Master’s Thesis MEMS-based step-up voltage conversion for comb-drive actuation by Jarno Groenesteijn Transducer Science and Technology Group University of Twente, The Netherlands Supervisors: Graduation Committee: Dr. Ir. L. (Leon) Abelmann Prof. Dr. M.C. (Miko) Elwenspoek Ir. J.B.C. (Johan) Engelen Dr. Ir. L. (Leon) Abelmann Dr. Ir. R.A.R. (Ronan) van der Zee Ir. J.B.C. (Johan) Engelen September 15, 2010 2 Title: MEMS-based step-up voltage conversion for comb-drive actuation Author: J. (Jarno) Groenesteijn Master’s Thesis defense: September 24, 2010 Carr´e, Ca2186 (Lecture Hall 2N) Graduation Commitee: Prof. Dr. M.C. (Miko) Elwenspoek University of Twente, EEMCS, TST Dr. Ir. L. (Leon) Abelmann University of Twente, EEMCS, TST Ir. J.B.C. (Johan) Engelen University of Twente, EEMCS, TST Dr. Ir. R.A.R. (Ronan) van der Zee University of Twente, EEMCS, ICD 3 4 Abstract This report is the result of the master’s thesis of Jarno Groenesteijn. Theoretical and practical research has been done on a voltage step-up converter based on a comb-drive to investigate the possibilities of this new kind of converter. An analytical analysis has been done to predict the operation of a converter like this. The analytical description is used to make a computer model topredictthemechanicalandelectricalbehaviouroftheconverter. Eightdifferentteststructures are designed and fabricated to test the operation of the converter and to improve the quality of the computer model. Measurement on the mechanical behaviour of the comb-drives have been combined with finite element simulations on the electrical behaviour of the comb-drives to find an accurate computer model of the voltage step-up converter. Analysis has been done to find the most important elements that reduce the performance of the converter and solutions to these problems have been proposed. 5 6 ABSTRACT Acknowledgements This project started of in the heads of my supervisors Johan Engelen and Leon Abelmann and without their enthusiastic support, the project would never have been this complete. I would also would like to thanks Theo Lammerink, Gijs Krijnen, Remco Wiegerink and Niels Tas for donating chip area on the wafers for the MEMS Design course to my project and Meint de Boer for fabricating more working chips then I could measure. CreditgoestoRemcoSandersforhisexpertiseandworkonthevibrometerandvacuumcham- ber. Withouthishelp,Iwouldnothavebeenabletospendsomuchtimeonactualmeasurements. And last but not least, I would like to thank my fellow TST students: Jaap Kokorian, Tjitte- JeltePeters,HenrideJong,MichelZoontjesandKoertVergeerfortheirtalk,theirinsightfulideas and for the group trips to the coffee machine. 7 8 ACKNOWLEDGEMENTS Contents Abstract 5 Acknowledgements 7 1 Introduction 11 2 Theory 13 2.1 Theoretical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.1.1 Output voltage and current . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3 Device design 19 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.2 Simulation and design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.2.1 The Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.2.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2.3 Simulation of final designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2.4 20Sim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.3 Simulations of the updated model. . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3.3.1 The Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3.3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.4 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.4.1 Design overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.4.2 Spring Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.4.3 Snap-in to the bulk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.4.4 The different mask designs . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.4.5 Final Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.4.6 Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.4.7 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4 Experiments 47 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 4.2 Output specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 4.3 Mechanical behaviour of the converter . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.4 Actuation electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.4.2 Parasitic impedances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.4.3 The Micro Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 4.4.4 Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 4.4.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.5 Leakage and parasitics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5 Discussion 59 9 10 CONTENTS 6 Conclusion 61 7 Recommendations 63 7.1 Improvements on current design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 7.1.1 Back-etch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 7.1.2 The wafer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 7.1.3 The electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 7.1.4 Other improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 7.2 Other options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 7.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Bibliography 67 A Tables 69 A.1 Device Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 A.2 Experiements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 B Figures 75 B.1 Device Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 C 20-Sim code 81 C.1 SignalGenerator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 C.2 Comb-Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 C.2.1 Version 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 C.2.2 Version 2: including stator capacitance. . . . . . . . . . . . . . . . . . . . . 84 C.3 Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 C.3.1 Version 1: Initial model (resistor only) . . . . . . . . . . . . . . . . . . . . . 87 C.3.2 Version 2: Variable resistors and capacitors . . . . . . . . . . . . . . . . . . 87 C.4 Cbuffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 C.5 Vsource . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 C.6 Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 C.7 Spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 C.8 Damper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 C.9 Rleakage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 C.10Roxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 D MatLab code for Comsol 91 D.1 find cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 D.2 build struct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 D.3 getpolyfit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 E MatLab code for CleWin 97 E.1 Build Comb Bank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 E.2 function Build DC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 E.3 function Build FF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 E.4 function DC Build Spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 E.5 function FF Build Spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 E.6 function Finger Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 E.7 function Lightning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 E.8 function PerfBox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 E.9 Set Comb variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 F MicroController code 117 F.1 Main . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
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