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Fabrication and characterization of InP semiconductor ring lasers Vafaei, Raha 2010-10-06

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Fabrication And Characterization ofInP Semiconductor Ring LasersbyRaha VafaeiB.Sc., The University of British Columbia, 2008A THESIS SUBMITTED IN PARTIAL FULFILMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMaster of Applied ScienceinThe Faculty of Graduate Studies(Electrical and Computer Engineering)The University Of British Columbia(Vancouver)October, 2010c￿ Raha Vafaei 2010AbstractThis thesis investigates the fabrication of 1550 nm emitting InP semicon-ductor racetrack resonator lasers (SRLs) via wet etching techniques. Themethod of choice for SRL fabrication is reported to be via dry etching. Dryetching is a complex, time consuming and expensive process which leaves rel-atively rougher surfaces and sidewalls compared to wet etching techniques.In this thesis, coupling, racetrack resonators, and edge emitter laser theorywere studied for the SRL design. Then, a fabrication process for the SRLswas developed via wet etching techniques. The light emitting diode (LED)characteristics of the fabricated devices were observed and successfully mea-sured. The spectrum of the device was also measured with optical spectrumanalyzer (OSA) and resonances were observed.However, lasing was not observed. The cleaving process is a major lim-iting step in the fabrication and it is being improved. In parallel to the wetetching fabrication at UBC, dry etching (the common method for SRL fab-rication) is being performed at the Centre de Recherche en Nanofabricationet Nanocaract´erisation (CNR2) at the Universit´e de Sherbrooke.iiPrefaceI am one of the co-authors in the Conference and Optics Letter titled, “RingResonator Reflector with a Waveguide Crossing” and “Design and Charac-terization of Microring Reflectors with a Waveguide Crossing” respectively.More specifically, I did the roundtrip loss value measurements and calcu-lations, rebuilt and improved the SOI setup used for the measurements,performed some of the temperature measurements, wrote the curve fittingcode which was modified by Wei Shi to fit his design and helped with thedesign procedure used to determine device parameters such as effective in-dex, group index and coupling coefficients. The manuscript was written byWei Shi.I am one of the co-authors in the paper published in the Journal of Light-waves titled, “Temperature Effects On Silicon-On-Insulator (SOI) RacetrackResonators: a Coupled Analytic and 2D Finite Difference Approach”. Morespecifically I did some of the design and analysis of the racetrack resonator,designed the SOI racetrack resonators that were fabricated and used to ver-ify the design procedure and to study the temperature effects. I also did thequality factor analysis. The manuscript was written by Dr. Nicolas Rouger.I am one of the co-authors in the SPIE paper titled, “Simulation of a1550 nm InGaAsP-InP transistor laser”, I researched the required p-typeiiiPrefacecontact composition for obtaining ohmic contact behaviour with the lowestreported resistance on the InGaAsP base contact layer. The manuscript waswritten by Wei Shi.I am one of the co-authors in the poster titled, “SOI Nanophotonic De-vices Analysis and Fabrication” presented at the Pacific Centre for AdvancedMaterials and Microstructures (PCAMM). More specifically I wrote the sec-tion on modelling and analysis of InP lasers, and provided the quality factorplot and the SOI racetrack resonator design.The list of publications include:1. Wei Shi, Raha Vafaei, Miguel´Angel Guill´en Torres, Nicolas A. F.Jaeger, Lukas Chrostowski, “Design and Characterization of MicroringReflectors with a Waveguide Crossing”, Optics Letters, vol. 35, issue17, pp. 2901-2903, 09/2010.2. Wei Shi, Raha Vafaei, Miguel´Angel Guill´en Torres, Nicolas A. F.Jaeger, Lukas Chrostowski, “Ring Resonator Reflector with a Waveg-uide Crossing”, 2010 International Conference on Optical MEMS andNanophotonics, 09/08/2010.3. Nicolas Rouger, Lukas Chrostowski, Raha Vafaei, “Temperature Ef-fects On Silicon-On-Insulator (SOI) Racetrack Resonators: a CoupledAnalytic and 2D Finite Difference Approach”, Journal of LightwaveTechnology, vol. 28, issue 9, pp. 1380–1391, 05/2010.4. Wei Shi, Zigang Duan, Raha Vafaei, Nicolas Rouger, Behnam Faraji,Lukas Chrostowski, “Simulation of a 1550 nm InGaAsP-InP transistorivPrefacelaser”, Photonics and OptoElectronics Meetings, Proc. SPIE, vol.7516, Wuhan, China, pp. 75160P-75160P-7, 08/2009.5. Miguel´Angel Guill´en Torres, Nicolas Rouger, Raha Vafaei, ShahroozM. Amin, Robi Boeck, Behnam Faraji, Brendan Francis, Alina Kulpa,Juan Mario Michaan, Lukas Chrostowski, Nicolas Jaeger, Dan Dep-tuck, “SOI Nanophotonic Devices Analysis and Fabrication”, PacificCentre for Advanced Materials and Microstructures (PCAMM) An-nual Meeting, 29/11/2008.vTable of ContentsAbstract ................................. iiPreface .................................. iiiTable of Contents ............................ viList of Tables ..............................viiiList of Figures .............................. ixList of Abbreviations ......................... xiAcknowledgements ...........................xiii1 Introduction ............................. 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Operating Principle . . . . . . . . . . . . . . . . . . . . . . . 21.3 Literature Review . . . . . . . . . . . . . . . . . . . . . . . . 31.4 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.5 Novel Contributions . . . . . . . . . . . . . . . . . . . . . . . 121.6 Thesis Organization . . . . . . . . . . . . . . . . . . . . . . . 122 InP Racetrack Laser Design . . . . . . . . . . . . . . . . . . . 152.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2 Mode Calculation . . . . . . . . . . . . . . . . . . . . . . . . 152.3 Racetrack Laser Coupler Design . . . . . . . . . . . . . . . . 182.4 Racetrack Laser Resonator Design . . . . . . . . . . . . . . . 292.5 Double Bus Passive Resonator . . . . . . . . . . . . . . . . . 373 Fabrication Via Wet Etching . . . . . . . . . . . . . . . . . . . 413.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2 InP Material Structure . . . . . . . . . . . . . . . . . . . . . 453.3 InP Edge Emitter Racetrack Resonator Laser . . . . . . . . . 49viTable of Contents3.3.1 Mask . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.3.2 Sample Preparation . . . . . . . . . . . . . . . . . . . 533.3.3 Photolithography . . . . . . . . . . . . . . . . . . . . 543.3.4 Wet Etching . . . . . . . . . . . . . . . . . . . . . . . 633.3.5 Planarization . . . . . . . . . . . . . . . . . . . . . . . 753.3.6 Metalization . . . . . . . . . . . . . . . . . . . . . . . 773.3.7 Cleaving . . . . . . . . . . . . . . . . . . . . . . . . . 813.3.8 Results . . . . . . . . . . . . . . . . . . . . . . . . . . 823.3.9 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 883.4 InP Edge Emitter Ridge Waveguide Laser . . . . . . . . . . . 903.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904 Fabrication Via Dry Etching . . . . . . . . . . . . . . . . . . . 925 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015.1 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . 102Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104AppendixA Clewin Matlab Script Used for the Mask Layout . . . . . . 109viiList of Tables2.1 Refractive Index and Lattice Constant Data . . . . . . . . . 162.2 Power Coupling for Rectangular Waveguides W = 2 µmH=2 µm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.3 Power Coupling for Rectangular Waveguides W = 3 µmH=2 µm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.4 Power Coupling for Trapezoidal Waveguide Wbottom=3µm,Wtop=1µm . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.5 Predicted Performance for Trapezoidal Waveguide Wbottom= 3, µmWtop=1µm, R = 300 µm and a 200 nm ThickPolyimide Planarization, Biased @ 100 mA . . . . . . . . . . 282.6 Predicted Performance for Trapezoidal Waveguide Wbottom=3 µmWtop=1µm R = 50µm and a 200nm Thick PolyimidePlanarization, Biased @ 100mA . . . . . . . . . . . . . . . . . 292.7 Simulation Parameters. Data taken from Lecture Notes ofDr.Lukas Chrostowski and Ref [1] . . . . . . . . . . . . . . . . 313.1 Overview of The Fabrication Steps . . . . . . . . . . . . . . . 433.2 R2 - 1550 nm FP-LD Epi-wafer Structure[2] . . . . . . . . . . 463.3 Solvent and Properties (Data from [3]) . . . . . . . . . . . . . 553.4 Etchants Tried . . . . . . . . . . . . . . . . . . . . . . . . . . 653.5 Wet Etch Monitor Table for Sample : R2 epi-wafer, ID: D04Date: Jul19/2010 . . . . . . . . . . . . . . . . . . . . . . . . . 703.6 Summary of Ohmic Contact Data . . . . . . . . . . . . . . . . 774.1 Etch Depth Analysis, W = 2 µm . . . . . . . . . . . . . . . . 984.2 Etch Depth Analysis, W = 3 µm . . . . . . . . . . . . . . . . 994.3 Predicted Performance for Waveguide W = 2 µm R = 300 µm 99viiiList of Figures1.1 Ring Laser Structure by Sorel and Group... . . . . . . . . . . 41.2 Ring Laser Spectrum by Sorel and Group... . . . . . . . . . 51.3 Micro-square Resonators... . . . . . . . . . . . . . . . . . . . 61.4 Microring Amplifier at University of Maryland... . . . . . . . 71.5 SEM of Dry Etched Test Coupler... . . . . . . . . . . . . . . 81.6 SEM of SRL By S. Park, S. S. Kim, L. Wang, and S. T. Ho... 91.7 Spectra of SRL By S. Park, S. S. Kim, L. Wang, and S. T.Ho... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.8 Hybrid Silicon Evanescent Device... . . . . . . . . . . . . . . 101.9 SRLwithTuneableCouplerbyG.MezosiS.FurstandM.Sorel... 112.1 Refractive Index Amplitude Profile... . . . . . . . . . . . . . 172.2 First and Second Order Mode Distributions... . . . . . . . . 182.3 Even and Odd Modes ... . . . . . . . . . . . . . . . . . . . . 202.4 Crossover Length vs. Separation ... . . . . . . . . . . . . . . 222.5 Power Coupling vs. Waveguide Spacing ... . . . . . . . . . . 232.6 Mode Distribution in Trapezoidal Waveguide... . . . . . . . . 262.7 Transverse Component of Photons in Laser... . . . . . . . . . 262.8 Refractive Index Distribution in the Coupler .... . . . . . . . 272.9 neffand ngvs. λ .... . . . . . . . . . . . . . . . . . . . . . . 302.10 Threshold Current vs. κ .... . . . . . . . . . . . . . . . . . . 332.11 Output Power vs. κ At 3 Bias Conditions.... . . . . . . . . . 332.12 Output Power vs. κ for a Varying Ring Radius with LowLosses.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.13 Output Power vs. κ for a Varying Ring Radius with HighLosses.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.14 Output Power vs. Current with High Losses.... . . . . . . . . 362.15 SOI Resonator.... . . . . . . . . . . . . . . . . . . . . . . . . 382.16 SOI Measurement Setup.... . . . . . . . . . . . . . . . . . . . 392.17 SOI Racetrack Resonator Drop Port Response.... . . . . . . 403.1 Fabrication Flow Diagram.... . . . . . . . . . . . . . . . . . . 43ixList of Figures3.2 The Epi-wafer PL Data .... . . . . . . . . . . . . . . . . . . . 483.3 Mask Layout.... . . . . . . . . . . . . . . . . . . . . . . . . . 503.4 Layer 1(a) Mask Layout.... . . . . . . . . . . . . . . . . . . . 513.5 Layer 1(b) Mask Layout.... . . . . . . . . . . . . . . . . . . . 513.6 Layer 2 Mask Layout.... . . . . . . . . . . . . . . . . . . . . . 513.7 Theoretical Resolution Versus Mask to Photoresist Gap... . . 593.8 1µm Resolution Lithography... . . . . . . . . . . . . . . . . . 603.9 Rough Edge Mask Defects... . . . . . . . . . . . . . . . . . . 613.10 Photoresist Impurity Defect... . . . . . . . . . . . . . . . . . 623.11 Stubborn Photoresist Residue... . . . . . . . . . . . . . . . . 623.12 3H2SO4:H2O2:H2O System... . . . . . . . . . . . . . . . . 663.13 H3PO4/HCl System... . . . . . . . . . . . . . . . . . . . . . . 663.14 HCl/ CH3COOH/H2O2System... . . . . . . . . . . . . . . . 673.15 HBr/H2O2/H2O/HCl System... . . . . . . . . . . . . . . . . 673.16 Etch Profile with HBr/H2O2/H2O/HCl System... . . . . . . 683.17 SphericalLens Formationwith HBr/H2O2/H2O/HClSystem... 693.18 Side Wall Wet Etch Profile... . . . . . . . . . . . . . . . . . . 723.19 PR Lifting Defect... . . . . . . . . . . . . . . . . . . . . . . . 733.20 Bubble Formation Defect... . . . . . . . . . . . . . . . . . . . 743.21 Planarization Flow Diagram... . . . . . . . . . . . . . . . . . 783.22 IV Characteristic of the N-type Contact ... . . . . . . . . . . 803.23 IV Characteristic of the P-type Contact ... . . . . . . . . . . 803.24 Cleaved Region Optical Image... . . . . . . . . . . . . . . . . 833.25 Final Fabricated Device... . . . . . . . . . . . . . . . . . . . 833.26 Edge Emitter Setup... . . . . . . . . . . . . . . . . . . . . . . 843.27 LED Characteristic... . . . . . . . . . . . . . . . . . . . . . . 853.28 LED Characteristic... . . . . . . . . . . . . . . . . . . . . . . 864.1 Mask Designed for Sherbrooke ... . . . . . . . . . . . . . . . 944.2 Cartoon Epi-wafer Structure and a Dry Etched SRL SideView.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 954.3 SEM Side View After Dry Etching ... . . . . . . . . . . . . . 964.4 Measured ARDE of Dry Etched InP Sample ... . . . . . . . 974.5 PredictedModeDistributioninCoupler RegionforWetEtchedSample ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004.6 PredictedModeDistributioninCoupler RegionforDryEtchedSample ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100xList of Abbreviations• Semiconductor Racetrack Laser (SRL)• Ridge Waveguide (RWG)• Photoresist (PR)• Infrastructure of Nanosctructures and Femtosciences (INRS)• Centre de recherche en nanofabrication et nanocaracterisation (CNR2)• Counter Clockwise (CCW)• Clockwise (CW)• Optical Injection Locking (OIL)• Vertical Cavity Surface Emitting Laser (VCSEL)• Finite Difference Time Domain (FDTD)• Quantum Well (QW)• Photoluminescence (PL)• Ultraviolet (UV)• Scanning Electron Microscope (SEM)xiList of Abbreviations• Plasma Enhanced Chemical Vapor Deposition (PECVD)• Electron Cyclotron Resonance (ECR)• Reactive Ion Etching (RIE)• Aspect Ration Dependent Etching (ARDE)xiiAcknowledgementsI would like to acknowledge the support, wisdom, leadership and friendshipof my supervisor Dr. Lukas Chrostowski. In addition, I would like to ac-knowledge the support of all of my lab-mates, specially Wei Shi, Dr. MarkGreenberg, Dr. Nicolas Roger, Behnam Faraji and Miguel´Angel Guill´enTorres for their daily discussions and brain storming sessions. I am verygrateful to have had the chance to work along side them and to have hadthe opportunity to learn about optics in such a dynamic group.Furthermore, I would like to acknowledge CMC Microsystems Inc. (Jes-sica Zhang, Dan Deptuck) for supporting the Silicon-On-Insulator racetrackresonator and InP semiconductor ring laser fabrication, and the Centre deRecherche en Nanofabrication et Nanocaract´erisation (CNR2) at the Uni-versit´e de Sherbrooke (Vincent Aimee, Marie-Jos´ee Gour, Jean-Francois Be-dard, Etienne Grondin) for fabricating the semiconductor ring lasers via dryetching. I thank the BC Innovation Council for funding this research. Also,I would like to thank the committee (Nicolas A. F. Jaeger, Alireza Nojeh,S. Tang) for taking the time to read my thesis and provide me with theirfeedback.I am very thankful to my family for all their love. My dad MasoudVafaei, mom Sara Montazemi, sister Mina S. Vafaei and brother Rod VafaeixiiiAcknowledgementshave been continually supportive throughout my life and eduction. It is theirunconditional love that has always been my biggest motivation for growthand learning.I would also like to thank all of my great friends, for being there for meand supporting me.xivChapter 1Introduction1.1 MotivationThe continual advancement in communications has placed a growing demandon the innovation of optical communication systems to proceed to deliverintegrated photonics and systems that are low cost, small sized and highbandwidth[4]. The region near 1550 nm emission is of interest, since opticalfibers are available with losses as small as 0.15dB/km at this wavelength,makingit agood candidate for long-distance optical communications [5]. Ac-tive ring resonators are desirable for optical monolithic integrated systems;they have been explored for applications such as optical amplifiers, opticalflip flops (memory), optical digital processing (digital logic and switching),filters, and lasers. These active resonators can solve many drawbacks thatpassive resonators have been experiencing such as 1) lack of tune-ability 2)high insertion losses and 3) high optical switching powers [6]. One excitingarea of interest is to use these active resonators as ring lasers for monolithicoptically injection-locked laser systems, without the use of an isolator [4].The modulation bandwidth (in GHz) of a directly modulated laser re-veals the communication systems maximum achievable data rate (Gb/s).The best modulation bandwidths achieved by edge emitting lasers are up1Chapter 1. Introductionto 40GHz [4]. Optical injection locking (OIL) can significantly improvethe performance of directly modulated lasers. Using SRLs in OIL allowsthe removal of optical isolators by relying on the lasers’ output asymmetry.Moreover, SRL cavity do not have any mirrors, gratings or cleaved facets;the rings’ physical structure (i.e. width, height, radius) determines the op-tical modes. It is these inherent qualities of SRL that make it an excitingcandidate for future optical interface technologies such as backplane andinter-chip communication [4].1.2 Operating PrincipleSRLs are made of two main components: 1) a straight bus waveguide and a2) bent waveguide forming a closed loop. The region where the bus waveg-uide is close enough to the ring waveguide such that the fields in the twowaveguides can interact is called the coupler region. The lasing action oc-curs in the bent waveguide resonator structure. Power is then coupled out ofthe resonator by the bus waveguide (power coupling factor affects the deviceperformance and is a function of the coupler region design). To couple thepower out of the bus waveguide, either a cleaved facet or a grating coupleris required.Laser systems require three major components: 1) a pump (optical orelectrical) to provide carriers required for light emission 2) an optical feed-back to provide oscillation and 3) a gain region to provide amplification.When current is applied to the ring/racetrack structure, lasing recombi-nation takes place in the higher refractive index, active quantum well (QW)2Chapter 1. Introductionregion where spontaneous emission, stimulated emission and absorption areall happening simultaneously. When the laser is biased below threshold,only spontaneous emission can be observed. Laser threshold is reached oncethe device is biased at high enough currents such that the gain equals thecavity losses; the laser turns on at threshold. Increasing the bias currentabove threshold results in higher output power as long as the device is notoverheating [5]. The wave-guiding layers provide orthogonal optical confine-ment. For a shallow etch design (where the waveguide structure is definedabove the QWs), lateral confinement is defined by the air/wave-guiding layerrefractive index contrast.Laser oscillation in most laser systems such as in a Fabry-Perot Etalonlaser, is achieved by the constructive interference of two counter propagatingwaves travelling between the two Fabry-Perot mirrors and forming a stand-ing wave in the cavity. In the ring laser, the light is confined to circulatearound the ring. However, two counter propagating waves can travel aroundthe ring structure. One of the two counter propagating waves can be sup-pressed by experiencing higher losses and thus a unidirectional output canbe obtained from the ring laser.1.3 Literature ReviewThe preferred laser fabrication method used in industry for regular ridgewaveguide (RWG) edge emitters and vertical Cavity surface emitting lasers(VCSELS) is wet etching or a combination of dry and wet etching. Wetetching techniques are preferred due to their lower processing costs, lower3Chapter 1. Introductionprocessing time and lower optical losses [7–10]. To the best of our knowledge,there has been no reports of SRLs fabricated via wet etching techniques.Figure 1.1: Geometry of the device showing the contact layout; IR,IW1,IW2indicate the current biases applied to the ring and to the two outputwave- guide contacts, respectively. The ring radius is 600 µm and the outputwaveguides are 800 µm long. Figure from Ref [11].There are a few groups making SRLs in InP based materials, all withdry etching. In 2002, M. Sorel and P. J. R. Laybourn at the University ofGlasgow and G. Giuliani and S. Donati at the University of Pavia publishedresults on unidirectional bistability in large-diameter semiconductor ringlasers fabricated by CH4H2reactive ion etching (RIE). Their design is shownin Figure 1.1 and had the following design parameters: ring radius of 600µm,total length of the coupled waveguide of 1.6 mm, ring to output waveguidespacing of 1 µm, waveguide width of 2 µm [11]. They reported continuouswave measurements at room temperature exhibiting a threshold current of125 mA. Figure 1.2 shows the optical spectrum of their device when biased4Chapter 1. IntroductionFigure 1.2: Showing a switching extinction ratio larger than 30 dB. Theresolution of the optical spectrum analyzer is 0.1 nm. Figure from Ref [11].at 175 mA.L. Bach, J. P. Reithmaier, A. Forchel, J. L. Gentner, and L. Gold-stein fabricated single-mode lasers by coupled micro-square resonators inGaInAsP-InP epitaxy; their stucture is shown in Figure 1.3. The deviceswere fabricated using Ar-Cl2electron cyclotron resonance (ECR)-RIE pro-cess and had width 4 µm, height 6 µm, radius 10-30 µm and thresholdcurrents as low as 48 mA [12].K. Amarnath, R. Grover, S. Kanakaraju and P. T. Ho from universityof Maryland, published results on 0.8-1 µm wide, 20 µm radius, 5-10 µmlong coupler length, deep etched, 1550 nm emitting microring lasers in 2005.Their design is shown in Figure 1.4. They were able to achieve thresholdvalues as low as 12-20 mA [13].5Chapter 1. IntroductionFigure 1.3: Schematic illustration of the device geometry with deeply etchedRWG section and a strongly MMI-coupled pair of ring resonators with dif-ferent diameters D1and D2. The rings have square-like geometry with 45 Cfacets of width e. The resonators are also covered by BCB like the left sideof the RWG. Figure from Ref [12].6Chapter 1. IntroductionFigure 1.4: Schematic cross section of microring amplifier shoing the layerstructure. Inset shows an SEM picture of a 20 µm radius microring device.The bus-waveguide (not visible) is shown as a dashed line. Figure from Ref[13].7Chapter 1. IntroductionFigure 1.5: A cross sectional view in the coupling region of the waveguidesetched down to the InAlAs layer and subsequently covered with 200 nmPECVD SiO2 (after removing the etching mask) for electrical passivation.The effect of small gaps on the etching: the coupler is not etched to theInAlAs layer. Figure from Ref [14]S. Furst, M. Sorel, A. Scire, G. Giuliani and S. Yu Pavia published detailson their high selectivity (between the upper cladding InP and the core layerAlInAs) RIE process in 2006. They achieved very smooth surfaces and acomplete etch depth in the gap between the waveguide couplers [14]. Theirdry etching recipe allowed precise control over the coupling (gaps as small as0.5 µm) as well as reduction in losses resulting in a lower threshold currentof 34 mA for a 150 µm ring radius for a structure emitting at 1300nm.They predicted theoretical values of 60 mA and 20 mA for rings of radii 100µm and 170 µm correspondingly for an epitaxy emitting at 1550 nm [14].Figure 1.5 shows the etching challenge of achieving a full etch depth in thegap region which was then solved by over etching with a selective recipe.S. Park, S. S. Kim, L. Wang, and S. T. Ho published results on high-8Chapter 1. Introductiondensity plasma etching of InP based SRLs using Cl2-N2based gas mixtures.The addition of N2had an excellent effect on the InP reaction chemistry;it helped reduce the surface roughness and lateral etching. They fabricatedSRLs with diameters 10 µm and 20 µm, gaps as low as 0.2 µm, waveguidewidth 0.8 µm, and threshold currents as low as 1.1 mA. In order to clear the0.2 µm gap, the devices were over etched to achieve a 3 µmetchdepthinthe gap region and a 5 µm etch depth outside the gap region [15]. Figure1.6 shows an SEM image of their fabricated SRL and Figure 1.7 shows thedevice spectrum.Figure 1.6: SEM image of a 0.8-µm-wide waveguide-coupled 20-µm-diameterring resonator etched by ICP with 10/35/10 sccm Cl2N2Ar plasma at 200-W ICP power, 350-V dc bias, 2.3-mtorr pressure, 250 C temperature, and400-nm/min etch rate. Figure from Ref [15].A. W. Fang, R. Jones, H. Park, O. Cohen, O. Raday, M. J. Paniccia,and J. E. Bowers at the university of California reported a SRL integrated9Chapter 1. IntroductionFigure 1.7: Spectra of scattered light from 20-µm-diameter microring laserand currentoptical output characteristics of a 20-µm-diameter microringlaser (inset). Figure from Ref [15].Figure 1.8: The hybrid silicon-evanescent device cross section structure.Figure from Ref [16].10Chapter 1. Introductionwith two photo-detectors on the hybrid AlGaInAs-silicon evanescent deviceplatform. The SRL had a 200 µm radius, 0.5 µm gap, and 600 µm parallelcoupler length. Running on continuous-wave at 1590 nm the laser had athreshold of 175mA, maximum total output power of 29 mW and a maxi-mum operating temperature of 60 C [16]. The device cross section is shownin Figure 1.8.Figure 1.9: SEM image of fabricated SRL. Figure from Ref [14].In 2008, Sorel and group made dry etched SRLs with tuneable evanescentcouplers. Their fabricated design is depicted in Figure 1.9. They reported acoupler tuning ratio from 90% down to 10-20% with current injection valuesas low as 10 mA [14].11Chapter 1. Introduction1.4 ObjectivesThe main objective of this thesis is to design, fabricate, and characterizeSRLs in InP based materials. A large part of the project was invested inthe fabrication of the SRLs. We aimed to develop a fabrication process forthe SRLs via wet etching techniques.1.5 Novel ContributionsThe LED characteristics of the fabricated devices via wet etching were ob-served and successfully measured. The spectrum of the device was alsomeasured with OSA and resonances were observed. The following were ac-complished in the fabrication process:1. A selective multiple step wet etch technique was developed to achieveInP waveguides with smooth surfaces, in-plane isotropy (equal etchrates in the x and y cross section), orthogonal to in-plane etch rateratio of 2.2. A planarization technique was developed using polyimide without anymasking or plasma exposure.1.6 Thesis OrganizationSRL design was divided into 3 main sections:1. Mode simulations were performed via Finite Difference Time Domain12Chapter 1. Introduction(FDTD) mode solver to determine the main propagating mode, con-finement factor, group index, crossover length and κ2. Plots to summarize the dependency of output power and thresholdcurrent on cavity length, κ and propagation loss were obtained usingMatlab.3. Double bus passive resonators were designed in silicon on insulator(SOI), fabricated by Interuniversity Microelectronics Centre (IMEC)to further study the racetrack resonator, test the design process, anddevelop a parameter extraction method. The drop port output wassuccessfully measured and the design parameters were extracted. Thepredicted design parameters matched the extracted parameters fromthe measurements, verifying the design method.After the design section the devices were fabricated at the Advanced Materi-als and Process Engineering Laboratory (AMPEL) Nanofabrication facilityin UBC. The following steps were taken and will be discussed in more detail:1. The required mask was designed via a script and purchased. A com-mercial InP epi-wafer was purchased to eliminate epi-wafer design chal-lenges.2. A photolithography process was developed to successfully achieve aresolution better than 1 µm.3. A selective multiple step wet etch technique was developed to achieveInP waveguides with smooth surfaces, in-plane isotropy (equal etchrates in the x and y cross section), orthogonal toin-plane etch rate ratio13Chapter 1. Introductionof 2. A new planarization technique was developed using polyimidewithout any masking or plasma exposure.4. Ohmic metal contacts with reasonably low resistance ( roughly 1Ω )were found for the n-type and p-type contact layers.5. SRL and RWG samples were fabricated via wet etching6. A suitable setup was made for the measurements of edge emittinglasers. Matlab codes were written (some based on previously availablecodes) to automate most of the measurement process.7. Devices were manually cleaved and the LED characteristics and opticalspectrum were successfully measured.8. Dry etching of SRLs were started in collaboration with CNR2 at theUniversit´e de Sherbrooke.14Chapter 2InP Racetrack Laser Design2.1 PurposeThe aim of this chapter is to study the laser theory and design the SRLphysical parameters such as waveguide dimensions, coupling spacings andpredict the threshold and output power.2.2 Mode CalculationThe design is based on a conventional InP laser epitaxy commercially avail-able from LandMark Optoelectronics Inc with details provided in Table 3.2.A simplified structure was used for FDTD mode simulations using Lumer-ical. Because the waveguiding layers are followed by relatively thicker InPlayers, only the waveguiding layers (layers 4 to 7 in Table 3.2) were in-cluded in the simulated structure and the rest of the material was assumedto be that of bulk InP. The index values for the layers were calculated usingthe material information depicted in Table 2.1 and Equations 2.1 and 2.2,all provided by Crosslight (a commercial software used for laser/active opticsimulations and design). For calculating the refractive index of the quantumwell (QW) and barrier layers, initial guesses for the x and y compositions in15Chapter 2. InP Racetrack Laser DesignIn1−x−yAlyGaxAs, based on the following references [17] [18], were inputedinto Equations 2.1 and 2.2 to achieve the material stress values of Table 3.2.The refractive index amplitude profile results are depicted in figure 2.1. TheQWs and wave-guiding layers have higher refractive indices and lower bandgaps compared to the InP n and p regions, thus, providing lateral opticalconfinement.nInAlGaAs= nInAs+x(nGaAs−nInAs)+y(nAlAs−nInAs) (2.1)aInAlGaAs= aInAs+x(aGaAs−aInAs)+y(aAlAs−aInAs) (2.2)Table 2.1: Refractive Index and Lattice Constant DataMaterial Refractive Index Lattice ConstantInP 3.164 5.8688GaAs 3.65 5.65325InAs 3.892 5.6605AlAs 2.92 6.0584The mode simulations via the FDTD mode solver showed 4 differentpossible modes; however, due to their distribution the higher order modeswill be subjected to higher losses (i.e. scattering and bending). Eventuallythe other modes will lose and only the first order mode will be significant tothe lasing. The main mode distribution is shown in Figure 2.2 (a) and the16Chapter 2. InP Racetrack Laser DesignFigure 2.1: The index amplitude profile for λ = 1550 nm simulated in FDTD.The QW refractive index = 3.5889 with In1−0.65−0.15Al0.15Ga0.65As. Thebarrier refractive index = 3.5130 with In1−0.12−0.36Al0.36Ga0.12As. Taking x= 0.03 and y=0.44 as provided in Table 3.2, the upper In0.53Al0.44Ga0.03Aslayer’s refractive index = 3.4254.second order mode distribution is shown in Figure 2.2 (b). The portion ofthe mode in the active region will experience gain. The field intensity datawas imported into Matlab in order to solve for the mode confinement factorΓ, for rectangular waveguides of width W and height H and trapezoidalwaveguide of top width WTand bottom width WB:• Rectangular waveguide: W = 2 µm, H= 2 µm ⇒ Γ = 8.9%• Rectangular waveguide: W = 3 µm, H= 2 µm ⇒ Γ = 8.8%• Trapezoidal waveguide: WT=1µm, WB=3µm, H = 2 µm ⇒ Γ=8.82%The trapezoidal structure was considered to take into account the effectof wet etching during fabrication as it will be discussed later in the Fab-17Chapter 2. InP Racetrack Laser Designrication chapter. However, it seems that the 3 considered structures havevery similar mode confinement factors. The gain experienced by the modevaries with 1/t, where t is the thickness of the active layer. The two mainfactors responsible for achieving low threshold in this semiconductor are: 1)confinement of injected carriers in the QW. and 2) confinement of opticalmode in the QW.Figure 2.2: (a) First order mode distribution for λ = 1550 nm simulatedin FDTD. It has an effective index of 3.276593. (b) Second order modedistribution for λ = 1550 nm simulated in FDTD. It has an effective indexof 3.238451. This mode will not lase since it will experience higher losses(i.e. bending and scattering losses) relative to the first mode.2.3 Racetrack Laser Coupler DesignThe purpose of this section is to design the coupler of the semiconductorracetrack laser (SRL) and derive the coupling relationship with respect tothe coupler’s physical parameters such as waveguide width, height, length18Chapter 2. InP Racetrack Laser Designand spacing.The SRL output coupling loss (analogous to the mirror loss), is calcu-lated using FDTD mode solver in combination with the super-mode couplingtheory.An exchange (coupling) of power between the modes of two waveguidesoccurs where the guided mode functions have a physical overlap region.Since the evanescent field has an exponential decay, there will always besome coupling between the guided modes, however at very large separationdistances between the waveguides the suffers coupling significantly. ThePower coupling between waveguides is dependent on many parameters suchas the waveguide structure (waveguide dimensions, separation (s), and ma-terial), wavelength, and the distance (z) travelled by the electromagneticwave along the waveguide [19].The following assumptions were made for the coupling calculations:• The couplers were rectangular, identical and phased matched• The coupling was mainly due to the parallel region of the waveguides.Coupling in the bent region leading to the parallel section was ignored.• The couplers were lossless• The coupling was unidirectional• The waveguide was transverse electric (TE) guided• The bent effects on the guided mode were ignored19Chapter 2. InP Racetrack Laser DesignFigure 2.3: The E intensity for the coupler with rectangular waveguides ofwidth=2 µm, gap=0.5 µm, λ=1.55 µm. (a) Symmetric/Even supermode.(b) Anti-symmetric/Odd supermode.20Chapter 2. InP Racetrack Laser DesignUsing the Lumerical software integrated with FDTD mode solver pack-age, the effective indices neff1and neff2for the even and odd supermodeswere extracted for waveguides of width W = 2 µm and W = 3 µmwithvarying coupler spacings. Figure 2.3 depicts an example of such simulationfor the even and odd modes for a coupler with W = 2 µm and Gap =0.5 µm. The rectangular waveguide structure used for the effective indexsimulations has the epi-wafer layer structure of a typical commercial InPepi-wafer which was purchased for our fabrication. This epi-wafer structuredetails are shown and discussed later in the InP Material Structure sectionof chapter 2.Let the two electromagnetic modes with power amplitudes Paand Pbrepresent the power in the input coupler waveguide and the power in thecoupled waveguide, respectively. The power coupling coefficient κ, equiva-lent to the square of the field coupling coefficient is defined byκ =PbPa(2.3)Onceneff1andneff2areextracted, usingthesuper-modetheorythecrossoverlength (coupler length at which 100% coupling occurs) Lx, and κ were cal-culated as [20]:β1=2πneff1(λ)λ(2.4)β2=2πneff2(λ)λ(2.5)where β is the propagation constant in units of rad/m. The cross over length21Chapter 2. InP Racetrack Laser Designoccurs at a distance travelled by the propagating fields where the two fieldshave a phase difference of π.Lx(λ,s)=λ2(neff1(λ)−neff2(λ))(2.6)Coupling per length = κ∗=πLx(λ,s)(2.7)Pb= Pasin(κ∗z)2(2.8)The exponential dependancy of Lxon the waveguide separation is de-picted in Figure 2.4.0.5 1 1.5 20123456789x 104Lx vs. Gap, W = 3µm H = 2µmGap (µm)Lx (µm)Figure 2.4: Lxas a function of separation between waveguides. This plotwas obtained for a 3 µm wide rectangular structure.Figure 2.5 shows the exponential relationship between the coupled mode22Chapter 2. InP Racetrack Laser Design0.5 1 1.5 210−510−410−310−210−1100Power Coupling vs. Gap, H = 2 µmGap (µm)κ  Lκ = 200 µm, W = 2 µmLκ = 250 µm, W = 2 µmLκ = 200 µm, W = 3 µmLκ = 250 µm, W = 3 µmFigure 2.5: The evolution of κ is shown as a function of coupler waveguidespacing for waveguides with coupler lengths 200 µm and 250 µm at 1550 nmwavelength. The case for a constant waveguide height of H = 2 µmwithwidths of W = 2 µm and W = 3 µm, are shown. The data was obtainedusing FDTD mode solver and the super mode theory.23Chapter 2. InP Racetrack Laser Designpower and the waveguide spacing for waveguides of widths 2 µm and 3 µm,with a fixed height of 2 µm at 1550 nm wavelength. Tables 2.2 and 2.3 showthe simulated coupling design parameters for the SRLs that were fabricatedin this project. These coupling values are rather small, which result in lowoutput power values in the µW ranges ( the detectors in the lab are verysensitive and can measure power down to nW ranges). To increase the powercoupling (this is analogous to the increase in the effective index differencebetweenneff1and neff2), smaller gaps are required (>0.5µm); however, thisis challenging for the fabrication process. These lower coupling factors areadvantageous for the initial devices since they result in a lower threshold.After the first devices are fabricated and their lasing characteristics aremeasured successfully, the mask could be re-designed and the fabricationprocess improved in order to optimize for power (while satisfying reasonablethreshold conditions) by increasing the power coupling coefficient.Table 2.2: Power Coupling for Rectangular Waveguides W =2 µmH=2µmGap (µm) neff1neff2κaκbLx(µm)0.5 3.257165 3.255365 0.444 0.625 4301 3.25653 3.256057 0.036 0.056 16381.5 3.256362 3.256239 0.00248 0.00388 63012 3.256485 3.256453 0.00017 0.00026 2421824Chapter 2. InP Racetrack Laser DesignTable 2.3: Power Coupling for Rectangular Waveguides W =3 µmH=2µmGap (µm) neff1neff2κaκbLx(µm)0.5 3.264419 3.263803 0.061 0.094 12581 3.26417 3.264023 0.0036 0.0055 52721.5 3.264131 3.264095 0.00021 0.00033 215272 3.26417 3.264161 0.00001 0.00002 86111As later will be discussed in the wet etching fabrication section of thisthesis, the 3 µm wide waveguide devices have a better chance of being suc-cessfully fabricated. Moreover, the waveguides end up with a more trape-zoidal structure rather than a rectangular structure after the wet etchingprocess. To take into account the wet etching effects, a trapezoidal struc-ture was made in the FDTD solver. The first order mode distribution for thetrapezoidal structure is shown in Figure 2.6 and its corresponding refractiveindex profile and field distribution are depicted in Figure 2.7.The trapezoidal structure results in even slightly lower coupling values.The coupling could be increased via deposition of a thin dielectric film. Thisdielectric film deposition will also provide planarization as will be explainedin the Fabrication chapter. Considering a roughly 200 nm thick layer of poly-imide with a refractive index of 1.7, trapezoidal structures were simulatedin the FDTD mode solver to obtain the coupling information by consider-25Chapter 2. InP Racetrack Laser DesignFigure 2.6: The first order mode distribution in the trapezoidal waveguidestructure with H = 2 µm, Wbottom=3µm and Wtop=1µmisshownhere.Figure 2.7: Aspects of the trapezoidal waveguide structure with H = 2 µm,Wbottom=3µm and Wtop=1µm : (a) the refractive index profile; (b) theelectric field profile for a mode traveling in the y-direction.26Chapter 2. InP Racetrack Laser Designing the wet etching fabrication process. Figure 2.8 shows this waveguidestructure for the case of a coupler with a 0.5 µm waveguide spacing.Figure 2.8: Trapezoidal waveguide structure with a uniform 200 nm thickpolyimide layer. The refractive index distribution for a coupler with a 0.5µm spacing is shown here.Table 2.4 gives a summary of the extracted coupling information andTable 2.5 gives a summary of the expected device performance for the largestpossible cavity design with bend radius R = 300 µm when biased at a currentof 100 mA ( this value is higher than the threshold current). Table 2.6 givesa summary of the expected device performance for the largest possible cavitydesign with bend radius R = 50 µm when biased at a current of 100 mA. In27Chapter 2. InP Racetrack Laser Designthe Tables, the subscripts a and b refer to a coupler length Lκa= 200 µmand a coupler length Lκb= 250 µm, respectively.Table 2.4: Power Coupling for Trapezoidal WaveguideWbottom=3µm, Wtop=1µmwith 200 nm Thick PolyimideGap (µm) neff1neff2κaκbLx(µm)0.5 3.264004 3.263390 0.061 0.094 12601 3.263763 3.263606 0.004 0.0063 49361.5 3.263763 3.263679 0.0012 0.0018 92262 3.263763 3.263753 0.00002 0.00003 77500Table 2.5: Predicted Performance for Trapezoidal Waveguide Wbottom= 3,µmWtop=1µm, R = 300 µm and a 200 nm Thick Polyimide Planarization,Biased @ 100 mAIth(mA) PoutµWGap (µm) 1a1bab0.5 89 85 365 4301 81 84.5 25 301.5 80 84 7 92 80 84 1 2One of the parameters for the SRL that can be measured is the freespectral range (FSR). Taking into consideration the effect of wavelength onthe effective index the FSR is [19] :28Chapter 2. InP Racetrack Laser DesignTable 2.6: Predicted Performance for Trapezoidal Waveguide Wbottom=3µmWtop=1µm R = 50µm and a 200nm Thick Polyimide Planarization,Biased @ 100mAIth(mA) PoutµWGap (µm) 1a1bab0.5 26 30 4300 55001 25 29 300 4001.5 25 29 140 1002 25 29 15 19FSR=∆λ =λ2ngd(2.9)where d is defined as the total cavity length of the racetrack resonator inEquation 2.10 [19], R is the radius of curvature and Lκis the parallel lengthin the coupler region.d =2πR+2Lκ(2.10)The parameter ngis called the group index and it takes into account theeffective index dependence on the wavelength. For further information onFSR and ngplease refer to the reference [19]. FDTD simulations showapproximately a linear dependency of the effective index and the groupindex on wavelength. Figure 2.9 shows this result.2.4 Racetrack Laser Resonator DesignWe are primarily interested in the LI characteristic of the SRL. First, theeffect of coupling (κ) and resonator length (d) on the output power and29Chapter 2. InP Racetrack Laser Design1.4 1.451.5 1.551.6 1.653.253.33.353.43.453.53.55neff and ng vs. λ, W = 2µm H = 2µmlambda(µm)ng and neff  ngneffFigure 2.9: Effective index and group index as a function of wavelength. Thecalculations for group index here take into account the waveguide dispersionsand the bulk InP material dispersion.30Chapter 2. InP Racetrack Laser Designthreshold current were investigated. Then, an estimated LI curve was pre-dicted for the designed SRLs. These analyses were all carried out usingMatlab. Table 2.7 lists the parameter values used.Table 2.7: Simulation Parameters. Data taken from LectureNotes of Dr.Lukas Chrostowski and Ref [1]Symbol Boiling Point (Description) Value or Rangeλ Wavelength 1550 nmB Gain slope (near transparency) 1.5×10−16cm2￿ Gain compression 2×10−7ηiQuantum efficiency 0.9NtrTransparency carrier density 3.3×1018cm−3τsCarrier lifetime 2nsThe equivalent for the mirror reflectivity R (as normally seen for a Fabry-Perot Etalon) for the racetrack resonator is (1-κ). Thus, the output couplingloss was calculated [5]:αm=(1d)ln√(1−κ) (2.11)The additional propagation losses were roughly chosen to be 5 cm−1.Thisis a pessimistic value based on taking a higher value compared to reportedvalues such as 0.34 cm−1[21].31Chapter 2. InP Racetrack Laser DesignThe threshold and output power for the laser can be expressed [5].Ith=ngc(ΓBτp)+NtrτsηiqV (2.12)Pout= ηiακα + ακhνq(I −Ith) (2.13)Where τpis the photon lifetime and α is the propagation loss here taken tobe 5 cm−1.τp=ngc[α−αm](2.14)Figure 2.10 shows the threshold current as a function of κ for fixed ringradii. τpincreases for larger ring radii given a fixed κ,whichinturnreducesthe threshold Ith; however, at the same time the volume is increasing forlarger ring radii, which will increase Ith. Overall the increase in volumeovercomes the increase in τpresulting in a linear relationship between thering radii and the threshold current Ith. Increasing κ for a fixed ring radiusallows more photons to escape in the coupling region; in other words, itreduces the photon lifetime; for high κ, this reduction in τpis followed byan exponential increase in Ith.Increasing the input current results in an increase in the unsaturatedgain and the intensity inside the laser; this increase in the bias effectivelyshifts the Poutvs. κ curve to higher values as depicted in figure 2.11. Thepeak of each curve corresponds to the optimum output coupling (κ). Forhigher bias points the output coupling needs to shift to slightly higher values32Chapter 2. InP Racetrack Laser DesignFigure 2.10: Ithof the laser is plotted as a function of κ for an increasing ringradius and a coupler length of 200µm. Ithhas an exponential dependencyon κ and almost a linear dependency with the ring radius. Round trip lossesare taken to be 5cm−10 0.2 0.4 0.6 0.8 11020304050κPout(mW)Pout vs K, Fixed Radius=150um  I=80mAI=90mAI=100mALoss = 5/cmLoss = 0.34/cmFigure 2.11: Poutof the laser is plotted as a function of κ at 3 different biasconditions for a device wtih a coupler length of 200 µm and radius of 150µm. Round trip losses are taken to be 5cm−133Chapter 2. InP Racetrack Laser Designin order to track the peak power. Moreover, Figure 2.11 shows the effect ofincreased losses from a low loss value of 0.34 cm−1[21] to a higher value of5cm−1. Increase in losses could be due to poor fabrication, high bendinglosses and/or mode conversion losses amongst other possibilities. From theseplots it is observed that for lower losses (excluding coupling losses), the peakof the power shifts to lower coupling values at the same time allowing a widerange of couplings to be used for delivering high values very close to the peakoutput power. Thus, reducing losses plays a major roll in the performanceof the device.Figure 2.12 shows the relationship between the output power, couplingefficiency κ and ring radius R. The output power has a rollover behaviourwith respect to κ for a fixed R. Increasing κ allows more photons to es-cape/couple out of the ring, resulting in a higher output power; however,too much coupling will prevent the intensity inside the ring from recover-ing, resulting in a net decrease of intensity inside the ring and thus a dropin the output power. The output power decreases for larger SRLs. Figure2.13 shows that with higher losses, the output power is more sensitive to κ;moreover, higher coupling is required to output more power from the laser.Finally we can predict the LI curve using the laser rate Equations [5].Figure 2.14 shows the LI curve for a SRL with the following specs: W = 3µm, H = 2 µm, G = 1 µm, R = 150 µm, Lκ= 250 µm.34Chapter 2. InP Racetrack Laser DesignFigure 2.12: Output power of the laser is plotted as a function of κ for avarying ring radius and a coupler length of 200 µm. Round trip losses areapproximate to be 0.34 cm−1.Figure 2.13: Output power of the laser is plotted as a function of κ for avarying ring radius and a coupler length of 200 µm. A higher round triploss is taken to be 5 cm−1in the case of poor fabrication, bending losses,and mode conversion losses amongst others.35Chapter 2. InP Racetrack Laser Design0 50 100 150 200 25000.10.20.30.40.50.6Light−OutputCurrent(mA)Pout(mW)Figure 2.14: Output power is plotted as a function of input current for aSRL with the following specs: W = 3 µm, H = 2 µm, G = 1 µm, R = 150µm, Lκ= 250 µm. Round trip losses are approximate to be 5 cm−1.Thisdevice has a threshold of 49 mA and a slope efficiency of 2.5 µW/mA.36Chapter 2. InP Racetrack Laser Design2.5 Double Bus Passive ResonatorThe purpose of this section is to further study the racetrack resonator struc-ture, test the design process and develop a parameter extraction method.Passive racetrack resonators were designed in silicon on insulator (SOI)technologyand fabricated byInteruniversityMicroelectronicsCentre(IMEC).Racetrack resonators were chosen instead of ring resonators in order to en-hance coupling as well as allowing more control over the coupling.The waveg-uide height is fixed to 0.22 µm according to IMEC fabrication steps and thewidth of the waveguide is determined to be 0.4 - 0.6µm to ensure single modeoperation at TE polarization. The buried oxide box is 2 µm thick. Dosagesweeping during the photolithography step provided a variation in the de-signed waveguide widths and spacings. To facilitate light coupling, verticalfiber couplers are chosen, as there is no need for cleaving, and althoughaligning is still challenging they are relatively easier to align. Couplers usedhere should have a coupling efficiency of 30%, with a 3 dB bandwidth of 60nm around 1550 nm as designed by IMEC. SOI racetrack resonators weredesigned with add and drop coupler waveguides [19]. An optical image of afabricated design is shown in Figure 2.15.A 2D finite difference methode integrated with Matlab developed by thePhotonics Research Lab at the University of Maryland is used to estimateneff1,neff2from which the coupling was calculated using the super-modeapproach.The equations [19] to predict the add and drop port responses are asfollows:37Chapter 2. InP Racetrack Laser DesignFigure 2.15: Depicted here is the optical image of a fabricated SOI device.Add : |σt1|2=t21+t22a2−2t1t2acos(δ)1+t21t22a2−2t1t2acos(δ)(2.15)Drop : |σt2|2=(1−|t1|2)(1−|t2|2)a1+t21t22a2−2t1t2acos(δ)(2.16),where t1and t2are the straight through field coefficients, δ = ωd/c [rad]with c [m/sec] being the phase velocity and ω [rad/sec] the angular frequency[19].High quality factor (Q) devices with varying parameters were designed.In the Double Bus design, the response at the drop port is similar to theresponse expected from the SRL output. Figure 2.17 shows the drop portmeasurement for a device with the following specifications: Lx= 391 µm,Ly= 75 µm, radius = 4 µm. Using both the Lumerical FDTD mode solverand the Maryland Matlab mode solver, coupling was calculated to comparethe two methods:• κ from FDTD mode solver = 0.07• κ from Matlab 2-D FDTD = 0.138Chapter 2. InP Racetrack Laser DesignA set up was originally built by Dr. Nicolas Rouger and Nadai Makan,which was then improved to include polarization maintaining lensed fibres,a wider fiber movement range and a 3D microscope movement. A pictureof the setup is shown in Figure 2.16.Figure 2.16: SOI Measurement Setup [22].κ,ng, and FSR were extracted by curve fitting as shown in Figure 2.17.The extracted values were:• κ from curve fitting = 0.14• ng= 4.474622• FSR = 0.56 nmThe κ extracted from measurements is higher than both predictions.This is expected since in the coupling predictions it was assumed that allthe power coupling is from the parallel length region of the couplers and thecontributions from the bent waveguide were ignored. The measured FSR is39Chapter 2. InP Racetrack Laser Designmatches 99% to the prediction by Equation 2.9. The device shows a Q of47000 and a 22 dB extinction ratio. The loss values extracted from thesemeasurements were used in publications by other students. This designapproach has been used for several silicon-on-insulator projects, some ofwhich have been published and presented at conferences [23] [24] [25].154815491550155115521553−65−60−55−50−45−40−35Wavelength [nm]Drop Transmission [dBm]Drop Transmission vs. Wavelength  Curve FitMeasurementFigure 2.17: Drop port response of a SOI racetrack resonator with Lx= 391µm, Ly= 75 µm, R = 4 µm. The solid line is the measurement data andthe dotted line is the curve fitted function from which the parameters κ,FSR and ngwere extracted to be 0.14, 0.56 nm, and 4.474622, respectively.40Chapter 3Fabrication Via Wet Etching3.1 IntroductionThis chapter presents the experiments and results of the investigation offabrication techniques via wet etching utilized for SRLs and ridge waveguideRWG edge emitting lasers in InP based material systems. The objective isto develop a new fabrication process via the wet etching approach for SRLs.To the best of our knowledge, there has been no publication on SRLs madevia wet etching in InP based materials.The preferred laser fabrication method used in industry for regular RWGedge emitters and VCSELS is wet etching or a combination of dry and wetetching. Wet etching techniques are preferred due to their lower process-ing costs, lower processing time and lower optical losses [7–10]. In order toachieve completely anisotropic sidewall profiles and zero in-plane etching,plasma and dry etching techniques have been mainly utilized for the fabri-cation of SRLs; these processes are usually very expensive, more complex,can leave relatively rough surfaces and may even damage the sample. Thisproject investigates the fabrication steps for InP semiconductor ring laserusing simpler, less costly, wet etching techniques with the aim of achievingsmoother sidewalls/surfaces in order to reduce the scattering from the rough41Chapter 3. Fabrication Via Wet Etchingsurface typically present from plasma etched ring lasers. This would improvethe efficiency of the laser and reduce the threshold. The wet etch processrequires some optimization of the process conditions in order to achieve:isotropic etch rate in the x and y cross section, flat surfaces, maximize theetch rate ratio of orthogonal etching to in-plane etching (for higher sidewallverticality) and reduced under cuts.There are 3 major sections in this chapter: InP Material Structure, InPEdge Emitter Racetrack Resonator Laser, InP Edge Emitter Ridge Waveg-uide Laser. In the first section the InP epi-wafer structure details are in-troduced; the same epi-wafer was used for the fabrication investigation ofboth the SRLs and RWGLs. The analysis of the SRLs fabricated via wetetching revealed a set of possible areas for improvements in the fabricationand design. To pin point the main challenge in the SRL fabrication a sim-pler design was made to fabricate RWG edge emitting lasers via wet etching.Finally we started processing dry etched SRLs in collaboration with the fab-rication facility at Sherbrooke. The mask designs and fabrication processesfor each of the devices are described in this chapter. Figure 3.1 show a flowdiagram for the fabrication steps required. Table 3.1 shows an overview ofthe material and equipments used for the fabrication processes.42Chapter 3. Fabrication Via Wet EtchingFigure 3.1: Depicts the fabrication steps required to process an SRL.Table 3.1: Overview of The Fabrication StepsProcess Materials Equipments Settings/ActionSolvent Clean-ingSample HotplateAcetone MicroscopeIsopropanol Waste BeakerDI Water, Com-pressed N2gasContinued on next page43Chapter 3. Fabrication Via Wet EtchingTable 3.1 – continued from previous pageProcess Materials Equipments Settings/ActionWet Etch Sample,H3PO4:HBr:H2O, DIWater, CompressedN2gas2 BeakerstweezersEtch ∼150nm of u-InPSpinning SampleAZ P4110 Spinner 1min @ 3000rpmSoft Bake Sample Hotplate 7min @ 100 COptical Pat-terningSample 320nm Optics 0 Gap contact modeMask Karl SussMask Aligner1 min exposurePattern Devel-opingSample, BeakersAZ400k Developer 1:490 secDI Water, MicroscopeWet Etch Sample, CompressedN2gas, DI Water,H2SO4:H2O2:H2O4 Beakers Etch InP and In-GaAs/InGaAsP fora total etch depth of∼2µmContinued on next page44Chapter 3. Fabrication Via Wet EtchingTable 3.1 – continued from previous pageProcess Materials Equipments Settings/ActionH3PO4:HBr:H2O Dektak Pro-filometer3.2 InP Material StructureThe basic layered structure is grown epitaxially on a crystalline InP sub-strate by metal organic vapour phase epitaxy (MOCVD) at LandMark Op-toelectronics Corp. in Taninan, Taiwan. The orientation of the substrateused is (100) +/- 0.1 deg. Table 3.2 shows the epi-wafer structure in detail.This design is a commercial product used for edge emitting lasers and weuse this proven design as a starting point for our fabrication.The substrate is n+Sulphur doped InP with layers grown on top asfollows: a lower cladding including a 500 nm n-type InP layer and a 185 nmtransition quaternary layer; a 100 nm waveguide core; an upper claddingincluding a 75 nm InAlGaAs transition quaternary layer, a 50 nm InAlAsetch stop layer, a total of 72 nm transition quaternary layers, a 1.8 µmP-type InP layer, 50 nm of InGaAsP transition quaternary layers and finallya p-type InGaAs contact layer.For 1550 nm emission:Eg =1.24eV1.55µm=0.8eV (3.1)45Chapter 3. Fabrication Via Wet EtchingTherefore, the bandgap of the QW is 0.8 eV. The bandgap energy is grad-ually increased as we move away from the QW with changes in the x and ycomposition ratios. Using InP as substrate a range of lattice-matched qua-ternaries extending from InP to the InGaAs ternary line are accommodated.Here the quaternary is specified by an x and y value, i.e., In1−xGaxAsyP1−y.Choosing x equal 0.46 y results in approximate lattice matching to InP [2].This structure includes 6 InAlGaAs QW layers each 6 nm thick, alter-nating with 7 barriers each 9 nm thick (In mode calculations the barriersare not included), sandwiched between two regions of varying layers (forsimplicity of mode calculations here considered to be just InP).LandMark provided some test data of this epi-wafer including a photo-luminescence (PL) measurement reporting a peak wavelength of 1512.5±5nm. The PL was measured again at UBC by Ph.D student Wei Shi and thedata are shown in Figure 3.2.Table 3.2: R2 - 1550 nm FP-LD Epi-wafer Structure[2]# Name Value Unit ThicknessAccuracy0 InP Substrate S-Doped2−8×1018cm−31 N-InP Buffer Layer 0.5 µm ±10%(Concentration) 1×1018cm−3±20%2 N −In0.53AlxGa0.47−xAs 0.01 µm ±10%Continued on next page46Chapter 3. Fabrication Via Wet EtchingTable 3.2 – continued from previous page# Name Value Unit ThicknessAccuracy(x:0.31→0.44) 1×1018cm−3±20%(Concentration)3 N −In0.52Al0.48As 0.1 µm ±10%(Concentration) 1×1018cm−3±20%4 U −In0.53AlxGa0.47−xAs 0.075 µm ±10%(x:0.44→0.3)6× InAlGaAs QW 5.5∼6/ nm(+1.0∼+1.15% CS)/5 7× InAlGaAs Barrier 9 nm ±10%(−0.45∼−0.55% TS) (1512.5) nm ±5%(λPL)6 U −In0.53AlxGa0.47−xAs 0.075 µm ±10%(x:0.3→0.44)7 U −In0.52Al0.48As 0.05 µm ±10%8 P-InP Layer 0.05 µm ±10%(Concentration) 5∼8×1017cm−39 P-InGaAsP Layer 0.022 µm ±10%(Concentration) 5∼8×1017cm−310 P-InP Layer 1.8 µm ±10%(Concentration) 1→1.5×1018cm−3Continued on next page47Chapter 3. Fabrication Via Wet EtchingTable 3.2 – continued from previous page# Name Value Unit ThicknessAccuracy11 P-InGaAsP Layer 0.025 µm ±10%(Concentration) > 3×1018cm−312 P-InGaAsP Layer 0.025 µm ±10%(Concentration) > 3×1018cm−313 P-InGaAs Layer 0.15 µm ±10%(Concentration) > 1×1019cm−314 U-InP Layer 0.15 µm ±10%Figure 3.2: The epi-wafer PL data at room temperature is shown here.Courtesy of Wei Shi.48Chapter 3. Fabrication Via Wet Etching3.3 InP Edge Emitter Racetrack Resonator Laser3.3.1 MaskOverviewOnce a range of possible SRL design parameters was determined, the re-quired mask was designed using a Matlab program integrated with Clewin(a drawing software). Mask was done using a script to facilitate varying theparameters.The mask was purchased from the Infrastructure of Nanosctructures andFemtosciences (INRS) Centre in Quebec.At INRS two types of direct writing techniques were available: a laserwriter, an e-beam writer. The e-beam mask was chosen because it providesthe highest possible resolution and that is necessary to obtain the 0.5 µmgaps in our design.The mask patterns were defined in chromium on a 5” × 5” glass platewith a thickness of 0.09”. The mask was later on cut down into a 4” × 4”plate in-order to fit the Karl Suss mask aligner used at the UBC AMPELNanofabrication facility. Figure 3.3 shows the mask design.The fabrication required two photolithography steps thus two mask lay-ers: Layer 1 to form the waveguide ridge (both the coupler waveguide andthe racetrack waveguide), Layer 2 to define the contact opening before met-allization. For Layer 1, two designs were printed on the mask. Design ofLayer 1(a) as shown in Figure 3.4 is simple and appropriate for wet etchingtechniques; however, the exposed area in this design is very large for dry49Chapter 3. Fabrication Via Wet EtchingFigure 3.3: The SRL mask layout. This is the final version of the masksubmitted. The metal layer is inverted.50Chapter 3. Fabrication Via Wet EtchingFigure 3.4: Layer 1(a) shows the etch mask layout used for wet etching.Figure 3.5: Layer 1(b) shows the etch mask layout. A trench design isapplied for the case of dry etching.Figure 3.6: Layer 2 shows the top contact mask layout. This is beforeinversion, thus the coloured region would be the contact area.51Chapter 3. Fabrication Via Wet Etchingetching techniques and causes reduction in the uniformity throughout thewafer. In order to reduce the exposed area and increase the reliability ofuniformity throughout the processed sample specially when dry etching, asecond design Layer 1(b) using trenches was drawn as shown in Figure 3.5.Figure 3.6 shows the layer 2 design before inversion.SRL Design VariationDue to the approximations in modelling (for example ignoring coupling dueto the bend region, estimating the overall losses, etc.), potential unexpectedimperfections and/or limitations in the fabrication process, and also for thepurpose of device characterization, the SRLs were designed with varying pa-rameters. The coupling is a function of the effective index, coupling spacingand coupler length. In order to study the coupling behaviour and its effectson quality factor, output power, resonance frequency, bandwidth and sen-sitivity, different coupling lengths each with several varying coupling gapswere designed. Over 80 different designs were included on the mask with acombination of the following parameters:• Bent Region Radius: 150 µm, 160 µm, 180 µm, 200 µm, 250 µm, 300µm• Coupling Gap: 0.5 µm,1 µm, 1.5 µm, 2 µm, 2.5 µm• Coupler Lengths: 200 µm, 250 µm• Waveguide Width: 2 µm, 3 µm52Chapter 3. Fabrication Via Wet EtchingEvery device included a label to identify its parameters. The labels haveaformatof LxxxRxxWxGxxx. Asanexample alabelreadingL200R150W2G1,identifies a device with coupler length 200 µm, bent radius of 150 µm, waveg-uide width of 2 µm and coupling gap of 1 µm.Alignment MarksThere are two important alignment steps required with the current maskdesign. First, when using Layer1 of the mask, the mask patterns shouldbe aligned to the edge of the sample in such a way so that the waveguidesrun parallel to an edge of the sample. This alignment is necessary for thecleaving step required to achieve the output facet of the laser. If the patternsare misaligned with the edge of the sample, when the sample is cleaved alongits crystallographic orientation, many devices can be damaged and thus thefabrication yield is reduced. The second critical alignment is between Layer1 and Layer 2 of the mask. This alignment has a large tolerance, roughly ±10µm since the metal pattern is very large. Large cross like alignment markerswere placed throughout the mask to assist with coarse alignment. For fineralignment, vernier scales were designed with 6 µm to 6.2 µm spacings toimprove the alignment accuracy up to ±2 µm.3.3.2 Sample PreparationInP is very brittle; compared to Si it is a few times easier to break or shatterInP. The cleaving process was done manually. A tip of a diamond cutter wasused to apply pressure to a point on the InP wafer. Once enough pressurewas applied, the sample broke along its crystallographic orientation. The 2”53Chapter 3. Fabrication Via Wet Etchingepi-wafer was cleaved into small pieces of roughly 1 cm by 1 cm.3.3.3 PhotolithographyCleaningInP based fabrication is costly; high yields are demanded for making prod-ucts more affordable. Cleanliness is a critical factor in achieving reliable,reproducible results with a high yield in all micro-fabrication processes in-cluding semiconductor laser fabrication. The cleaning process refers to theremoval of any undesirable material from the sample surface prior to anyfurther processing. Examples of such undesirable materials include chemicalresidues from previous steps, debris from cleaving or particles due to poorenvironmental control. Insufficient cleanliness leads to degradation of theprocess quality or sample integrity (because it can crack during alignment).Photoresist (PR) spin-coating and patterning, wet or dry etching, polymerapplication and oxidation for passivation and planarization, and metal ad-hesion are all affected by the initial cleanliness conditions of the samplesurface. The environment cleanliness is defined by the size and number ofparticles present in the fabrication environment. The UBC yellow room fa-cility is a class 1000, which means the maximum concentration of airborneparticles larger than 0.5 µm in diameter is controlled to be less than 1000particles/ft3. Moreover, the cleanroom atmosphere is both temperature andhumidity controlled [26].Organic solvents were used to remove any organic material such as PRand oil, grease or wax residues from the InP surface. Acetone was used54Chapter 3. Fabrication Via Wet Etchingto rinse the sample surface; unfortunately it may also leave some residuebehind. Isopropanol or methanol was used to rinse away any residues leftbehind after the acetone rinse, then the sample was rinsed by de-ionizedwater and dried with a compressed nitrogen gas. Some properties of thesolvents used are shown in Table 3.3.Table 3.3: Solvent and Properties (Data from [3])Solvent BoilingPoint (◦C)FlashPoint(◦C)WaterSolubilitySafetyAcetone 56.3 -16 100% FlammablePropanol-2 82.3 22 100% Flammable(Isopropylalcohol)Methanol 64.7 12 100% FlammableThe solvents used were disposed of in the appropriate non-halogenatedwaste containers [27].After rinsing the sample surface and drying with the nitrogen gun, thesample was heated at 100◦C on a hotplate for 2 minutes to ensure that thesurface was perfectly dried before the PR spinning step. After drying thesample on the hotplate, the sample was left for 2 minutes to cool before PRspinning. The sample surface was then observed under the microscope toensure that it was sufficiently clean. The solvent cleaning procedure was55Chapter 3. Fabrication Via Wet Etchingrepeated as many times as necessary until the surface was acceptably clean.Photoresist Spin-coatingThe next step in the process of photolithography involves PR, a photosen-sitive organic material that is applied to sample surfaces a thin film. It isselectively subjected to UV light that creates exposed areas. Photolithog-raphy in short is a patterning technique used in semiconductor processing.There are two general types of PR, negative and positive. When positiveresist is exposed to UV light, it gains enough energy to change its chemi-cal structure. This chemical change makes the resist sensitive to chemicaletchants and more soluble in the developer. Therefore, any areas where thepositive resist is exposed to UV light are being washed away in the devel-oper, exposing the underlying InP layer. Negative PR works in the oppositeway. Exposure to UV polymerizes the PR and therefore the exposed areasbecome resistant to chemical etchants and do not dissolve easily in the de-veloper. In the negative resist, it is the unexposed portions that are removedduring the developing process. In our fabrication a positive PR was used.A thin layer of roughly 1.4 µm thick PR was applied uniformly on thedie surface via spin coating. The final thickness of the PR film is determinedby the material of the stage surface, the viscosity of the PR and the speedat which the wafer spins. Given the spinning speed, spinning longer than acertain amount of time will not change the final thickness of the PR. Thethickness was determined by a profilometer.After making sure that the sample was completely clean, dry and cool,the wafer was placed onto a spinner. A pipette was used to dispense 2-3 PR56Chapter 3. Fabrication Via Wet Etchingdrops onto the sample which covered the entire surface. The PR was spunat 3000 rpm for 1 min.A successful spin coat should result in a uniform layer of PR. If the layerwas non-uniform or there were dots, lines or areas that were not covered byPR, the process was started from the beginning.Soft BakeThe amount of water present in the PR film determines how soluble theUV exposed area becomes in the developing step. Optical exposure causesa chemical reaction in the PR film to form a ketone. If water is present, theketone will react to form a product (indene carboxylic acid)which is highlysoluble in the developer. In the absence of water, the degraded componentsform cross-linkages with other molecules and solubility decreases. Soft bak-ing removes some of the water and organics in the PR film which is criticalto the PR exposure and developing steps. Some baking is required to hardenthe film, reduces its thickness and improves its adhesion. If the PR is overbaked, it loses its sensitivity to light and thus degrades its solubility in thebasic developer. However, if it is under-baked and there is still a significantamount of organics remained, PR resistance in the developer would degraderesulting in overdevelopment [28] and the sample’s PR can stick to the maskduring the subsequent contact lithography step. Recipe for soft baking isgiven in the steps below[29]:• Adjust the hotplate temperature to 100 C• Place sample on the hotplate for 7 mins57Chapter 3. Fabrication Via Wet Etching• Remove sample from hotplate and keep away from UV light sources• Allow 3 minutes for the surface to cool before exposureOptical PatterningThe AZ P4110 PR absorption spectrum is matched to the emission spectrumof mercury. When the PR is exposed to Hg emission, it gains energy whichcauses a chemical reaction necessary for the developing process. Using theappropriate photon energy and H2O concentration in the resist after softbake, the PR development rate increases. For the positive PR the trans-parent patterns on the mask are the parts that will be exposed and laterremoved in the developing process [28].The Karl Suss mask aligner used at the UBC fabrication facility is amanual system with 320 nm optics, which focuses UV light produced by amercury lamp via a system of lenses onto the mask. This machine is limitedto a maximum wafer diameter of 3” and a maximum mask plate size of 4”by 4”. The distance between the mask and the wafer, the intensity of theUV light, and the exposure time could all be adjusted to achieve the desiredresults [30].The lateral resolution is limited by the emission wavelength and the gapbetween the mask and the sample. The lateral resolution d of an imagetransferred from the mask pattern to the PR has a relationship with thewavelength λ and the distance between the mask and PR g as shown inEquation 3.2 [28]. Figure 3.7 shows the relationship of Equation 3.2.58Chapter 3. Fabrication Via Wet Etchingd =(λg)12(3.2)Figure 3.7: Theoretical maximum resolution that can be achieved as a func-tion of the distance between the mask and the PR surface is shown here.[28]Since the mask has a high resolution demand on the lithography (small-est coupling gap is 0.5 µm), it is very critical to reduce the distance betweenthe mask and the PR surface sufficiently (g<0.5 µm). Profilometer datarevealed that the PR height close to the edges of the sample could be asmuch as roughly 1 µm to 1.5 µm higher than the uniform PR height ev-erywhere else on the sample. This height difference becomes an issue forpatterns requiring lateral resolutions below 2 µm. To resolve this issue, be-fore transferring the mask patterns onto the PR surface a pre-exposure stepwas applied to remove the PR from the edges and corners of the sample.The sample was exposed in contact mode (the gap between the wafer andthe mask was manually closed), for 1min.59Chapter 3. Fabrication Via Wet EtchingPattern DevelopingA low concentration developer solution was prepared with 1 part AZ400kdeveloper and 4 parts DI water[29]. Developing refers to the process ofagitating the sample in the developer solution for the appropriate amountof time needed for the exposed PR to dissolve in the solution. Developingfor 1 min was found sufficient. Thanks to the corner and edge bead removalstep as shown in Figure 3.8 a resolution of 1um was achieved.Figure 3.8: Shows a 1µm resolution achieved by optical lithography using themanual Karl Suss mask aligner and applying the edge beading compensationtechnique.Yeild AnanlysisWith no unexpected issues in the above steps, a 100 percent yield wasachieved on devices with 1µm resolution and higher. Since a near perfect60Chapter 3. Fabrication Via Wet Etchinglithography step is possible under expected conditions, if any defects wereobserved at this point in the fabrication the sample was cleaned and theprocess was repeated once the occurred defects were investigated. Here aresome factors that may contribute to defects and lower the yield:Figure 3.9: Defect revealed after developing due to rough edges present onthe mask. Some of these mask damages were present originally due to themask fabrication quality and some damages developed over time throughusage.Particles inthePRduetocontaminatedsubstrate(insufficient cleaningorinherent defects), insufficient cleanroom conditions, contaminated PR con-tainer or pipette (PR dries very quickly and even reusing the same pipettecould introduce some dry chunks), expired PR, mask contamination(by par-ticles or previous PR), bubbles introduced when dispensing the PR dropletsonto the surface, edge bead. These problems will cause a g>0. Also overor under baking and exposure would effect the developing results. Figures3.10, 3.9 and 3.11 show optical images of some different defects.61Chapter 3. Fabrication Via Wet EtchingFigure 3.10: Defect revealed after developing due to presence of impuritiesin the PR. This problem was resolved by using fresh PR and also ensuringthat the sample surface was cooled before PR spinning.Figure 3.11: PR residue left over after cleaning including the following ap-proaches: over-flooding the PR with the UV and developing, two-solventprocess at room temperature, two-solvent process at close to boiling tem-perature. This is most likely due to expired PR and/or over-baking the PR.A PR stripper solution can usually remove the residues.62Chapter 3. Fabrication Via Wet Etching3.3.4 Wet EtchingWet etching refers to the process of removing layers from the epi-wafersample through chemical reactions that occur at the surface of the materialwhen exposed to liquid etchants. Once the sample has been patterned duringthe photolithography process, some areas are protected by PR from theetchant while the unprotected areas are exposed to the etchant and couldbe removed. Wet etching is used here to form mesa structures for deviceisolation, to define the waveguide structures, and to remove the protectiveu-InP layer.The wet etch requirements for the SRL fabrication are:• High selectivity: An acceptable selectivity allows the etchant to per-fectly remove the desired layer without damaging the underlying layersor completely removing the mask layer. In other words, the etch ratefor the layer to be removed must be higher than that of the mask andthe underlying layers. Selective etchants are needed to control the etchdepth.• In-plane isotropy: Perfect isotropic in-plane etching results in equalerosion of the material in the x and y cross-sections. This is importantfor achieving the same waveguide dimensions everywhere on the SRLincluding the bend regions. Typical InP wet etchants are anisotropic.• Orthogonal to in-plane anisotropy: Ideally, a perfectly straight side-wall profile is desired with no undercuts, however, that is impossiblewith wet etching techniques. It is desired to maximize the etch rate63Chapter 3. Fabrication Via Wet Etchingratio of orthogonal to in-plane etching.Suitable EtchantsThe approach was to adjust the etchant concentrations in already reportedInP etchant recipes to find the most suitable etchant recipe. A suitablerecipe should achieve:• The highest possible in-plane isotropy• Flat bottom• Smooth surfaces• Maximize the sidewall verticality• Minimize the amount of mask undercutThere are various wet etching recipes previously reported by other re-searchers including defect revealing etchants, material selective etchants oretchants for applications that were not highly sensitive to in-plane isotropyas reported in the review paper of Ref. [31] and Ref. [32].These recipes weretested on dummy InP wafer with the help of Dr. Mark Greenberg in orderto determine the most suitable recipe for our application.Table 3.4 shows a summary of a number of approaches taken to achievethe wet etching requirement; the table lists the different etchant systemsthat were tried and the observations made during fabrication testing.64Chapter 3. Fabrication Via Wet EtchingTable 3.4: Etchants TriedEtchant System EtchedMaterialSelective Observations1 H2SO4/H2O2/H2O InP No Large undercut2 H3PO4/HCl InP No in-plane anisotropy3 HCl/ CH3COOH/H2O2InP No In-plane isotropyachieved but veryslanted sidewall profile4 HBr/H2O2/H2O/HCl InP No in-plane isotropyachieved but non-flatbottom profile5 H3PO4/HBr/ H2InP Yes, <4 in-plane isotropyachieved,6 H2SO4/H2O2/H2O InGaAsPand In-GaAsYes, <4 smooth surfacesFigures 3.12 to 3.15 show optical images of test samples etched with theetchants of approach 1 to 4 listed in Table 3.4.Itwas initiallydecidedthatApproach4listedinTable3.4, theHBr/H2O2/H2O/HClsystem, was the most suitable for SRL fabrication. However, after the firstbatch of devices were fabricated, no light was observed from the cavity.Using the Dektak profilometer, the etching profile revealed that there is a65Chapter 3. Fabrication Via Wet EtchingFigure 3.12: 3H2SO4:H2O2:H2O system is highly corrosive (etch ratemainly proportional to Sulphuric acid concentration) and results in largeamount of undercut. Not suitable for our masking patterns.Figure 3.13: H3PO4/HCl systems are not suitable for our masking patterns.(a) High concentrations of phosphoric acid seems to result in holes andnonplanar bottoms. (b)Reducing the phosphoric acid concentration seemsto result in smoother surfaces.66Chapter 3. Fabrication Via Wet EtchingFigure 3.14: Etching results with HCl/ CH3COOH/H2O2systems (a)HighHCl and acetic acid concentrations enhance the in-plane anisotropic etch-ing behaviour. (b)Reducing the HCl concentration improved the in planeisotropy. (c) Reducing the acetic acid concentration to the HCl and hy-drogen peroxide concentration by a factor of 2 resulted in perfect in-planeisotropy; however, the sidewall profile was very slanted.Figure 3.15: Etching results with HBr/H2O2/H2O/HCl systems. (a) Revealsvery non-flat bottom and sidewall profiles with high undercuts.(b)Dilutingthe recipe reduced the etch rate and the undercut behaviour; however, thebottom and sidewall profile is still very non flat. (c)The HBr concentrationwas reduced to help improve the flatness issue and then the recipe wasdiluted 12 times. This recipe was still inappropriate for our masking patternsdue to the bottom and sidewall non-flatness.67Chapter 3. Fabrication Via Wet Etchinghigher etch rate near the waveguide edges, as shown in Figure 3.16. Thisnon uniform etch depth is due to a higher concentration of Br2moleculesnear the edges compared to far away from the edges. Br2molecules diffuseto the mask corners since they can’t react with the substrate in the maskedregions; moreover, due to their low mobility they have a higher probabilityof consumption near the mask corner rather than far away from the cornersas shown in Figure 3.17 [33]. The HBr/H2O2/H2O/HCl systems are com-monly used for spherical lens formation and are not suitable for the SRLfabrication.Figure 3.16: Etching profile with HBr/H2O2/H2O/HCl system shows theBr2-based diffusion limited etching effect observed from the higher etchdepth near the mask boundary relative to far away from it.68Chapter 3. Fabrication Via Wet EtchingFigure 3.17: A schematic illustration of the semiconductor microlens fabri-cation process using Br2-based diffusion-limited etching [33].The most suitable selective (higher etch rate of InP to InGaAs and In-GaAsP) recipe for wet etching InP was observed to be approach 5 of Table3.4, H3PO4: HBr : H2O solution. It resulted in a vertical etch rate thatwas twice as fast as the in-plane etch rate. Similar to approach 4, approach5 system also uses HBr; however, it does not have the non-uniform etchdepth problem of approach 4 since there are no H2O2molecules oxidizingHBr molecules and thus no Br2molecules are introduced into the solution.Theepi-wafer includessomeInPlayersalternatingwithInGaAs/InGaAsPlayers. For removing the InGaAs and InGaAsP layers selectively and stop-ping at the InP layer, approach 6 of table 3.4 was used [34] [35]. This ap-proach resulted in very smooth surfaces as observed with optical microscopyand profilometer measurements.The wet etching process for the epi-wafer of Table 3.2 required switchingbetween approach 5 and approach 6 solutions for a total of 5 steps in order69Chapter 3. Fabrication Via Wet Etchingto selectively remove the 3 InP layers (including the protective layer on top)and the 2 InGaAs/InGaAsP layers and stop at the InAlGaAs layer abovethe QWs. This switching makes use of etch stop layers to allow more controlover the etch depth. Etch depth measurements with the Dektak profilometerafter each etch step was carried out to further monitor the etching process.Table 3.5 shows an example of the data collected to monitor the wet etchingprocess. As seen in table 3.5 the typical total etch time was roughly 5minutes + 40 seconds for the desired etch depth of ∼ 2 µm. However, itwas observed that for every 2 µm etched in the vertical direction, therewas a 1 µm undercut from each side of the mask. This places a limit onthe minimum width of our structures for the desired etch depth, as well asresulting in relatively low steep sidewall profiles (ideally straight sidewallsare desired: slope of infinity). The side profile in the SEM image of a wetetched InP sample in Figure 3.18 shows this 1:2 ratio. Figure 3.19 showsthe device condition as the wet etching time is increased for both 2 µmwidewaveguides and 3 µm wide waveguides.Table 3.5: Wet Etch Monitor Table for Sample : R2 epi-wafer, ID: D04 Date: Jul19/2010DektakSetting[µm/sample]EtchTime[min]PR+MetalHeight[µm]Etchant Step’s EtchRate [µm/min]0.128 0 1.582Continued on next page70Chapter 3. Fabrication Via Wet EtchingTable 3.5 – continued from previous pageDektakSetting[µm/sample]EtchTimePR+MetalHeight[µm]Etchant Step’s EtchRate [µm/min]0.128 2.5 1.763 H2SO4:H2O2:H2O 0.0760.128 2.5 3.502 H3PO4:HBr:H2O 0.6980.128 0.5 3.53 H2SO4:H2O2:H2O 0.030.128 0.117 3.593 H3PO4:HBr:H2O 0.53Overall Etch Rate =Total Etch DepthTotal Etch Time=2.015.62=0.36µm/min.H2SO4:H2O2:H2O and H3PO4:HBr:H2OEtchSystemResultsWith no unexpected issues in the above etching steps, roughly a 30% yieldwas achieved; devices with a 3 µm waveguide width and coupling gaps of 1µm and higher survive. Here are some factors that may contribute to defectsand lower the yield further:• The PR lifting from the the 2 µmwidedevicesintroducesPRstripsthat are free to move around the sample and sit anywhere on thesurface for a random amount of time during the etching process. Thiscauses additional undesired masking effects and could be reduced byavoiding drying the sample with a nitrogen gun (or via applying verylow pressure) in between etching steps.71Chapter 3. Fabrication Via Wet EtchingFigure 3.18: SEM image showing the side wall profile of InP after wet etch-ing.72Chapter 3. Fabrication Via Wet EtchingFigure 3.19: Shows the PR lifting effect due to increasing under cuts as thesample is etched for a longer time. (a)Shows the start of PR lifting at time1 for waveguides of width 2µm. (b) Shows complete PR lifting at time 2> time 1for waveguides of width 2µm. (c)Shows no PR lifting problems attime 1 for waveguides of width 3µm. (d) Shows no PR lifting problems attime 2> time 1for waveguides of width 3µm.73Chapter 3. Fabrication Via Wet EtchingFigure 3.20: Shows defects due to formation of bubbles during wet etching.Also the PR lifting effect is observed in the 2µmwidedevices.74Chapter 3. Fabrication Via Wet Etching• Bubbles forming when using some etchants containing hydrogen per-oxide may cause non uniform etching and introduce undesired maskingeffects as seen in Figure 3.20. This problem can be reduced by agita-tion.• HBr oxidation occurs in a solution that has been used and kept for arelatively longer time. This oxidation results in non uniform etchingobserved due to Br2-based diffusion-limited etching. A change in thecolor of this solution from clear to more reddish is an indicator of thisproblem. A fresh solution of H3PO4:HBr:H2O should always be used.3.3.5 PlanarizationThe aim here is to develop a planarization technique. A successful planariza-tion step must satisfy the following requirements:• Providedevice isolation: The dielectricfilmshouldpreventun-intendedelectrical connections. Thus the film must provide coverage withoutany cracks and sufficiently low porosity.• Providecontactsupport: The dielectricfilmshouldminimizethe heightmismatches throughout the sample to increase contact stability.At first, SiNxand SiO2dielectric film growth followed by an etch backstep was considered for the planarization and passivation process. SiO2filmgrowth and etch processes were successfully developed using the PECVDmachine at the UBC AMPEL Nanofabrication facility. However, this ap-proach required an additional mask layer with an alignment tolerance better75Chapter 3. Fabrication Via Wet Etchingthan 1 µm and it was due to this fabrication limitation that this techniquewas not applied.An alternative to the dielectric film solution is the use of polyimide.Polyimide is a photosensitive dielectric. It has been reported that UV sen-sitive polyimide can be lithographically masked as a dielectric film for pla-narization usage. However, an additional mask layer would be required, alsoaligning this mask layer to the small coupler gap would be very challeng-ing and not reliable with the current mask aligner available. Instead of thetraditional masking of polyimide, a new ’etch-back’ process in a developerwithout UV irradiation was developed. Figure 3.25 shows an image of asample planarized with Polyimide.The process steps are the following:Polyimide HD8220:• Spinning: (1) 60 sec, 500 rpm, (2) 60 sec, 5000 rpm• Softbake: 3 min at 120C on a hotplate• Developing: ∼ 5min in 10% TMA developer for the ’etch back’ pro-cess, in a few steps, checked under microscope until the top of thewaveguides were exposed as monitored by the colour change• Hardbaking; 30 min at 320 C at a hotplatePolyimide spinning process allows a non-uniform film deposition in sucha way that a higher thickness of polyimide is deposited in the trenchescompared to the thickness deposited on top of the waveguides. Spinningpolyimide allows sidewall coverage and partial filling of gaps. This ’filling76Chapter 3. Fabrication Via Wet Etchingeffect’ reduces the height difference throughout the sample, although it doesnot result in a perfectly flat surface. Figure 3.21 shows a flow diagram ofthe planarization step.3.3.6 MetalizationOhmic contacts are needed to allow the flow of electrical current into and outof the laser. A good contact is one that contributes very little to resistance, itis stable and has a linear I-V characteristic. The contact compositions chosenfor the n-type InP and p-type InGaAs layers for our laser were chosen basedon previously reported results by other groups[36] [37][38]. A summary ofthe ohmic contact data used for n-type InP and p-type InGaAs are shownin Table 3.6.Table 3.6: Summary of Ohmic Contact DataMetalization Anneal Doping(cm−3)ρc(Ωcm2)N-InPNi(30nm),Au/Ge(60nm), Au(80nm)10min @ 400C in5%H295%N22−8×10182 × 10−7[36]P-InGaAsPd(10nm), Ti(35nm),Pd(35nm), Au(80nm)10min @ 400C in5%H295%N21×10197.7 × 10−6[37] [38]77Chapter 3. Fabrication Via Wet EtchingFigure 3.21: A flow diagram showing the polyimide planarization step. Oncethe polyimide was spinned onto the sample, an etch back process in thedeveloper solution exposed the top of the waveguides. The next step was toevaporate the contacts.78Chapter 3. Fabrication Via Wet EtchingThe metal contacts were applied to the device by the e-beam evaporatoravailable at the UBC cleanroom facility. A crystal thickness monitor wasused to control the film thickness during the evaporation process [39]. Thecontacts were then annealed for 10 min at 400 C in 5%H295%N2.The contact quality was tested by measuring the IV characteristics oftwo separated contact regions. The contact geometry was a large circleof roughly 600 µm2area and the current path was as follows: Probe 1→ metal/substrate(doped InP) interface layer → substrate(doped InP) →Probe 2Thus, the measured resistance was a total resistance RTincluding re-sistance from the wirings Rw, probes Rp, contacts Rcand the InP materialRInP.The N-type back side contact quality was tested by probing two sepa-rated metal strips and measuring the IV characteristic as shown in Figure3.22. A total resistance of RT= 1.7 Ω was obtained. The P-type top contactwas tested in the same way after removal of the U-InP protective layer; theIV characteristic is shown in Figure 3.23. A total resistance of RT= 1.5 Ωwas obtained.The achieved contact quality was acceptable since it satisfied the follow-ing conditions:• The contacts exhibited reasonably low resistances, RT< Rdiode,whereRdiode= 37 Ω is shown in Figure 3.27.• The contact characteristics were ohmic79Chapter 3. Fabrication Via Wet EtchingFigure 3.22: The IV characteristic of the N-type ohmic contact evaporatedon the InP contact layer is shown here. A resistance of roughly 1.7 Ω.Figure 3.23: The IV characteristic of the p-type ohmic contact evaporatedon the InGaAs contact layer is shown here. A resistance of roughly 1.5 Ω.80Chapter 3. Fabrication Via Wet Etching3.3.7 CleavingSince these devices are edge emitters, the samples need to be cleaved in-orderto provide an output port for measurements. The cleaving requirements forour mask and design are:• Provide a smooth and flat facet free of any damages such as scratchesor cracks• Separate devices, which means a cleaving pitch of about 200 µmThe cleaving process was done manually. Similar to when the wafer wascleaved into small die pieces during the sample preparation step, a tip of adiamond cutter was used to brake the sample along its crystallographicorientation line. Placing a damper such as clean wipes underneath thesample when cleaving helps the process.Looking at the side wall of the cleaved samples as shown in the opti-cal images of Figure 3.24 and cleaved facets in the SEM image of Figure3.18, many defects and damages are observed as the result of the cleavingtechnique and process.The device yield after the final cleaving step is very low for the followingreasons:• Cleaving location is not well controlled and results in destruction ofdevices,• Coupler waveguides are damaged and/or terminated before the cleavedoutput edge81Chapter 3. Fabrication Via Wet Etching• Due to large cleaving pitch and limited location control, a number ofresonators maybe left coupled to the same coupler waveguide. Testingdevices away from the cleaved facet allows more losses due to defects(such as other devices coupled to the same output waveguide or dam-ages to the output waveguide) along the path to the output.• The cleaved facet has a poor quality as observed in Figure 3.18 andFigure 3.24• A good cleaved output facet would significantly improve the amountof light coupled out as well as easing the task of fiber alignment whendoing measurements.A good facet is specially critical for the RWG edge emitter lasers sinceit forms the cavity as further discussed in Section 5.4. Suggestions andcurrent efforts to improve and to replace this manual cleaving technique areexplained in the final analysis section.3.3.8 ResultsFigure 3.25 shows an optical image of the fabricated devices.A suitable setup was made with the help of Wei Shi, in order to measurethe diode and laser characteristics of t he edge emitting devices. A pictureof the setup is shown in Figure 3.26. After cleaving, the samples were fixedusing a carbon tape onto a 4 cm × 4cm× 2 mm copper sheet which wasplaced on top of a 3-axis stage. The temperature of the copper sheet wascontrolled by a temperature controller through a Peltier cooler. The bus top82Chapter 3. Fabrication Via Wet EtchingFigure 3.24: Optical images showing the side view of the wafer after cleaving.These samples were not thinned.Figure 3.25: The final fabricated SRLs are displayed before cleaving. Thewaveguides, top contacts and the polyimide regions are all shown in thisFigure.83Chapter 3. Fabrication Via Wet Etchingcontact was kept at a fixed low bias while the ring contact current was swept.A Matlab program (LIVdownloader.m) was used to retrieve the data.Figure 3.26: The set up used for measuring edge emitter lasers.The IV and LI measurements of Figures 3.27(a) and 3.27(b) show thata good IV characteristic as well as light emission was successfully achieved.The device’s optical spectrum behaviour for varied bias conditions wasmeasured using an optical spectrum analyzer (OSA), an example of such84Chapter 3. Fabrication Via Wet EtchingFigure 3.27: (a)IV Characteristic of a fabricated SRL with a diode resistanceRdiode= 37Ω(b)LI characteristic of a fabricated SRL.85Chapter 3. Fabrication Via Wet Etching1500 1520 1540 1560 1580 1600−80−78−76−74−72−70−68−66Spectrum of InP SRL Below ThresholdWavelength (nm)Light Output (dBm)  R=500mA R=800mA R=1000mA R=1200mA R=1500mA Figure 3.28: (a)Optical spectrum of a fabricated SRL biased at 500 mA, 800mA, 1000 mA, 1200 mA, and 1500 mA. The sample was cooled to roughly5 degrees.86Chapter 3. Fabrication Via Wet Etchingmeasurement is shown in Figure 3.28. Lasing, however was not observed.Going to higher currents showed that the device overheated and the powerrolled off before lasing could occur.The voltage at 500mA was 6 V. Thus the power consumed by the devicewas:Pconsumed= IV = (1500mA)(6V)=9W (3.3)The output power as measured by the OSA was:Pout= IV = 10−65dBm/10=0.3nW (3.4)Device area:Area =(3µm)(2π200µm+2×250) = (3µm)(1756.6µm)≈5300µm2(3.5)The lasing wavelength shifts with a varying bias current. The lasing wave-length is dependent on the length (d) of the cavity and the index of refraction(n).λ =ngd0m(m =1,2,3,4...) (3.6)The gain-peak shows a shift of ≈ 20 nm. This shift is due to the hightemperature effects.87Chapter 3. Fabrication Via Wet Etching3.3.9 AnalysisThe spectrum measurements of Figure 3.28 show the gain spectrum, someresonances and the effect of increasing the bias current on the emission spec-trum below threshold. Below threshold, increases in bias current contributemostly to spontaneous emission, as also shown in the LI curve of Figure3.27(b); there is almost no stimulated emission. An increase in the pump-ing rate directly contributes to an increase in the population inversion andthe amplitude of the gain spectrum increases, but since there is no lasing,the emission spectrum is wide. Below threshold, the cavity losses exceedthe gain coefficient, which means none of the emissions in the spectrum canexperience a large enough gain to start lasing. The losses mainly due tofabrication have resulted in a very large threshold.Some sources contributing to the cavity losses could be:• Defects along the ring waveguide such as dirt and scratches could causetoo much scattering or completely interfere with the light propagation.• Polyimide residues leftover on some areas of the ring waveguide (beforethe contact evaporation step) would result in areas where there is nopumping and thus no inversion occurs.• Metal deposition on the waveguide sidewalls could result in increasedpropagation losses, specially if the metal is closer to the bottom of thewaveguide where it can have a higher affect on the field distribution.• Losses due to bending, mode conversion losses88Chapter 3. Fabrication Via Wet Etching• Self-heating effect: the device’s resistance could lead to heating, whichwould reduce the optical gain.• Another possibility could be problems with the epitaxy. The PL andthe contacts are reasonable, so there is no evidence for this hypothesis,but it has not been ruled out.The major limiting step in the fabrication process was the cleaving. Dueto the very low yield after cleaving, many devices did not have the chanceto be properly tested. This process is currently under improvement. Thecommon industry practice is to thin the wafers from their typical roughly1 mm thickness down to 100 µm - 200 µm thickness before cleaving. Wehave tested the thinning idea, by manually thinning a sample on a sandpaper and much better cleaving results were observed. However, doing thismanually takes several hours of sanding by hand and it is not a reliablemethod. This wafer thinning process is done with automatic equipment inindustry. We are currently putting parts together to test a wafer thinningmachine at UBC.Once the wafer is thinned, still a better cleaving tool is required. Thediamond tips are too wide, a better choice are surgery knives. Surgery knivesare not the common option for cleaving InP wafers, however after thinningnot much force is required and the surgery knife can easily be used to cleavedevices.89Chapter 3. Fabrication Via Wet Etching3.4 InP Edge Emitter Ridge Waveguide LaserDemonstrating a RWG edge emitting laser in InP would be a step closertowards fabricating InP SRLs. The same epi-wafer used to fabricate theSRLs was used for the fabrication of the RWG lasers. A mask was designedusing Clewin’s Matlab script; the mask patterns consisted of long straightrectangular waveguides with varying waveguide widths. There were 2 masklayers: Layer 1 for etching and Layer 2 for contact evaporation. The etchmask was drawn wider in order to compensate for the under-etch effect. Theetch mask included the following variations:Waveguide width [µm]: 6, 8, 12, 17, 22, 27, 32, 37, 47Waveguide contact [µm]: 2, 4, 8, 10, 15, 20, 25, 30, 40The fabricated devices were tested and LED characteristics similar to theSRL results were obtained; however, lasing was not observed. This is againmost likely due to the cleaving technique leaving a poor quality facet whichis very critical for the RWG laser since the cleaved facets form the cavitymirrors. Moreover, the cleaving pitch is also currently limited to roughly 1mm and higher. Thus the cavity lengths are large, providing a larger areafor inversion and thus having higher threshold currents.3.5 ConclusionA commercial 1550 nm epi-wafer was purchased. The required mask wasdesigned and purchased. The following fabrication steps were successfullyachieved:90Chapter 3. Fabrication Via Wet Etching• A semi-automated mask drawing script• A photolithography process with a resolution better than 1 µm thanksto edge bead removal• Aselective wetetchingrecipe givingsmoothsurfaces, in-planeisotropy,and an orthogonal to in-plane etch rate ratio of 2• A planarizing technique via polyimide spinning, which does not requireany extra masking step• Reasonably low resistance, ohmic contacts for n and p contactsAn appropriate setup was made for testing edge emitters. The diode char-acteristics of the device was successfully measured. Light emission was alsomeasured. The spectrum measurements were performed and the gain spec-trum and some resonances were observed. However, the output power wasvery low, in the tenths of nWs, and lasing did not occur. The cleavingprocess is being improved in order to allow further testing.91Chapter 4Fabrication Via Dry EtchingDry etching techiques used for SRL fabrication are well published and moremature, thus we wanted to make some SRLs via dry etching to test thedesigns. We originally planned to pursue dry and wet etching techniques inparallel at the UBC Nanofabrication facility.The most common dry etching techniques are reported to be high tem-perature Cl2based or hydrocarbon-based plasma. Usually the hydrocarbon-based dry etching is preferred for InP laser fabrication. The main reasonfor which hydrocarbon-based plasma is preferred over Cl2-based electroncyclotron resonance (ECR) plasma is that Cl2 etching rate of InP is tem-perature dependent [40]. In low temperatures of 80deg etching rate is <0.05µm/min (this is due to the nonvolatile InCl reaction products, which alsoresult in grassy-roughened surfaces), for higher temp of 230deg the rate issaturated at more than 1 µm/min [40]. This higher temperature requiresa hard mask ( SiO2/or SiN). To achieve smoother side-walls and bottomsurfaces high ion voltage of 900 V is reported and Cl2gas pressure of 7E-5torr [40]. Meanwhile the CH4plasma etching is done at room temperatureand it has been reported that addition of O2 decreases the etch rate but itgives selectivity of 60 (higher etching rate of InP over InAlAs) [40]. This92Chapter 4. Fabrication Via Dry Etchingselectivity is explained by the oxidation of InAlAs layer, forming a thin layerof Al2O3 which could also provide electrical passivation. Addition of O2 alsoimproves the side-wall verticality because it partially removes the polymer,which forms during the InP etching[40].The UBC AMPEL Nanofabrication facility has an ECR machine in work-ing condition. This machine was not suitable for CH4gas; however, it hasCl2gas available but the machine was not able to provide the required hightemperatures. Investigation was done to replace the heating chuck in orderto supply higher temperatures but this turned out to be an overly expensivetask.The UBC facility also has an reactive ion etcher (RIE) machine whichcould be used for the preferred CH4etching; however, this machine is not inworking conditions. The machine was being repaired for over 1.5 years butit was decided that it was not going to reach working conditions. Thus westarted to look dry etching fabrication outside UBC.Dry etching fabrication of the InP SRL has started at CRN2 facility ofUniversity of Sherbrooke funded by CMC. The high resolution SRL maskwas redesigned in order to take into account the dry etching approach andinclude a planarization step. The planarization step will be done via SiO2deposition and etch back process instead of the previous polyimide spinningtechnique. Figure 4.1 shows the 3 different mask layers with layer 1 for dryetching process, layer 2 for planarization and layer 3 for metal evaporation.Figure 4.2 shows a cartoon like cross section of the epi-wafer and SRL viadry etching.They already have a recipe developed for InP etching using a plasma93Chapter 4. Fabrication Via Dry EtchingFigure 4.1: The three different mask layers used to first define the trench fordry etching, next to make a contact opening for the oxide planarization pro-cess and finally to define the metalization region for the top p-type contacts.The n-type contact is evaporated on the backside.94Chapter 4. Fabrication Via Dry EtchingFigure 4.2: Cartoon of the epi-wafer structure and the side view of a dryetched SRL.95Chapter 4. Fabrication Via Dry Etchingetcher STS ICP III-V. They have completed the first dry etching testsand tuned their recipe for our heterostructure; the etch depth is monitoredwith a laser end point detection system. An aspect ratio dependent etching(ARDE) was observed as depicted in Figure 4.3, which shows an SEM im-age of the cross section for the latest sample that was dry etched with theirsystem.Figure 4.3: SEM image revealing the etch profile and etch depth in thecoupler region and the trench region. Courtesy of Sherbrooke.The ARDE was summarized in Figure 4.4. The ARDE can be translatedin terms of the thickness between the bottom of the etched trench and theetch stop layer.96Chapter 4. Fabrication Via Dry EtchingFigure 4.4: A plot of etching rate as a function of the trench size is shownhere to capture the ARDE for the dry etching of the InP samples. Courtesyof Sherbrooke.A study was done to analyze the designs etch depth tolerance. Struc-tures were simulated in FDTD mode solver and the corresponding couplingbehavior was then extracted for the etch depths predicted by the graph ofFigure 4.4. The results show much stronger field interaction between thewaveguides and thus much shorter crossover coupling lengths as depicted inthe data of Table 4.1 and Table 4.2. In the Tables, the subscripts a and brefer to a coupler length Lκa= 200 µm and a coupler length Lκb= 250 µmrespectively. The corresponding threshold current and output power valuesfor the devices of Table 4.1 are predicted and presented in Table 4.3 for adevice with a bend region of radius R = 300 µm. The etch rate dependenceof the dry etching on the gap size results in much shorter crossover lengths.However these Lxvalues are now shorter than the current designed coupler97Chapter 4. Fabrication Via Dry Etchinglengths which could make the coupling determination more challenging, butit will provide more devices at higher powers to measure and analyze. Com-paring the results of Table 4.2 to Table 2.3, the achieved Lxvalues are verydifferent. These two Tables present results for 3 wide devices, the onlydifferences are the etch depths and the waveguide shapes (rectangular vs.trapezoidal). The Lxvalue for the trapezoidal device with an etch depthof 2 µm is 86111 µm, meanwhile for the rectangular device of etch depth1.64 µm it is 171.1 µm. This can be explained by comparing the effect ofthe etch depth on the mode distributions for the structures. Comparingthe first order mode distribution for the trapezoidal structure as shown inFigure 4.5 to the dry etched waveguide of etch depth 1.64 µm as depicted inFigure 4.6, a much stronger field interaction is observed between the couplerwaveguides.They are currently working to improve their recipe and perhaps tochange into a selective recipe.Table 4.1: Etch Depth Analysis, W = 2 µmGap(µm)EtchDepth(µm)neff1neff2κaκbLx(µm)0.5 1.1 3.268150 3.256780 0.99 0.249 68.160.8 1.26 3.268454 3.258455 0.625 0.88 77.511 1.36 3.268499 3.259402 0.27 0.99 85.19Continued on next page98Chapter 4. Fabrication Via Dry Etching1.5 1.54 3.267857 3.261085 0.149 0.082 114.42 1.64 3.266478 3.261949 0.931 0.561 171.1Table 4.2: Etch Depth Analysis, W = 3 µmGap(µm)EtchDepth(µm)neff1neff2κaκbLx(µm)0.5 1.1 3.270061 3.264313 0.5 0.052 134.80.8 1.26 3.270126 3.264932 0.74 0.238 149.21 1.36 3.270069 3.265295 0.873 0.437 162.31.5 1.54 3.269434 3.265993 0.97 0.97 225.22 1.64 3.268362 3.266283 0.557 0.755 372.8Table 4.3: Predicted Performance for Waveguide W = 2 µm R = 300 µmIth(mA) Pout(mW)Gap (µm) 1a1bab0.5 80 58 9 30.8 60 69 8.5 10.51 56 83 4 7.91.5 55 57 2 1.12 71 61 11.8 7.299Chapter 4. Fabrication Via Dry EtchingFigure 4.5: The mode distribution is shown for a trapezoidal structure withuniform 2 µmetchdeptheverywhere.Figure 4.6: The mode distribution is shown for a structure with a 1.64 µmetch depth in the gap region and 2 µm etch depth on the outside walls ofthe waveguides.100Chapter 5ConclusionThis thesis discussed the design process and process development for thefabrication of SRLs based on a commercial InP epitaxy. Unlike the welldeveloped dry etching approach taken by other groups, we developed a newselective multi step wet etching technique for the fabrication of SRLs. Thiswet etching approach is a much simpler, cheaper and faster process comparedto the dry etching methods. Moreover, achieving smooth surfaces by dryetching is possible however it adds more complexity to the etching processwhere as wet etching inherently results in smooth surfaces. Using this wetetching approach SRLs with waveguide widths as small as 3 µm can befabricated. This limitation on the width of the device is due to the 1 to 2relationship of the in-plane etch rate to the orthogonal etch rate.Also a polyimide planarization technique was developed that didn’t re-quire any additional masking, UV exposure or plasma exposure.We were able to measure the spectrum and LED characteristics of thefabricated SRLs.101Chapter 5. Conclusion5.1 Future WorkFurther design, fabrication and measurement work are required in future toimprove the SRL design model and optimize for device performance.For a more accurate SRL modelling, a more detailed loss model shouldbe incorporated into the simulations which considers:• Bending losses: This occurs in the bend waveguide region and is afunction of the bend radius.• Modeconversion losses: Thisoccurswhenlighttravelsfromthe straightwaveguide region to the bend waveguide region as well as when lighttravels into the coupling region. Amongst other parameters, this lossis a function of the effective index difference between the regions alongthe light’s path of travel.• Scattering losses: This loss is due to interaction of the field at crystalimperfections and surface roughness. It is a function of the amount offield intensity at the waveguide walls as well as the waveguide surfaceroughness root mean square (rms) amongst other parameters.Moreover, the power coupling prediction accuracy can be increased by con-sidering power coupling contributions from the bend regions near the cou-pler region of the SRL device. Other considerations contributing to the SRLmodelling accuracy are:• The refractive index changes due to varying bias conditions since thecoupling and the losses are refractive index dependent102Chapter 5. Conclusion• The effect of bias conditions and surface roughness on the dominatingresonant mode (CW/CCW/bistable)• Mode competition analysis and effectsDeveloping a cleaving process via wafer thinning (thinning the wafersdown to 100 µm to 200 µm thickness) that results in high quality facetsand allows high cleaving accuracy (allowing device separation) would sig-nificantly increase the fabrication yield and reduce the thermal effects tothe substrate resistance. Moreover, in order to optimize for output power,higher power coupling coefficients are required. To achieve a higher powercoupling coefficient, the higher lithography resolution (better than 300nm)needs to be increased to reduce the coupling gap during fabrication.Measurement future work:• Pulsed Measurement, self heating could be reduced by this technique• Modification of the set up to include a high magnification in order to beable to see 2 µm and 3 µm wide waveguides. 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Available: http://www.nanofab.ubc.ca/equipment view[40] Wada and Hasegawa, Eds., InP-based Materials and Devices.Wiley-Interscience, 1999.108Appendix AClewin Matlab Script Usedfor the Mask Layout%This script will create the etch mask layer% Due to Clewin file script size limitation, separate scripts had to be written and positioned%after each other to make the whole mask.% Each of the scripts titled Script 01a to Script 01f will make 3 rows of the mask.%All scripts are the same except for varying ring\radius and gaps.%%%%%%%%%%%%%%%%%%%%%%%%% Script 01a %%%%%%%%%%%%%%%%%%%%%%%%%%Create an angle vector between -90deg to 90deg, ex:100 anglesangle = linspace(-pi/2,pi/2,100);offset = 80;yshift = 0;yshiftNext = 0;xshift = 0;%Create a radius vector giving a radius for each angle:rInner = [300,250,200,180,160,150,100];%Radius for the inner circlelenR = length(rInner);width = [2,3];%rOuter-rInner;lenw = length(width);gapArray1 = [0.5,0.5,1,1,1.5,1.5,2,2,1,1,1.5,1.5,2,2];gapArray2 = [1.5,1.5,2,2,2,1.5,1,1];109Appendix A. Clewin Matlab Script Used for the Mask LayoutCouplingL = 200;LofDye=6500;x1Shift_init=0;for iR = 1:3;%lenRiG=1;for iw = 1:lenww = width(iw);if w==2gapArray=gapArray1;lenG = length(gapArray);elsegapArray = gapArray2;lenG = length(gapArray);endrOuter = width(iw)+rInner; %Radius for the outer circle for a given width%Calculate the nodes of a inner circle and an outer circle,nodesInnery = rInner(iR).*sin(angle)+yshift;nodesOutery = rOuter(iR).*sin(angle)+yshift;lenN = length(nodesInnery);Rnodes = [];Lnodes = [];LofRaceTrack = rOuter(iR)*2+CouplingL+offset;CurrentNumofRaceTracks= round(LofDye/(LofRaceTrack));%Calculate the nodes of a half ring,%i.e. a matrix with 2 columns and 1000 rows:for iRx = 1:CurrentNumofRaceTracks/2+1if iRx < CurrentNumofRaceTracks/2 && iR==2 && iRx==1xshift = xshift+50;endgap = gapArray(iRx);110Appendix A. Clewin Matlab Script Used for the Mask Layoutif gap == 0.5 && w==2shiftUp = (gap - 0.5)+2*w;elseshiftUp = (gap - 0.5)+2*w;endnodesInnerx = rInner(iR).*cos(angle)+xshift;nodesOuterx = rOuter(iR).*cos(angle)+xshift;for index = 1:lenNif (nodesOuterx(index)-nodesInnerx(index))>= 0nodesy(index) = nodesOutery(index)+shiftUp;Rnodesx(index) = nodesOuterx(index)+CouplingL/2;Rnodes = [Rnodesx(:), nodesy(:)];Lnodesx(index) = nodesOuterx(index) - 2.*nodesOuterx(index)- CouplingL/2+2*xshift;Lnodes = [Lnodesx(:), nodesy(:)];endlenNr = length(Rnodes(:,1));%calculate nodes of coupling lengthx2couplingT = Rnodes(lenNr,1);x1couplingT = x2couplingT - CouplingL;y2couplingT = Rnodes(lenNr,2)+w/2;y1couplingT = y2couplingT - w;x2couplingB = Rnodes(1,1);x1couplingB = x2couplingB - CouplingL+ x1Shift_init;y2couplingB = Rnodes(1,2)+w/2;y1couplingB = y2couplingB - w;x1Line_init = -400;x1Line = x1Line_init + xshift;x2Line = x1Line_init+LofRaceTrack+10; %*CurrentNumofRaceTracks;y2Line = y1couplingB - gap;111Appendix A. Clewin Matlab Script Used for the Mask Layouty1Line = y2Line- w;iG = iG+1;endif iRx > CurrentNumofRaceTracks/2 && iR == 2 && w==2rectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB-100,y2Line);rectangle(x2couplingB-100, y1Line-0.5,x2couplingB+rOuter(iR)+offset,y2Line+0.5);xshiftNext = 2*rOuter(iR)+CouplingL+offset-400;xshift = xshift+xshiftNext;elsewire(1,w,Rnodes); %Right half ringrectangle(x1couplingT,y1couplingT,x2couplingT,y2couplingT);rectangle(x1couplingB,y1couplingB,x2couplingB,y2couplingB);wire(1,w,Lnodes); %Left half ringrectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB+rOuter(iR)+offset*0.5,y2Line);xshiftNext = 2*rOuter(iR)+CouplingL+offset;xshift = xshift+xshiftNext;endif iRx==1 && w==2rectangle(-700, y1Line,x1couplingB-rOuter(iR)-offset*0.5,y2Line);elseif iRx>=CurrentNumofRaceTracks/2 && w==3rectangle(x2couplingB+rOuter(iR)+offset*0.5, y1Line,7600,y2Line);endendendxshift = 0;if iR<lenRyshiftNext = rOuter(iR)+ rOuter(iR+1)+gap+2*w+offset;yshift = yshift + yshiftNext;end112Appendix A. Clewin Matlab Script Used for the Mask Layoutend%%%%%%%%%%%%%%%%%%%%%%%%% Script 01b %%%%%%%%%%%%%%%%%%%%%%%%%Create an angle vector between -90deg to 90deg, ex:100 anglesangle = linspace(-pi/2,pi/2,100);offset = 80;yshift = 0;yshiftNext = 0;xshift = 0;%Create a radius vector giving a radius for each angle:rInner = [180,160,150,100];%Radius for the inner circlelenR = length(rInner);width = [2,3];%rOuter-rInner;lenw = length(width);gapArray1 = [0.5,0.5,1,1,1.5,1.5,2,2,1,1,1.5,1.5,2,2];gapArray2 = [1,1.5,1.5,1.5,2,2,1,1];CouplingL = 200;LofDye=6730;x1Shift_init=0;for iR = 1:3;%lenRiG=1;for iw = 1:lenww = width(iw);if w==2gapArray=gapArray1;lenG = length(gapArray);elsegapArray = gapArray2;lenG = length(gapArray);end113Appendix A. Clewin Matlab Script Used for the Mask LayoutrOuter = width(iw)+rInner; %Radius for the outer circle for a given width%Calculate the nodes of a inner circle and an outer circle,nodesInnery = rInner(iR).*sin(angle)+yshift;nodesOutery = rOuter(iR).*sin(angle)+yshift;lenN = length(nodesInnery);Rnodes = [];Lnodes = [];LofRaceTrack = rOuter(iR)*2+CouplingL+offset;CurrentNumofRaceTracks= round(LofDye/(LofRaceTrack));%Calculate the nodes of a half ring,%i.e. a matrix with 2 columns and 1000 rows:for iRx = 1:CurrentNumofRaceTracks/2+1if iRx < CurrentNumofRaceTracks/2 && iR==2 && iRx==1xshift = xshift-50 Remove -700 lineendgap = gapArray(iRx);if gap == 0.5 && w==2shiftUp = (gap - 0.5)+2*w;elseshiftUp = (gap - 0.5)+2*w;endnodesInnerx = rInner(iR).*cos(angle)+xshift;nodesOuterx = rOuter(iR).*cos(angle)+xshift;for index = 1:lenNif (nodesOuterx(index)-nodesInnerx(index))>= 0nodesy(index) = nodesOutery(index)+shiftUp;Rnodesx(index) = nodesOuterx(index)+CouplingL/2;Rnodes = [Rnodesx(:), nodesy(:)];Lnodesx(index) = nodesOuterx(index) - 2.*nodesOuterx(index)- CouplingL/2+2*xshift;114Appendix A. Clewin Matlab Script Used for the Mask LayoutLnodes = [Lnodesx(:), nodesy(:)];endlenNr = length(Rnodes(:,1));%calculate nodes of coupling lengthx2couplingT = Rnodes(lenNr,1);x1couplingT = x2couplingT - CouplingL;y2couplingT = Rnodes(lenNr,2)+w/2;y1couplingT = y2couplingT - w;x2couplingB = Rnodes(1,1);x1couplingB = x2couplingB - CouplingL+ x1Shift_init;y2couplingB = Rnodes(1,2)+w/2;y1couplingB = y2couplingB - w;x1Line_init = -400;x1Line = x1Line_init + xshift;x2Line = x1Line_init+LofRaceTrack+10; %*CurrentNumofRaceTracks;y2Line = y1couplingB - gap;y1Line = y2Line- w;iG = iG+1;endif iRx > CurrentNumofRaceTracks/2 && iR == 2 && w==2rectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB-100,y2Line);rectangle(x2couplingB-100, y1Line-0.5,x2couplingB+rOuter(iR)+offset,y2Line+0.5);xshiftNext = 2*rOuter(iR)+CouplingL+offset-250;xshift = xshift+xshiftNext;elsewire(1,w,Rnodes); %Right half ringrectangle(x1couplingT,y1couplingT,x2couplingT,y2couplingT);rectangle(x1couplingB,y1couplingB,x2couplingB,y2couplingB);wire(1,w,Lnodes); %Left half ringrectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB115Appendix A. Clewin Matlab Script Used for the Mask Layout+rOuter(iR)+offset*0.5,y2Line);xshiftNext = 2*rOuter(iR)+CouplingL+offset;xshift = xshift+xshiftNext;endif iRx==1 && w==2rectangle(-700, y1Line,x1couplingB-rOuter(iR)-offset*0.5,y2Line);elseif iRx>=CurrentNumofRaceTracks/2 && w==3rectangle(x2couplingB+rOuter(iR)+offset*0.5, y1Line,7600,y2Line);endendendxshift = 0;if iR<lenRyshiftNext = rOuter(iR)+ rOuter(iR+1)+gap+2*w+offset;yshift = yshift + yshiftNext;endend%%%%%%%%%%%%%%%%%%%%%%%%% Script 01c %%%%%%%%%%%%%%%%%%%%%%%%%%Create an angle vector between -90deg to 90deg, ex:100 anglesangle = linspace(-pi/2,pi/2,100);offset = 80;yshift = 0;yshiftNext = 0;xshift = 0;%Create a radius vector giving a radius for each angle:rInner = [100];%Radius for the inner circlelenR = length(rInner);width = [2,3];%rOuter-rInner;lenw = length(width);116Appendix A. Clewin Matlab Script Used for the Mask LayoutgapArray1 = [0.5,0.5,1,1,1.5,1.5,2,2,1,1,1.5,1.5,2,2];gapArray2 = [0.5,1,1,1.5,1.5,1.5,2,2,1,1];CouplingL = 200;LofDye=6730;x1Shift_init=0;for iR = 1:lenRiG=1;for iw = 1:lenww = width(iw);if w==2gapArray=gapArray1;lenG = length(gapArray);elsegapArray = gapArray2;lenG = length(gapArray);endrOuter = width(iw)+rInner; %Radius for the outer circle for a given width%Calculate the nodes of a inner circle and an outer circle,nodesInnery = rInner(iR).*sin(angle)+yshift;nodesOutery = rOuter(iR).*sin(angle)+yshift;lenN = length(nodesInnery);Rnodes = [];Lnodes = [];LofRaceTrack = rOuter(iR)*2+CouplingL+offset;CurrentNumofRaceTracks= round(LofDye/(LofRaceTrack));%Calculate the nodes of a half ring,%i.e. a matrix with 2 columns and 1000 rows:for iRx = 1:CurrentNumofRaceTracks/2+1if iRx < CurrentNumofRaceTracks/2 && iR==2 && iRx==1xshift = xshift-50;117Appendix A. Clewin Matlab Script Used for the Mask Layoutendgap = gapArray(iRx);if gap == 0.5 && w==2shiftUp = (gap - 0.5)+2*w;elseshiftUp = (gap - 0.5)+2*w;endnodesInnerx = rInner(iR).*cos(angle)+xshift;nodesOuterx = rOuter(iR).*cos(angle)+xshift;for index = 1:lenNif (nodesOuterx(index)-nodesInnerx(index))>= 0nodesy(index) = nodesOutery(index)+shiftUp;Rnodesx(index) = nodesOuterx(index)+CouplingL/2;Rnodes = [Rnodesx(:), nodesy(:)];Lnodesx(index) = nodesOuterx(index) - 2.*nodesOuterx(index)- CouplingL/2+2*xshift;Lnodes = [Lnodesx(:), nodesy(:)];endlenNr = length(Rnodes(:,1));%calculate nodes of coupling lengthx2couplingT = Rnodes(lenNr,1);x1couplingT = x2couplingT - CouplingL;y2couplingT = Rnodes(lenNr,2)+w/2;y1couplingT = y2couplingT - w;x2couplingB = Rnodes(1,1);x1couplingB = x2couplingB - CouplingL+ x1Shift_init;y2couplingB = Rnodes(1,2)+w/2;y1couplingB = y2couplingB - w;x1Line_init = -400;x1Line = x1Line_init + xshift;118Appendix A. Clewin Matlab Script Used for the Mask Layoutx2Line = x1Line_init+LofRaceTrack+10; %*CurrentNumofRaceTracks;y2Line = y1couplingB - gap;y1Line = y2Line- w;iG = iG+1;endif iRx > CurrentNumofRaceTracks/2 && iR == 2 && w==2rectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB-100,y2Line);rectangle(x2couplingB-100, y1Line-0.5,x2couplingB+rOuter(iR)+offset,y2Line+0.5);xshiftNext = 2*rOuter(iR)+CouplingL+offset-250;xshift = xshift+xshiftNext;elsewire(1,w,Rnodes); %Right half ringrectangle(x1couplingT,y1couplingT,x2couplingT,y2couplingT);rectangle(x1couplingB,y1couplingB,x2couplingB,y2couplingB);wire(1,w,Lnodes); %Left half ringrectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB+rOuter(iR)+offset*0.5,y2Line);xshiftNext = 2*rOuter(iR)+CouplingL+offset;xshift = xshift+xshiftNext;endif iRx==1 && w==2rectangle(-700, y1Line,x1couplingB-rOuter(iR)-offset*0.5,y2Line);elseif iRx>=CurrentNumofRaceTracks/2 && w==3rectangle(x2couplingB+rOuter(iR)+offset*0.5, y1Line,7600,y2Line);endendendxshift = 0;if iR<lenRyshiftNext = rOuter(iR)+ rOuter(iR+1)+gap+2*w+offset;119Appendix A. Clewin Matlab Script Used for the Mask Layoutyshift = yshift + yshiftNext;endend%%%%%%%%%%%%%%%%%%%%%%%%% Script 01d %%%%%%%%%%%%%%%%%%%%%%%%%%Create an angle vector between -90deg to 90deg, ex:100 anglesangle = linspace(-pi/2,pi/2,100);offset = 80;yshift = 0;yshiftNext = 0;xshift = 0;%Create a radius vector giving a radius for each angle:rInner = [250,200,180,160,150,100];%Radius for the inner circlelenR = length(rInner);width = [2,3];%rOuter-rInner;lenw = length(width);gapArray1 = [0.5,0.5,1,1,1.5,1.5,2,2,1,1,1.5,1.5,2,2];gapArray2 = [1,1.5,1.5,1.5,2,2,1,1];CouplingL = 250;LofDye=6600;x1Shift_init=0;for iR = 1:4;%lenRiG=1;for iw = 1:lenww = width(iw);if w==2gapArray=gapArray1;lenG = length(gapArray);elsegapArray = gapArray2;120Appendix A. Clewin Matlab Script Used for the Mask LayoutlenG = length(gapArray);endrOuter = width(iw)+rInner; %Radius for the outer circle for a given width%Calculate the nodes of a inner circle and an outer circle,nodesInnery = rInner(iR).*sin(angle)+yshift;nodesOutery = rOuter(iR).*sin(angle)+yshift;lenN = length(nodesInnery);Rnodes = [];Lnodes = [];LofRaceTrack = rOuter(iR)*2+CouplingL+offset;CurrentNumofRaceTracks= round(LofDye/(LofRaceTrack));%Calculate the nodes of a half ring,%i.e. a matrix with 2 columns and 1000 rows:for iRx = 1:CurrentNumofRaceTracks/2+1if iRx < CurrentNumofRaceTracks/2 && iR==2 && iRx==1xshift = xshift+50;endgap = gapArray(iRx);if gap == 0.5 && w==2shiftUp = (gap - 0.5)+2*w;elseshiftUp = (gap - 0.5)+2*w;endnodesInnerx = rInner(iR).*cos(angle)+xshift;nodesOuterx = rOuter(iR).*cos(angle)+xshift;for index = 1:lenNif (nodesOuterx(index)-nodesInnerx(index))>= 0nodesy(index) = nodesOutery(index)+shiftUp;Rnodesx(index) = nodesOuterx(index)+CouplingL/2;Rnodes = [Rnodesx(:), nodesy(:)];121Appendix A. Clewin Matlab Script Used for the Mask LayoutLnodesx(index) = nodesOuterx(index) - 2.*nodesOuterx(index)- CouplingL/2+2*xshift;Lnodes = [Lnodesx(:), nodesy(:)];endlenNr = length(Rnodes(:,1));%calculate nodes of coupling lengthx2couplingT = Rnodes(lenNr,1);x1couplingT = x2couplingT - CouplingL;y2couplingT = Rnodes(lenNr,2)+w/2;y1couplingT = y2couplingT - w;x2couplingB = Rnodes(1,1);x1couplingB = x2couplingB - CouplingL+ x1Shift_init;y2couplingB = Rnodes(1,2)+w/2;y1couplingB = y2couplingB - w;x1Line_init = -400;x1Line = x1Line_init + xshift;x2Line = x1Line_init+LofRaceTrack+10; %*CurrentNumofRaceTracks;y2Line = y1couplingB - gap;y1Line = y2Line- w;iG = iG+1;endif iRx > CurrentNumofRaceTracks/2 && (iR == 2 || iR ==1 || iR==4) && w==2rectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB-100,y2Line);rectangle(x2couplingB-100, y1Line-0.5,x2couplingB+rOuter(iR)+offset,y2Line+0.5);if iR==1AlignmentG=500elseAlignmentG=250;endxshiftNext = 2*rOuter(iR)+CouplingL+offset-AlignmentG;122Appendix A. Clewin Matlab Script Used for the Mask Layoutxshift = xshift+xshiftNext;elsewire(1,w,Rnodes); %Right half ringrectangle(x1couplingT,y1couplingT,x2couplingT,y2couplingT);rectangle(x1couplingB,y1couplingB,x2couplingB,y2couplingB);wire(1,w,Lnodes); %Left half ringrectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB+rOuter(iR)+offset*0.5,y2Line);xshiftNext = 2*rOuter(iR)+CouplingL+offset;xshift = xshift+xshiftNext;endif iRx==1 && w==2rectangle(-700, y1Line,x1couplingB-rOuter(iR)-offset*0.5,y2Line);elseif iRx>=CurrentNumofRaceTracks/2 && w==3rectangle(x2couplingB+rOuter(iR)+offset*0.5, y1Line,7600,y2Line);endendendxshift = 0;if iR<lenRyshiftNext = rOuter(iR)+ rOuter(iR+1)+gap+2*w+offset;yshift = yshift + yshiftNext;endend%%%%%%%%%%%%%%%%%%%%%%%%% Script 01e %%%%%%%%%%%%%%%%%%%%%%%%%%Create an angle vector between -90deg to 90deg, ex:100 anglesangle = linspace(-pi/2,pi/2,100);offset = 80;yshift = 0;123Appendix A. Clewin Matlab Script Used for the Mask LayoutyshiftNext = 0;xshift = 0;%Create a radius vector giving a radius for each angle:rInner = [200,180,160,150,100];%Radius for the inner circlelenR = length(rInner);width = [2,3];%rOuter-rInner;lenw = length(width);gapArray1 = [0.5,0.5,1,1,1.5,1.5,2,2,1,1,1.5,1.5,2,2];gapArray2 = [1,1.5,1.5,1.5,2,2,1,1];CouplingL = 150;LofDye=6600;x1Shift_init=0;for iR = 1:3;%lenRiG=1;for iw = 1:lenww = width(iw);if w==2gapArray=gapArray1;lenG = length(gapArray);elsegapArray = gapArray2;lenG = length(gapArray);endrOuter = width(iw)+rInner; %Radius for the outer circle for a given width%Calculate the nodes of a inner circle and an outer circle,nodesInnery = rInner(iR).*sin(angle)+yshift;nodesOutery = rOuter(iR).*sin(angle)+yshift;lenN = length(nodesInnery);Rnodes = [];Lnodes = [];124Appendix A. Clewin Matlab Script Used for the Mask LayoutLofRaceTrack = rOuter(iR)*2+CouplingL+offset;CurrentNumofRaceTracks= round(LofDye/(LofRaceTrack));%Calculate the nodes of a half ring,%i.e. a matrix with 2 columns and 1000 rows:for iRx = 1:CurrentNumofRaceTracks/2+1gap = gapArray(iRx);if gap == 0.5 && w==2shiftUp = (gap - 0.5)+2*w;elseshiftUp = (gap - 0.5)+2*w;endnodesInnerx = rInner(iR).*cos(angle)+xshift;nodesOuterx = rOuter(iR).*cos(angle)+xshift;for index = 1:lenNif (nodesOuterx(index)-nodesInnerx(index))>= 0nodesy(index) = nodesOutery(index)+shiftUp;Rnodesx(index) = nodesOuterx(index)+CouplingL/2;Rnodes = [Rnodesx(:), nodesy(:)];Lnodesx(index) = nodesOuterx(index) - 2.*nodesOuterx(index)- CouplingL/2+2*xshift;Lnodes = [Lnodesx(:), nodesy(:)];endlenNr = length(Rnodes(:,1));%calculate nodes of coupling lengthx2couplingT = Rnodes(lenNr,1);x1couplingT = x2couplingT - CouplingL;y2couplingT = Rnodes(lenNr,2)+w/2;y1couplingT = y2couplingT - w;x2couplingB = Rnodes(1,1);x1couplingB = x2couplingB - CouplingL+ x1Shift_init;125Appendix A. Clewin Matlab Script Used for the Mask Layouty2couplingB = Rnodes(1,2)+w/2;y1couplingB = y2couplingB - w;x1Line_init = -400;x1Line = x1Line_init + xshift;x2Line = x1Line_init+LofRaceTrack+10; %*CurrentNumofRaceTracks;y2Line = y1couplingB - gap;y1Line = y2Line- w;iG = iG+1;endif iRx > CurrentNumofRaceTracks/2 && iR==4 && w==2rectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB-100,y2Line);rectangle(x2couplingB-100, y1Line-0.5,x2couplingB+rOuter(iR)+offset,y2Line+0.5);xshiftNext = 2*rOuter(iR)+CouplingL+offset;xshift = xshift+xshiftNext;elsewire(1,w,Rnodes); %Right half ringrectangle(x1couplingT,y1couplingT,x2couplingT,y2couplingT);rectangle(x1couplingB,y1couplingB,x2couplingB,y2couplingB);wire(1,w,Lnodes); %Left half ringrectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB+rOuter(iR)+offset*0.5,y2Line);xshiftNext = 2*rOuter(iR)+CouplingL+offset;xshift = xshift+xshiftNext;endif iRx==1 && w==2rectangle(-700, y1Line,x1couplingB-rOuter(iR)-offset*0.5,y2Line);elseif iRx>=CurrentNumofRaceTracks/2 && w==3rectangle(x2couplingB+rOuter(iR)+offset*0.5, y1Line,7600,y2Line);endend126Appendix A. Clewin Matlab Script Used for the Mask Layoutendxshift = 0;if iR<lenRyshiftNext = rOuter(iR)+ rOuter(iR+1)+gap+2*w+offset;yshift = yshift + yshiftNext;endend%%%%%%%%%%%%%%%%%%%%%%%%% Script 01f %%%%%%%%%%%%%%%%%%%%%%%%%%Create an angle vector between -90deg to 90deg, ex:100 anglesangle = linspace(-pi/2,pi/2,100);offset = 80;yshift = 0;yshiftNext = 0;xshift = 0;%Create a radius vector giving a radius for each angle:rInner = [150];%Radius for the inner circlelenR = length(rInner);width = [2,3];%rOuter-rInner;lenw = length(width);gapArray1 = [0.5,0.5,1,1,1.5,1.5,2,2,1,1,1.5,1.5,2,2];gapArray2 = [0.5,1,1.5,1.5,1.5,2,2,1,1];CouplingL = 150;LofDye=6600;x1Shift_init=0;for iR = 1:lenRiG=1;for iw = 1:lenww = width(iw);if w==2127Appendix A. Clewin Matlab Script Used for the Mask LayoutgapArray=gapArray1;lenG = length(gapArray);elsegapArray = gapArray2;lenG = length(gapArray);endrOuter = width(iw)+rInner; %Radius for the outer circle for a given width%Calculate the nodes of a inner circle and an outer circle,nodesInnery = rInner(iR).*sin(angle)+yshift;nodesOutery = rOuter(iR).*sin(angle)+yshift;lenN = length(nodesInnery);Rnodes = [];Lnodes = [];LofRaceTrack = rOuter(iR)*2+CouplingL+offset;CurrentNumofRaceTracks= round(LofDye/(LofRaceTrack));%Calculate the nodes of a half ring,%i.e. a matrix with 2 columns and 1000 rows:for iRx = 1:CurrentNumofRaceTracks/2+1if iRx < CurrentNumofRaceTracks/2 && iR==2 && iRx==1xshift = xshift-50;endgap = gapArray(iRx);if gap == 0.5 && w==2shiftUp = (gap - 0.5)+2*w;elseshiftUp = (gap - 0.5)+2*w;endnodesInnerx = rInner(iR).*cos(angle)+xshift;nodesOuterx = rOuter(iR).*cos(angle)+xshift;for index = 1:lenN128Appendix A. Clewin Matlab Script Used for the Mask Layoutif (nodesOuterx(index)-nodesInnerx(index))>= 0nodesy(index) = nodesOutery(index)+shiftUp;Rnodesx(index) = nodesOuterx(index)+CouplingL/2;Rnodes = [Rnodesx(:), nodesy(:)];Lnodesx(index) = nodesOuterx(index) - 2.*nodesOuterx(index)- CouplingL/2+2*xshift;Lnodes = [Lnodesx(:), nodesy(:)];endlenNr = length(Rnodes(:,1));%calculate nodes of coupling lengthx2couplingT = Rnodes(lenNr,1);x1couplingT = x2couplingT - CouplingL;y2couplingT = Rnodes(lenNr,2)+w/2;y1couplingT = y2couplingT - w;x2couplingB = Rnodes(1,1);x1couplingB = x2couplingB - CouplingL+ x1Shift_init;y2couplingB = Rnodes(1,2)+w/2;y1couplingB = y2couplingB - w;x1Line_init = -400;x1Line = x1Line_init + xshift;x2Line = x1Line_init+LofRaceTrack+10; %*CurrentNumofRaceTracks;y2Line = y1couplingB - gap;y1Line = y2Line- w;iG = iG+1;endif iRx > CurrentNumofRaceTracks/2 && iR == 1 && w==2rectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB-100,y2Line);rectangle(x2couplingB-100, y1Line-0.5,x2couplingB+rOuter(iR)+offset,y2Line+0.5);xshiftNext = 2*rOuter(iR)+CouplingL+offset-250;xshift = xshift+xshiftNext;129Appendix A. Clewin Matlab Script Used for the Mask Layoutelsewire(1,w,Rnodes); %Right half ringrectangle(x1couplingT,y1couplingT,x2couplingT,y2couplingT);rectangle(x1couplingB,y1couplingB,x2couplingB,y2couplingB);wire(1,w,Lnodes); %Left half ring%rectangle(x1Line,y1Line,x2Line,y2Line);rectangle(x1couplingB-rOuter(iR)-offset*0.5, y1Line,x2couplingB+rOuter(iR)+offset*0.5,y2Line);xshiftNext = 2*rOuter(iR)+CouplingL+offset;xshift = xshift+xshiftNext;endif iRx==1 && w==2rectangle(-700, y1Line,x1couplingB-rOuter(iR)-offset*0.5,y2Line);elseif iRx>=CurrentNumofRaceTracks/2 && w==3rectangle(x2couplingB+rOuter(iR)+offset*0.5, y1Line,7600,y2Line);endendendxshift = 0;if iR<lenRyshiftNext = rOuter(iR)+ rOuter(iR+1)+gap+2*w+offset;yshift = yshift + yshiftNext;endend130

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