STUDIES ON MICROMACINED STRUCTURES FOR RF APPLICATIONS

Transcription

1 STUDIES ON MICROMACINED STRUCTURES FOR RF APPLICATIONS by PREETI SHARMA Centre for Applied Research in Electronics (CARE) Submitted in fulfillment of the requirements of the degree of Doctor of Philosophy to the INDIAN INSTITIUTE OF TECHNOLOGY DELHI (IITD) NEW DELHI , INDIA FEBRUARY 2009

2 I. 1. T. DELHI. LL JRY Acc. No. Th.t:31 6 I.I.T. DELHI 0

3 DEDICATED TO MY FAMILY

4 This is to certify that the thesis entitled, "Studies on Micromachined Structures for RF Applications", being the submitted by Ms. Preeti Sharma for the award of the degree of Doctor of Philosophy (Ph.D.) to the Indian Institute of Technology Delhi, New Delhi, is a record of original bonafide research work carried out by her under our guidance and supervision. In our opinion, the thesis has reached the standard of fulfilling the requirements of all the regulations related to the degree. The results contained in this thesis have not been submitted in part or full, to any other university or institute for the award of any degree or diploma. We certify that she has pursued the prescribed course of research r. ta Prof. Shiban K. Koul Centre for Applied Research in Electronics (CARE) Indian Institute of Technology Delhi New Delhi , INDIA Prof. Sudhir Chandra Centre for Applied Research in Electronics (CARE) Indian Institute of Technology Delhi New Delhi , INDIA

5 ACKNOWLEDGEMENTS I would like to express my sincere appreciation and gratefulness to my supervisors Prof. Shiban Koul and Prof. Sudhir Chandra for their valuable guidance, constant motivation and generous support throughout this research work. Prof. Koul has a vision for future state of the art compact RF modules. He with his expertise in microwave and phase shifter technology motivated me to initiate the research in RF MEMS area that would translate his vision into reality. His main thrust is on the development of these high-performance compact systems within our country. Prof. Chandra's expertise and broad knowledge in device fabrication related issues proved to be important in the realization of this work. I gained lot of scholastic knowledge from the invaluable discussions held with Prof. Chandra. I was indeed blessed and honoured to be their student. I am also grateful to Prof. Ananjan Basu and Prof. Mahesh P. Abegaonkar for sharing their expertise and knowledge in antenna technology. I would like to thank Prof. Suneet Tuli and Prof. Arun Kumar for serving on my committee as Ph.D. program coordinators. I would like to recognize the company of my fellow researchers at CARE, Dr. Nagendra Pathak, Dr. Prem Pal, Dr. Ravindra Singh, Dr. Jyoti Prakash, Dr. Ravi Babu, Manoj Singh Parihar, Madhur Deo Upadhayay, Sandeep Chaturvedi, K. Srinivasan and Vivekanand Bhatt who made useful contribution to this work by discussions and suggestions from time to time. I would also like to appreciate Abhishek Kumbhat (M. Tech. student) for his valuable suggestions. I would like to thank all CARE faculty and staff members who helped me directly or indirectly during stay in CARE for successful completion of my research work. I am highly thankful to Mr. S. P. Chakraborty, Mr. Govind Ram and Mr. Chana for providing technical support for operating and maintaining the equipments. My deepest gratitude is reserved for

6 Mr. Ashok Pramanilc, Purshottam Das and Govind Prasad (Laboratory Attendants) for providing support in processing the devices during the course of the work. I would like to express thanks to Gallium Arsenide Technology Enabling Center (GATEC), Hyderabad, India, for extending its facilities for the fabrication and testing of surface-micromachined MEMS components. The gratitude is incomplete without conveying my thankfulness to Dr. P. Datta (Sc. F), Mr. Praveen Kumar and other members of Silicon Microwave Division (SMD), SSPL, India. Dr. Datta is the one who inspired me to take up research as a career. I tacitly learnt the meaning of "being a scientist" from his dedication and persistence towards work. A part of research presented in this thesis was supported by Institute of Electronics and Telecommunication Engineers (IETE) under the GOLDEN JUBILEE RESEARCH FELLOWSHIP programme. I gratefully acknowledge the support and appreciate the endurance of Cdr. A. P. Shanna of IETE, New Delhi and his team for periodically reviewing the status of the research. Last but certainly not least I have to express my everlasting gratitude to my Grandfather, Father and Mother who have inspired me to be the best that I can be. They gave me all the tools in life to be able to succeed, and taught me never to stop achieving, and to never settle for less than best. I derive my strength from the strength they exuded through all their battles in life. I am eternally indebted to my sisters, Neeti and Aarti, my brother, Ambuj my best friends who remained with me throughout the grueling past few years; they made me believe that hard work never goes in vain. I would also like to recognize my husband Anuj who made the last tough stages a breeze. Preeti Sharma ii

7 ABSTRACT Comprehensive studies on Radio-Frequency Micro-Electro-Mechanical-Systems (RF MEMS) components are presented in this thesis. The work aimed at developing and optimizing RF MEMS including antennas, filters, switches and phase shifters on silicon and gallium arsenide substrates for future integrated microwave communication systems. Development and optimization include device design, fabrication and microwave circuit demonstrations. To start with, dielectric films of silicon dioxide and silicon nitride have been investigated to realize stress-flee large-size membranes for membrane-supported RF circuits. Dielectric films are deposited on silicon substrates using RF sputtering techniques and their respective membranes are released by silicon bulk-micromachining. The emphasis has been put on fabricating these membranes using low-temperature processes which ensure their compatibility with MMIC circuit realization. Based on electrical properties and mechanical buckling, the best dielectric film composition has been selected for realizing membranesupported RF circuits. Low-loss microstrip antennas and filters have been demonstrated on the selected dielectric membrane material. RF characteristics of various patch shapes including square, circular, triangular and coplanar have been presented in this work. The proposed micromachined antennas promise to deliver wider bandwidth with good radiation characteristics on high permittivity silicon substrate. Bulk-micromachining is also been applied to realize microstrip filters with improve RF characteristics. An attempt is also made to investigate the performance of few freely air-suspended filters and comparisons have been done with their membrane-suspended counterparts. iii

8 Surface-micromachined RF MEMS switches have been designed for wide-band, lowactuation and tuning applications such as phase shifters, wireless handsets and impedance tuners. Detailed processing techniques and fabrication concerns of these switches are discussed to understand their effect on device performance. In addition, few test structures have been proposed to extract circuit parameters and identify the cause of deviation in the performance of these switches. Switches with insertion loss < 0.5 db and isolation better than db in the range of 1-40 GHz have been demonstrated in this work. The maximum capacitance ratio of a switch reported in this thesis is 20 and actuation voltage is less than 30 V. Based on air-bridges architecture, performance of distributed transmission line MEMS phase shifters and wide-band switches are also investigated to demonstrate the viability of integrated RF front-end modules on gallium arsenide substrates. The thesis concludes by suggesting the scope of further research in this area. iv

9 TABLE OF CONTENTS ACKNOWLEDGEMENTS ABSTRACT iii TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF ABBREVIATIONS AND SYMBOLS xii xxvi CHAPTER 1: Introduction 1.1 Introduction to RF MEMS Overview of Membrane-supported RF Circuits Overview of a Typical RF MEMS Switch Development of RF MEMS Fabrication of RF MEMS Objectives and Scope of the Work Organization of Thesis 7 CHAPTER 2: Dielectric Membranes for RF Circuits 2.1 Introduction Methods of Dielectric Film Deposition Choice of Anisotropic Etchant Choice of Membrane Material Fabrication Characterization of Deposited Films Surface Roughness Study Elemental Composition 22

10 2.6.3 Electrical Characterization Characterization of Fabricated Membranes Curvature Analysis of the Membranes Application of Millimeter-size Dielectric Membranes Conclusions 30 CHAPTER 3: Micromachined Antennas and Filters 3.1 Introduction Fabrication of Antennas and Filters on Micromachined Silicon 33 Substrates 3.3 Design of Micromachined Microstrip Line and Coplanar 35 Waveguide Microstrip Line Coplanar Waveguide Micromachined Patch Antennas Circular Patch Sensitivity Analysis Experimental Results Square Patch Experimental Results Triangular Patch Experimental Results Coplanar Patch Experimental Results Performance Comparison Micromachined Microstrip Filters Low Pass Filter Stepped Impedance LPF 65 vi

11 Stub Line LPF Band Stop Filter Wide Band Spurline Filter Conclusions 80 CHAPTER 4: RF MEMS Capacitive MEMS Shunt Switches 4.1 Introduction Architecture and Working Principle of MEMS Shunt Capacitive 82 Switches 4.3 Wide-Band Switch Electrical Design Mechanical Design Parametric Study Variation of Aspect Ratio of the FGC Line Variation of Width of the Bridge Variation of Thickness and Permittivity of the 97 Dielectric Layer Sensitivity Study S-parameters Measurement Set-up Fabrication of the Switch and Novel Test Structures to 103 Identify the Failure Mechanisms and Common Measurement Errors Fabrication of Proposed Switches and Test 104 Structures Test Structure to Identib, the Failure Mechanism and Model the Series Inductance of the Switch Experimental Results 4.4 Low Actuation Voltage Switch Electrical Design vii

12 4.4.2 Mechanical Design Experimental Results Inductive Switch Experimental Results Conclusions 129 CHAPTER 5: DMTL Phase Shifter and Wideband Switches 5.1 Introduction Distributed MEMS Transmission Line Phase Shifter Transmission Line Model Design of a Unit Cell Variation of Bridge-width Variation of Periodic Spacing Variation of Dielectric Layer Thickness Variation as a function of Number of Bridges Fabrication of Multi-bridge DMTL Results and Discussions Wideband Switches Modeling of Proposed Switch Varying Periodic Spacing Varying Down-state Capacitance Varying Series Inductance Varying Series Resistance Experimental Results and Discussions Conclusions 161 CHAPTER 6: Conclusions 6.1 Summary of the Results 163 viii

13 6.2 Future Scope 165 REFERENCES 168 APPENDICES 177 PUBLICATIONS 181 BIO-DATA 184 ix