual sensor designs fabricated are variations of this basic structure. 3.1 Fabricated sensors PMMA (Polymethylmethacrylate) is a polymeric material that is sensitive to electron bombardment. It is a common electron-beam (e-beam) resist that hardens (crosslinks) upon exposure to high electron dosages. At moderate dosages, PMMA decomposes into chains of lower molecular weight fragments and becomes soluble in certain solvents (developers) in which it was originally insoluble. PMGI (polydimethylglutarimide), similarly, is an e-beam sensitive resist that uses a different developer and remover than that of PMMA. PMGI is often used as an undercut layer for liftoff processes. An e-beam lithography and liftoff process consists of A) coating a substrate with e-beam resist and using a beam of electrons to create patterns on the resist, B) developing the resist to obtain 'windows' that clears through the resist layer to the substrate, C) depositing a layer of material on the substrate and D) removing the resist layer, leaving behind window patterns of the desired material. A diagram illustrating the general idea behind the lithography and liftoff process is shown in Figure 3.1. A B C D Figure 3.1 Lithography and liftoff process: A) Coating with resist and patterning with electron beam; B) developing exposed resist; C) depositing the desired material for the pattern and D) removing the resist layer and leaving behind the patterned layer. Our present study uses the above process to fabricate H 2 sensors. The earliest sensor design consisted of about 30 parallel wires spaced approximately 1pm apart. None of these samples withstood the PMMA development process. Subsequent designs are shown from Figure 3.2 through Figure 3.5. In these designs, spacing between wires is wider, and two additional sensor designs were fabricated. The first design, shown in Figure 3.4, is a symmetric U-shaped structure that may potentially produce a gap across the Pd traces close enough 23 together to form a mechanical switch from lattice expansion. The second is a platform structure, shown in Figure 3.5, which was originally designed to investigate crosslinked P M M A as a negative resist. A s wi l l be discussed at a later chapter, there is insufficient data to distinguish differences in H 2 response among these sensors designs. SE 08-May-06 061243 WD13.1nmi 2 0 . O k V x l ^ V °25UIII Figure 3.2 Typical early design sensor showing many-wire Pd array across Au/Cr contacts approximately 65um apart. S4700 5.0kV 6.1mm x1.10kSE(U) 3/21/07 ' 50.0um ' Figure 3.3 Later design sensor with five-wire Pd array. These sensors have improved liftoff yield and higher overall resistance than many-wire arrays. 24 Figure 3.4 Sensor wi th U-shaped symmetr ic structure. Instead of pat terning gaps along a wire by tu rn ing off the e-beam, these samples are patterned by having the e-beam travel ha l f of the gap and then re turning to the same side. A U shape was used on each side (symmetr ical ly) to lessen overexposure of the resist at the tu rn ing points. At tempts at generating gaps in this manner were unsuccessful. F igure 3.5 F ive -wi re device sensor wi th crossl inked polymer pla t form. The presence of imperfections along these wires have less effects on the overal l resistance than in other sensors developed. The above sensor structures are fabricated via e-beam lithography and liftoff of Pd. These structures are patterned between and spans across etched Au contact pads on oxidized Si substrates. The fabrication process of these sensors will be described in the following section. 25 3.2 Process description 3.2.a Substrate preparation 50.8mm (2") p-type Si(l 11) wafers about 279um ± 25u.m thick (Polishing Corporation of America) are RCA cleaned and thermally oxidized at the Institute of Micromachine and Microfabrication Research at the SFU School of Engineering Science. The oxide layer for samples fabricated vary from 200nm to 600nm in thickness and serves as an insulating barrier on which the sensors are built. A thin layer of Cr is sputtered briefly onto the oxidized wafer as an adhesion layer for the subsequent Au deposition (SFU). Thicknesses between 174.5nm and 213.5nm are measured for the Au layer using a travelling-probe profdometer (Alpha-Step) at the UBC Nanofabrication Facility. The metal layer is photolithographically patterned and subsequently etched in aqua regia to produce contact pads for the sensor. The earliest version of these contacts consists of pairs of 1.8mm x 1,975mm rectangular pads spaced 80um apart. Later versions are smaller and are designed with alignment marks to facilitate sample navigation during the e-beam writing process. An SEM image of the latest version of the sensor contacts is shown in Figure 3.6, and the corresponding mask used to fabricate them is shown in Figure 3.7. Figure 3.6 Au contact pads of a sample, shown with bonded Au wires. Crosses are alignment marks to ease navigation of electron beam during sample exposure. 26 Figure 3.7 M a s k for photol i thography of A u / C r contact pads for fabricated sensors. Pad sizes are • l m m . The patterns are rotated to al ign wi th the cleave planes of S i ( l l l ) wafers. l m m x Dots between the contacts shown in the SEM image originate from imperfections on the acetate masks as received from the print shop (Digital People Inc.). Resolution of the printer was set at 3600dpi, which corresponds well to the edge roughness observed (<10um) and is sufficient for our work. Masks are printed as positives as they are used directly in the mask aligner to expose portions of the photoresist to be etched. Patterns are rotated to align with one of the cleave planes of Si(l 11) wafers. The earliest samples fabricated were done on Cr sputtered contacts. Subsequently, Au films on a Cr adhesion layer were used instead of Cr to permit bonding with conventional ceramic packages. About half of the 27 wafers prepared this way are subsequently spin-coated with a 200nm or 400nm layer (4,000rpm and 2,000rpm for 45s respectively) of polydimethylglutarimide (PMGI SF5 from MicroChem) and heated to a temperature between 458K and 473K on a hotplate for 5min. Fabrication work up to this point, aside from design specifications, film characterization and lithography mask development, is performed by Eva Czyzewska of the NRC-IFCI at the SFU microfabrication facilities. Both PMGI-coated and bare substrates are subsequently spin-coated at UBC with a 200nm layer of polymethylmethacrylate (PMMA C4 MW950k from MicroChem Corp). Existing data for the thickness as a function of spin rate was not available for PMMA on PMGI polymer. Cleaved cross-sections of bilayer samples confirm that the resist thicknesses are very close to expected values for spinning on bare Si. A cross-section SEM image of the bilayer resist is shown in Figure 3.8. ^ M l l Figure 3.8 B i l aye r resist cross-section on S i 0 2 . The upper layer is P M M A , the middle layer is P M G I and the bottom is S i 0 2 . Thicknesses of the resist layers are about 200nm each. Wafers are subsequently diced into dies of 3 to 4 samples each, about 6mm x 8mm in size. Unused dies are kept in wafer boxes stored in a lithography room to minimize resist degradation. Selected dies (chips) are individually exposed in an e-beam lithography SEM at UBC (JEOL 840 with NPGS e-beam writing control software). 28 3.2.b Sensor patterning Patterning the chips typically requires navigation of an e-beam over the chip, thus exposing a path. Care must be taken to prevent accidental exposure of critical areas on the chip, as well as unexpected electrical 'shorts' across different sections of the sample. Patterns with an overall size of approximately 90um x 90um are'drawn and saved as DesignCAD fdes. Pattern settings such as the pattern name, electron dosage, line spacing, line widths and beam currents are entered in the Nanometer Pattern Generation System (NPGS) program. NPGS calculates the beam 'dwell time' for each point based on the entered data, and controls the e-beam accordingly. For this study, dosages are entered as "line doses" with centre-to-centre spacing of 2.98nm. Centre-to-centre spacing refers to the distance between exposure points making up a line, and should ideally be set to about half of the smallest feature size. One of the original goals of this work was to introduce gaps on the order of several nm along the wire patterns. As will be shown at a later paragraph, the resolution needed to achieve these gap sizes using direct writing is likely beyond the capability of the instrument. The small centre-to-centre spacing was kept in later work for consistency. PMMA and PMGI are both electron-beam sensitive resists. There are a number of studies reported in literature that investigate electron-PMMA interactions. Energy dissipation of incident electrons on PMMA peaks at a characteristic depth from the surface depending on the electron flux, resist density and the incident beam potential [104, 105]. An accelerating voltage of 20kV was used for patterning our sensors, which provides a fairly uniform beam exposure throughout the resist layer(s). The e-beam is first optimized on reference Au clusters, after which sample heights are adjusted to the working distance of the e-beam. Some SEM and NPGS program user settings are listed in Table 3.1 and Table 3.2. Table 3.1 J E O L 840 S E M Settings Accelerating Voltage 20kV Working Distance 15mm Filament Current 240uA Emission Current 50uA , Physical Aperture 4 Probe Current 3 x 10""A Measured Beam Current (Faraday Cup) lOpA Chamber Vacuum < 1.0 x lO-'torr 29 Table 3.2 NPGS e-beam lithography program settings Magnification lOOOx (90um x90um writing area) Center-to-center Distance 30A ( Line Spacing 500A Coarse Beam 1A Measured Beam Current As measured at Faraday Cup Line Dose 0.5 to 4nC/cm Electron dose in lithography refers to the amount of electrons per spatial unit that a sample is subject to, and is typically reported in literature as charge per unit area (eg. C/m2). Doses entered in the NPGS program can be a point, line or area dose (fC, nC/cm, and uC/cm2 respectively) while the program itself calculates the corresponding dwell time of the beam at each point. Equivalent area dose calculated by NPGS for the lithography settings used in the present work are tabulated in Table 3.3. Table 3.3 Equivalent line and area dose from NPGS program Line Dose Area Dose (NPGS) 0.5nC/cm 100u€/cm2 1 .OnC/cm 200uC/cm2 3.0nC/cm 600uC/cm2 lO.OnC/cm 2000uC/cm2 Hoole et al. [106] observed and estimated an area vs. line dose relationship described by Equation (3.1), assuming a Gaussian distribution for each point along a line. D line (3.1) C7V7T where Darea and Du„e are the equivalent area and line dose, and o the Gaussian width. Using the values listed in Table 3.3, expected exposed PMMA linewidth can be estimated by the full width at half maximum (FWHM) of a Gaussian curve, approximately 67nm in our case. Given that FWHM = 2V21n2cr (3.2) 30 The calculated 'expected' linewidth is a clear indication that patterning gaps on the order of several nm along the wires is not possible with the present setup. Finer features may be achievable on the instrument at a higher magnification at the expense of a smaller pattern writing area. 3.2.C Developing the resist Our observed line widths of fully developed lines exposed at 3nC/cm of electron dose are on the order of 150nm. Finer lines were observed earlier in the development process, though samples were generally developed for a longer time to ensure the PMMA is cleared through to the substrate. Later trials with development times show that PMMA is indeed cleared through much earlier. As such, it is quite conceivable that existing equipment may achieve developed line patterns on the same order of magnitude as the FWHM value estimated earlier. A thinner PMMA resist layer (such as PMMA in anisole instead of chlorobenzene solvent) may also help to produce thinner lines. Off-focusing of the electron beam also plays a role, since the focus plane of each sample is currently estimated from focusing on adjacent Au edges. We expect this effect to be small compared to the line-widening effects from extended resist development, as our focusing method was checked against spot focusing on the PMMA on several occasions and the resulting sample height from both methods were in good agreement. The present development process for sensor fabrication uses 1:3 ratio of MIBK:IPA (methyl isobutyl ketone: iso-propyl alcohol) for developing the PMMA resist. It is well known that moderately-exposed PMMA becomes soluble in MIBK due to degradation of the polymer into fragments of lower molecular weight. Exposure to much higher doses, however, causes the polymers to crosslink and become a very tough material that typically requires plasma cleaning to remove. Confirmation that the resist is working as a positive resist with our e-beam settings is clear after Pd deposition, where developed lines can be seen in the form of grooves. An SEM image showing partial liftoff of a Pd pattern is shown in Figure 3.9. 31 Figure 3.9 Partial Liftoff of Pd from e-beam exposure of PMMA. This shows PMMA working as a positive resist at moderate electron-beam dose. One set of samples was fabricated using PMMA both as a positive and a negative resist. Line patterns were exposed at the usual dosage, after which a vertical line with a much larger dose was written down the centre of the pattern, forming a platform at the centre of the gap. An example of this structure was shown earlier in Figure 3.5. Exposed PMMA samples are sonicated in the MIBK:IPA solvent mixture for 3-4 minutes. Bilayer resist samples use PMGI as the 'undercut' layer. These samples are further developed with sonication in 2.2% tetramethylammonium hydroxide (TMAH, Rohm & Haas Electronic Materials MF319). As described in datasheets from the supplier, the development rate of the PMGI layer depends largely on the amount of leftover solvent in the resist, and is consequently dependent on the heat treatments and storage history of the sample. As a result, no correlation was observed for development rates across samples on different chips. It is clear, however, that for a given chip, the undercut development rate of the PMGI layer increases with electron dosage and/or the size of the PMMA opening. It was observed that the structural integrity of the PMMA is compromised as undercut cavity widths grow beyond lum. An example of patterned PMMA delaminating from the substrate is shown in Figure 3.10. It was concluded that the PMGI development progress should be monitored to make sure resist structures remain intact. 32 Figure 3.10 Effects of overdeveloping the PMGI undercut layer. The PMMA layer becomes entirely delaminated from the substrate. Development progress for a sample with bilayer resist is shown in the following images. E-beam dosage for this sample is 4.0nC/cm, and is chosen for the best image contrast. Similar progression is seen for samples exposed with dosages down tol .OnC/cm (in 1 .OnC/cm decrements). Exposure at 0.5nC/cm was not sufficient to completely clear samples with a 200nm layer of PMMA developed in 1:3 MIBK:IPA for 5minutes. Progression of MIBK development of PMMA exposed at 4nC/cm 50nm Figure 3.11 PMMA development: IminOOsec, 1:3 MIBK:IPA. Very little development is observed at this point. 33 Figure 3.12 P M M A development: 2min00sec, 1:3 M I B K : I P A . Fa in t traces corresponding to developed lines begin to appear. 50nm Figure 3.13 P M M A development: 3min00sec, 1:3 M I B K : I P A . 50nm Figure 3.14 P M M A development: 4min00sec, 1:3 M I B K : I P A . Patterns in P M M A are now easily dist inguishable. 34 Figure 3.15 P M M A development: 5min06sec, 1:3 M I B K : I P A . E n d of the M I B K development process. TMAH development of PMGI layer following development of PMMA Figure 3.16 P M G I development: 0min20sec 2 .2% T M A H . Patterns begin to broaden. « M 50nm Figure 3.17 P M G I development: 0min40sec 2 .2% T M A H . Cavi t ies under the P M M A layer (dissolved P M G I ) begin to appear. 35 Figure 3.18 P M G I development: IminOOsec 2 .2% T M A H . M i l d delaminat ion (circled) of the P M M A begins to appear. 50nm Figure 3.19 P M G I development: lmin20sec 2 .2% T M A H . Fu r the r widening of P M G I cavity. In the above images, the gradual emergence of written patterns can be observed. The bright regions on the two sides of each image are edges of the Au contact pads. The edge roughness is attributed to the mask used to pattern these contacts, which is simply a piece of acetate film printed at a local print shop. MIBK development appears to be generally consistent with e-beam dosages, and the development / line-widening rate is greater for lines written at higher exposure. A control piece was cleaved off and kept after each development process, and a method was developed to cleave samples through the 60um gap in order to view the cross-sections. Unfortunately, even when cleaves run successfully through the gap between the contacts without extensive debris being generated, delamination and peeling of the resist layers occurred. Attempts at grinding samples held in epoxy with standard metallurgical techniques were also largely unsuccessful due to the excessive debris generated at patterned areas. It is possible that air trapped in the 36 small channel features was not completely removed before the epoxy cured, leading to structural damage during the grinding and polishing process. Images of samples cleaved through the pattern gap are shown in Figure 3.20 (top view) and Figure 3.21 (side view). Figure 3.21 PMMA Resist peeling off is clearly visible from sample cross-sectioned area. There was one sample that produced three consecutive, successful cross-sections in which the progression of PMGI cavity sizes as e-beam dosage varies can be observed (despite PMMA peeling). An SEM image of one 37 of these cross-sections is shown in Figure 3.22, followed by a chart beam dosage increased. showing the increase in cavity width as e-Figure 3.22 Cross-section of developed P M M A / P M G I bi layer . The layer to the left is P M M A , which can be seen wi th a large flap/extension left behind from the cleaving process. The layer wi th the cavity is the P M G I layer, sandwiched between the P M M A and the S i 0 2 . Width of developed PMGI Cavity 1 0 0 0 i Dosage (nC/cm) Figure 3.23 C h a r t showing the wid th of the developed P M G I cavities as a function of electron dosage. O n l y a few data points are available because of difficulties in obta ining intact cross-sections of polymer layers. 38 3.2.d Sputtering and liftoff Exposed and developed chips are brought to SFU Micromachining facilities where a 200nm ± 30nm thick Pd layer is sputtered on. Thicknesses are measured from 'control' glass slides that were sputtered at the same time. Measurements were made using an Alpha-step and a Talysurf profilometer (located at UBC and NRC-IFCI respectively), and vary from 170nm to 235nm. One series of bilayer resist samples were sputtered with a thicker layer of Pd (measured to be approximately 290nm ± 26nm). Liftoff for samples with a single layer of PMMA resist is done by sonicating samples in acetone for about 3minutes. Bilayer PMMA/PMGI resist samples are first soaked in l-methyl-2-pyrrolidinone (Shipley Remover 1165) for 30minutes, then briefly sonicated in the Remover and rinsed in DI water. Problems were encountered during the liftoff process for a number of samples in which residue resist layers remain despite extended immersion in the remover solvent. Contamination is suspected to be responsible as the same protocols at a different site (UBC) with dedicated glassware for the liftoff process did not encounter the same problems. Examples of successful sensor structures fabricated were shown at the beginning of the chapter in Figure 3.2 through Figure 3.5. Gaps directly patterned along wire structures could not be seen in the resist during development, nor were they visible after the Pd liftoff process. A comparison of post-liftoff Pd wires shows a marked difference between wires fabricated with a single PMMA layer resist and those fabricated with PMMA/PMGI bilayer resist. An example of each is shown in Figure 3.24 and Figure 3.25. Figure 3.24 Pd wires after liftoff from single layer resist 39 Figure 3.25 P d wi re after liftoff from bi layer resist. P d wires fabricated from the bi layer process are thinner than wi th the single layer P M M A process. Figure 3.24 and Figure 3.25 are tilted projections of Pd wires fabricated from PMMA-only liftoff and PMMA/PMGI liftoff respectively. The former yields wires that are rougher and thicker (close to the full deposition thickness). In single-layer PMMA samples, removal of the PMMA layer causes the Pd fdm to fracture at the wire edges (as sputtering is not directional). There are a number of samples where entire patterns were removed, presumably when the tensile strength of the fdm overcomes adhesion of the Pd fdm to the SiC»2 substrate. From the processing point of view, liftoff from a single layer resist at feature sizes down to lOOnm is not a reliable process if a non-directional deposition method is used. Later samples are fabricated with bilayer resist. Pd wires deposited through bilayer resist windows are thin and relatively wide. These wire structures have a very smooth and gradual height profde. Liftoff of the bilayer samples originally appeared to have removed all patterns when viewed through an optical microscope! A more careful inspection of the samples showed that there were indeed Pd structures remaining. Energy Dispersive X-ray spectroscopy (EDX) measurements were made to confirm the presence of Pd (Hitachi S3500N at NRC-IFCI) and are shown in the following Figures. 40 Figure 3.26 Secondary Electrons image of a thin P d wi re sputtered through bi layer resist. F igure 3.27 Elec t ron counts associated wi th S i obtained from E D X measurements. Sl ight ly lower S i count density is observed across the lower half of the image corresponding to the location of the P d wire . F igure 3.28 Elec t ron counts associated wi th P d obtained from E D X measurements. Pd is present as a very faint bar of higher count density across the lower ha l f o f the image. These measurements prompted further character izat ion of the bi layer samples wi th a field emission scanning electron microscope ( F E S E M ) and an atomic force microscope ( A F M ) , wh ich determined unambiguously the presence of the P d wires as a continuous thin film. 41 Subsequent investigation with a Field Emission Scanning Electron Microscope (FESEM) produced better quality images. It was determined that these wires are intrinsically difficult to image due to the lack of a clear Pd / oxide boundary. The wires' height is believed to be self limiting from closure of the PMMA windows as sputtered Pd accumulates at the window edges. Top views of Pd wires fabricated with single-layer and bilayer resist, obtained from a high resolution FESEM (UBC Bioimaging Facility), are shown in Figure 3.29 and Figure 3.30 for comparison. S4700 5.0kV 6.1mm x90.0k SE(U) 3/21/07 i i I i T 500nm Figure 3.29 FESEM micrograph of a Pd wire on oxide fabricated with single-layer resist. Figure 3.30 FESEM micrograph of a Pd wire on oxide fabricated with bilayer resist. 42 The Pd height profde of a bilayer sample was obtained using an Atomic Force Microscope (AFM). This profde was obtained by Andras Pattantyus-Abraham from UBC Physics and Astronomy. A plot of the wire cross-section is shown in Figure 3.31. 0 i r 500 100 150 200 • Width (nm) 250 300 Figure 3.31 Cross-section of Pd mesowire sputtered through bilayer resist. Fitted line follows the Gaussian form given by W0+W,*exp(-((x-W2AV3)2). The profde is Gaussian and is fitted with the function h = W,+Wx-e x-W2 (3.3) where h is the height, Wn are constants and x is the lateral distance across the wire. A fitted curve is shown as a continuous line on the same plot. The height is approximately 90nm and the FWHM width is around 260nm. Best-fit parameters used to calculate these values are listed in the following Table. Table 3.4 Gaussian best-fit parameters for Pd wire cross-section. Constant Value w0 -0.32 W] 91 W2 1400 w3 150 43 Edge-to-edge widths for wires from bilayer resist samples are obtained from FESEM images. The image contrast had to be manually adjusted to identify the threshold where Pd is detected. These widths, as expected, increase with e-beam dosage. An example of these images is shown in Figure 3.32, and a summary of these widths is provided in Figure 3.33. i i i i i i i i i i i S4700 5.0kV 6.1mm x90.0k SE(U) 3/21/07 SOOnm Figure 3.32 T o p view of P d wire deposited through bi layer resist. Sample was exposed wi th l . O n C / c m electron beam dose, and subsequently developed in 1:3 M I B K : I P A and M F - 3 1 9 for 5m06s and lm20s respectively. C5 = 1 H 0.8 •g c E o c T J D_ 1.2 0.6 0.4 Pd width vs Dosage 2 3 Dosage (nC/cm) Figure 3.33 Sputtered P d wi re widths wi th increasing e-beam dosage. Wid ths are normal ized wi th the measured wid th o f wires exposed wi th 4 .0nC/cm electron dose. 44 The performance of some of these sensors will be described in detail at a later chapter. The thicker sensors fabricated with single-layer liftoff tend to delaminate readily in the presence of H 2 . Delamination sometimes leads to localized heating and causes sensor failure. The thinner, bilayer liftoff sensors are more robust, but have sometimes been observed to cluster and form disconnected beads upon exposure to H 2 . These results will be described in Chapter 1. 3.2.e Packaging Chips that are successfully fabricated are mounted onto 24pin ceramic DIP packages (Global Chip Materials) using conductive double-sided tape. Contact pads on the samples are gold-bonded to bonding pads using a wedge wirebonder (WestBond 7400A) at the UBC Nanofabrication Facility. Figure 3.34 shows a photograph of two packaged samples, ready for H 2 testing. 3.3 Sensor testing Sensors fabricated in the manner described above are tested in H 2 gas by measuring the resistance across the sensor contact pads. It was generally observed that sensors fabricated using PMMA/PMGI bilayers produced better quality results - both in terms of mechanical stability and electrical signal-to-noise. Results of these tests will be discussed in Chapter 1 Figure 3.34 Packaged, bonded chips for sensor testing. 45 Chapter 4 Development of a High Concentration H 2 Test Bench 4.1 Application requirements A facility for testing high-concentration H 2 sensors is ideally one where H 2 is either isolated from oxidants and sources of heat, or sufficiently diluted and purged quickly to ensure that explosive or flammable mixtures will not build up. The equipment should provide accurate measurement and control of gas concentrations, and allow gas mixtures to be tested. Pressure, humidity, temperature, vibrations, flow velocities and the presence of other chemical species are key environmental parameters that affect sensor response. H 2 is known to cause embrittlement in numerous materials. Materials for a H 2 test facility should therefore take into consideration its chemical compatibility with H 2 , and the mechanical or structural function of each component. 4.2 Description of the facility A H 2 sensor test bench was designed and assembled to provide accurate control over gas concentrations with minimal system response time. Initial sensor test results have been obtained on this test bench, but are mostly limited to that of a few previously characterized Pd-MOS capacitor sensors. The development of this apparatus was nonetheless a major component of the thesis work, and is thus described here for completeness. At the present time, the test bench controls the composition of H 2 /N 2 mixtures accurately where N 2 is present as a trace gas. The test bench is divided into 3 modules that can be used independently of one another: the gas (mixing) manifold, the test chamber (TC), and the data acquisition electronics (DAQ). 4.2.a Gas manifold The gas manifold module takes N 2 and H 2 gases from the building supply and routes them to a network of mass flow controllers (MFCs). A valve panel downstream from the MFCs allows the user to selectively mix the monitored gases in two separate lines, LI and L2. These lines afe electrically controlled downstream with solenoid valves in which one of either LI or L2 is open to the test chamber (TC) at any one time. This line-switching mechanism is designed to allow gas mixtures to stabilize before being supplied to the test chamber, and warrants accuracy of the system response time and gas concentration. A separate line connected to N 2 and compressed air is routed to the test chamber directly through a high flow rate MFC for test chamber purging (up to 50 Standard Litres per Minute (slpm)). A schematic of the gas manifold and test chamber modules is shown in Figure 4.1. 46 V TO e re a S3 cya 3 3 | Q. BS 3 a. re sr S3 O 3" re 3 S3 re' 3" ST re C re 3 re ET L E G E N D J\j Check Valve |^ j Ball Valve (lever) Gas Filler •ifi 3-Way Ball Valve Gale Valve Junction Flash Arrester ] Cap 0 Thermocouple fpl Pressure Tranaduc'ei.Gauge (MFC| ^ a s s ^ o w Conlrdler . A B B R E V I A T I O N S Pressure Regulator TC Tesl Chamber RV ReliefValve ReliefValve PV Purge Valve PRV Purge ReliefValve Solenoid Valve II,V, NjV Hydrogen, Nitrogen Venl Valves NC.NO Normally Closed, NormallyOpen 3-Way Solenoid Valve L1V.L2V Line 1, Line 2 Venl Valves (Co EXHAUST! L1S.L2S Solenoid Valves, electrically connected loswap either L] or L2 to L1NE0UT or EXHAUST March 12,2007 manifold schematic v4.dwg B E N C H M A N I F O L D (RACK) (RACK) II,JN RV2 JPPLY PV1 ^ & I - H & — a 1 II, SUPPLY PV1 A > (RACK) N, SUPPLY •ni l—o N.1N RVT 0 (RACK) (RACK) PURGE N,PURGE PV5 Q P (RACK) AIR SUPPLY H,MAiN SNC NC T 0, MAIN RELIEF RESET N,l 0, M:. r-tir--1(3)1-,, m U V LIS LINEOUT RA™ T i A T TNC IIUMIDiFIER r^jn (I! T 1 EXHAUST PURGE -|MFC| j\J-TEST" C H A M B E R TC OVERPRESSURE VENT CET— rH©H TC PURGE TESTOIAMBER - K M TC RELIEF RESET L^-T>M TC EXHAUST I 4.2.a.l Supply to MFC controllers Main gas shutoff valves are provided by the building's gas supply racks. Present gas services to the supply rack include H 2 , N 2 , compressed air and domestic water, along with an actively ventilated copper vent for combustible gases and a separate vent for oxidants. Provisions (additional labelled lines) exist for other gases such as 0 2, CO, C0 2 , CH 4 and Heliox8. These gases are not presently used in the test bench but are readily available to the system if required. A three-way valve is installed at the rack connecting N 2 supply to the H 2 lines to facilitate system-wide purging. All lines that may potentially carry H 2 (such as the H 2 SUPPLY lines to the test bench and the exhaust lines) are stainless tubing or grounded, flame-resistant hose. N 2 supply lines are connected via Teflon tubing. Flexible supply and exhaust lines allow for some mobility for the test bench during installation and servicing. All other tubing, fittings and wetted components used in the gas manifold are made of brass, stainless steels suitable for H 2 service [107], and H 2 compatible sealing materials such as EPDM, VITON and TEFLON [108, 109]. The test bench uses Swagelok® fittings and valves, Burkert solenoid valves rated for potentially explosive environments, and mass flow controllers from Advanced Specialty Gas Equipment. Building supplied gases are first passed through step down regulators and relief valves to limit pressure throughout the test bench to below 275.8kPa (40PSI). An emergency (push button) shutoff solenoid valve is installed in the H 2 supply line to provide an easily accessible means to terminate H 2 flow. N 2 and H 2 mains are each split into three feed lines, each feed line is controlled separately by a pressure regulator and MFC combination. Some characteristics of the MFCs used on the test bench are.listed in Table 4.1. The step down regulators immediately upstream from each MFC stabilizes pressure fluctuations in the line that may be caused by disturbances from other equipment connected to the building gas supply. This translates to a more stable output from the MFCs. s Heliox is a mixture of He and 0 2 gas 48 ( Table 4.1 Mass flow controller characteristics H 2 (test gas) N 2 or Air (test gas) N 2 (purge gas) Maximum flow 500sccm 20sccm 50slpm Minimum flow lOsccm 0.4sccm l.Oslpm (alarm at 12slpm) Accuracy ±1.0sccm ±0.2sccm ±0.8slpm Repeatability ±0.25% reading ±0.25% reading ±0.13slpm 4.2.a.2 MFCs to LINEOUT Downstream from the N 2 and H 2 MFCs, the feed lines merge at a mixing panel into two mixed-gas lines LI and L2 (see Figure 4.1). Feed lines may be shut off manually using ball valves on the mixing panel. Additional gas-handling devices such as a humidifier may be installed in-Hne with LI. LI and L2 are each fitted with a flash arrestor, a temperature probe and a pressure transducer (from Specialty Gas Equipment, Valax Manufacturing and GP:50 respectively) before being routed to two 3-way solenoid valves which are mechanically and electrically coupled to one another. The coupled operation of these valves is outlined in Table 4.2. Table 4.2 Operation of line-selecting solenoid valves Switch Position LI power L2 power To LINEOUT To Exhaust ON: (L2) On On L2 LI OFF: (LI) Off Off LI L2 The coupled solenoid valves are used as a gas line-selector, allowing gas flows to stabilize in each line before being 'switched' to enter LINEOUT, the supply line for the test chamber (see Figure 4.1). Solenoid valves heat up significantly when they are powered on. This is not ideal for gas sensor testing as gas temperatures will tend to increase over the course of the experiments. They are, however, fast, readily available, and are easy to integrate with electronics for data acquisition if needed. Solenoid latch valves would have been ideal, but ones that satisfy our safety considerations and budget could not be easily acquired. As a result, the above line-selector mechanism was chosen to minimize heating from ** Data according to manufacturer specifications 49 these valves. One line is open to the test chamber at any one time unless mechanical overrides on the solenoid valves are activated. In this configuration, valves can be left in the OFF position when testing LI, and can be manually overridden if L2 must be left on for testing over an extended period of time. 4.2.a.3 Purge line and safety alarms A separate N 2 and compressed air supply line is connected to a MFC that is set to deliver 20slpm of N 2 or compressed air for purging while samples are tested in localized, high concentrations of H 2 inside the test chamber. Two combustible gas monitors (Sierra Monitor Corporation) set to alarm at 400ppm H 2 are installed in the immediate vicinity of the test bench and test chamber, and an alarm buzzer is attached to the purge MFC to warn users of insufficient purge flow rates during testing. 4.2.b Test chamber The test chamber consists of a vacuum-fitted stainless steel drum. Pressure transducers, valves and other fittings for mounting and testing sensor specimens are attached to the body of the drum via numerous ports. A photo of this chamber is shown in Figure 4.2. The test chamber can be operated at pressures ranging from 0.027Pa (2 x 10"4 torr) to 120kPa (3PSIG). Some physical data for the test chamber and its affiliated components are listed in Table 4.3. Figure 4.2 Test chamber showing t est sensors, pressure transducers and vacuum pumps mounted on numerous ports. 50 Table 4.3 Test chamber physical data Approximate Volume . 6.2 x 104cm3 Purge inlet ball valve flow coefficient C v 0.6TT TC Check valve maximum C v / cracking pressure 1.68 / 2.3kPa differential (0.333PSI) n Overpressure outlet plug valve C v 1.6TT Vacuum vent device delay / vent timer 15s/4min Cavity size of 24pin test chip packages 0.965cm x 0.965cm x 0.1143cm 4.2.b.l Vacuum system ^ The test chamber is fitted with a Varian Turbomolecular Pump and a Multi-gauge controller that provides pressure readout from a number of pressure transducers. Repeatedly evacuating the test chamber to below 0.03Pa (2 x 10"4 torr) and backfilling with N 2 to close to atmospheric pressure minimizes the possibility of having pockets of air present inside the test chamber. The pressure transducers attached to the test chamber (P T a and PT C2 in Figure 4.1) are not suitable for H 2 service, and must only be used during the initial evacuation procedures. A gate valve between the Turbo pump and the test chamber isolates the test chamber from potential leaks across the Turbo and Mechanical pumps. The system is also fitted with a vent device which vents the pump side of the gate valve to atmospheric pressure after the pump stops to prevent backflow of oil into the system. 4.2.b.2 Sensor mount The sensor mount fitting was designed by Tom Vanderhoek at NRC-IFCI, and was built with support from shop personnel there. The fitting features an 8-wire electrical feedthrough and three gas ports to: 1. Route mixed gas (LINEOUT) to mounted sensors 2. Monitor pressure (mechanical gauge) in the test chamber during sensor testing 3. Exhaust H 2 gas mixtures from the test chamber to the H 2 vents The fitting is designed with an optional cap that closes off the fitting from the rest of the chamber and allows the fitting to be used as a nested chamber. This option is not used in the present study as all experiments are performed at close to atmospheric pressure. A check valve and a manual plug valve (TC Data from Swagelok Company 51 CHECK and TC OVERPRESSURE in Figure 4.1) connect the test chamber to the EXHAUST line. These are installed to keep the system from holding substantial positive pressure. On the inside of the fitting is a spring-loaded test socket for inserting standard Dual In-line Packaged chips up to 24pins. A maximum of 8 pins can be connected at any one time to the electrical feedthrough wires for connection to external circuitry. A short piece of polyurethane hose with an inner diameter of 1.59mm (0.0625") connects the gas inlet (LINEOUT) to a slit nozzle (rectangular orifice) mounted on the sensor. A diagram illustrating the nozzle and sensor configuration is shown in Figure 4.3. CFD simulations of gas flow through a slit nozzle were done by Yanghua Tang at NRC-IFCI. The model structure is shown in Figure 4.4. Simulations using estimated inlet gas velocity of 1 m/s blowing into the sensor cavity show flow and pressure uniformity across the sensor surface. Results of these simulations are illustrated in Figure 4.5 and Figure 4.6. (side view) Gas entrance through slit Gas Out 4-(To Test Chamber) 1 Gas In i s i i l i '.-lev,-; Sample in DIP (end riew} Figure 4.3 Diagram of slit nozzle. Gas entering through the port at one side of the nozzle is distributed across a slit orifice close to the sample. 52 4mm the centre of the bottom Figure 4.4 Simulation model structure for slit nozzle. Gas enters through the circular inlet on the left and exits to the right through a slit. Samples would be located at the bottom surface (extrusion) of the model. U - m/s 0BS72 -0 0856 Figure 4.5 Vertical section of simulated gas flow through a slit nozzle. Flow velocity is uniform across much of the sensor cavity. Sensors are attached at the centre of the bottom surface. P - I M / m ^ 50 -2<H -23.63 Figure 4.6 Horizontal section of simulated pressure in a slit nozzle. Pressure appears to be uniform across the sensor cavity. Sensors are attached at the centre of the bottom surface. 53 4.2.D.3 Operation of the gas manifold and test chamber The test chamber serves as a large N 2 reservoir in which high-concentration H 2 gas mixtures from the gas manifold are introduced through a small slit nozzle (see Figure 4.3), blowing directly onto test sensors before exiting and entering the rest of the chamber. This enables gas sensors to be tested in a small, localized 'pocket' of test gas within an otherwise inert reservoir. Test gas mixtures entering through this nozzle are controlled by H 2 and N 2 (or Air) MFCs described in section 4.2.a. 1, while purge N 2 is forced into the test chamber at 20slpm through a separate inlet (TC PURGE in Figure 4.1). A check valve (TC CHECK) is installed close to the H 2 inlet and a plug valve (TC OVERPRESSURE) is installed at the top of the test chamber to pipe exhaust gases (a mixture of purge N 2 and test H2) to the building H2-safe vents. The test chamber is maintained at a slight positive pressure to ensure air from the outside does not migrate into the test chamber while H 2 testing is in progress. The purge MFC alarm is set to buzz at N 2 flow rates below 12slpm based on the following calculation: . / \ max(a>„) Where min((pp„rge) is the lowest acceptable purge flow rate, max(cpH) is the maximum H 2 flow rate, and LELH is the lower explosive limit of H 2 in air, about 4% H 2 . By maintaining this minimum flow rate, and by having exhaust outlets close to areas where H 2 is likely to be present at higher concentrations (close to the H 2 inlet and near the top of the test chamber), we ensure that incoming H 2 gas will be sufficiently diluted and will not accumulate to dangerous levels inside the test chamber. 4.2.c Data acquisition electronics H 2 sensors inside the test chamber can be accessed electrically using 8 TEFLON insulated feedthrough cables. A number of options are available for powering the sensors and recording their response characteristics. A voltage-logging program was written in LabVIEW to monitor and record voltage readings from a data acquisition card (DAQ, National Instruments PCI 607IE) to ASCII files, and a simple resistive circuit was built to power the sensors. / 54 \ 4.2.C.1 Sensor excitation and signal detection circuitry A simple, battery-operated resistive circuit is used to power test sensors and connect them with the data acquisition board. The main function of the circuit is to provide and limit the power applied to the sensors, and protect the data acquisition board from unexpected shorts and other sensor failures. This DC excitation circuit uses rechargeable NiMH batteries to power the sensors. Rechargeable batteries are also used to power current amplifiers that measure current through the sensors. The use of batteries and shielded cabling lowers the peak-to-peak noise detected by at least an order of magnitude. Background noise in current measurements of this data acquisition unit was around ± 0.6nA. A schematic for the circuit used for sensor excitation and measurement is shown in Figure 4.7. Battery • DAQProbe .Sensor. 100 CurrentArnp (o) . . . , 0 Figure 4.7 Excitation and DAQ interface circuit to power resistive sensors and measure changes in resistance. Resistance values in the circuit are modified based on the sensors' initial resistance to optimize the detected signals. Typical values of R used to test early versions of fabricated sensors with low resistances are on the order of 10Q. DAQProbe and CurrentArnp are probes connected either directly or via a current amplifier to the DAQ. 4.2.C.2 LabVTEW data logging program A LabView data logging program was developed to monitor and record time and voltage measurements from a National Instruments PCI-6071E data acquisition card. The main objective for developing this program is to create a DAQ module that allows users to modify the rate at which data is logged without the need to stop the acquisition, reconfigure, and restart settings on the DAQ board. This was deemed necessary to maintain an accurate time-stamp on the recorded data. The program records time measurements extracted from the DAQ hardware, which should be accurate to the DAQ card's specifications and not dependent on the control computer's system clock or processing load. Such a module is useful for logging data in systems where both transient and steady-state information exist at intermittent intervals. Gas sensor testing is a classic example of such a system. The resulting module's control panel permits users to enter the file name to be saved, the desired sampling frequency of the DAQ hardware (i.e. the maximum sampling frequency), and the number of points sampled for each data point to be logged and saved. Users may also enter the edit mode and increase the number of physical channels to monitor. At the present time, no processing is performed on the acquired (raw) data, though simple signal processing functions can be easily added into the program module that extracts the logged data. The program outputs a text file containing a header specifying the device, date, start time and scan rate of the DAQ hardware. A scrolling chart on the control panel allows users to monitor the measured signals. The acquisition start time is displayed on the control panel, as is a numerical monitor of the amount of data 'backlogged' in the acquisition board buffer. This monitor is important for ensuring that the acquisition settings do not overwhelm the computer's available memory space. An example of data acquired using this DAQ setup is shown in Figure 4.8. This sensor was mounted in a commercial gas sensor test station (Greenlight Power Systems) and is electrically connected to a simple voltage divider. The voltage signal measured across the sensor is normalized with the open circuit (supply) voltage. This sensor was fabricated with a single layer of PMMA resist, and is marked by a large number of defects along the Pd wires. The fabrication process for this sensor' was described in Chapter 1, and observations on its test results will be described later in Chapter 1. Sensor Response (W04_3N25AB) Run2 1.004 g, 1 000 [ii"" irfijiptjnfn|tiiT > 0.996 •a M | 0.992 Z 0.988 -I 0.984 f 100% 80% 60% I - Signal -H2 0% 20 40 60 80 100 120 140 Time (s) Figure 4.8 Sample measurement obtained from LabvTEW-controIled DAQ setup Screen outputs of the LabVIEW control panels and the source-code diagrams are included in Appendix A. 4.3 Commissioning and testing with the test bench 4.3.a Test bench commissioning At the time of writing, the H 2 sensor test bench had just commenced operation. Existing MFCs were calibrated against new units from the manufacturer, and resulting calibration charts are included in Appendix B. Relief valve settings were calibrated using an N 2 MFC with a readout accuracy of 0.2sccm and a digital pressure gauge (Cole Parmer) with an accuracy of 0.69kPa differential (0.1PSI). This calibration was necessary to minimize leakage across the relief valves below 380kPa (40PSIG). The test bench is leak tested to 380kPa (40PSIG). There are known, small leaks across two valves in the system into the H 2 exhaust vent. The leak rate is sufficiently small and is satisfactory for H 2 testing to proceed. The volume of hose connecting the LINEOUT solenoid valves (LIS and L2S, refer to Figure 4.1) to the test sample is estimated to be 221cm3 ± 4cm3 by measuring the volume of gas flow through the line (using MFCs) to increase the line pressure by 101.3kPa. The use of building-supplied flame-resistant hoses and present equipment fixation requirements at the laboratory account for the use of such a large volume. At present, estimated system response time with H 2 MFCs fully open (500sccm) is approximately 27s. This test volume will drop substantially when alternative provisions and modifications are made for the test bench. » 4.3.b Testing with the test bench There are numerous ways of mixing gases for sensor testing [110]. The test system described in this chapter uses a dual-line buffer system where a stabilized mixed-gas line is fed to the test chamber while another is routed to the exhaust for concentration and flow rate 'preparation'. Coupled solenoid valves are then used to switch the two lines. MFC settings can be chosen to match the total flow in each line. This results in less disturbances in the test gas. ( Total flow from each line serves as the starting point from which the flow rates of component gases are calculated and set, that is, <Pi=CtO (4.2) i. where 57 (4:3) (p; is the flow setpoint of constituent gas /, C, is the gas' concentration and O is the total flow rate in the line. The present selection of MFCs is optimized for two types of tests: one in which small changes in high concentrations of H 2 gas can be delivered (useful for fine resolution gas testing at high concentrations), and another in which high concentrations of H 2 gas flow alternates with low H 2 concentration flow. Initial sensor response data from this test station is shown in Figure 4.9 and Figure 4.10. Changes in resistance were observed using this test bench for one of our wire sensors described earlier in this thesis (Figure 4.9). Testing was performed by Weimin Qian and Cheng Hu at NRC-IFCI. Results from the test bench are compared to response obtained from a commercial high flow rate test station (Greenlight Power Technologies), an example of which is shown in Figure 4.11. Figure 4.10 and Figure 4.11 are response curves obtained from the same acquisition unit testing the same sample. The sample was a MOS sensor obtained from a different project [111]. Average response times for both test facilities are obtained from testing and comparing the response of a number of MOS sensors, and results are tabulated in Table 4.4. The observed response time for the test bench is less than our original estimated time by about 10s, which suggests an inaccurate test volume measurement that can be attributed to system leaks. 149.6 .g. 149.4 .c O — 149.2 a> o £ | 149.0 "55 0) * 148.8 148.6 Test Bench Response (W05_2N24AB) -— Resistance — H2 % 100% 98% .1 + 96% g u c 94% g 92% CM X 90% 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Time (s) Figure 4.9 Sample response of a wire sensor fabricated with bilayer resist at very high concentrations of H 2. 58 rr 9.50E-11 g 9.30E-11 (0 o 9.10E-11 H Q-O 8.90E-11 8.70E-11 -| 8.50E-11 T e s t b e n c h s a m p l e r e s p o n s e ° Capacitance response — H2 % 100 80 e 60 c 40 g o o CM + 20 200 400 600 800 1000 1200 1400 1600 1800 Time (s) Figure 4.10 Sample response of a capacitor sensor tested with the test bench. Response of these sensors are compared with response of the same sensors tested with a commercial test station. 1.61E-10 1.6E-10 1.59E-10 £ 1 . 5 8 E - 1 0 81.57E-10 | l . 5 6 E - 1 0 21.55E-10 O1.54E-10 1.53E-10 1.52E-10 1.51E-10 G r e e n l i g h t Tes t S t a t i o n s a m p l e r e s p o n s e 600 Time (s) 100 Figure 4.11 Sample response of the same sample as in Figure 4.10, tested in the Greenlight commercial sensor test station. Table 4.4 Average response times for commercial test station and test bench Greenlight Test Station Test Bench Total Flow rate lOOslpm 400sccm Average Response Time 13s ± 7s 16s ± 9s 59 The primary difference between the operation of the sensor test bench and the commercial test station is the amount of manual operation required, and the range of flow rates and flow concentrations achievable. The Greenlight test station features automatic control and adjustment of gas flow rates in a mixture given the total flow and corresponding % of constituent gases. For the test bench, however, individual flow rates for gas mixtures in each line (out of two) must be 'prepared' in advance by entering flow setpoints manually into the flow controller. When the resulting mixed gas stream has stabilized in one line, it can be switched over to flow to the test chamber for sensor testing using a line-selector. The Greenlight test station is designed for high flow rate applications, however, and is unable to provide accurate concentration changes of less than 2 % of total flow. Minimum flow requirements for the MFCs also limit the use of the Greenlight in trace concentration testing, as in the case of H 2 leak tests (trace concentrations of H2), and fuel stream monitoring tests (trace concentrations of N 2 or air). Outside of these concentration ratios, the Greenlight test station offers excellent versatility and flexibility for sensor testing. It is exactly at these concentrations that the operation of our test bench is optimized. In this way, the two test facilities complement one another in terms of testing capabilities. When modifications to minimize the test bench system response times are implemented, the test bench will also provide a means to measure much faster sensor response than what is possible with the present systems. 60 Chapter 5 Results and Analysis This chapter reports on test results for gas sensors of which the fabricated by the process described in Chapter 1. A test bench, described in Chapter 1, was developed as part of the thesis work. Due to time constraints, however, the sensors developed have not been extensively tested with this system. Instead, sensors were tested in a commercial high flow rate sensor test station (Greenlight Power Technologies G7830 Gas Sensor Test Station). Sensor testing is performed by Shuozhi Yang and Cheng Hu at NRC-IFCI. Sensors are tested in H 2 /N 2 mixtures at a total gas flow of lOOslpm at atmospheric pressure and room temperature. The data acquisition unit used in conjunction with this test station is a commercial . impedance analyser (Solartron SI 1287 and SI 1260), although some early testing was performed with a LabVIEW controlled DAQ station described in the previous chapter. The DAQ station was developed to capture sub-millisecond changes in DC signals that is beyond the output time-resolution of the Solartron impedance analyser. After initial tests of these sensors, however, it became apparent that the physical response of the Greenlight test station is much slower, allowing the more refined Solartron unit to be used for later tests. 5.1 Test results 5.1.a Early sensors The original plan for sensor development was to investigate the use of an alternative top-down processing method to generate mesowires that operate based on the percolation mechanism described in the introductory chapters. This was desirable since the use of top-down fabrication techniques permit greater ease of sensor integration with electronics and other system-components. In addition, it has the potential to facilitate simpler alloying of the sensing material. Sensors fabricated early on consisted of arrays of 20 to 30 wires, and were fabricated using a single layer of PMMA as the e-beam resist. It was clear from initial testing that the sensors were indeed sensitive to H 2 , and two general types of response were obtained. These are shown in Figure 5.1 and Figure 5.2. Images of sensors corresponding to these tests are shown in Figure 5.3 and Figure 5.4 61 10 E .c 0) o c re *-» t/> '5> a> 7 4 Signal Response (W04_4N01CD) 1000 2000 3000 Time (s) 4000 100 80 60 40 20 0 5000 o c <u o c o O X Figure 5.1 Resistance response from an intact array of many Pd wires. A very slow and gradual response was observed, in addition to a large hysteresis. S e n s o r R e s p o n s e ( W 0 4 _ 3 N 2 5 A B ) R u n 2 1.004 -i 1 1 -r 100% 1.000 cs •4-* > 0.996 •a at N = 0.992 E Z 0.988 0.984 -— Signal — H2 80% c o T 60% £ s o 40% o CM I 4- 20% 0% 20 40 60 80 100 120 140 Time (s) Figure 5.2 Voltage across defect-ridden Pd wire array. Response is indicative of percolation effects. Wires that are open circuits in air become conductive at 100% H 2 . 62 Figure 5.3 Intact wire array sample corresponding to test results shown in Figure 5.1 Figure 5.1 shows the response of a sensor with few defects along the Pd wires (Figure 5.3), and Figure 5.2 shows one with many defects (Figure 5.4). The former shows resistance measurements recorded by the Solartron AC Impedance Analyser, using a probe voltage of lOOmVpp at 1kHz. The latter shows DC voltage measurements from the DAQ station, which can be easily configured to determine whether sensors exhibiting infinite resistance would become conductive when exposed to H 2 . Voltages are normalized to the open circuit voltage. Resistance measurements from intact sensor wires consistently show a large hysteresis, in which the final resistance after purging of H 2 remains at much higher values 63 than prior to the wires' exposure to H 2 . Observations and analysis of these response curves will be described in later sections. Defect-ridden sensors, though exhibiting behaviour that resemble percolation-type operation (lattice expansion in wires causing wire segments to connect and become conductive, refer to section 2.4), generated response that were too unstable, and tended to fail readily during H 2 tests. SEM images of these sensors after H 2 tests typically showed portions of the sensor wires had melted. Response showing this instability and breakdown from the same sensor as that in Figure 5.2 is shown in Figure 5.5 and Figure 5.6, tested immediately afterwards. S e n s o r R e s p o n s e ( W 0 4 _ 3 N 2 5 A B ) R u n 3 1.000 0.998 £ 0.996 -j •a N re 0.994 E o Z 0.992 0.990 Signal • H2 concentration 100 200 300 100% 80% 40% 20% 0% 400 500 600 700 800 900 Time (s) c o 60% £ c u c o U Figure 5.5 Unstable response of the defect-ridden many-wire array of Figure 5.4. 0-100% H 2 Step response of this sensor was shown in Figure 5.2 64 11 o 0.998 co CO +-» £ 0.996 TJ Q> N 're 0.994 E i— o z 0.992 0.99 S e n s o r Fa i lu re ( W 0 4 _ 3 N 2 5 A B ) R u n 4 Signal -H2 200 400 600 800 Time (s) 100% 80% c o 60% ^ c V o 40% O O CM X 20% 0% 1000 1200 1400 Figure 5.6 Failure response of the defect-ridden sensor of Figure 5.4. This test was performed immediately after the sensor test shown in Figure 5.5 Response from intact wires, on the other hand, seemed sluggish, and changes at high H 2 concentrations were not easily discernible through the system noise. Despite the large background however, some of these sensors exhibited a secondary, smaller response that followed changes in concentration closely. An example of this small signal response is shown in Figure 5.7. This sample consists of an array of 32 Pd wires that were each approximately 1 pm in width, 200nm thick and spaced 2um apart. 6.8 ^ 6.75 0) o c re (A 0) 6.7 6.65 4 6.6 S e n s o r R e s p o n s e ( W 0 4 _ 4 N 0 1 A B ) R u n 2 (detail) 100 + 80 g_ c o 60 * + 40 20 c o c o U CM X 0 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 Time (s) Figure 5.7 Small signal dependence of resistance on H 2 concentration: very small changes. 65 To amplify these changes in resistance, the overall resistance of the wire sensors must be much higher. This early work led to subsequent sensor designs which consist of 5 or less patterned wires across the contact pads. Bilayer resists were also used to achieve thinner wires to increase the overall resistance and improve the signal-to-noise. The resulting response of these sensors will be described in section 5. I.e. 5.1.b Failure mechanisms The defect-ridden sensor in Figure 5.4 was fabricated with a single PMMA resist layer. The sensor was patterned with a gap running down the centre perpendicular to the wires. This sensor illustrates a typical example of early sensors that showed good initial H2 response: they generally correspond to ones where a large number of defects exist along the wires. In the case of the sensor shown, some of these defects were introduced by patterning lOOnm gaps during the e-beam lithography process, while others were simply results of poor liftoff and processing. The nature and the origin of these defects point to two failure modes of these sensors: - » 1. Pd lattice does indeed expand and contract upon exposure and removal from H 2 . However, the extent to which subsequent expansion occurs is insufficient to reconstruct a continuous, conductive wire. This failure mode is the most common among all sensors tested. Sensors that suffered from this mechanism were generally functional immediately after their H 2 tests, but became open circuits the following day, presumably after hydrogen atoms have desorbed completely from the a-hydrides region and the Pd host lattice has contracted. This may be due to the release of residual tensile stresses in the sputtered Pd during lattice expansion and contraction, causing the contracted wire segments to be shorter than pre-H2 exposure lengths. 2. Defects that were not the result of direct patterning point to poor adhesion of the Pd film to the oxide surface. These defects may have originated from strong (thick) Pd layers that stripped portions of the patterned wires during liftoff, or they may simply have resulted from poor surface quality of the oxide.^ Proper adhesion of the wire to the surface of the substrate is critical since delamination from the substrate causes the sensor to overheat and melt at much lower probing voltages. This was illustrated by heat transport models described in section 2.5.d. A sensor that shows the effects of overheating is shown before voltage measurements in Figure 5.8 (intact), and after sensor excitation in Figure 5.9 (damaged). Irregularities on the oxide surface are clearly seen in this example, and correspond to the exact locations where overheating from delamination occurs. 66 Figure 5.8 Sample with imperfections on the underlying oxide layer, as fabricated. The compromised region is the darker patch of oxide between the contact pads, to the right. SE Oa-May-06 061241 WD13.3mm 20. O k W l !2k° °2°5um Figure 5.9 Failure of the sample of Figure 5.8. Portions of wires spanning oxide with imperfections can be seen to have melted. The adhesion between the wires and the oxide layer at these areas was compromised, which led to wire delamination and overheating. A third failure mode is suspected to exist for these sensors, in the case where the Pd lattice expansion in the presence of H2 causes wire structures to buckle (and consequently delaminate). This failure mechanism may explain the response behaviour of sensors that become open circuits during the initial exposure to H 2 (but were otherwise intact when tested in air). 67 Thick wire structures from single-layer resist liftoff that had good adhesion to the substrate were tested to withstand excitation voltages up to 7VDC, comparable to that predicted by temperature simulations discussed earlier. Subsequent work with bilayer resist samples resulted in a similar voltage limit before changes in the wires' resistance were observed. Figure 5.10 shows a Joule heating I-V curve for a 25-wire sample fabricated with bilayer resist. 0.08 0.07 0.06 < 0.05 I 0.04 1— § 0.03 0.02 0.01 0 Joule heating of Pd wires 10 12 Voltage (V) Figure 5.10 I-V curve for Pd wires with good adhesion to the oxide substrate. Trend line is a linear regression of I=0.008*V based on data up to 6.25VDC. 5.1.c Sample response curves Response characteristics of the bilayer resist sensors are generally similar to that of the single-layer resist samples. The sensors themselves generally survive 2-3 rounds of testing. This compares favourably to the single-layer resist samples, all of which failed after repeated H 2 cycling during their first H 2 test. General response shape is similar across all wire sensors fabricated using the bilayer resists regardless.of structure. These sensors appear to be less prone to the delamination-caused overheating mentioned earlier, except for the sensor structure where a Pd platform exists between the contact pads, shown earlier in Figure 3.5. Failure points for this set of platform sensors were generally at the edge of the thick, Pd coated, crosslinked PMMA platform, where portions of the connecting Pd wires are not in contact with the oxide layer. 1kHz AC probing voltages are kept at lOOmVAC to minimize temperature effects while 68 maintaining reasonable signal-to-noise. Typical response curves for these sensors with their corresponding W2 concentrations are shown in the following Figures. 370 360 H 300 S e n s o r R e s p o n s e ( W 0 5 _ 2 N 1 6 A B ) 1000 2000 3000 Time (s) 4000 100 80 c o k_ •*-» C <d 40 £ "o u CM + 20 5000 Figure 5.11 Response from bilayer PMMA/PMGI resist sample. This sample is one with a platform structure. See text for circled property Figure 5.12 Step response portion of Figure 5.11 in detail. See text for circled properties Resistance Reponse (W07_2D2AB) Time (s) Figure 5.13 Response from sample fabricated with the single PMMA resist process, 5 wire structure. See text for circled property 770 o 750 a) u c co en a) or 740 730 720 Resistance Reponse (W07_2D2AB) (detail) Signal -H2 2000 2500 3000 3500 Time (s) 4000 100 80 60 + 40 20 c o c a> u c .— O *9, o — c a> O) O "D X 0 4500 Figure 5.14 Step response portion of Figure 5.13 in detail. 70 Figure 5.15 Repeated testing of a platform sensor. Several general observations could be made from the sensors' response data and tests: 1. 2. The shape of the response curves are similar regardless of geometry of Pd wires. H 2 response of these Pd structures appear to be a superposition of two or more response mechanisms (marked with their corresponding letters in the Figures): a. A large step increase between 0% and 10% H 2 (covering the entire [H]/[Pd] range for a-hydrides and portions of the (3-hydrides), followed by a decay that disappears after the first few 0-100% H 2 cycling, b. An increase in resistance as H 2 decreases from 20% to 0%H2, causing a hysteresis or (in some cases) permanent change in the sensors' resistance. c. A smaller step response that follows closely the changes in H 2 concentration. d. In a few of the sensors tested, a step decrease in resistance as H 2 concentration increase. 3. Starting resistances of each test for a given sample increase from repeated H 2 testing 4. All concentration steps beyond 10% H 2 should be well into the disordered fj-hydrides-only region according to established (p-C) isotherms, but R/Ro values are much less than reported resistance ratios for P-hydrides. 5. Time response of bilayer resist samples is several times faster than response of single-layer resist samples. Further testing is needed to confirm and determine this relationship, as only two single-layer resist samples were tested with the present test protocols, and both had relatively poor signal-to-noise. 6. Signal-to-noise is low for samples with many wires in parallel. This is expected since placing wires in parallel lowers the overall resistance of the wires. It is difficult to detect resistance changes when the overall resistance is very low, as these changes become comparable in magnitude to system noise. 7. The platform sensor design shown in Figure 3.5 was observed to be more robust and reliable than non-platform wire sensors, in that effects from imperfections along the wiresare averaged out over sections of parallel wires that are in series with one another. This maintains a large overall resistance across the sensor, and translates to better signal-to-noise that is less dependent on sensor imperfections. Close inspection of the small signal step response led to conclusions that this change in resistance could be logarithmic. This is of interest since the (p-C) isotherm in our region of interest (high [H]/[Pd] ratios) i can be approximated by a logarithmic relation. Table 5.1 shows some values extracted from (p-C) isotherms reported in literature. 72 Table 5.1 Reported (p-C) isotherm values of p Pd hydrides from literature Concentration (% H 2 at atmospheric pressure) [H]/[Pd] ratio Temperature (K) Reference 1% 0.62 293K [94] 10% 0.68 293K [94] 100% 0.73 293K [94] 10% 0.63 323K [95] 100% 0.68 323K [95] By inspection of reported data in literature, [H]/[Pd] values for H 2 concentrations of over 10% in an inert carrier gas can be estimated with the above data by [H]/[Pd] = A + Bln[p] (5.1) where [P] is % H 2 from 0 to 100. Table 5.2 summarizes values for these coefficients at 293K and 323K. Table 5.2 Summary of pressure interpolation constants for determining [H]/[Pd] ratios Temperature (K) A B 293 K 0.621± 0.006 0.025 ± .003 323K 0.58 ±0.02 0.022 ±.007 We will see in the following analysis section that resistance changes, in [3-hydrides follow a similar logarithmic behaviour. 5.2 Response analysis 5.2.a Small signal step response The small signal step response from each sample was extracted by measuring the step sizes in the overall response, integrating them, then using fitted values to generate estimated resistance step responses from H 2 concentrations. The estimated response is further translated such that the response has a reference value of 0 for all H 2 concentrations at or below 10%. R s, eps is the portion of normalized resistance attributed to the small signal response. A sample fit of this small signal response is shown in Figure 5.17 and is fitted with the function 73 tf, = C l + C2-ln[P] (5.2) + ^ 1 1 1 1 1 : | | 30 40 50 60 70 80 90 100 H2 Concentration (%) Figure 5.17 Logarithmic Fit of small signal resistance measured as a function of H 2 concentration. The resulting small signal step response fit is shown with a vertical offset in Figure 5.18 over the normalized response. This signal can be subtracted from the overall response. The leftover background is shown in Figure 5.19 and is mostly smooth for the majority of sensors tested. Deviations exist at datapoints immediately after each concentration change, as the delay in system time response was not taken into account during the subtraction. This delay is typically 20s and corresponds roughly to the test station's system response time. 0 1000 2000 3000 4000 Time (s) ' Figure 5.18 Sample response along with small signal component for H 2 concentrations over 10%. 74 I I I I I 1000 2000' 3000 4000 5000 Time (s) Figure 5.19 Sample background response left over after small signal component is removed. More observations of the response were made over the course of fitting data in this manner: 1. The background resistance change is attributed to more than one irreversible response mechanism that may include physical damage (delamination and melting) during H 2 absorption. The traditional R/RQ ratio, as such, is not a meaningful normalization for comparison of fitted coefficients across different samples. Instead, the stabilized resistance at 100% H 2 for each sample, just prior to desorption, was chosen as the reference resistance (Rref) with which the sensor response is normalized. Figure 5.20 shows the Rref/Ro ratios for data presented in this analysis. The average value of this ratio is 1.2 ± 0.2, and is extracted from the data simply for rough comparisons with literature reported values. This ratio is clearly much lower than the reported maximum R/Ro values of 1.8 reported in literature for atmospheric H 2 . The samples marked with t before the identifying labels on the x-axis represent film-like sensors that were the result of poor (bilayer) liftoff. The sample marked with * represents a many-wired sample fabricated with single-layer resist. Results from these sensors are not included in the average. 75 Reference Resistance/Initial Resistance Test Samples Figure 5.20 Reference resistances for samples analysed with respect to their initial resistance. Samples marked with f before their labels did not liftoff properly. Sample marked with * was an older, many-wire sensor with compromised signal-to-noise. These sensors (the three on the right) were not used to calculate averages. 2. Small signal step response was set to 0 at concentrations below 10% H 2 , as the [H]/[Pd] vs. (In p[H2]) approximation (Equation (2.2)) is not valid at lowr [H]/[Pd] ratios in the P-hydride region. There is, as a result, an additional uncertain (but constant) portion of the background response attributed to the P-hydrides. 3. The observed resistance increase during desorption through 20% to 0% H 2 clearly exhibits a hysteresis (about -5,.5 x 10"3 for the sample shown) in the a and a-P regions. 4. Absorption response of resistance for P-hydrides can be considered linear to the hydrogen atomic ratio, but not so for desorption. This is in clear agreement with Lewis' summary [20] and less apparent with Sakamoto's [95] results, in which the slope of R/Ro decreases greatly at close to 40% H 2 . It is important to note however that Sakamoto's results were reported for testing at 323K, while sensors were tested at room temperature for the present study. Fit coefficients extracted are summarized in Figure 5.21 and appear to be roughly constant among samples analysed. The average value of C/.and C2 are listed in Table 5.3. As in the case for the Rre/R-o plot shown in Figure 5.20, averages are taken only from bilayer resist samples that went through successful liftoff, and exclude samples marked with | and * on their labels. 76 .5. c c> o o O 0 02 -i 0 01 -0 --0 01 --0 02 --0 03 --0 04 --0 05 --0 06 -CQ < CD 21 c CN 3 i ce: o Fitted Coefficients of < R s t e P S = C1 + C2 Liri[P]> < CD T - CN Z c CN 3 i Q : in o Q Q a O O o CD CD CD T— T— T - CN T - CO ~Z. <= CN 3 CN ^ CN 3 i if) m IT) o p g $ 5 CQ < CN T -Z c CN 3 I tt. m p LU co _ co § p CQ < CN a T -C N C P X co p CQ < O _ P Test Samples Figure 5.21 Coefficients (arbitrary normalized units) extracted from small signal response fitting. Again, samples marked with f before their labels did not liftoff properly. Sample marked with * was an older, many-wire sensor with compromised signal-to-noise. These sensors (the three on the right) were not used to calculate averages. Table 5.3 Average values for fit coefficients CI (a.u.) C2 (a.u.) (-2.4 ± 0.9) x 10"2 (7.9 ± 2.3) x 10"J Coefficients CI and C2, in conjunction with the coefficients A and B from interpolating the p region (p-C) isotherm in Equation (2.2), relate the normalized resistance to atomic hydrogen content as follows: [H/Pd]=CA+CB~ (5.3) where 77 (5.4) and It can be seen that the uncertainty in the background for the fitted step resistances will have an effect only in C A . Taking into account the average Rre/Ro ratio of 1.2 ± 0.2, the slope of our R/R0 vs. [H]/[Pd] curve for P-hydrides then, at 293K, is around 2.7 ± 0.9. This result is higher than reported values in Lewis' summary of about 0.87 [20], and much less than the slope estimated from Sakamoto's results of 8.3 for 323K [95]. Our estimated errors are large, and fitted constants for the same sensor vary with each H 2 test. It is likely that a substantial portion of the errors stem from the need to subtract a background response which, at present, is not well characterized and understood. Other sources of error include changes and differences in temperature, H 2 response of other sensor components, and the response of Pd to N 2 or the lack of 0 2. Changes in the background resistance were observed during tests when the sensors are purged with N 2 . This was thought to be due to the removal of oxygen atoms from the Pd, and was assumed constant in our analysis. It should be clear, however, that should the signal be the result of sensor cross-sensitivity to N 2 , this change in resistance will not be constant. Identification and characterization of these effects will require further tests and studies. 5.2.D Background response The background portion of the response extracted from data analysis typically shows a large step increase followed by a slow decay that stabilizes in time scales on the order of 1000s. Fits of the response with 1/t and exponential decays did not converge well and were not consistent with the measured data. As such, further tests and analysis are needed to determine a mathematical form of the decay mechanism occurring here. The subsequent discussion will present some qualitative interpretations of the background response. 78 5.2.b.l Lattice and grain boundary effects The decay portion of the background response vanishes after repeated H 2 cycling, and is likely to be related to dislocations, stresses and cracks in the Pd. Resistivity is generally considered to be inversely proportional to the mean free path of electrons in a metal as follows [112]: m vr P = — T • <5-6> where p is the resistivity, / is the mean free path (effective path) of an electron, e is the electron electronic charge, m, n and Vf are the electron mass, carrier density and Fermi velocity of the conduction electrons. A higher density of dislocations and grain boundaries are introduced in the form of deformations and cracks as the Pd lattice expands and contracts. This translates to a shorter mean free path for conduction electrons before they are scattered by these scattering centres. These scattering effects are amplified as physical dimensions of the conduction structure itself diminish and approaches dimensions comparable to the electrons' mean free path in the material. Microstructural changes from the formation of disordered P-hydrides are permanent upon repeated H 2 cycling [97], and accounts for non-repeatable peaks and decays in the background resistance when H 2 is introduced to a sample for the first time. Lattice stresses'and expansion of Pd hydrides in the stochiometric a region, however, is reversible and repeatable. Electron scattering from these lattice changes and associated volume-induced deformations likely accounts for the repeatable portion of the resistance hysteresis. These effects would be observed in the resistivity of the material at the a and a-P regions regardless of the absorption history of the sample. 5.2.b.2 Percolation effects In several cases, a large step decrease in resistance was observed during H 2 absorption. The resistances of samples in these cases return to close to their original value on H 2 desorption. The background for such a sample is illustrated in Figure 5.22. 79 Background Reponse (W07 2D2AB) Time (s) Figure 5.22 Percolation-like effects in resistance response. Change in resistance due to (a) expansion of Pd wire segments, forming an additional conductive path and (b) contraction of Pd segments. This behaviour likely originates from percolation effects occurring in the Pd wire structures. The magnitude of change in resistance from a change in the number of identical conducting wires in parallel is as follows: Ai? R AR Nr (5.7) where N. = (5.8) In the above, «/and n, are the final and initial number of conducting wires. For a single wire to two identical wires transition, Nr reduces the normalized resistance by (a maximum of) 50%. 80 5.2.b.3 Auxiliary effects A different process that may contribute to the background response characteristics is the response of other materials present in the device. Although Pd is by far one of the most sensitive materials to H 2 , it is well known that characteristics of Au (such as the work function) and Si02 (such as barrier height at the Pd-Si02 interface) also change in the presence of H 2 . Such auxiliary processes are worthy of further investigation to determine their relevance in our sensor structure. 81 Chapter 6 Conclusion A resistive Pd nanowire sensor was fabricated to address application needs for on-line monitoring of H 2 fuel cell systems. The objective of the study was to design a device that monitors concentration changes between 90% and 100% H 2 in N 2 . The original work plan included developing a top-down fabrication method for percolation-based wire sensors. Top down methods use existing silicon chip fabrication technologies, which facilitate integration of sensors with signal processing electronics. Modifications of material and sensor structure (for response tuning, for example) are also generally easier with these fabrication techniques. 6.1 Test bench development summary In parallel with the sensor development was the design and assembly of a test bench and data acquisition system capable of detecting fast sensor response, since commercial test stations presently available have a system lag time of about 13s. The test bench was commissioned and initial test data comparable to the commercial test station were successfully obtained. Due to time constraints, sensors developed in this work have not yet been characterized in this test bench, and a number of modifications to the test bench are still pending. These include the following: 1. Addition of an extra controller console to take advantage of the two additional gas supply lines to improve versatility of the gas mixing. Present use of these extra MFCs requires physical re-wiring of cables and jumper settings on the controller console. 2. Reducing gas volume between the line-selector and the test sample in order to reduce the system response time to ~ 1 s. Initial test results obtained from the test bench showed comparable signal results to those obtained from the commercial gas test station. The average time delay observed in these sensor signals was about 16s ± 9s, while the same sensors tested in the commercial test station showed an average delay of about 13s ± 7s. Compared to the commercial test station, the test bench requires much more user involvement in gas routing and settings, as the majority of controls are manual. The test bench setup features a line-switching mechanism which, when the system is optimized, has the potential to supply accurate concentration changes of H 2 /N 2 gas mixtures much more quickly than existing commercial test stations. i 82 6.2 Sensor development summary Sensors developed in this thesis use pure Pd sputtered onto patterned PMMA or PMMA/PMGI resists for liftoff. The use of PMMA/PMGI bilayer resists resulted in less defects in sensor wire structures, and the developed fabrication protocols were more reliable than those using a single PMMA layer for liftoff. Wire thicknesses of bilayer samples were on the order of 80nm and were much thinner than those fabricated with a single layer of PMMA (about 200nm). The use of bilayer resist introduces a 'deposition stop', in which metallization accumulating at the upper resist layer closes off the layer's opening, and no further thickness is added to the patterned cavity. Further work in this area may allow post-liftoff thickness of the structures to be accurately controlled and replicated depending on the electron beam dose and development times of the resist layers. Multi-wire structures were fabricated and tested in a commercial H 2 sensor test station. Results were analysed as a superposition of a fast, small signal response attributed to p Pd hydrides, and a slow, large decay and hysteresis corresponding to the existence / coexistence of a-hydrides in the system. The small signal response can be modeled as a logarithmic function of absorbed hydrogen content, and the slope of the resulting R/R0 vs. [H]/[Pd] relationship is estimated to be 2.7 ± 0.9 for concentrations of H 2 over 10%. It is clear from our study that between 90% and 100% H 2 , the small signal resistance response can be used to monitor changes in H 2 concentration. Despite the existence of a large and relatively slow 'background' transient, the small signal response can be extracted quite easily and follows changes in concentration about as quickly as the present gas test system will allow. Further insights to the time-response characteristics of the sensors may be obtained when pending changes for the test bench are in place. These modifications will allow the system to trigger fast concentration changes and measure sensor response at acquisition rates on the order of 10,000samples/s. 6.3 Significance of results and comparison with literature Pd film resistance measurements are reported in the literature with system settling times varying from minutes to hours [95]. A number of Pd film sensor studies report observing faster response times for higher concentrations of H 2 , but results for concentrations beyond the a-hydrides region are generally not presented, making a direct comparison with these studies difficult. The presence of a large hysteresis over the hydrides phase transition is well documented, and partly accounts for the lack of Pd film sensor developments over concentrations above several percent H 2 . At the present time, a number of groups are investigating the percolation mode of operation for thin film Pd on a variety of substrates (such as 83 polymeric cluster-nucleation promoters [99], anodic alumina [113], etc.), while others are studying Pd alloy compositions to eliminate this phase change entirely such that only the a-hydride forms [25]. Of the resistance response observed in our thin Pd wire sensors, the P-hydride resistance provides the most consistent and repeatable means of determining H 2 concentrations over 10% regardless of exact geometries of the sensors. Comparatively, sensor mechanisms based on percolation is novel, and has a great potential for excellent sensitivities and robustness over low- to mid-H2 concentrations. Percolation effects in a device, however, are strongly dependent on its fabricated geometry as well as its material properties. This may result in stringent fabrication tolerances in order that accurate response analysis can be performed in sensing applications. Normalized resistance of the P-hydrides at a given temperature, on the other hand, is a material properly that is relatively invariant across sensors regardless of geometry, making its use more forgiving, simple and economical for mass manufacturing. With advancements in signal processing technologies and fuzzy logic algorithms-that compensate for superimposed signals and sensor cross-sensitivities, extracting p-hydride response may prove to be a viable means of measuring high concentrations of H 2 accurately. Our study shows that geometry does not play a significant role in affecting hydride resistance. It does, however, affect the robustness of the sensors. Inserting platform structures spanning across sensor wires improves the robustness of the sensors by averaging out effects of imperfections over several parallel wire segments, while maintaining good signal-to-noise by having these parallel sections in series with one another. Pd films are known to blister and delaminate from exposure to H 2 . Delamination is a primary source of failure for our wire sensors as well, especially after repeated exposure to H 2 . The emphasis of the present work has been on designing a reliable fabrication process that yields intact wire structures. More sensor tests will be needed to determine whether thin Pd wires in H 2 exhibit better adhesion than thick Pd films. As spatial dimensions of these sensors become smaller, the long response times associated with Pd resistive devices diminish and the initial resistances of our sensors increase. It is generally agreed that smaller physical dimensions translate to larger surface area to volume ratios (more surface coverage) and shorter diffusion lengths for atomic hydrogen migration. This is generally favourable for speeding up sensor response. In addition, we can expect at least a comparable (if not speedier) response for nano-scale hydride resistance measurements with percolation film measurements for high-concentration monitoring. The former involves chemical absorption processes that should be faster than the physical lattice expansion dominant in the latter mechanism. Although there is insufficient data to establish a 84 quantitative correlation between response times and our sensors' physical geometries, thicker Pd wires fabricated from single-layer PMMA liftoff exhibited slower reponse times in our tests, on the order of minutes, as opposed to about 20s for bilayer resist Pd wire sensors. Wire sensors, as mentioned earlier, also feature higher initial resistance and improved signal-to-noise over comparable fdm sensors. Although characteristic parameters are described in terms of the ratio of resistances, wire sensors with high initial resistance were generally easier to work with. Since the total resistance of fdm and many-wired sensors are on the order of a few ohms, sensor signals (changes in resistance) are easily overwhelmed by background noise from the rest of the system! In one example, noise from a fdm sensor was on the order of half the resistance step size corresponding to the concentration increase from 80% to 100% H 2 . Five-wired sensor devices however, have signal-to-noise ratios that are far superior to those of fdm sensors. As such, it is recommended that wire based patterns such as those presented in this study or serpentine structures be used in place of fdm structures for resistive sensor fabrication. 6.4 Additional recommendations We observed that scaling down of the device caused amplification of dislocation and grain boundary effects, as a-P transition peaks are much sharper and stronger than those observed in many-wired and film samples. No conclusive fit was achieved for the background unfortunately, and further work in identifying physical (expansion) and chemical (absorption and desorption) processes involved in background response studies is recommended. The sensors studied in this work may be successfully incorporated into sensor packages where processing circuitry exists to extract sensor data froni an array of different sensors' response. The sensor should be integrated with ones optimized for low H 2 concentration measurements (corresponding to a and a-(3 hydrides). To achieve H 2 monitoring and detection throughout all H 2 concentrations with a single Pd-based sensing material or structure however, one must address challenges caused by the a-P hydride transition. In this regard, promising progress is noted in Pd percolation films and Pd-alloy films. Some preliminary work on creating a Ni target mask has been performed for the present study, but results are not reported here as this work has progressed only as far as confirming the uniformity and accuracy in Pd/Ni content for resulting sputtered films. As existing reports in literature on alloy studies have already achieved sufficient sensitivity, continuation of alloy studies should place emphasis on reducing response time via design of the sensor geometry. 85 Future work to continue direct patterning of defects along Pd nanowires is not recommended unless access to higher resolution patterning instruments, or alternative deposition methods such as angled deposition systems become available. Studies into depositing wires of thicknesses close to the percolation limit can be pursued, however, especially due to its apparent ease of implemention with existing capabilities. Such future work will benefit from existing literature on nucleation promoters such as siloxane self-assembled monolayers [99]. Work recommended in this area includes controlling deposited wire thicknesses by controlling 'window' (pinhole) widths of developed PMMA in PMMA/PMGI bilayers, as such a technique may facilitate operation range 'tuning', allowing multiple sensors optimized at different concentrations to be fabricated on the same chip with the same fabrication processes. The sensor work described in this document covers the development of fabrication protocols and records a number of observations in tested sensors. 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LabVIEW control panels and block diagrams Control panels and source diagrams of vi's and subvi's written in LabVIEW 7.1 for acquiring voltage signals are shown here. max scans/sec file path 10000 j \% C:\Lilian_Labview_060119\Controls\DAQData\t:est (#sampled/datapoint r J j l O bean backlog Saved Data Chart 0~ scans/read 1000 start time 100:00:00.000 Pr J D D / M M / Y Y Y Y error code no error | source I STOP Figure A.l Main control program front panel (MultiChannelDAQ.vi) 95 Convert scan rate info to string for file header [Hlf scan backJogl -sl5] LpTJ |Read acquired data in Buffer 1 is /read | n o i o c t t d D a D D D D D a a b ' B n o o ' D n n n n p ^ p q p t i a D a D D D D D D - g Discard excess datapomts (take only 1 point out of every #sampled/datapomt) [Build time (relative) array | — B n P & f l a f l Q r J ' Q D a D D Q D Q D D O a D D Q O OD u DTP •DDQDDDDQQDDP [Merge time & data into ZD array] Llears | DAQ K create or replace w\ MultiChannelDAQ.vi Lilian Fan Aug 18, 2005 Code to acquire data from multiple channels (default 3) at a variable rate. Modified from MultiChannelDAQ_10.vi. Inserted comments and rearranged blocks Program generates data with 1 time column only: i.e. does not take into account intrinsic acquisition delay between channel reads, Inputs; max scans/sec [unsigned integer], #sampled/datapoint [unsigned integer], file path [path], stop [boolean] Function: 1) Acquires data to buffer from DAQ board channels specified on Block Diagram at rate <max scans/sec> 2) Reads <scans/read> (set to equal <max scans/sec> for now) # of points from buffer (i.e. ~1 read per second) 3) Writes header information with <max s c a n s / s e o , date, <start time> 4) Generates time axis based on <start time> and <dt> (from DAQ board) 5) Extract 1 data point out of every <#sarnpled/datapoint> points to plot and save to file (at 6 significant figs in Exp, notation) (this to minimize the amount of data storage space needed) 6) Repeat until <stop> is true During execution, the only control that can be changed is <#data/DAQ stream> and <stop>. Saved data is shown on <5aved Data Chart> and is updated once per read. <scans/read> indicator is pretty useless at the moment but is placed on front panel to make it easy to change to a control. <scan backlog> indicator is for monitoring how well the program is clearing the DAQ buffer. <start time> shows the time/date when the acquisition starts. <error> indicator shows if there are any errors with the DAQ board. Data in the following format is saved to file specified in <file path>: > # [date] / / [time] / / Max scan rate: [max scans/sec] scans/sec > x_0 [tab] y i _ 0 [tab] y2_0 [tab] y3_0 [tab] > x _ l [tab] y l _ l [tab] y2_ l [tab] y3_ l [tab] > x_2 [tab] y l _ 2 [tab] y2_2 [tab] y3_2 [tab] For reference, datafile size at 3 channels of 17,000 scans/sec (61.2ml time points) extrapolates to about 2.4Gb per hour, Figure A.2 Block diagram of main control program (MultiChannelDAQ.vi) 96 Figure A.3 Time reference case in main control program (MultiChannelDAQ.vi) 1 False T£T Figure A.4 Case for no plot data in main control program (MultiChannelDAQ.vi) • 1 -in Buffer New Buffer array r.i'O • •> * r • 0 r j 0 -;.o ^ffcT" ,: o r.'' 0 New Arra j Q Q fj 0 1,0 0 0 o . Q (J 0 0 0 I D o I 0 J 0 0 G I 0 |0 : 0 0 0 • 0 1 o -foT^  0 Figure A.5 Front Panel of module for parsing and extracting data to be logged (Array2DExtract01.vi) 97 [at ray | j jmt"j-PuFferj Eh:; Length of Array | ft EH Rate! . R 04 tew Buffer" > mi) 3 J^ew Array] Inputs: Y [2D array]. Buffer [2D array], Rate [unsigned integer] Outputs: New Y [2D array], New Buffer [2D array] Usage: Appends <Buffer> to the beginning of <Array>, Selects 1 column out of <Rate> # of columns in <Array> and build <New Array> from it. Generate <New Buffer> out of remaining elements to be appended to the following set of data. Rate must be > Q. Range set to coerce to 1 if Rate <= 0 , Figure A.6 Block Diagram of module for parsing and extracting data to be logged (Array2DExtract01.vi) 2D array 7!:0 I/P waveforms O/P lAiflVftf nrm«; |i;Cl ,:o~_H 0 |M0O:0O:0OPM I,, 1 J HDD/MM/YYYY I dt m f^i .uuuuuu to v | | o 1 1 00:00:00 PM ; 1 DD/MM/YYYY dt 1 p. . 000000 Figure A.7 Front Panel for plotting module. Plots scrolling data (MultiPlof2DArray.vi) 98 Same as MultiPlot2DArray_02.vi, Lilian Fan, Aug 17, 2005 Plots a 2D array, works for multiple traces with the same x-axis. Input array format as follows: x_0 x_l x_2 .,, Y1_0 Yl_ l Yl 2 ... Y2_0 y2_l Y2_2 ... Figure A.8 Block Diagram for plotting module (MultiPlot2DArray.vi) 99 Appendix B. Calibration charts for gas sensor test bench Calibration charts for MFCs (Advanced Specialty Gas Equipment) used in the test bench are provided here. These MFCs are calibrated against one another, using readings from MFCs newly acquired from the manufacturer as reference (where applicable). MFCs to be calibrated are placed upstream and set the gas flow rate through reference MFCs that are fully open downstream. Calibration tests are done in N 2 at 446kPa (50PSIG). Care was taken to ensure that an older H 2 MFC is calibrated to be consistent with a brand new H 2 MFC. For reference however, equivalent flow of gas through an MFC that is calibrated for a different gas remains linear and can be approximated with the ratio of the gases' specific heats. The approximating equation and factors are given by the manufacturer, as follows'1'*: _ . Conv ersionFactorR OutputA * — = Output B Conv ersionF actorA Equation B. 1 Table B.l Conversion factors for MFCs Gas Conversion Factor Hydrogen 1.01 Nitrogen 1.00 Air 1.00 A represents the gas for which the MFC is calibrated, and B represents the gas flowing through the MFC. A cross-reference chart for two brand new N 2 MFCs, followed by the calibration curve for readings from an older H 2 MFC calibrated against a new unit, are shown in the following Figures. { J Data from Advanced Specialty Gas Equipment. Instructions for Mass Flow Sensors and Mass Flow Control Modules. 100 Nitrogen MFC's X-Reference 20 16 ? u o <£, 12 3 O T3 -^ y = 0.9667x-0.1501 R 2 = 0.9999 0 10 15 N2 readout (seem) 20 25 Figure B.l Cross reference chart for N 2 MFCs, received new from manufacturer (ASGE) Actual Flow vs. H2 Flow Readout, 50PSI y = 1.0078x + 37.854 R 2 = 0.9999 100 150 200 250 300 H2 readout (seem) 350 400 450 500 Figure B.2 Real flow conversion chart for an older H 2 MFC. Flow readings from MFCs used in the test bench generally follow closely the setpoints entered in their control console except for the older hydrogen MFC. The actual flow through this unit plotted against the console's setpoint is shown in the following Figure. 101 Actual Flow vs H2 Flow Setting, 50PSI 600 -i 0 -I : , 1 : 1 ' 1 : ' i 0 100 200 300 r 400 500 600 H2 Flow Setting (seem) Figure B.3 Actual flow of older H 2 MFC as the console setpoints vary. A Cole Parmer MFC is also used in the test bench for N 2 purging. This MFC comes with its own readout display. Cross reference measurements with another high flowrate MFC show discrepancies of about 2slpm and are shown in the following Figure. Purge flow rates are thus set to well above the estimated required minimum. Advanced vs. Cole Parmer MFC N2 X-Reference o -I 1 1 1 1 0 5 10 15 20 25 Cole Parmer 50SLPM MFC flow rate (slpm) Figure B.4 Cross Reference curve for Cole Parmer MFC with ASGE MFC. Discrepancies of about 2slpm were observed between the two MFCs. 102 

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