@prefix vivo: . @prefix edm: . @prefix ns0: . @prefix dcterms: . @prefix skos: . vivo:departmentOrSchool "Applied Science, Faculty of"@en, "Electrical and Computer Engineering, Department of"@en ; edm:dataProvider "DSpace"@en ; ns0:degreeCampus "UBCV"@en ; dcterms:creator "Motieian Najar, Mohammad Hadi"@en ; dcterms:issued "2010-09-24T21:15:28Z"@en, "2010"@en ; vivo:relatedDegree "Master of Applied Science - MASc"@en ; ns0:degreeGrantor "University of British Columbia"@en ; dcterms:description """Capacitive Micromachined Ultrasound Transducers (CMUTs) have been recently introduced as a viable substitute to piezoelectric transducers in medical ultrasound imaging. CMUT possesses advantages such as allowing high frequency, having wide bandwidth, high sensitivity, low cost, CMOS compatibility and being easy to fabricate. This thesis is motivated by movement towards a better detection of breast tumors using ultrasound imaging techniques, which CMUTs have promised to achieve. Therefore, CMUTs were designed to fulfill requirements of this application in terms of resonant frequency, pull-in voltage and geometrical dimensions. The entire design and analysis were performed considering that the CMUTs are to be fabricated using PolyMUMPs technology, for this technology being precise, accurate and well established in the micro-electromechanical systems (MEMS) community. CMUTs were first analytically modeled and designed by exploiting the parallel-plate capacitor equations. A behavioral model was developed in VHDL-AMS, which, unlike previous models, incorporates the non-linear electromechanical relations of the CMUT. The behavioral model has the advantage of being more time efficient than finite element models (FEM) and more accurate than analytical models. Prior to fabrication, a 3D FEM was developed in COMSOL Multiphysics® software. Resonant frequency analysis determined the frequency response and eigenfrequencies of the CMUT, which could not be determined using previous models. Parametric analysis determined the pull-in voltage, the spring constant and spring softening effect, the variation in capacitance and the electromechanical efficiency of the CMUT. The CMUT resonated at 5.868MHz frequency and the collapse voltage was determined at 275V using FEM results, which were close to analytical modeling results and in excellent agreement with behavioral modeling results. The thickness and the radius of the circular CMUT membrane were found to be 1.5μm and 32μm, respectively. The air/vacuum gap distance was 0.75μm and the insulation layer was 0.6μm. The CMUTs were fabricated in cell and array form. An array of 128 elements each containing 118 cells were fabricated to be compatible with existing ultrasound probes. Unfortunately, due to mal-fabrication by the company, which was experimentally proved, the experimental results were not as successful."""@en ; edm:aggregatedCHO "https://circle.library.ubc.ca/rest/handle/2429/28690?expand=metadata"@en ; skos:note """ Design and Analysis of Capacitive Micromachined Ultrasound Transducer by Mohammad Hadi Motieian Najar B.A.Sc., The University of British Columbia, 2008 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in The Faculty of Graduate Studies (Electrical and Computer Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) September 2010 © Hadi Najar, 2010   ii Abstract   Capacitive Micromachined Ultrasound Transducers (CMUTs) have been recently introduced as a viable substitute to piezoelectric transducers in medical ultrasound imaging. CMUT possesses advantages such as allowing high frequency, having wide bandwidth, high sensitivity, low cost, CMOS compatibility and being easy to fabricate. This thesis is motivated by movement towards a better detection of breast tumors using ultrasound imaging techniques, which CMUTs have promised to achieve. Therefore, CMUTs were designed to fulfill requirements of this application in terms of resonant frequency, pull-in voltage and geometrical dimensions. The entire design and analysis were performed considering that the CMUTs are to be fabricated using PolyMUMPs technology, for this technology being precise, accurate and well established in the micro-electromechanical systems (MEMS) community. CMUTs were first analytically modeled and designed by exploiting the parallel-plate capacitor equations. A behavioral model was developed in VHDL-AMS, which, unlike previous models, incorporates the non-linear electromechanical relations of the CMUT. The behavioral model has the advantage of being more time efficient than finite element models (FEM) and more accurate than analytical models. Prior to fabrication, a 3D FEM was developed in COMSOL Multiphysics® software. Resonant frequency analysis determined the frequency response and eigenfrequencies of the CMUT, which could not be determined using previous models. Parametric analysis determined the pull- in voltage, the spring constant and spring softening effect, the variation in capacitance and the electromechanical efficiency of the CMUT. The CMUT resonated at 5.868MHz frequency and   iii the collapse voltage was determined at 275V using FEM results, which were close to analytical modeling results and in excellent agreement with behavioral modeling results. The thickness and the radius of the circular CMUT membrane were found to be 1.5µm and 32µm, respectively. The air/vacuum gap distance was 0.75µm and the insulation layer was 0.6µm. The CMUTs were fabricated in cell and array form. An array of 128 elements each containing 118 cells were fabricated to be compatible with existing ultrasound probes. Unfortunately, due to mal-fabrication by the company, which was experimentally proved, the experimental results were not as successful. Abstract   iv Table of Contents Abstract.......................................................................................................................................... ii Table of Contents ......................................................................................................................... iv List of Tables ................................................................................................................................ vi List of Figures.............................................................................................................................. vii Acknowledgements ...................................................................................................................... xi Dedication .................................................................................................................................... xii 1 Introduction............................................................................................................................... 1 1.1 Background on CMUTs and Ultrasounds .......................................................................... 2 1.2 Advantages of CMUTs over Piezoelectric Materials ......................................................... 4 1.3 Research Motivation........................................................................................................... 5 1.4 Objectives and Methods ..................................................................................................... 6 2 Modeling of CMUT................................................................................................................... 9 2.1 Principle of Operation of CMUT........................................................................................ 9 2.2 Analytical Modeling of CMUT ........................................................................................ 11 2.2.1 Collapse Voltage Calculation..................................................................................... 15 2.2.2 Resonant Frequency Calculation................................................................................ 20 2.2.3 Spring Softening Coefficient...................................................................................... 25 2.2.4 Electromechanical Coupling Effect ........................................................................... 29 2.3 Electromechanical Equivalent circuit of CMUT .............................................................. 31 3 Simulation................................................................................................................................ 36 3.1 Finite Element Simulation of CMUT ............................................................................... 36 3.1.1 CMUT Model Setup................................................................................................... 37 3.1.2 Eigen Frequency Analysis and Frequency Response................................................. 41 3.1.3 Parametric Analysis.................................................................................................... 44 3.1.3.1 Pull-in Phenomenon .............................................................................................. 44 3.1.3.2 Spring Softening Effect ......................................................................................... 49 3.1.3.3 Electromechanical Coupling Coefficient............................................................... 52   v 3.2 Behavioral Modeling with VHDL-AMS of CMUT ......................................................... 53 3.2.1 Frequency Analysis .................................................................................................... 55 3.2.2 Pull-in Analysis .......................................................................................................... 56 4 Fabrication............................................................................................................................... 59 4.1 Previously Fabricated Processes....................................................................................... 59 4.1.1 Surface Micromachining ............................................................................................ 60 4.1.2 Bulk Micromachining ................................................................................................ 63 5 Testing and Characterization ................................................................................................ 71 5.1 Experimental Setup........................................................................................................... 71 5.1.1 Electrical Testing Setup ............................................................................................. 71 5.1.2 Mechanical Testing Setup .......................................................................................... 73 5.1.3 Acoustical Testing Setup............................................................................................ 75 5.2 Experimental Results ........................................................................................................ 77 6 Conclusions and Future Work............................................................................................... 82 6.1 The Road Ahead ............................................................................................................... 84 References.................................................................................................................................... 86 Appendices................................................................................................................................... 92 Appendix A – COMSOL Multiphysics Simulation Setup for Parametric Analysis ................. 92 Appendix B – Images of CMUT Masking Layouts, Fabricated Devices, and Pin Configuration ................................................................................................................................................... 98   vi List of Tables Table 1. Combinations of layers in PolyMUMPs to fabricate CMUT ......................................... 18   Table 2. List of important parameter values ................................................................................. 23   Table 3. List of important parameters to design the CMUT calculated analytically.................... 35   Table 4. Summary of FEM and behavioral results and their relative errors................................. 58   Table 5. Rectangle parameters in COMSOL ................................................................................ 93   Table 6. Boundary settings for electrostatics in COMSOL .......................................................... 95   Table 7. Boundary settings for moving mesh in COMSOL ......................................................... 95   Table 8. Subdomain settings for solid, stress-stain in COMSOL................................................. 96     vii List of Figures   Figure 1.Cross-section of a basic unit of a CMUT ....................................................................... 10   Figure 2. Principle of operation of a CMUT cell in (a) transmission mode and (b) receiving mode.............................................................................................................................................. 11   Figure 3. First order lumped electromechanical model of a CMUT............................................. 12   Figure 4. Electrostatic and spring forces as a function of gap distance in analytical modeling of CMUT when VDC Structural Mechanics > Solid, Stress-Strain. Click Add. 3- From the list of application modes, select COMSOL Multiphysics > Deformed Mesh > Moving Mesh (ALE) and then click Add. 4- In the Multiphysics list on the right side of the dialog box, select Frame (ale) then add MEMS Module > Electrostatics > Electrostatics to that list. 5- Click OK to close the Model Navigator. Options and Settings 1- From the Draw menu, select Work-Plane Settings. 2- Click the Quick tab. 3- Choose the x-y plane, and for z use a value of 0. 4- Click OK.   93 Geometry Modeling 1- From the Draw menu, select Work-Plane Settings. 2- Shift-click the Ellipse/Circle (Centered) button on the Draw toolbar to create a circle. 3- Put 32e-6 for Radius and for x and y positions put 0 and 0. 4- Click the Zoom extents from the Main toolbar at the top. 5- While holding down the Shift key, click the Rectangle/Square button on the Draw toolbar on the far left side of the graphical user interface; this action opens the Rectangle dialog box. Enter dimensions as in the following table, and then click OK. Property Value Width 70e-6 Height 35e-6 X -35e-6 Y 0 Table 5. Rectangle parameters in COMSOL 6- Click Control+A to select both geometries. 7- Click the intersection button on the Draw toolbar. 8- From the Draw menu select Extrude to open the extrude option box. 9- While CO1 is selected, type -0.6e-6 in the Distance box. 10- Under Extrude Object Name type Nitride Layer. 11- Change the work plane to Geom2 from the top taps. 12- Open the Extrude box as in step 8 and while CO1 is selected, type 0.75e-6 in the Distance box. 13- In Extrude Object Name type Gap distance. 14- Change the work plane to Geom2, again. Appendix A. COMSOL Multiphysics simulation setup for parametric analysis   94 15- Open the Extrude box for the third time as explained in step 8. 16- While CO1 is selected, type 1.5e-6 in the Distance. 17- In Extrude Object Name type Membrane. 18- Make sure Membrane geometry is selected and click on Move button from left toolbar. 19- Enter 0.75e-6 in Z Displacement. 20- Change the work plane to Geom2. 21- Shift-click the Rectangle/Square button on the Draw toolbar. 22- Entre dimension values as in table 5. 23- Open the Extrude box as in step 8 and ensure R1 is selected. 24- Enter 3.5e-6 in the Distance, and in Extrude Object Name type Medium. 25- While the Medium rectangle is selected, click on Move button from left toolbar and enter -0.6e-6 in Z Displacement. Physics Settings Subdomain Settings – Electrostatics 1- Go to the Multiphysics menu and make sure that Electrostatics (emes) is selected. 2- Open the Physics>Subdomain Settings dialog box. 3- Select Subdomain 2 and entre 9.7 for relative permittivity of nitride, εr. 4- Select Subdomain 4 and entre 4.5 for relative permittivity of poly-silicon, εr. 5- Click the Force tab. In the first row enter Fes. The software automatically generates the variables Fes_nTx_emes and Fes_nTy_emes for the electrostatic force components. Later on you use these variables to define the boundary load in the Plane Strain application mode. 6- Click OK. Appendix A. COMSOL Multiphysics simulation setup for parametric analysis   95 Boundary Conditions – Electrostatics 1- From the Physics menu, open the Boundary Settings dialog box. 2- Check the Interior boundaries box and then specify conditions as follows: Settings Boundaries 1-5, 7, 8, 10, 13, 19 Boundary 14 Boundary 8 Rest of Boundaries Boundary Condition Zero charge/Symmetry Electric Potential Ground Continuity V0 V_in Table 6. Boundary settings for electrostatics in COMSOL 3- Click OK. Subdomain Settings – Moving Mesh 1- From the Multiphysics menu, select Moving Mesh (ale). 2- In the Subdomain Settings dialog box select Subdomain 2 and choose No displacement setting. 3- Select Subdomain 4 and choose Physics induced displacement. For the displacement variables, dx, dy and dz, enter u and v and w, respectively. 4- Click OK. Boundary Conditions – Moving Mesh 1- In the Boundary Settings dialog box for the mesh displacements, enter these settings: Settings Boundaries 14, 15 Boundaries 10, 2 All other Boundaries dx u Not selected 0 dy v 0 0 dz w Not selected 0 Table 7. Boundary settings for moving mesh in COMSOL Appendix A. COMSOL Multiphysics simulation setup for parametric analysis   96 2- Click OK. Subdomain Settings – Solid, Stress-Strain 1- Go to the Multiphysics menu and ensure that the Solid, Stress-Strain (smsld) application mode is selected 2- Open Subdomain Settings and select Subdomains 1 and 3 and clear the Active in this subdomain check box. 3- Fill out the properties of Subdomains 2 and 4 according to the following: Properties Values of Subdomain 1 Values of Subdomain 2 E 160e9 250e9 ν 0.23 0.23 α 4.15e-6 2.3e-6 ρ 2330 3100 Table 8. Subdomain settings for solid, stress-stain in COMSOL Boundary Conditions – Solid, Stress-Strain 1- Open Boundary Conditions and choose all the boundaries. From the Constraint condition choose Fixed. 2- Select Boundaries 13 and 7, and change the Constraint condition to Symmetry Plane. 3- Select Boundaries 14 and 15 and change the Constraint condition to Free. 4- Select Boundary 14 and change to Load tab. For FX, FY and FZ values enter Fes_nTx_emes, Fes_nTy_emes, and Fes_nTz_emesi, respectively. 5- Click OK. 6- To obtain more precise results select, select Properties from the Physics menu and set the Large Deformation property option to On. Appendix A. COMSOL Multiphysics simulation setup for parametric analysis   97 Mesh Generation Click the Initialize Mesh button on the Main toolbar. The mesh consists of roughly 22061 elements. Computing the Solution 1- Click the Solver Parameters button from the Main toolbar; the Solver Parameters dialog box then opens. 2- In the dialog box, change the Analysis to Parametric and the Solver to Parametric Segregated. 3- For Parameter name entre V_in, and for Parameter values enter [5:5:300]. 4- In Segregated Groups enter V for Group 1, “u v w” for Group 2 and “x y z lm4 lm5 lm6” for Group 3. 5- Click OK to close the dialog box 6- Click Solve button on the Main toolbar to solve for parametric analysis. It takes about a few hours. Appendix A. COMSOL Multiphysics simulation setup for parametric analysis   98 Appendix B – Images of CMUT Masking Layouts, Fabricated Devices, and Pin Configuration Figure 40. (a) Masking layout and, (b) fabricated device of a CMUT cell with a layer of metal atop and using substrate as the ground electrode Figure 41. (a) Masking layout and, (b) fabricated device of a CMUT cell structure using the substrate as the ground electrode (a) (b) (a) (b) 70µm 70µm   99 Figure 42. (a) Masking layout and,(b) fabricated device of CMUT cells with different radii and using substrate as the ground electrode (a) (b) 70µm Appendix B. Images of CMUT Masking layouts, fabricated devices, and pin configuration   100 Figure 43. (a) Masking layout and, (b) fabricated device of CMUT arrays and the ground pad (a) (b) 350µm Ground Pad Appendix B. Images of CMUT Masking layouts, fabricated devices, and pin configuration   101 Figure 44. (a) Masking layout and, (b) fabricated device of CMUT arrays and their interconnecting tracks (a) (b) 200µm Appendix B. Images of CMUT Masking layouts, fabricated devices, and pin configuration   102 Figure 45. Photos of CMUT cells with burnt tracks after high current I-V characterization indicating presence of short-circuit on the dies 70µm 70µm Appendix B. Images of CMUT Masking layouts, fabricated devices, and pin configuration   103 Figure 46. Pin configuration and the interconnecting tracks to the bond pads Appendix B. Images of CMUT Masking layouts, fabricated devices, and pin configuration   104 Figure 47. PGA 209 package pin layout"""@en ; edm:hasType "Thesis/Dissertation"@en ; vivo:dateIssued "2010-11"@en ; edm:isShownAt "10.14288/1.0071310"@en ; dcterms:language "eng"@en ; ns0:degreeDiscipline "Electrical and Computer Engineering"@en ; edm:provider "Vancouver : University of British Columbia Library"@en ; dcterms:publisher "University of British Columbia"@en ; dcterms:rights "Attribution-NoDerivs 3.0 Unported"@en ; ns0:rightsURI "http://creativecommons.org/licenses/by-nd/3.0/"@en ; ns0:scholarLevel "Graduate"@en ; dcterms:title "Design and analysis of capacitive micromachined ultrasound transducer"@en ; dcterms:type "Text"@en ; ns0:identifierURI "http://hdl.handle.net/2429/28690"@en .