High-Q AlN Contour Mode Resonators with Unattached, Voltage-Actuated Electrodes

222 Pages·2015·6.72 MB·English

High-Q AlN Contour Mode Resonators with
Unattached, Voltage-Actuated Electrodes
Robert Schneider
Electrical Engineering and Computer Sciences
University of California at Berkeley
Technical Report No. UCB/EECS-2015-247
http://www.eecs.berkeley.edu/Pubs/TechRpts/2015/EECS-2015-247.html
December 17, 2015
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High-Q AlN Contour Mode Resonators with Unattached, Voltage-Actuated
Electrodes
by
Robert Anthony Schneider
A dissertation submitted in partial satisfaction of the
requirements for the degree of
Doctor of Philosophy
in
Engineering–Electrical Engineering and Computer Sciences
in the
Graduate Division
of the
University of California, Berkeley
Committee in charge:
Professor Clark T.-C. Nguyen, Chair
Professor Kristofer S. J. Pister
Professor Liwei Lin
Fall 2015

High-Q AlN Contour Mode Resonators with Unattached, Voltage-Actuated
Electrodes
Copyright 2015
by
Robert Anthony Schneider
1
Abstract
High-Q AlN Contour Mode Resonators with Unattached, Voltage-Actuated Electrodes
by
Robert Anthony Schneider
Doctor of Philosophy in Engineering–Electrical Engineering and Computer Sciences
University of California, Berkeley
Professor Clark T.-C. Nguyen, Chair
High-Q narrowband filters at ultra-high frequencies hold promise for reducing noise and
suppressing interferers in wireless transceivers, yet research efforts confront a daunting chal-
lenge. So far, no existing resonator technology can provide the simultaneous high-Q, high
electromechanical coupling (k2 ), frequency tunability, low motional resistance (R ), stop-
eff x
band rejection, self-switchability, frequency accuracy, and power handling desired to select
individual channels or small portions of a band over a wide RF range. Indeed, each technol-
ogy provides only a subset of the desired properties.
Recently introduced “capacitive-piezoelectric” resonators, i.e., piezoelectric resonators
with non-contacting transduction electrodes, known for achieving very good Q’s, have re-
cently emerged (in the early 2010’s) as a contender among existing technologies to address
the needs of RF narrowband selection. Several reports of such devices, made from alu-
minum nitride (AlN), have demonstrated improved Q’s over attached electrode counterparts
at frequencies up to 1.2 GHz, albeit with reduced transduction efficiency due to the added
capacitive gaps. Fabrication challenges, while still allowing for a glimpse of the promise of
thistechnology, have, untilnow, hindered attempts atmorecomplexdevicesthanjustsimple
resonators with improved Q’s.
This thesis project demonstrates several key improvements to capacitive-piezo technol-
ogy, which, taken together, further bolster its case for deployment for frequency control
applications. First, new fabrication techniques improve yields, reliability, and performance.
Second, design modifications now allow k2 ’s on par even with attached-electrode contour-
eff
mode devices, while most importantly, achieving unprecedented Q-factors for AlN. Third,
a new electrode-collapse based resonance-quenching capability allows ON/OFF switching of
resonators and filters, such as would be useful for a bank of parallel filters. Fourth, an in-
tegrated voltage-controlled gap-reduction-based frequency tuning mechanism permits wide
frequency tuning of devices and thus much improved frequency accuracy. Gap actuation
also allows for the decoupling of filters in the OFF state. And fifth, switchable and tunable
capacitive-piezo narrow-band filters are demonstrated for the first time.
2
This thesis is divided into eight parts. In the first chapter, context is provided to demon-
strate the purpose of this work. RF channel selection is introduced and a survey of currently
available technology is presented. The second chapter explains key operating principles for
MEMS resonators so a novice reader can be better equipped to fully understand the design
choices made in later chapters. Chapter 3, on high-performance capacitive-piezo disk res-
onators, introduces the fundamental device of this thesis, providing examples of performance
and design optimization, experimental results, simulation methods, and modeling. Chap-
ter 4 introduces capacitive-piezoelectric disk arrays as a method to increase the area and
thereby reduce the motional resistance of the unit disk resonator. Chapter 5 discusses volt-
age controlled gap actuation of the capacitive piezoelectric transducer’s top electrode, which
enables voltage controlled frequency tuning and on/off switching. Chapter 6 takes a thor-
ough look at the fabrication technology needed to make capacitive-piezo devices, including
lessons learned on how to avoid certain pitfalls. Chapter 7, on filters, contains both theory
and measurement results of filters. Chapter 8 concludes the thesis by summarizing the key
achievements of Chapters 3 through 7, highlighting key areas needing further development,
and discussing implications of this technology for the future.
i
For my family.
ii
Contents
Contents ii
List of Figures ix
List of Tables xv
1 Introduction 1
1.1 Radios for Wireless Communication . . . . . . . . . . . . . . . . . . . . . . . 1
Operating Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Heterodyne Receiver Architecture . . . . . . . . . . . . . . . . . . . . . . . . 2
Requirements For a Radio Link . . . . . . . . . . . . . . . . . . . . . . . . . 3
Operate on an Unoccupied Channel . . . . . . . . . . . . . . . . . . . 3
Meet Signal to Noise Demands . . . . . . . . . . . . . . . . . . . . . . 5
Suppress and Manage Interferers . . . . . . . . . . . . . . . . . . . . 5
Avoid Jammers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2 On-chip High-Q Filters and Oscillators for Radio . . . . . . . . . . . . . . . 6
MEMS as a Quartz Alternative for Local Oscillators . . . . . . . . . . . . . . 6
The Importance of Resonator Q in Local Oscillators . . . . . . . . . . 7
On-Chip High Q MEMS Filters . . . . . . . . . . . . . . . . . . . . . . . . . 8
High-Q Narrowband Filters for Improved Rejection of Interferers . . . 8
RF Channel Selection Requires Switchability and/or Tunability . . . 9
High-Q Resonators for Low Insertion Loss . . . . . . . . . . . . . . . 10
High-Q Resonators for Narrow Fractional Bandwidths and Steep Rolloff 10
Low Impedance Resonators for Low Termination Resistance . . . . . 11
Temperature Stability of Filters . . . . . . . . . . . . . . . . . . . . . 12
Minimized Capacitive Feedthrough for Strong Stopband Rejection . . 12
Additional Aspects of Filter Design . . . . . . . . . . . . . . . . . . . 12
1.3 Survey of Band- and Channel-Select Filter Technologies at 100MHz-3GHz . . 12
SAW Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
MEMS-Based Quartz VHF and UHF Filters . . . . . . . . . . . . . . . . . . 14
BAW Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
FBAR Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
iii
Contour Mode AlN Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Biased Capacitive-Gap Transduced Micromechanical Filters . . . . . . . . . 18
Ultra-High Q Resonance . . . . . . . . . . . . . . . . . . . . . . . . . 18
A Path Towards Stronger Electromechanical Coupling, k2 = Cx . . 19
eff C0
A Path Towards Lower Motional Resistances . . . . . . . . . . . . . . 19
Frequency Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
On/Off Switchability . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Channelizer Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1.4 Outline of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2 MEMS Resonator Fundamentals 23
2.1 Lumped Element Modeling of Vibrations . . . . . . . . . . . . . . . . . . . . 23
Lumped Component Analogues in the Electrical and Mechanical Domains . 24
Principles of Vibratory Motion . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Single-, Multi-, and Infinite-Degree of Freedom (DOF) Systems . . . 26
Single Degree of Freedom Mass-Spring System . . . . . . . . . . . . . 26
Modal Analysis for multi-DOF Systems . . . . . . . . . . . . . . . . 27
Modal Analysis for Continuous (Infinite Degree of Freedom) Systems 29
2.2 Single Degree of Freedom Description of Mechanical Resonance . . . . . . . . 30
2.3 Vibrational Mode Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Modal Velocity of a Mechanical Resonator . . . . . . . . . . . . . . . . . . . 31
Kinetic Energy of a Mechanical Resonator . . . . . . . . . . . . . . . . . . . 32
2.4 Effective Mass of a Mechanical Resonator . . . . . . . . . . . . . . . . . . . . 32
2.5 Potential Energy and Effective Stiffness of a Mechanical Resonator . . . . . . 33
2.6 Electromechanical Transduction . . . . . . . . . . . . . . . . . . . . . . . . . 34
Electromechanical Transformers . . . . . . . . . . . . . . . . . . . . . . . . . 35
Equivalent Physical Interpretations of η . . . . . . . . . . . . . . . . 35
Modal Force: Calculating η For a Vibrational Mode . . . . . . . . . . 36
η for a Capacitively Transduced Resonator . . . . . . . . . . . . . . . . . . . 37
η for a Piezoelectrically Transduced Resonator . . . . . . . . . . . . . . . . . 39
η for a Capacitive-Piezoelectrically Transduced Resonator . . . . . . . . . . . 39
Equivalent Electrical Components via Transducer Absorption . . . . . . . . . 40
2.7 Butterworth-Van Dyke (BVD) Equivalent Circuit Model . . . . . . . . . . . 40
Series and Parallel Resonance Frequencies of the BVD Circuit . . . . . . . . 41
2.8 Electromechanical Coupling Coefficient, k2 = C /(C +C ) . . . . . . . . . 42
eff x x 0
2.9 Multi-Transducer Resonant Systems . . . . . . . . . . . . . . . . . . . . . . . 43
Frequency Tuning Without Affecting Motional Impedance . . . . . . 43
Two-Port Capacitive Feedthrough Reduction Via I/O Separation . . 44
Differential Operation for Further Capacitive Feedthrough Reduction 44
2.10 Coupling Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2-Port Models for Extensional Coupling Beams . . . . . . . . . . . . 45