In today’s connected world, our lives depend heavily on information obtained from networks of sensor nodes and mobile devices. Billons of mobile devices are in communication over these networks at any given time. The trillion sensor vision calls for low-power, low-cost and small-sized wireless sensor nodes, posing ever-increasing constraints on the power consumption of wireless networks. Radio Frequency (RF) Microelectromechanical Systems (MEMS) offer one path towards nano-watt wireless communications.

This dissertation presents the first demonstration of an all-mechanical wireless receiver that employs a micromechanical resonant switch (“resoswitch”) to consume zero quiescent power while listening and ~30nW while actively receiving. Here, high-Q mechanical resonance offers frequency selectivity, while mechanical impact switching provides amplification. The mechanical receiver successfully detects and demodulates (in an On-Off-Keying (OOK) fashion) Frequency-Shift-Keying (FSK)-modulated input signals down to -68dBm, a promising sensitivity for low-speed and low-power wireless applications. Because this receiver consumes no power while listening in standby, it obviates the sleep/wake cycles often used by low power sensor networks to save energy. It thus eliminates the need for continuously operating clocks that govern sleep/wake periods, thereby eliminating their power consumption.

If on the other hand one prefers to use more conventional mote transceivers that consume tens of milli-watts when on, then the sleep/wake strategy is paramount to save energy, and the clock sets the power bottleneck. A commercially available real time clock (RTC) typically consumes 1µW of power, which is considerably more than the tens of nano-watts more applicable for giant sensor networks. Fortunately, even when not used as a wireless transceiver, the all-mechanical circuit herein provides a solution to the clock problem, as well.

Specifically, the second part of this dissertation presents an RF powered wireless clock that feeds on a received FSK-modulated or Continuous-Wave input and generates a local clock output signal, while consuming only 13nW of battery power, which is more than 50 times lower than an off-the-shelf RTC. Calculations further predict a three-order-of-magnitude power reduction if an on-chip next stage inverter replaces the off-chip discrete component in the current demonstration. The input capacitance of the off-chip inverter is 1.5pF, which translates to nano-watts of power for kilohertz clock frequencies. Use of an on-chip inverter with a few femto-Farads of input capacitance would reduce the power consumption to pico-watts.

Resoswitches as receivers or as clock generators, while very compelling, face several challenges that must be further explored before they can be commercially viable. For instance, the frequency stability of a resoswitch directly determines the error rate of a receiver or the stability of a clock. This frequency stability depends on perturbations of mechanical resonance impact dynamics that create unstable switching and instantaneous frequency jitter, a phenomena referred to as squegging. The quality factor, the contact material property, and the external excitation voltage all affect the squegging dynamics, but controlling these factors is not an easy task. On the other hand, manipulating the electrode configuration can also alter the squegging behavior, and is much easier to design and implement. This dissertation models and experimentally verifies observed squegging phenomena. The methods and models presented here provide a path to improving the stability of resoswitches towards commercial viability for use in future ultra-low power wireless networks.

Recent advancements in micro-optical and micro-mechanical resonator technologies have allowed researchers to exploit coupling between the optical field and mechanical motion in an optical cavity to affect cooling or amplification of mechanical motion. Cooling the mechanical motion of microscale objects has been of high scientific interest, since it facilitates observation and exploration of certain quantum phenomena, e.g., the standard quantum limit of detection. On the other hand, amplification of the mechanical motion allows realization of microscale devices for practical applications, such as light driven low phase noise signal generation by radiation pressure driven Opto-Mechanical Oscillators (OMO’s).

The ability to achieve self-sustained oscillation with no need for feedback electronics makes an OMO compelling for on-chip applications where directed light energy, e.g., from a laser, is available to fuel the oscillation, such as Chip Scale Atomic Clocks (CSAC’s). Indeed, an OMO can substantially reduce power consumption of a CSAC by replacing its power-hungry conventional quartz-based microwave synthesizer but this requires that the OMO output is sufficiently stable, as gauged over short time spans by its phase noise.

Pursuant to identify phase noise mechanisms, this thesis presents a new phase noise model for OMO’s by deriving an OMO oscillator model with intuitive engineering understanding of its operation consistent with the established OMO theory. Phase noise theory suggests that attaining high mechanical-Q (Qm) is crucial to lower the phase noise while high enough optical-Q (Qo) is required for reasonably low-power operation. This motivates a focus on achieving a high-Qm OMO to have low phase noise while maintaining a high enough Qo for low power operation–a challenge in previous OMO’s that had to trade-off Qm and Qo mainly because they use a single material that sets both.

The work in this thesis demonstrates integrated MEMS-cavity optomechanical oscillators that combine the best properties of optical and MEMS resonators in single composite multi- material OMO structures to simultaneously optimize mechanical and optical Q’s. The multi- material coplanar ring OMO structure using a high-Qo silicon nitride optical ring and a high- Qm polysilicon ring simultaneously achieves high Qm > 22, 000, which is more than 2× higher than that of previous-best silicon nitride OMO, and high Qo > 280, 000 on par with single silicon nitride ring demonstrations. With its high Qm, the coplanar ring OMO exhibits a best-to-date phase noise of -114 dBc/Hz at 1 kHz offset and -142 dBc/Hz at 1 MHz offset from its 52-MHz carrier–a 12 dB improvement from the previous best by an OMO constructed of silicon nitride alone. The doped polysilicon structure and electrodes additionally allow tuning of the OMO’s oscillation frequency via voltage control and harmonic locking to an external source, enabling future deployment of the multi-material OMO as a locked oscillator in a target low-power CSAC application. A second integrated OMO structure, dubbed stacked- ring OMO, is also demonstrated using similar silicon nitride and polysilicon ring resonators but this time coupled in a vertical fashion, allowing easy integration with sidewall sacrificial layer defined gap MEMS process technology to achieve high electromechanical coupling in the composite OMO.

Enabled by the MEMS integration that allows electrically coupled input-outputs, a new optical communications application based on an OMO is introduced. A super-regenerative optical receiver detecting on-off key (OOK) modulated light inputs has been demonstrated that harnesses the radiation-pressure gain of the electrically-sustained integrated OMO to render its oscillation amplitude as a function of the intensity of light coupled into the oscillator. Unlike previous electronic super-regenerative receivers, this rendition removes the need to periodically quench the oscillation signal, which then simplifies the receiver architecture and increases the attainable receive bit rate. A fully functional receiver with a compact ∼ 90 μm OMO comprised only of silicon-compatible materials demonstrates successful recovery of a 2 kbps bit stream from an OOK modulated 1550 nm laser input. By removing the need for the expensive III-V compound semiconductor materials often used in conventional optical receivers, this OMO-based receiver offers a lower cost alternative for sensor network applications.

Power consumption, form factor and more importantly cost, are major challenges for today’s wireless communication systems that hinder realization of the Internet of the Things and beyond, e.g., the Trillion Sensor vision. This dissertation explores micromechanical methods that enable RF channel-selection to simplify receiver architectures and considerably reduce their power consumption.

In particular, strong interfering signals picked up by the antenna impose strict requirements on system nonlinearity and dynamic range, which translate to higher power consumption in the RF front-end and the baseband circuitry. Removal of these unwanted signals relaxes dynamic range requirements and reduces power consumption. Rejection of all interferers, if possible, could potentially lift any nonlinearity requirements on the receiver and considerably reduce power consumption. This work first investigates the requirements for RF channel selection, then demonstrates that capacitive-gap transduced micromechanical resonators possess the high quality factor and strong electromechanical coupling needed for successful demonstration of channel selection at RF.

This dissertation specifically focuses on clamped-clamped beam (CC-beam) micromechanical resonators as building blocks for channel-select filters. Here, a small-signal equivalent model developed for a general parallel-plate capacitive transducer and then refined for CC-beam resonators predicts very strong electromechanical coupling. Experimental measurements on the fabricated CC-beam resonators confirm these predictions and demonstrate coupling strengths greater than 10%. CC-beam resonators with such a strong coupling and equipped with inherent high quality factor enabled by capacitive transducers are a suitable choice for realization of narrow-bandwidth filters at HF, as confirmed by experimental results.

The filter design procedure presented in this dissertation and refinements to narrow mechanical coupling beam modeling pave the way for better understanding of mechanical circuits and comprehensive study of filter transfer function. This dissertation illustrates the importance of coupling beam design for the optimum filter realization. The refinements to coupling beam formulation also expands our understanding of extensional- and flexural-mode beams, and demonstrate the creation and manipulation of system poles by coupling beam design.

Taking advantage of different theories presented and developed here, the 3rd- and 4th-order micromechanical filters of this work exploit bridging between non-adjacent resonators to insert and control transfer function loss poles that sharpen passband-to-stopband roll-off. Measurement of these filters demonstrates very sharp roll-offs, as evidenced by 20dB shape factors as small as 1.84 for filters with narrow bandwidths of 0.1% to 0.3%, centered at 8MHz. The high-Q CC-beam resonators constituting the filters enable insertion loss of only 1dB in a properly terminated filter.

RF channel selection eliminates unwanted signals sufficiently to relax the nonlinearity requirements on the following stages. Consequently, the micromechanical filter becomes a significant contributor to the nonlinear performance of the overall system. This work investigates different nonlinear phenomena in capacitive-gap transducers and predicts nonlinear performance sufficient for today’s wireless system requirements. Experimental measurements on bridged filters confirms these expectations. Specifically, a 4th-order bridged filter has a third-order intercept point (IIP3) of +31.8dBm, which translates to an ample dynamic range of 88dB.

To fully harness the strong electromechanical coupling and high quality factor offered by CC-beam resonators, this dissertation demonstrates a 7th-order bridged micromechanical filter with very sharp passband-to-stopband roll-off, marked by a 20dB shape factor of 1.45, the best shape factor reported so far for any on-chip channel-select filter. This high-order filter with +31.4dBm of IIP3 for 200kHz tone spacing offers the essential framework for the realization of channel selection and the receiver performance enhancement it promises.

Finally, this work addresses concerns on the electromechanical coupling strength of capacitive resonators at higher frequencies. The specialized fabrication processes herein to (1) deposit low-stress polysilicon layers, (2) etch the polysilicon structure with sufficiently smooth sidewalls, and (3) deposit a conformal and uniform thin oxide layer, enable capacitive-gap transducers with gap spacings as small as 13.2nm. Such a small gap spacing delivers strong electromechanical coupling greater than 1.6% in a 60-MHz wine-glass disk resonator.

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.

On chip capacitive-gap transduced micromechanical resonators constructed via MEMS technology have achieved very high Q’s at both VHF and UHF range, making them very attractive as on-chip frequency selecting elements for filters in wireless communication applications. Still, there are applications, such as software-defined cognitive radio, that demand even higher Q’s at RF to enable low-loss selection of single channels (rather than bands of them) to reduce the power consumption of succeeding electronic stages down to levels more appropriate for battery-powered handhelds.

This dissertation focuses on improving the performance of MEMS capacitive-gap resonators to the degree which can be used to build the aforementioned RF channel-select filters. It first aims to enhance quality factor of MEMS capacitive-gap resonators by suppressing vibration energy loss via device substrate, which will lead to low insertion loss in RF channel selection. Then, in order to reduce an RF front-end filter’s bandwidth and termination resistance, it explores the method of building micromechanical resonator array composites that include large number of mechanically coupled resonators. Finally, the dissertation presents an experimentally demonstrated RF narrowband filter built upon mechanically coupled high-Q resonator array composites.

Pursuant to further increasing Q at UHF for low insertion loss RF channel select application, the thesis develops an equivalent circuit model of a radial contour mode disk resonator that can analytically predict anchor loss dominated Q. Indicated by this improved equivalent circuit model, this work “hollows” the stems supporting all-polysilicon micromechanical disk resonators to effectively squeeze the energy conduit between the disk structure and the substrate, thereby suppressing energy loss and maximizing Q. By using the same fabrication process flow from the conventional all-polysilicon devices, the use of hollow stem support enhances Q with minimal increase in fabrication complexity. Measurements confirm Q enhancements of 2.6× for contour modes at 154 MHz and 2.9× for wine glass modes around 112 MHz over values previously achieved by full stem all-polysilicon disk resonators with identical dimensions. Measured Q’s as high as 56,061 at 329 MHz and 93,231 at 178 MHz for whispering gallery modes further attest to the efficacy of this approach.

This dissertation also employs mechanically coupled disk array composites to increase resonator stiffness and lower motional resistance, which are both highly desired for RF front-end channel-select filters. By using half-wavelength coupling beams and proper electrode phasing design, measurements confirm that a 215-MHz 50-resonator disk array achieves 46.5× Q-normalized Rx reduction, with no observation of other undesired vibration modes. Notably, as indicated by the newly developed negative-capacitance equivalent circuit model, such array composite also shows enhanced frequency stability against dc-bias voltage fluctuations because of its large electrode-to-resonator overlap capacitance.

Finally, the thesis demonstrates a 75MHz 3rd order 210 kHz bandwidth (0.3%) filter with a sharp roll-off of 20dB shape factor of 1.46. This filter employs three high-Q disk array composites connected by quarter-wavelength rotational coupling beams to achieve a weak coupling for narrowband selection. Each array composite itself includes seven flexural disk resonators coupled by strong quasi-zero length beams to enforce desired response. By using electromechanical analogies, the equivalent electrical circuit model of this filter can accurately capture the device’s response and provide insights for filter designers.

Most importantly, the accuracy of the described equivalent circuit model in predicting quality factor, frequency stability, and filter response encourages the design of even more complex micromechanical circuits to come, for example, as would be needed in an all-mechanical RF front-end.

Wireless technology, which already plays a major part in our daily lives, is expected to further expand to networks of billions of autonomous sensors in coming years: the so-called Internet of Things. In one vision, sensors employing low-cost, low-power wireless motes collect and transmit data through a mesh network while operating only on scavenged or battery power. RF MEMS provides one approach to the stringent power and performance required by sensor networks.

This dissertation presents improvement to these MEMS technologies and introduces new approaches for wireless communication in low power wireless networks. First, this work presents oscillators based on the capacitive-gap transduced MEMS resonator. As wireless radio needs at least one such oscillator, the space and power savings offered by these MEMS oscillators make them compelling alternatives over bulky quartz-based devices. The high quality factors (Q) > 100,000 possible in these on-chip resonators allow for phase noise performance of the oscillator exceeding even the challenging GSM specifications using less than 100 uW of power consumption. Despite their small size and tiny capacitive gaps, MEMS-based oscillators are found to be insensitive to vibration and achieve only a few ppm shift in frequency over 10 months of measurement: the performance shown is on par or better than the off-the-shelf crystal oscillators. Interestingly, exploiting nonlinearities in the MEMS resonators also allows multiple simultaneous oscillation frequencies using one amplifier. Combined with electrical stiffness-based frequency tuning, this enables Frequency-Shift Keyed modulation of the output waveform, offering a space and power-efficient multichannel transmitter, as desired for mobile applications requiring long battery life.

Intrinsically, oscillator systems involve positive feedback loops, which regeneratively amplify signals in the loop. Taking advantage of this property, MEMS oscillator systems may be used for other wireless signal processing applications. This dissertation explores such systems applied to: 1) a narrow channel-select filter with low insertion loss unachievable using passive resonators only and 2) a super-regenerative amplification-based channel-selecting radio transceiver. Finally, this dissertation presents two capacitive-gap transduced micromechanical resonator designs which can achieve the high Q at GHz frequencies needed for many wireless communication standards. The methods and solutions provided here pave a path towards realization of future low-power wireless technologies.

The ever increasing need for regional and global roaming together with continuous advances in wireless communication standards continue to push future transceivers towards an ability to support multi-mode operation with minimal increases in cost, hardware complexity, and power consumption. RF channel-select filter banks pose a particularly attractive method for achieving multiband reconfigurability, since they not only provide the needed front-end reconfigurability, but also allow for power efficient and versatile transceiver designs, e.g., software-defined radio. Such channel-select filters, however, impose requirements on their constituent resonators that are not yet achievable on the micro-scale. Specifically, capacitively-transduced micromechanical resonators achieve high Q, but suffer from high impedance; while piezoelectric micromechanical resonators offer low impedance, but with insufficient Q. This dissertation demonstrates four new techniques to address the issues in both technologies.

Two of the methods recognize that sub-30 nm gap spacing enables electrostatic resonators to achieve acceptably low impedance. Unfortunately, however, such small gaps with the needed high aspect ratios are difficult to achieve via wafer-level batch processing. Two new methods are proposed and experimentally verified for forming sub-30 nm gaps: 1) partial-filling of electrode-to-resonator gaps with atomic layer deposition (ALD) of high-k dielectric; and 2) generating gaps via the volume reduction associated with a silicidation reaction. Among the many benefits provided by a silicide-based approach to gap formation is speed of release, where sub-30 nm gaps can be formed and high-aspect-ratio microstructures can be released via anneals lasting from seconds to a few minutes, regardless the lateral dimensions of the devices. Silicide-induced gap formation further does not require any etching and is applicable to a wide range of applications, from electronics to vacuum packaging.

The next two methods seek to circumvent the fact that AlN thin-film resonators have historically been measured with much lower Q than capacitive ones at similar frequencies. As a result, it was commonly accepted that the AlN thin films sputtered at low temperatures are to blame for the lower Q. This dissertation provides experimental evidence that it is not AlN material loss that restricts the Q of conventional AlN resonators, but rather the losses associated with their contacting electrodes. Specifically, a new transducer dubbed the "capacitive-piezoelectric" transducer is introduced that lifts the electrodes away from a piezoelectric resonator by tiny nanometer scale gaps that retain strong electric fields for good electromechanical coupling, while eliminating electrode-derived losses. After removing the electrode losses, the Q of piezoelectric AlN resonators rise by up to 9 times. A new surface-micromachining fabrication process has been developed for the capacitive-piezoelectric resonators, where the metal electrodes are separated from the AlN resonators by small air (or vacuum) gaps. The second approach for tapping the material Q of AlN uses Q-boosting mechanical circuits, where the electrode-equipped AlN resonators are mechanically coupled to electrode-less ones to form a composite-array. In this structure, the energy shared among all of the resonators in the composite-array effectively boost the Q of the electrode-equipped resonators. The Q of electrode-less resonators are extrapolated from the measurement data to be from 14,040 to 15,795. Both methods achieve measured Q exceeding 10,000, posting the highest reported Q for resonators constructed of sputtered AlN and confirming that AlN is indeed a high-Q material.

Abstract

A Micromechanical RF Channelizer

by

Mehmet Akgul

Doctor of Philosophy in Engineering - Electrical Engineering and Computer Sciences

University of California, Berkeley

Professor Clark T.-C. Nguyen, Chair

The power consumption of a radio generally goes as the number and strength of the RF signals it must process. In particular, a radio receiver would consume much less power if the signal presented to its electronics contained only the desired signal in a tiny percent bandwidth frequency channel, rather than the typical mix of signals containing unwanted energy outside the desired channel. Unfortunately, a lack of filters capable of selecting single channel bandwidths at RF forces the front-ends of contemporary receivers to accept unwanted signals, and thus, to operate with sub-optimal efficiency.

This dissertation focuses on the degree to which capacitive-gap transduced micromechanical resonators can achieve the aforementioned RF channel-selecting filters. It aims to first show theoretically that with appropriate scaling capacitive-gap transducers are strong enough to meet the needed coupling requirements; and second, to fully detail an architecture and design procedure needed to realize said filters. Finally, this dissertation provides an actual experimentally demonstrated RF channel-select filter designed using the developed procedures and confirming theoretical predictions.

Specifically, this dissertation introduces four methods that make possible the design and fabrication of RF channel-select filters. The first of these introduces a small-signal equivalent circuit for parallel-plate capacitive-gap transduced micromechanical resonators that employs negative capacitance to model the dependence of resonance frequency on electrical stiffness in a way that facilitates the analysis of micromechanical circuits loaded with arbitrary electrical impedances. The new circuit model not only correctly predicts the dependence of electrical stiffness on the impedances loading the input and output electrodes of parallel-plate capacitive-gap transduced micromechanical device, but does so in a visually intuitive way that identifies current drive as most appropriate for applications that must be stable against environmental perturbations, such as acceleration or power supply variations. Measurements on fabricated devices in fact confirm predictions by the new model of up to 4× improvement in frequency stability against DC-bias voltage variations for contour-mode disk resonators as the resistance loading their ports increases. By enhancing circuit visualization, this circuit model makes more obvious the circuit design procedures and topologies most beneficial for certain mechanical circuits, e.g., filters and oscillators.

The second method enables simultaneous low motional resistance (Rx < 130 Ω) and high Q (>70,000) at 61 MHz using an improved ALD-partial electrode-to-resonator gap filling technique that reduces the Q-limiting surface losses of previous renditions by adding an alumina pre-coating before ALD of the gap-filling high-k dielectric. This effort increases the Q over the ~10,000 of previous renditions by more than 6× towards demonstration of the first VHF micromechanical resonators in any material, piezoelectric or not, to meet the simultaneous high Q (>50,000) and low motional resistance Rx (< 200Ω) specs highly desired for front-end frequency channelizer requirements in cognitive and software-defined radio architectures. The methods presented in this chapter finally overcome the high impedance bottleneck that has plagued capacitively transduced micro-mechanical resonators over the past decade.

The third method introduces a capacitively transduced micromechanical resonator constructed in hot filament CVD boron-doped microcrystalline diamond (MCD) structural material that posts a measured Q of 146,580 at 232.441 kHz, which is 3× higher than the previous high for conductive polydiamond. Moreover, radial-contour mode disk resonators fabricated in the same MCD film and using material mismatched stems exhibit a Q of 71,400 at 299.86 MHz. The material used here further exhibits an acoustic velocity of 18,516 m/s, which is now the highest to date among available surface micromachinable materials. For many potential applications, the hot filament CVD method demonstrated in this work is quite enabling, since it provides a much less expensive method than microwave CVD based alternatives for depositing doped CVD diamond over large wafers (e.g., 8") for batch fabrication.

The first three methods described so far focus on a single vibrating disk resonator and improve its electrical equivalent modeling, Cx/Co, and Q. Once we craft the resonator that meets the challenging design requirements of RF channel-select filters, the last method presents a design hierarchy that achieves the desired filter response with a specific center frequency, bandwidth, and filter termination resistance. The design procedure culminates in specific values for all mechanical geometry variables necessary for the filter layout, such as disk radii, and beam widths; and process design variables such as resonator material thickness and capacitive actuation gap spacing.

Finally, the experimental results introduce a 39nm-gap capacitive transducer, voltage-controlled frequency tuning, and a stress relieving coupled array design that enable a 0.09% bandwidth 223.4 MHz channel-select filter with only 2.7dB of in-band insertion loss and 50dB rejection of out-of-band interferers. This amount of rejection is more than 23dB better than previous capacitive-gap transduced filter designs that did not benefit from sub-50nm gaps. It also comes in tandem with a 20dB shape factor of 2.7 realized by a hierarchical mechanical circuit design utilizing 206 micromechanical circuit elements, all contained in an area footprint of only 600μm×420μm. The key to such low insertion loss for this tiny percent bandwidth is Q's>8,800 supplied by polysilicon disk resonators employing for the first time capacitive transducer gaps small enough to generate coupling strengths of Cx/Co ~0.1%, which is a 6.1× improvement over previous efforts. The filter structure utilizes electrical tuning to correct frequency mismatches due to process variations, where a dc tuning voltage of 12.1 V improves the filter insertion loss by 1.8 dB and yields the desired equiripple passband shape. An electrical equivalent circuit is presented that captures not only the ideal filter response, but also parasitic non-idealities that create electrical feed-through, where simulation of the derived equivalent circuit matches the measured filter spectrum closely both in-band and out-of-band.

The combined 2.7dB passband insertion loss and 50dB stopband rejection of the demonstrated 206-element 0.09% bandwidth 223.4-MHz differential micromechanical disk filter represents a landmark for capacitive-gap transduced micromechanical resonator technology. This demonstration proves that the mere introduction of small gaps, on the order of 39 nm, goes a long way towards moving this technology from a research curiosity to practical performance specs commensurate with the needs of actual RF channel-selecting receiver front-ends. It also emphasizes the need for tuning and defensive stress-relieving structural design when percent bandwidths and gaps shrink, all demonstrated by the work herein.

Perhaps most encouraging is that the models presented in dissertation used to design the filter and predict its behavior seem to be all be spot on. This means that predictions using these models foretelling 1-GHz filters with sub-200Ω impedances enabled by 20nm-gaps might soon come true, bringing this technology ever closer to someday realizing the ultra-low power channel-selecting communication front-ends targeted for autonomous set-and-forget sensor networks. Work towards these goals continues.

Micromechanical resonant switches ("resoswitches") that harness the high-Q resonance and nonlinear dynamical properties of micromechanical structures are demonstrated that can achieve higher switching speed, better reliability (even under hot switching), and lower actuation voltage, all by substantial factors, over existing RF MEMS switches. Various mechanical structures (low to high resonance frequencies) with different structural and contact materials (medium to low resistances) are designed and fabricated to verify the advantages predicted by theory. Ample amounts of data were gathered and analyzed and new phenomena have been discovered.

The first prototype resoswitch was based on a 61MHz wine-glass mode disk con-structed in doped polysilicon. As the pioneer demonstration vehicle, this device requires an actuation voltage of only 2V to switch with sub-nanosecond switching times. It further achieved over 16.7 trillion hot-switched cycles, made possible in part by a large restoring force (>160mN) afforded it orders of magnitude larger stiffness than conventional RF MEMS switches. The contact resistance is on the kilo ohm range but reasonable considering the fact that doped polysilicon is the contact interface. The already impressive 16.7 trillion hot-switched cycle life time might have been much longer if not for undesirable impacting at the drive port that occurred with this initial device.

An immediate solution to the high contact and series resistances of the polysilicon resoswitch is to construct everything in metal instead. Therefore, an electroplated nickel surface micromachining process was developed that allows for unequal electrode-to-resonator gap spacings in order to eliminate drive port impacting. The process uses only metal materials to keep the process temperature below 80°C in an effort to permit post CMOS process compatibility. This process led to demonstration of a 25MHz wine-glass mode nickel disk resoswitch that delivered 17.7dB of power gain in a simple power am-plifier topology, where the power gain comes about mainly through substantial reduction of contact resistance. Despite the introduction of unequal electrode-to-resonator gaps, endurance did not improve, mainly because the method used introduced an asymmetry that lowered the pull-in voltage of these devices.

To address this issue, a 153MHz digitally specified displacement amplifier was then designed that employs asymmetrical mechanical coupling to generate larger displacement along the switch axis than that along the drive axis. The prototype device used polysilicon structural material, so did not have a lower enough contact resistance to demonstrate an actual power amplifier. However, the device did greatly extend the hot-switched cycle life time, which now achieves 173.9 trillion cycles using an open-loop drive circuit. If driven by a closed-loop drive circuit, an even larger count is expected.

Finally, by synthesizing a group of comb-driven resoswitches, MEMS-based charge pumps were demonstrated with promise to eventually remove the diode voltage drop and junction breakdown issues that plague conventional transistor versions, allowing them to transfer charge with any input voltage level and achieve much higher voltages, perhaps eventually as high as 200V. A number of different topologies were successfully implemented, including single and multiple stages of Dickson's and charge transfer series volt-age doubling designs, which proves the feasibility and agility of this technology.

This dissertation describes a MEMS-based frequency-selective power amplifier that performs both signal filtering and power amplification, while consuming zero power when there is no input, i.e., zero-quiescent power consumption. The frequency-selective power amplifier employs a micromechanical resonant switch (resoswitch) as a key building block similar to those recently used for zero-quiescent power radio receivers, but capable of handling higher powers. This document details the design, fabrication, and characterization of these higher frequency and higher power micromechanical resoswitches, and employs them as power amplifiers. Here, the mechanical Q of the resoswitch largely governs the threshold input level that instigates power gain. Theoretical and experimental studies of Q, as well as Q enhancement techniques and high-Q structural design, are discussed. Further, post-fabrication laser trimming addresses the frequency accuracy of the vibrating devices. A model that replaces laser blasted holes with stiffness-modifying cracks captures well the frequency shift dependence on laser blast location. The accuracy of this theory further enables a deterministic trimming protocol that specifies the laser targeting sequence needed to achieve a required amount of frequency tuning with minimal Q reduction.

The resoswitch used to demonstrate the frequency-selective power amplifier employs slots to engineer stiffness along orthogonal axes of a wine-glass disk resonator structure. The slots realize displacement amplification—a larger displacement magnitude along the output than the input axis—allowing impact switching only to the output electrodes, not the input, all of which improves reliability. A finite element analysis (FEA)-based model predicts the displacement gain as a function of disk size and slot dimension/location.

To improve impact contact resistance, this work employs metal for the slotted-disk resoswitch, achieved via a CMOS-compatible surface micromachining process that essentially replaces normally-used polysilicon material with aluminum metal, which serves as both structural and interconnect material. This not only improves contact resistance, but also reduces parasitic resistance, which in turn reduces feedthrough currents. When embedded in a switched-mode power amplifier circuit, the Al displacement-amplifying resoswitch performs signal filtering and power amplification that first filters an incoming signal with channel-like selectivity and then amplifies the signal with a power gain of 13.8 dB. More importantly, unlike transistor-based circuits, the resoswitch-based frequency-selective power amplifier consumes zero power while in standby.

Measurements indicate that sputtered aluminum has high mechanical Q at low frequencies, as a folded-beam capacitive-comb-driven micromechanical resonator fabricated via the aforementioned process achieved a Q up to ∼20,000. Unfortunately, the higher frequency slotted-disk used in the displacement amplifier did not fare as well, as its Q was only on the order of 1,000. An effort to study the Q limits of different resoswitch designs included an experimental study of intrinsic loss mechanisms using cryogenic operation to enhance resonator Q and better elucidate important energy loss mechanisms. Here, operation of a 61-MHz wine-glass disk resonator at temperatures as low as 5K reduces temperature-dependent energy loss and raises Q to as high as 362,000, likely devoid of thermoelastic friction. On the other hand, introduction of slots into a wine-glass disk structure (to effect displacement gain) reduces Q by introducing thermoelasting damping, which this document models in detail. A displacement-amplifying elliptic disk solves this problem by deriving gain-induced stiffness differences from geometric ratioing rather than Q-degrading slots, which permits simultaneous displacement gain and high-Q.

Finally, this work includes a study of laser trimming to trim power amplifier frequency to a desired range. Here, laser trimming offers flexible bidirectional frequency tuning with minimal effect on Q. Unlike other frequency tuning methods, laser trimming does not require added process steps, e.g., sputtering of frequency-shifting metal materials on structures. Rather, laser trimming offers precise post-fabrication frequency adjustment via blasts of controlled size and location on a given resonator. A model that captures the frequency shift dependency on laser blast location shows good agreement with measured results on micromechanical clamped-clamped (CC)-beam resonators. The theory provides a modeling framework for laser trimming applicable to other types of beam resonators, e.g., free-free beams, cantilevers, and even disks or rings by altering the boundary condition matrices.