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

Schneider, Robert
Technical Report Identifier: EECS-2015-247
December 17, 2015

Abstract: 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 challenge. So far, no existing resonator technology can provide the simultaneous high-Q, high electromechanical coupling (k_{eff}^2), frequency tunability, low motional resistance (R_x), stopband 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 technology 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 recently 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 aluminum 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 this technology, have, until now, hindered attempts at more complex devices than just simple resonators with improved Q's.

This thesis project demonstrates several key improvements to capacitive-piezo technology, 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 k_{eff}^2's on par even with attached-electrode contour-mode devices, while most importantly, achieving unprecedented Q-factors for AlN. Third, a new electrode-collapsed 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 integrated 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.

This thesis is divided into eight parts. In the first chapter, context is provided to demonstrate 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 resonators, introduces the fundamental device of this thesis, providing examples of performance and design optimization, experimental results, simulation methods, and modeling. Chapter 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 voltage 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 thorough 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.