Electrostatic Low Actuation Voltage RF MEMS Switches for Telecommunications

247 Pages·2005·9.7 MB·English

PhD Dissertation
International Doctorate School in Information and
Communication Technologies
DIT - University of Trento
Electrostatic Low Actuation Voltage RF
MEMS Switches for Telecommunications
KAMAL JIT RANGRA
Advisor:
Prof. Giovanni Soncini
Universit`a degli Studi di Trento
February 2005

Abstract
The present study is inspired by the superior performance of electrostatic
RF MEMS switches over the contemporary state-of-the-art solid-state de-
vices and the potential applications in communications field. The prevalent
high voltage actuation mode, limits the reliability and applications espe-
cially, in wireless communication, therefore the study focuses on the real-
ization of electrostatic low actuation switches with main emphasis on the
pull-in voltage and RF response. The actuation voltage optimization is
achieved by analyzing the flexure design, beam topology, actuation elec-
trodes and gap height, using analytical models validated by numerical sim-
ulations. The RF performance enhancement is done by incorporating the
floating metal design, active beam-area reduction and minimization of the
associated parasitics. The fabrication is based on surface-micromachining,
metal-electroplating and standard IC processing steps. The main features
of the fabricated capacitive shunt and ohmic contact switches are: the ac-
tuation voltage 3-15V, isolation ≥ 25dB, and insertion loss ≤0.2dB, at
1-25GHz, for three basic configurations. As a conclusion of the study,
an innovative switch design called ‘Symmetric Toggle Switch’ is presented.
The microtorsion-actuator based design has actuation voltages of 7-10V,
with good RF performance from 8-20GHz. In addition to improved relia-
bility in switching applications, the device can be configured as a MEMS
varactor with higher capacitance range or as a tunable filter over a narrow
bandwidth. The agreement of 20-60%, between the measurements and simu-
lations is expected to improve further with fabrication process optimization.
Keywords: RF MEMS Switch, Ohmic contact switch, Symmetric Toggle
Switch, micro-machining, torsion actuator, Residual stress.

.
dedicated to
H H S S B

Acknowledgements
During this work, I received cooperation, guidance and inspiration from
many individuals here in Italy and back home in India. I wish to express
my gratitude to them all. I begin by expressing my thanks, to Prof. Gio-
vanni Soncini my supervisor and Dr. Mario Zen, Director, ITC-irst, for
introducing me to the fascinating world of RF MEMS and whole hearted
support at every step during this work. I am especially thankful to Dr.
Benno Margesin, my supervisor in ITC-irst for his invaluable suggestions,
original ideas and encouragement. I wish to express my sincere thanks to
Dr Leandro Lorenzelli, for the invaluable discussions and his moral sup-
port. I also gratefully acknowledge the help from Dr Flavio Giacomozzi
and all the members of the process team.
I am deeply indebted to Prof. Gian-Franco Dalla Betta, Electronics
Group, UNITN, for the discussions, financial support to conferences and
encouragement when I needed it most. My thanks are also due to Dr
Lorenza Ferrario for the discussions on structural simulations and to all
other colleagues of MIS, for making the work a pleasure during three long
years. I am also thankful to Prof. R. Marcelli, CNR-IMM, Rome, Dr
Roberto Gaddi and Ms Laura Del Tin of University of Bologna for access
to measurements facility and simulations. I also thank the, staff members
of ICT Secretariat and International Student’s Office, UNITN, for their
cooperation through out this period. My special thanks to Prof. Chiara,
Mr Ricardo and their family, where I saw and experienced the Italian
hospitality at its best.
Finally, I am thankful to my wonderful children Aarushee and Subeer,
my wife Mithlesh, my parents and in-laws without whose love and support
it would have been impossible to complete this work.

Contents
1 Introduction 1
1.1 MEMS Technology . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 RF MEMS Switch . . . . . . . . . . . . . . . . . . 4
1.2 The Context .. . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 The Problem ... . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Structure of the Thesis . . . . . . . . . . . . . . . . . . . . 9
2 R F MEMS 17
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Basic RF MEMS . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.1 RF MEMS Switch . . . . . . . . . . . . . . . . . . 18
2.2.2 MEMS and Semiconductor Switches
- Basic Difference . . . . . . . . . . . . . . . . . . . 21
2.2.3 Contact and Circuit Configuration . . . . . . . . . 22
2.2.4 Devices based on Alternative Actuation Modes . . . 28
2.2.5 RF MEMS Tunable Capacitors . . . . . . . . . . . 29
2.2.6 MEMS Inductors . . . . . . . . . . . . . . . . . . . 30
2.2.7 General Fabrication Process and Materials . . . . . 32
2.2.8 Application Areas of RF MEMS . . . . . . . . . . . 34
2.3 Problem Areas . . . . . . . . . . . . . . . . . . . . . . . . 36
2.3.1 Device Packaging . . . . . . . . . . . . . . . . . . . 37
2.3.2 Power Handling and Reliability . . . . . . . . . . . 37
i
3 Mechanical Design ... 39
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.2 Mechanical Equations... . . . . . . . . . . . . . . . . . . . 40
3.3 Squeeze-Film Damping . . . . . . . . . . . . . . . . . . . . 44
3.3.1 Viscous flow . . . . . . . . . . . . . . . . . . . . . . 45
3.3.2 One dimensional Analysis . . . . . . . . . . . . . . 46
3.4 Micro-Machined.... . . . . . . . . . . . . . . . . . . . . . . 47
3.4.1 Spring Constants for Simple Beams . . . . . . . . . 49
3.4.2 Nonlinear Effects . . . . . . . . . . . . . . . . . . . 50
3.4.3 Spring Constant for the cases specific . . . . . . . . 51
3.4.4 Contribution of Residual Stress in Spring Constant 54
3.4.5 Support Flexure Design for Low Spring Constant
Bridge Structures . . . . . . . . . . . . . . . . . . . 58
3.4.6 Linear Spring Constant of Serpentine Flexures . . 59
3.4.7 Spring Constant: Comparison of Analytical
and FEM Simulated k . . . . . . . . . . . . . . . . 63
z
3.5 Electrostatic Actuation... . . . . . . . . . . . . . . . . . . . 66
3.5.1 Pull-in Voltage . . . . . . . . . . . . . . . . . . . . 67
3.5.2 Pull-out Voltage . . . . . . . . . . . . . . . . . . . 69
3.5.3 Power Handling . . . . . . . . . . . . . . . . . . . . 71
3.5.4 Effects of Non-periodic Excitation, Gravitational . . 72
3.5.5 Voltage Break Down in MEMS Switches . . . . . . 73
3.5.6 Dynamic Response of the Beams . . . . . . . . . . 74
4 Electromagnetic Design 79
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.2 Coplanar Wave Guide . . . . . . . . . . . . . . . . . . . . 80
4.2.1 Characteristic Impedance - Synthesis Approach . . 80
4.2.2 Thickness of CPW and Mobile Beams . . . . . . . 82
ii