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

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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-2016-4 http://www.eecs.berkeley.edu/Pubs/TechRpts/2016/EECS-2016-4.html January 6, 2016 Copyright © 2016, by the author(s). All rights reserved. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission. 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 iv Special lengths of mechanical couplers . . . . . . . . . . . . . . . . . 48 l = nλ,n ∈ N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 beam l = λ +nλ,n ∈ N . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 beam 4 l = λ +nλ,n ∈ N . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 beam 2 l = 3λ +nλ,n ∈ N . . . . . . . . . . . . . . . . . . . . . . . . . . 50 beam 4 3 AlN Contour Mode Disk Resonators with Unattached Electrodes 52 3.1 Introduction to Capacitive-Piezoelectric Transduction . . . . . . . . . . . . . 52 Two Common Methods for MEMS Transduction . . . . . . . . . . . . . . . . 53 Capacitive Transduction . . . . . . . . . . . . . . . . . . . . . . . . . 53 Piezoelectric Transduction . . . . . . . . . . . . . . . . . . . . . . . . 54 Comparitive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Piezoelectric Resonators with Non-Contacting Electrodes . . . . . . . . . . . 55 Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 3.2 Capacitive-Piezoelectric Radial Contour Mode Disk Resonators . . . . . . . 57 3.3 Radial Contour Mode Eigenfrequency and Mode Shape . . . . . . . . . . . . 58 3.4 Disk Resonator Circuit Model . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Electromechanical Model Component Values . . . . . . . . . . . . . . . . . . 60 BVD Model Component Values . . . . . . . . . . . . . . . . . . . . . . . . . 63 Capacitive-Piezo Voltage Coupling Efficiency, α . . . . . . . . . . . . . . . . 63 Capacitive-Piezo Resonance Frequency Tuning Factor, β . . . . . . . . . . . 64 Electromechanical Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 3.5 MATLAB Computation of Resonator Parameters . . . . . . . . . . . . . . . 65 Radial Contour Mode Eigenfrequency Calculation . . . . . . . . . . . . . . . 66 1st Radial Contour Mode Shape Function Definition . . . . . . . . . . . . . . 66 Meshing of the Domain for Numerical Integrations . . . . . . . . . . . . . . . 66 Effective Mass Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Electromechanical Transformer Coefficient Calculation . . . . . . . . . . . . 67 Resonator Impedance and Admittance . . . . . . . . . . . . . . . . . . . . . 68 Simulated S Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 21 3.6 Two-Port Characterization Method for Capacitive-Piezoelectric Resonators . 69 Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Rationale for Using Two Ports . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Parameter Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Unloaded Q and R . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 x k2 Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 eff C Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 0 3.7 Capacitive-Piezo Disk Resonator Performance Optimization . . . . . . . . . 73 Anchor Loss Minimized 300 MHz AlN Capacitive-Piezo Disk Resonator with Q=8.8k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Remark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

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tegrated voltage-controlled gap-reduction-based frequency tuning mechanism age controlled gap actuation of the capacitive piezoelectric transducer's top electrode, which .. 2.16 Normalized series and shunt impedance magnitudes for the T-network extensional . Q drops as downforce increases.
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