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High-Q Strong Coupling Capacitive-Gap Transduced RF Micromechanical Resonators


This dissertation presents a hierarchical, intuitive, and technology agnostic procedure for design-ing RF channel-select filters, followed by an actual demonstration solidly confirming the validity of the design method. Two distinct methods then follow that aim to increase the resonator electrome-chanical coupling coefficient, which substantially improves the functionality of the demonstrated filter for future applications, e.g., ones that require higher-order with sharper roll-off characteristics and less passband ripple. To increase functionality even further, the remaining chapters of this the-sis introduce a fabrication and post-processing method using CMOS-compatible ruthenium metal that allows integration of micromechanical devices, such as the aforementioned RF filters, atop CMOS.

Chapter 2 introduces design, fabrication, and experimental demonstration of a differential in-put/output RF channel-select micromechanical disk filter consisting of 96 mechanically coupled capacitive-gap-transduced polysilicon disk resonators, centered at 224MHz with only 0.1% (9kHz) bandwidth, all while attaining 2.7dB insertion loss and more than 50dB out-of-channel stopband rejection. Combined with inherent high-Q’s of capacitive-gap disk resonators, sub-40nm transduc-tion gaps enabled by the sidewall sacrificial layer fabrication technology and defensive design strat-egies employing buffer disks against fabrication residual stress were instrumental in obtaining this impressive performance with decent yield and RF-compatible 590 filter termination impedance. Perhaps most encouraging, the equivalent circuit model developed for this complicated structure based on mechanical and electrical parameters was spot on in capturing not only the ideal filter re-sponse, but also the parasitic nonidealities that might distort the filter performance.

Having presented an initial RF channel-select filter demonstration, Chapters 3-5 then focus on design methods and fabrication techniques that could raise the filter performance one step further by substantially increasing the electromechanical coupling coefficient of its constituent resonators. Specifically, Chapter 3 introduces a new type of a resonator formed via hollowing out a capacitive-gap transduced radial mode disk resonator that achieved a measured electromechanical coupling strength (Cx/Co) of 0.75% at 123 MHz without the need to scale the device’s meager 40-nm elec-trode-to-resonator gap. This is almost 7× improvement in Cx/Co compared with a conventional ra-dial contour-mode disk at the same frequency, same dc bias, and same gap. Cx/Co increases like this should improve the passbands of channel-select filters targeted for low power wireless trans-ceivers, as well as lower the power consumption of MEMS-based oscillators.

Considering the dependence of the electromechanical coupling on the actuation gap is inverse cubic compared to the linear dependence on mass, Chapters 4-5 attempt to obtain strong coupling by reducing the actuation gaps to below 10nm from their current 37nm. To realize this, one must first overcome fabrication-related hurdles such as precise thin film residual stress control, smooth post-etch sidewalls free of asperities, and sub-10nm sacrificial layer conformal deposition. Chapter 4 attacks the first hurdle by introducing an on-chip strain measurement device that harnesses preci-sion frequency measurement to precisely extract sub-nm displacements, allowing it to determine the residual strain in a given structural film with best-in-class accuracy, where stress as small as 15MPa corresponds to 2.9nm of displacement. The importance of attaining such accuracy mani-fests in the fact that knowledge of residual strain might be the single most important constraint on the complexity of large mechanical circuits, such as RF channel-select filters.

Chapter 5 then addresses the remaining hurdles for achieving sub-10nm gaps by using a modi-fied polysilicon etch recipe that generates considerably smoother sidewalls and an atomic layer de-posited (ALD) 8nm-thick conformal SiO2 sidewall sacrificial layer. The single-digit-nanometer electrode-to-resonator gaps demonstrated in this chapter have enabled 200-MHz radial-contour mode polysilicon disk resonators with motional resistance Rx as low as 144 while still posting Q’s exceeding 10,000, all with only 2.5V dc-bias. The tiny motional resistance, together with (Cx/Co)’s up to 1% at 4.7V dc-bias and (Cx/Co)-Q products exceeding 100, propel polysilicon ca-pacitive-gap transduced resonator technology to the forefront of MEMS resonator applications that put a premium on noise performance, such as radar oscillators. Simultaneous high-Q and strong electromechanical coupling (Cx/Co) makes this technology attractive for future sharp roll-off, flat passband RF channel-select filters targeted for low power receivers as well as wide band filters targeted for the LTE bands.

The decent resonator performance offered by polysilicon structural material with Q’s exceeding 10,000 at 200MHz comes with a drawback that LPCVD polysilicon with deposition temperatures of 590-615C is not directly integrable atop CMOS due to thermal budget constraints. Pursuant to mitigating this issue, Chapter 6 introduces a fabrication and post-processing method using CMOS-compatible ruthenium metal that allows integration of micromechanical devices atop CMOS. Spe-cifically, introduction of tensile stress via localized Joule heating has yielded some of the highest metal MEMS resonator Q’s measured to date, as high as 48,919 for a 12-MHz ruthenium micro-mechanical clamped-clamped beam. The low-temperature ruthenium metal process, with highest temperature of 450°C and paths to an even lower ceiling of 200°C, further allows for MEMS pro-cessing over CMOS wafers offering a promising route towards monolithic realization of CMOS-MEMS circuits needed in communication transceivers.

Finally, Chapter 7 fulfills the promise of this dissertation in metals, i.e., simultaneous high-Q and strong coupling, by employing a 20-nm-gap CMOS-compatible flexural-mode square-plate resonator constructed in thermal-annealed ruthenium metal that posts quality factors (Q’s) exceed-ing 5,000 and an impressive transducer strength Cx/Co (equivalent to kt2) of up to 71% intrinsic and 36% with 55fF of bond capacitance loading, which in turn permits more than 46% voltage-controlled resonance frequency tuning (from 18.005 to 9.713MHz) with a voltage excursion from 0.5 to 2.8V. The 36% Cx/Co is 75 times larger than the 0.48% of published AlN piezoelectric mate-rial in this HF frequency range. With processing temperatures potentially below 350°C (using lo-calized annealing), this metal resonator is amenable to integration directly over even advanced node CMOS, making this technology attractive for single-chip widely tunable filter and oscillator appli-cations, e.g., for wireless communications.

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