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Diffraction gratings are commonly used in optical instruments such as in spectrometers, monochromators, and wavelength division multiplexing devices. The diffraction efficiency is directly related to the ratio of the grating spacing or pitch to the wavelength of the light. In a sinusoidal grating (or holographic grating) the efficiency is generally lower than a blazed grating with a saw tooth profile, but the spectral coverage can be broader. To address a large spectral range, advanced spectrometers use a number of diffraction gratings with a fixed pitch. In this paper, we show for the first time, the use of a tunable-frequency surface acoustic wave (SAW) delay line as a broad-band sinusoidal diffraction grating. To tune the frequency, we tune the effective spacing of the interdigitated transducers (IDTs) by applying specific phases to each IDT finger. The prototype SAW grating covers frequency range of 86 MHz to 360 MHz corresponding to optical range of $1.3~\mu \text{m}$ to $9~\mu \text{m}$ . [2020-0189]

In this paper, we present ultra-high quality factor ( ${Q}$ ) SAW resonators fabricated on aluminum gallium nitride on gallium nitride on silicon (AlGaN/GaN/Si) heterostructures with ${Q}$ exceeding 6000 at room temperature. We characterize their temperature response in a broad range of temperature (−196°C to 500°C or 77 K to 773 K). The effect of doping on the ${Q}$ and temperature coefficient of frequency (TCF) of the GaN-based SAW resonators is analyzed, for the first time, using un-intentionally doped GaN (UID-GaN), carbon doped GaN (C-doped), and Si doped GaN. The ${Q}$ value for UID-GaN and C-doped GaN is similar and decreases from 2000 to 1000 as the temperature is increased from 77 K to 773 K. The ${Q}$ value of Si-doped GaN is higher than UID and C-doped GaN by a factor of 3 (6622 at 77 K) and decreases with increase of temperature. This value of ${Q}$ at 1.8 GHz is the highest reported amongst aluminum nitride (AlN) or GaN-based SAW resonators. For extreme high temperatures (≥573 K) the TCF value is half the TCF value of UID and C-doped GaN, which shows the possibility of engineering the TCF by tuning the doping concentration. For low temperatures (≤150 K) C-doped and UID-GaN show a turn over point in TCF curve which shows their promise for cold clocks. [2020-0163]

This paper reports on the investigation of anomalous low quality factors ( ${Q}\text{s}$ ) in AlN thin film-based length extensional (LE)-mode resonators using finite element method (FEM), analytical modeling, and experimental techniques. Different heterostructures of LE resonators having Al/AlN/Si, Al/AlN, AlN/Si, and Si are designed and fabricated using standard MEMS processes. Experimental ${Q}\text{s}$ along with resonances are obtained using electrical and optical readout and are compared with the modeled and analytically obtained ${Q}\text{s}$ . The results show that the thermoelastic damping (TED) in metal electrode, anchor loss, and charge redistribution loss are not the dominant loss mechanisms. With the help of Mason’s network modeling, we investigated the dielectric loss, which is also observed to be negligible for these resonators. An internal mechanical loss, originating from the columnar growth of AlN and observed using Mason’s model, is proved to be the dominant loss mechanism. This observation is confirmed and reinforced by the COMOSL’s FEM analysis of TED in AlN, having columnar growth effect and by experimental results. [2018-0258]

Silicon carbide (SiC) has been extensively investigated in the last decade, specifically for applications in harsh environments. However, most SiC sensors require an external power supply, which cannot operate at high temperatures. This letter develops a new sensing technology in a SiC platform based on near field communication to eliminate the requirement for wired power sources. The 3C-SiC temperature sensors were fabricated from a SiC-on-insulator substrate formed by anodic bonding. The sensors functioned based on the thermoresistance of the SiC films with the high TCR of −13 000 ppm/K at 300 K and −3 000 ppm/K at 600 K. The resistance change of the sensors was wirelessly measured using a reading coil placed outside of the heating chamber, showing a significant resonant-frequency-shift (−400 ppm/K at 600 K) of the coupling impedance under temperature variation. The proposed technique is promising for the development of wireless wide-band-gap sensors used in extreme conditions.

Emerging technologies based on tailorable interactions between photons and phonons promise new capabilities ranging from high-fidelity microwave signal processing to non-reciprocal optics and quantum state control. While such light-sound couplings have been studied in a variety of physical systems, many implementations rely on non-standard materials and fabrication schemes that are challenging to co-implement with standard integrated photonic circuitry. Notably, despite significant advances in integrated electro-optic modulators, related acousto-optic modulator concepts have remained relatively unexplored in silicon photonics. In this article, we demonstrate direct acousto-optic modulation within silicon waveguides using electrically-driven surface acoustic waves (SAWs). By co-integrating SAW transducers in piezoelectric AlN with a standard silicon-on-insulator photonic platform, we harness silicon's strong elasto-optic effect to mediate linear light-sound coupling. Through lithographic design, acousto-optic phase and single-sideband amplitude modulators in the range of 1-5 GHz are fabricated, exhibiting index modulation strengths comparable to existing electro-optic technologies. Extending this traveling-wave, acousto-optic interaction to cm-scales, we create electrically-driven non-reciprocal modulators in silicon. Non-reciprocal operation bandwidths of >100 GHz and insertion losses

This paper reports on tunable 2- and 3-pole bandpass filters with a wide frequency tuning range (tuning ratio >3) and a constant bandwidth using switchable varactor-tuned resonators. The wide center frequency tuning range is obtained using p-i-n diodes to switch in and out quarter-wavelength ( $\lambda $ /4) or half-wavelength ( $\lambda $ /2) resonators for low-band or high-band modes, without increasing the tuning capacitance range of the varactors. A combination of electric and magnetic coupling is utilized to realize a near constant absolute bandwidth across the tuning range. A switchable feed line with a fixed matching capacitance is used to realize the external coupling. Two filters are designed and fabricated on a Duroid substrate with $\varepsilon _{r} = 2.2$ and $h =0.787$ mm. For the 2-pole filter, the center frequency is tuned from 550 to 1900 MHz while maintaining a 3-dB bandwidth of 92 ± 6 MHz, insertion loss of 3.2~4.4 dB, and return loss of better than 15 dB. For the 3-pole filter, the center frequency is tuned from 540 to 1800 MHz while maintaining a 3-dB bandwidth of 89 ± 4 MHz, insertion loss of 4~5.4 dB, and return loss of better than 12 dB. For both filter types, the third-order intercept point and 1-dB compression point ( $P_{\mathrm {1~dB}})$ are 11 and 7 dBm, respectively. The rejection level at 200-MHz offset frequency from the passband center frequency is better than 25 and 41 dB for 2- and 3-pole filters, respectively, across the entire tuning range. To the best of our knowledge, this planar bandpass filter exhibits the widest tuning range with a near-constant bandwidth.

The 20th in the series of Solid-State Sensors, Actuators, and Microsystems Hilton Head Workshops was scheduled for May 31–June 4, 2020, at the Sonesta Resort on Hilton Head Island, SC, USA. Unfortunately, like most academic conferences in 2020, it was canceled due to the COVID-19 pandemic. The Hilton Head 2020 Workshop ( https://www.hh2020.org/ ) was designed to continue to represent the science and engineering excellence of the MEMS community while introducing a number of innovations to the structure of this historic meeting. Some of these changes included the workshop becoming an international meeting for the first time while remaining committed to its key mission of excellence in showcasing recent innovative work and offering strong networking opportunities. We would have had new representatives from multiple regions in North America on our Technical Program Committee (TPC) and a strong focus on cultivating the impact of MEMS on global issues and grand challenges (including a full-day short course on “Microsystems Frontiers in the IoT Era”).

In this paper, design, analysis, and implementation of a piezoelectric microelectromechanical systems (MEMS) oscillator on an ovenized microplatform is presented. An oxide-refill process is used to compensate the first-order temperature coefficient of frequency of MEMS resonators, as well as to realize thermal isolation structures. The technology enables fabrication of low-power ovenized device fusion platforms using standard silicon on insulator wafers. Utilizing the intrinsic frequency-temperature characteristic of two MEMS resonators, temperature sensing and closed-loop oven-control is realized by phase-locking two MEMS oscillators at an oven-set temperature. The design of the phase-lock control loop is studied using multidomain linear models. Control loop dynamics, noise properties, and nonideal effects are analyzed. Low-power and low-noise phase-locked loop-based control circuitry is designed in 0.18- $\mu \text{m}$ CMOS to interface with the MEMS resonators. Using the developed technology, an oven controlled MEMS oscillator exhibits an overall frequency drift of

In this letter, we present ultra-high frequency coupled ring resonators realized in a suspended AlGaN/GaN heterostructure. The piezoelectric transducers consist of back-to-back nickel (Ni) Schottky diodes deposited on a 20-nm-thick AlGaN layer and a floating 2-D electron gas (2-DEG) sheet at the AlGaN/GaN interface. The dispersion characteristics of symmetric and anti-symmetric Lamb waves in ring resonators are characterized. Higher order resonance harmonics are targeted with very high mechanical quality factors ( $Q\text{s}$ ) through optimized design of the coupling beam. Such a scheme minimizes the acoustic energy leakage to the substrate, and thus increases anchor $Q$ . We report on a high-order breathing resonance mode at 900.7 MHz with very high mechanical $Q$ of 8350 measured at room temperature and ambient pressure, achieving a record high frequency $\times Q$ value of $7.5\times 10^{12}$ . [2017-0064]

Harvesting sustainable energy resources from surrounding environments to power small electronic devices and systems has attracted massive research attention. Herein, we develop a novel technology to harvest light energy to self-power and simultaneously sense mechanical acceleration in a monolithic structure. When the photonic gate is illuminated the operation mode of the device changes from conventional mode to light harvesting and self-powered operation. The light illumination provides a gradient of majority carrier concentration on the top semiconductor layer, generating a lateral photovoltage, which is the output voltage of the sensor. Under acceleration, the mechanical inertial force induces stress in the sensor material leading to the change of mobility of the charge carriers, which shifts their diffusion rate, and hence changes the gradient of the majority carriers and the lateral photovoltage. The sensitivity at 480 lx light illumination was measured to be 107 μ V / g , while it was approximately 30 μ V / g under the ambient light illumination without any electrical power source. In addition, the acceleration sensitivity is tunable by controlling parameters of the photonic gate such as light power, light spot position and light wavelength. The integration of sensing and powering functions into a monolithic platform proposed in this work eliminates the requirement of external power sources and offers potential solutions for wireless, independent, remote, and battery-free sensing devices and systems.

Semiconductor nanowires are widely considered as the building blocks that revolutionized many areas of nanosciences and nanotechnologies. The unique features in nanowires, including high electron transport, excellent mechanical robustness, large surface area, and capability to engineer their intrinsic properties, enable new classes of nanoelectromechanical systems (NEMS). Wide bandgap (WBG) semiconductors in the form of nanowires are a hot spot of research owing to the tremendous possibilities in NEMS, particularly for environmental monitoring and energy harvesting. This article presents a comprehensive overview of the recent progress on the growth, properties and applications of silicon carbide (SiC), group III‐nitrides, and diamond nanowires as the materials of choice for NEMS. It begins with a snapshot on material developments and fabrication technologies, covering both bottom‐up and top‐down approaches. A discussion on the mechanical, electrical, optical, and thermal properties is provided detailing the fundamental physics of WBG nanowires along with their potential for NEMS. A series of sensing and electronic devices particularly for environmental monitoring is reviewed, which further extend the capability in industrial applications. The article concludes with the merits and shortcomings of environmental monitoring applications based on these classes of nanowires, providing a roadmap for future development in this fast‐emerging research field., Wide bandgap semiconductor nanowires with unique properties and advanced fabrication/integration technologies set a new class of nanosensing and electronic applications. Implementation of these smart devices such as gas sensors, photodetectors, wearable devices, strain sensors, and energy harvesters into environmental monitoring systems offers promising solutions for critical environmental challenges.

Tuning the natural frequency of a resonator is an innovative approach for the implementation of mechanical resonators in a broad range of fields such as timing applications, filters or sensors. The conventional electrothermal technique is not favorable towards large tuning range because of its reliance on metallic heating elements. The use of metallic heaters could limit the tuning capability due to the mismatch in thermal expansion coefficients of materials forming the resonator. To solve this drawback, herein, the design, fabrication, and testing of a highly-doped SiC bridge resonator that excludes the use of metallic material as a heating element has been proposed. Instead, free-standing SiC structure functions as the mechanical resonant component as well as the heating element. Through the use of the Joule heating effect, a frequency tuning capability of almost∆f/fo≈80% has been demonstrated. The proposed device also exhibited a wide operating frequency range from 72.3kHz to 14.5kHz. Our SiC device enables the development of highly sensitive resonant-based sensors, especially in harsh environments.

This work describes a novel architecture to realize high-performance gallium nitride (GaN) bulk acoustic wave (BAW) resonators. The method is based on the growth of a thick GaN layer on a metal electrode grid. The fabrication process starts with the growth of a thin GaN buffer layer on a Si (111) substrate. The GaN buffer layer is patterned and trenches are made and refilled with sputtered tungsten (W)/silicon dioxide (SiO2) forming passivated metal electrode grids. GaN is then regrown, nucleating from the exposed GaN seed layer and coalescing to form a thick GaN device layer. A metal electrode can be deposited and patterned on top of the GaN layer. This method enables vertical piezoelectric actuation of the GaN layer using its largest piezoelectric coefficient (d33) for thickness-mode resonance. Having a bottom electrode also results in a higher coupling coefficient, useful for the implementation of acoustic filters. Growth of GaN on Si enables releasing the device from the frontside using isotropic xenon difluoride (XeF2) etch and therefore eliminating the need for backside lithography and etching.

Gallium nitride (GaN) is a wide bandgap semiconductor material and is the most popular material after silicon in the semiconductor industry. The prime movers behind this trend are LEDs, microwave, and more recently, power electronics. New areas of research also include spintronics and nanoribbon transistors, which leverage some of the unique properties of GaN. GaN has electron mobility comparable with silicon, but with a bandgap that is three times larger, making it an excellent candidate for high-power applications and high-temperature operation. The ability to form thin-AlGaN/GaN heterostructures, which exhibit the 2-D electron gas phenomenon leads to high-electron mobility transistors, which exhibit high Johnson's figure of merit. Another interesting direction for GaN research, which is largely unexplored, is GaN-based micromechanical devices or GaN microelectromechanical systems (MEMS). To fully unlock the potential of GaN and realize new advanced all-GaN integrated circuits, it is essential to cointegrate passive devices (such as resonators and filters), sensors (such as temperature and gas sensors), and other more than Moore functional devices with GaN active electronics. Therefore, there is a growing interest in the use of GaN as a mechanical material. This paper reviews the electromechanical, thermal, acoustic, and piezoelectric properties of GaN, and describes the working principle of some of the reported high-performance GaN-based microelectromechanical components. It also provides an outlook for possible research directions in GaN MEMS.

This paper presents the analysis, design, fabrication, and the first measured results demonstrating the use of gallium nitride (GaN)-based micromechanical resonator arrays as high-sensitivity, low-noise infrared (IR) detectors. The IR sensing mechanism is based on monitoring the change in the resonance frequency of the resonators upon near IR radiation. The resonators are characterized for their RF and thermal performance and exhibit a radiant responsivity of 1.68%/W, thermal time constant on the order of 556 μs, and an average IR responsivity of -1.5% when compared with a reference resonator, for a 100 mK radiation-induced temperature rise. An analysis of the design of the devices is presented as a path toward better design, specifically, for low thermal noise equivalent temperature difference in the long wavelength IR spectrum.

A multigigahertz AlGaN/GaN resonant body transistor (RBT) is reported, wherein the mechanical resonance and electrical signal modulation are achieved simultaneously. A 175-Å-thick AlGaN layer is used as the piezoelectric transduction layer, and the 2-D electron gas present at the AlGaN/GaN interface is employed as the bottom electrode as well as the transistor conducting channel. The carrier concentration of the channel is modulated when the device undergoes acoustic strain. A quality factor of 250 and acoustic transconductance of 25 μS is achieved at resonance frequency of 4.23 GHz, marking the highest frequency and highest transconductance reported to date for GaN-based RBTs. The frequency×Q of this device is among the best reported for GaN-based resonators.

We report on the characterization of thin-film near and short wavelength infrared absorbers comprised of carbon nanotubes dispersed in a polymer. Charged nanodiamond particles are used to effectively and uniformly disperse the carbon nanotubes in the polymer matrix, leading to a very homogenous film. Using this new technique, we demonstrate an infrared absorption of up to 95% in films with thicknesses . This remarkably high absorption is the result of low reflection off the surface and high absorption across the film thickness. The complex refractive index of the films is extracted using an effective media approximation. Calculations show the film has a wide angle for high absorption and is polarization independent. These films are easy to fabricate, robust and damage-resistant, and are compatible with post-processing techniques. These films can be used as the coating layer to boost the efficiency of uncooled infrared sensors and solar-thermal energy harvesters.

In this paper, we explore the piezoelectric transduction of in-plane flexural-mode silicon resonators with a center frequency in the range of 1.3-1.6 MHz. A novel technique utilizing oxide-refilled trenches is implemented to achieve efficient temperature compensation. These trenches are encapsulated within the silicon resonator body so as to protect them during the device release process. By using this method, we demonstrate a high-Q (> 19 000) resonator having a low temperature coefficient of frequency of

In this, the second of two papers, we present numerical simulations and comprehensive analysis of acoustically coupled thickness-mode AlN-on-Si filters. We simulate the scattering parameters of such acoustically coupled filters using commercially available finite element analysis software and compare the simulation results with a set of measurements. The simulations are in good agreement with the measurements, allowing the optimization of filter characteristics. We analyze the filter response under varying geometric parameters and demonstrate that variations in the top electrode geometry allow the design of low-loss filters (insertion loss

This paper reports a continuously tunable lumped bandpass filter implemented in a third-order coupled resonator configuration. The filter is fabricated on a Borosilicate glass substrate using a surface micromachining technology that offers hightunable passive components. Continuous electrostatic tuning is achieved using three tunable capacitor banks, each consisting of one continuously tunable capacitor and three switched capacitors with pull-in voltage of less than 40 V. The center frequency of the filter is tuned from 1 GHz down to 600 MHz while maintaining a 3-dB bandwidth of 13%-14% and insertion loss of less than 4 dB. The maximum group delay is less than 10 ns across the entire tuning range. The temperature stability of the center frequency from -50°C to 50°C is better than 2%. The measured tuning speed of the filter is better than 80 s, and the is better than 20 dBm, which are in good agreement with simulations. The filter occupies a small size of less than 1.5 cm × 1.1 cm. The implemented filter shows the highest performance amongst the fully integrated microelectromechanical systems filters operating at sub-gigahertz range.

This paper reports on the implementation of miniaturized ultra-wideband filters integrated with tunable notch filters using a silicon-based integrated passive device technology. An ultra-wideband bandpass filter is realized on a micromachined silicon substrate, showing an insertion loss of 1.1 dB, return loss of better than 15 dB, and attenuation of more than 30 dB at both lower and upper stop-bands, with a spurious-free response up to 40 GHz. The filter occupies only 2.9 mm 2.4 mm of die area. To address the in-band interference issues associated with ultra-wideband communication, very compact tunable notch filters are monolithically integrated with the bandpass filters. A two-pole tunable notch filter integrated with an ultra-wideband filter provides more than 20 dB rejection in the 5-6 GHz range to reject U-NII interferences, with a total footprint of 4.8 mm 2.9 mm. The power handling, linearity, and temperature stability of filters are characterized and presented in this paper.

This paper presents a fully integrated tunable lumped filter on silicon using a low-temperature silver micromachining process. A prototype 836-MHz bandpass filter with a 3-dB bandwidth of 5.9% and insertion loss of 4.8 dB is demonstrated in a second-order coupled-resonator configuration. Continuous tuning of 50 MHz is achieved by electrostatically actuating the lateral air-gap capacitors of the filter. To control the bandwidth while tuning the center frequency, reconfigurable termination impedance is proposed. As a proof of concept, a low-noise amplifier with tunable input impedance is designed to interface with the bandpass filter. The tunable impedance is realized at the input of the low-noise amplifier using a shunt positive metal-oxide-semiconductor transistor. The fabrication, design, and measurement results of the filter are detailed, and future research directions to improve the performance of such filters are discussed.

This paper presents a new implementation of integrated tunable inductors using mutual inductances activated by micromechanical switches. To achieve a large tuning range and a high quality factor, silver was used as the structural material, and silicon was selectively removed from the backside of the substrate. Using this method, a maximum tuning of 47% at 6 GHz is achieved for a 1.1 nH silver inductor fabricated on a low-loss polymer membrane. The effect of the quality factor on the tuning characteristic of the inductor is investigated by comparing the measured result of identical inductors fabricated on various substrates. To maintain the quality factor of the silver inductor, the device was encapsulated using a low-cost wafer-level polymer packaging technique.

A thickness-mode GaN resonator is presented in this work, showing fourth-order thickness-mode resonance at 8.7 GHz with an extracted quality factor (Q) of 330, exhibiting the highest resonance frequency measured to date on GaN micromechanical resonators with a high frequency × Q value of 2.87 × 10 12 . With the goal of implementing an all-GaN multi-GHz oscillator, the resonator is monolithically integrated with an AlGaN/GaN high electron mobility transistor (HEMT). The results reported in this paper mark critical steps in development of high-frequency, high-power GaN microsystems.

This work reports on a multi-resonator system capable of generating a temperature-stable frequency reference across a wide temperature range. The system involves the use of a minimum of three temperature-compensated oscillators having a slightly different turnover temperature. The oscillator output frequency undergoes frequency multiplication and mixing in two stages to achieve a temperature-stable frequency output. The sensitivity of the clock frequency is analyzed as a function of temperature induced measurement errors. AlN-on-silicon ring resonators actuated piezoelectrically are proposed as the three frequency setting components. Their turnover temperature is controlled through the placement of oxide within the resonator volume. A total frequency shift of less than 10 ppm is estimated across the temperature range of -40 °C to 85 °C with this implementation.

This work presents the first comprehensive investigation of phonon-electron interactions in bulk acoustic standing wave (BAW) resonators made from piezoelectric semiconductor (PS) materials. We show that these interactions constitute a significant energy loss mechanism and can set practical loss limits lower than anharmonic phonon scattering limits or thermoelastic damping limits. Secondly, we theoretically and experimentally demonstrate that phonon-electron interactions, under appropriate conditions, can result in a significant acoustic gain manifested as an improved quality factor (Q). Measurements on GaN resonators are consistent with the presented interaction model and demonstrate up to 35% dynamic improvement in Q. The strong dependencies of electron-mediated acoustic loss/gain on resonance frequency and material properties are investigated. Piezoelectric semiconductors are an extremely important class of electromechanical materials, and this work provides crucial insights for material choice, material properties, and device design to achieve low-loss PS-BAW resonators along with the unprecedented ability to dynamically tune resonator Q.

This paper introduces a novel planetary exploration system capable of functioning in most solar planets and moons. The Moball Network is an agile, self-powered, intelligent, and reconfigurable distributed in-situ exploratory system. We compare our proposed system with state of the art planetary exploration systems and concepts and suggest several novel mission architectures specifically for planetary bodies with an atmosphere such as Mars, or those whose surface includes lakes and ice sheets such as Titan or the Earth's northern and southern caps, as well as smaller bodies with no atmosphere such as the Moon or asteroids. We also describe the current development status of the system and the proof of concept for the ideas presented.

A multi-gigahertz AlGaN/GaN resonator is introduced, where the fundamental thickness-mode resonance at 2.1 GHz is excited exhibiting a quality factor (Q) of 105. For the first time, acoustic strain in the vertical direction is excited and sensed through a two-dimensional electron gas (2DEG), induced at the AlGaN/GaN interface. The 2DEG sheet is used as the bottom electrode for piezoelectric actuation, as well as the transistor conduction channel for acoustic sensing. In this design, acoustic resonance is sensed by modulation of the HEMT drain current (ID); thus, the transistor is biased in the linear region of operation. Here, we study the dependency of the acoustic transconductance on the readout HEMT biasing and show that the read-out HEMT senses the drain current modulation only when the 2DEG channel is not pinched. To use the full potential of the HEMT intrinsic amplification, the transistor needs to be biased in the saturation region, which would require a modified RB-HEMT design.

This paper presents piezoelectric transduction and frequency trimming of silicon-based resonators with a center frequency in the low megahertz regime. The temperature coefficient of frequency (TCF) of the resonators is reduced using both passive and active compensation schemes. Specifically, a novel technique utilizing oxide-refilled trenches is implemented to achieve efficient temperature compensation while maintaining compatibility with wet release processes. Using this method, we demonstrate high-Q resonators having a first-order TCF as low as 3 ppm/°C and a turnover temperature of around 90 °C, ideally suited for use in ovenized platforms. Using active tuning, the temperature sensitivity of the resonator is further compensated around the turnover temperature, demonstrating frequency instability of less than 400 ppb. Such devices are ideally suited as timing units in space applications where size, power consumption, and temperature stability are of critical importance.

In this, the first of two papers, we present the working principle and the implementation of laterally acoustically coupled thickness-mode thin-film piezoelectric-on-substrate (TPoS) filters. This type of filter offers low insertion loss and small bandwidth in a broad frequency range--from a few hundred megahertz up to a few gigahertz--and occupy a small chip area. In this paper, we discuss several design concerns, including the choice of materials for TPoS filters. We demonstrate a design for an air-suspended AlN-on-Si filter, which offers a low insertion loss of 2.4 dB at 2.877 GHz. The bandwidth of this filter is 12 MHz with a return loss of better than 30 dB. In Part II of this paper, we present a comprehensive analysis of the effect of physical layout parameters on the frequency response of TPoS filters.

This Letter presents an innovative design of an electro-optical modulator using germanium telluride (GeTe) phase change material with an integrated nano-heater. The refractive index and the electrical conductivity of GeTe significantly change as the GeTe goes though the crystallographic phase change. Amorphization and crystallization of GeTe is achieved using the Joule heating method by passing current through an array of metal gratings, where GeTe fills the slits between the metal lines. These metal slits also increase the contrast between the amorphous (on) and crystalline (off) phases of the modulator by having extraordinary transmission and reflection response based on interactions of surface plasmon polaritons (SPPs) with the incoming light. The modulator is designed for 1550 nm wavelength, where GeTe is transparent in the amorphous phase and provides high optical on/off contrast. The metal-insulator-metal (MIM) is designed in such a way to only support SPP excitation when GeTe is crystalline and slit resonance when it is amorphous to increase the modulation index. The modulator is stable in both phases with higher than 12 dB change in transmission with zero static power consumption at room temperature.

In this paper, a novel electrostatic switch is reported that exhibits an ON/OFF capacitance ratio of 72, an ON capacitance value of 90 pF, and a pull-in voltage of less than 5 V within an area of 750 × 750 μm. These characteristics are achieved by utilizing a slit-shape middle metal layer in-between the top and bottom metal plates. This design makes the capacitance ratio and the ON state capacitance tolerant against fabrication and temperature induced residual stress in the suspended metal membrane, without significantly degrading the electrostatic force between the top and bottom metals. In this three-metal configuration, at the ON state, the ohmic contact between the top membrane and the middle metal layer enables direct charge transfer between these two electrodes. This characteristic is attractive for device applications in solar energy harvesting. As a proof of concept, charging of an external storage capacitor is shown for an exemplary switch, when a constant current is supplied to the top electrode. INTRODUCTION MEMS switches are an attractive alternative to existing switch solutions and have thus been a topic of extensive research in the past few decades [1], [2]. Conventionally, an electrostatic MEMS switch is made using two electrodes separated by an air gap and a suitable dielectric. One of the electrodes is a moving membrane, and it can be pulled onto the fixed electrode with the application of an electrostatic potential. For a capacitive switch with a given device area, the up-state air gap and the down state effective dielectric thickness decide the ON and OFF capacitance, respectively. In many applications where the MEMS device is used as a switched capacitor, the switch is not only required to have a high ON/OFF capacitance ratio but also a sufficient ON capacitance value. Scaling up the device area to achieve a high ON capacitance is not beneficial as larger devices also exhibit an undesirably high OFF capacitance. In addition, the ON capacitance which is defined by the effective insulator thickness of the switch does not scale linearly with the device size because of warping in the movable membrane (caused by residual or thermal stress) and the surface roughness of the contacts [3]. In order to resolve these issues, a modified switch structure is presented that makes use of an additional metal layer to achieve both a high ON/OFF capacitance ratio and a high ON capacitance value (Fig. 1). The three-metal switch design makes the ON capacitance invariant to stress-induced membrane warping. A micromachined capacitive switch exhibiting an ON/OFF capacitance ratio of 72 and an ON capacitance value of 90 pF within an area of 750 × 750 μm is demonstrated. A low pull-in voltage of less than 5 V is realized by designing the middle metal electrode in a slit type configuration ensuring sufficient electrostatic actuation force on the top suspended membrane. With such properties, the proposed threemetal switch design compares favorably with the state of the art MEMS switches [4]. In the ON state, the top metal makes an ohmic contact with the middle metal layer, allowing the transfer of electrostatic charge from the top membrane to the un-biased middle electrode. Such charge transfer can be useful when employing the switch as a solar energy harvester. We investigate this application and demonstrate charging of an external capacitor connected to the middle metal layer. Another unique feature of this three-metal switch when used as a harvester is the ability to deliver high-voltage charging pulses under heavy capacitive load without the need for an electrical boost converter. This is a significant advantage over conventional photovoltaic devices [5]. DEVICE CONCEPT AND FABRICATION The three dimensional model of the proposed switch is shown in Fig. 1. The MEMS switch is actuated by applying an electrostatic potential between the top and the bottom metal layers, whereas the middle metal layer is left floating. When the top membrane pulls in, it makes an ohmic contact with the middle metal layer. Thus, the ON capacitance of the switch is mainly determined by the capacitance between the middle and bottom metal layers, and is unaffected by the curvature of the top membrane. Therefore, a repeatable and large ON capacitance can be achieved even if the top membrane is warped due to the inevitable fabrication and thermally induced residual stresses. Figure 1: Schematic of the device structure with a top electrode section cut out to reveal the middle metal layer. The slit type design of the top and middle electrode can be clearly seen. Figure 2: Cross-section of the proposed switch structure. The floating middle metal layer can be used as an additional terminal. Note that in this configuration, the substrate acts as the ground electrode. To minimally affect the electric field distribution between the top and bottom metal layers, the metal layers are designed in a slit configuration (Fig. 2) and the middle metal layer is shifted laterally (Fig. 3). From the electrostatic simulations shown in Fig. 3, the slit-shaped middle metal layer is seen to have no significant effect on the electric field between the top and bottom metal layers. As a result, the electrostatic force on the suspended top metal remains unaffected. This allows us to design the membrane with a sufficient restoring force to overcome stiction, while still actuating the device with relatively small voltages. The fabrication process flow for the proposed devices is depicted in Fig. 4. Starting with a nominal silicon wafer, a 500 nm thick aluminum (Al)/ chrome (Cr) layer is lift-off patterned as the first metal layer (or ground electrode). A 150 nm thick aluminum oxide (Al2O3) layer is deposited using atomic layer deposition (ALD) as the dielectric layer. This material and its thickness along with the metal area define the ON state capacitance of the device. The dielectric layer is patterned to enable contact to the bottom metal layer. A 100 nm thick gold is evaporated and lift-off patterned as the middle floating electrode with Cr as the adhesion layer. Poly-methyl-methacrylate (PMMA) is spun as the sacrificial layer with a thickness of 1.7 μm which defines the air gap between the top and the floating electrode. Using titanium (Ti) as the etch mask, PMMA is patterned using a low power O2 plasma. Post PMMA etching, the Ti mask is removed and a 10/900 A TiW/silver layer is sputtered as a seed layer for electroplating. The plating mold is created using AZ 9260 photoresist and an 8 μmthick top electrode is subsequently electroplated. In the final step, the devices are released by removing the PMMA sacrificial layer and dried using critical point drying (CPD) to prevent stiction. The SEM images of a fabricated device are shown in Fig. 5. Figure 3: Simulated (a) electric potential and (b) electric field distribution of the proposed electrode layout for a two finger section. From (b) a uniform electric field is seen in between the top and bottom electrodes. The electric field is not disturbed significantly due to the addition of the middle metal layer. The two scale bars plot the range of voltage and electric field. Figure 4: Process flow of the proposed MEMS capacitive switch. SWITCH DEVICE DESIGN There are two primary design objectives in released membrane type electrostatic switches: low pull-in voltage and large restoring force. Both can be controlled by optimizing the stiffness of the springs, and the membrane shape and area. Using the process shown in Fig. 4, the membrane and the spring thickness is constrained to be the same to reduce the fabrication complexity. The spring stiffness is varied using lithographically defined dimensions, as shown in Fig. 6. A crab leg type spring design is implemented to allow self-compensation against fabrication induced in-plane stresses. While such a design is robust against in-plane stresses, the membrane is prone to bending under a vertical stress gradient commonly observed in most electroplated metals [6]. Figure 7 plots the simulated and measured top membrane bow due to a stress gradient in the top electroplated gold layer. Using a close to optimum plating condition, a membrane bow of ~ 2 μm was measured. From finite element simulations, a stress gradient of ~ 10 MPa/μm in the top membrane was found to fit the measured bow. Using other plating conditions, higher stress gradients have been measured. With such values of inevitable stress in the top membrane, designing a traditional MEMS capacitive switch with large capacitance values is challenging [7]. In a typical two electrode switch, the membrane bow necessitates application of a voltage higher than pull-in to flatten the membrane and achieve a full area contact [8]. In the proposed design, the capacitance between the middle metal and the bottom metal layer is set by the layout of the two electrodes and the dielectric material thickness in between. As such, a high ON state capacitance is obtained even when the top membrane is only partially in contact with the middle metal layer. Therefore, although warping of the top membrane makes a uniform contact difficult, the ON capacitance value remains unaffected. This is clearly seen from the measurement results shown in Fig. 8. Figure 6: Modal simulation results for (a) double beam (b) single beam (c) single fold beam and (d) double fold beam designs. The color spectrum plots the total displacement. The frequency shown is a measure of maximum switching speed. (a) (b) (c) (d) f ~ 2.5 kHz f ~ 2.02 kHz f ~ 1.38 kHz f ~ 1.04 kHz (a) (b) Figure 5: (a) Top SEM view of a micro-fabricated MEMS switch and (b) incline view of the top membrane revealing the air gap. Figure 7: (a) Simulated 2 μm top metal membrane bow with a stress gradient of 10 MPa/μm (b) Actual membrane bow measured using optical interferometry. The measured bow is ~2 μm, indicating the stress gradient in gold is ~ 10 MPa/μm.

In this paper, we present acoustically coupled filters capable of inducing a high-Q notch within the filter passband. The origin of the notch is a result of charge cancellation across the output electrodes, which is a direct consequence of exciting higher order lateral modes with a displacement profile out-of-phase with the primary modes of the filter. We show multiple filters with a passband notch and demonstrate the viability of using top electrode layout to adjust the placement of the notch within the filter passband. A modified equivalent circuit is used to intuitively explain the response of a bandpass filter with an integrated notch. Such filters can have applications as pre-select bandpass filters capable of rejecting in-band interfering signals.

This paper reports on the fabrication and characterization of thin-film nanocomposites comprised of tangled carbon nanotubes in a polymer matrix. The concentration of nanotubes in the polymer was significantly increased using detonation nanodiamonds. Nanodiamonds reduce the surface forces between the polymer and the nanotubes and mitigate the agglomeration problem of nanotubes in polymer. This resulted in thinner and more uniform networks that are efficient absorbers of infrared energy over a broad spectrum, ranging from the visible to the mid-wavelength infrared. An infrared absorbance of 97% was achieved for a 1.6 μm thick nanocomposite film across the spectral range of 714 nm to 5 μm. The films are mechanically and thermally stable up to 300 °C, and can be integrated with microbolometers to enhance their responsivity.

This paper reports on micromachined tunable bandstop filters that exhibit an octave frequency tuning range with high stopband rejection and low out-of-band loss. In addition to continuous frequency tuning, stopband switch on-off capability is realized by employing ohmic switches along with tunable capacitors. The fully re-configurable filters in this work are fabricated using a silicon-based integrated passive device technology and are the smallest-size bandstop filters reported in the low SHF range.

We discuss the contribution of phonon interactions in determining the upper limit of f.Q product in micromechanical resonators. There is a perception in the MEMS community that the maximum f.Q product of a microresonator is limited to a “frequency-independent constant” determined by the material properties of the resonator [1]. In this paper, we discuss that for frequencies higher than ω τ = 1/τ, where τ is the phonon relaxation time, the f.Q product is no longer constant but a linear function of frequency. This makes it possible to reach very high Qs in GHz micromechanical resonators. Moreover, we show that 〈100〉 is the preferred crystalline orientation for obtaining very high Q in bulk-acoustic-mode silicon resonators above ∼750 MHz, while 〈100〉 is the preferred direction for achieving high-Q at lower frequencies.

Fully integrated low insertion loss micromachined band-pass filters are designed and fabricated on the silicon substrate (ρ = 10‐20 � cm, er = 11.9) for UHF applications. Filters are made of silver, which has the highest conductivity of all metals, to minimize the ohmic loss. A detailed analysis for realizing low insertion loss and high out-of-band rejection filters using elliptic magnitude characteristics is presented, and a comprehensive model to take into account inductive parasitics of the interconnects is developed. Temperature characteristics of the filters are measured and show stable performance. The presented filters are different from the previously reported lumped element filters in that all filters are fully integrated on silicon substrate and occupy a remarkably smaller die area. Two filters are fabricated using the silver micromachining technique with center frequencies at 1.05 and 1.35 GHz. The filters have a constant 3 dB bandwidth of 300 MHz (28.6% and 22.2%) and an insertion loss of 1.4‐1.7 dB. The low insertion loss and CMOS compatibility make the presented filters suitable candidates for radio frequency integrated circuits. (Some figures in this article are in colour only in the electronic version)

This paper presents the theory and measured results of a lownoise un-cooled infrared (IR) detector, which uses a combination of piezoelectric, pyroelectric, electrostrictive, and resonant effects to achieve high sensitivity. The sensor consists of a high-Q gallium nitride (GaN) micro-mechanical resonator coated with an IR absorber layer. The IR absorber converts the IR energy into heat. The generated heat causes a shift in the frequency characteristics of the GaN resonators. IR detection is achieved by sensing the shift in the resonance frequency of the exposed GaN resonator as compared to a reference resonator. A prototype GaN-based IR sensor is implemented, showing a frequency shift of 400 Hz per μW of absorbed power. The change in the beat frequency upon IR radiation is 1830 ppm/μW, making it possible to sense IR radiation in nano watts range. INTRODUCTION The detection of infrared radiation is generally accomplished using either photonic or thermal detectors, both of which have generated a large amount of research over the last two decades [1]. The relative disadvantages of thermal detectors in terms of detectivity and response time are balanced by the fact that they do not require cooling systems, reducing the system weight, power consumption, and form factor. Furthermore, thermal detectors are better suited for radiation extending into the far wavelength IR, which is extremely useful for optical astronomy. While bolometric devices have been the most popular thermal sensors commercially, there is an increasing interest in resonant IR detectors [2]-[4]. This work presents a GaN-based resonant IR sensor that utilizes the unique material properties of GaN to achieve high sensitivity. A reference resonator is included in the system for self-calibration (Fig. 1). Fig. 2 shows a representative scanning electron micrograph (SEM) of a GaN detector. Figure 1: Schematic of a resonant sensor and a reference pair with interdigitated finger electrodes. The sensor is coated with an absorber layer. The reference resonator is either not coated with the absorber or the absorber layer on the reference resonator is blocked using the top electrode.