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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.

We demonstrate coupled bulk and surface acoustic wave (BAW/SAW) resonators with significantly improved coupling efficiency as compared to SAW only devices. Two sets of stacks are investigated: one uses a thin film piezoelectric material with a bottom electrode on GaN substrate (i.e., AlN/Mo/GaN) and another includes a piezoelectric thin film with bottom metal on a Si substrate (i.e., AlN/Mo/Si). A vibrating BAW transducer induces a SAW in the substrate and between two sets of interdigitated transducers improving the coupling efficiency by a factor of ${\geq }\, \mathsf {8}{x}$ as compared to the conventional SAW. The performance of the coupled BAW/SAW resonators using each of the Si and GaN substrates is compared using FEM analysis. By using FEM, coupling efficiency of the hybrid BAW/SAW with AlN/Mo/Si stack is 2.4%, whereas it is 3.2% for the SAW using AlN/Mo/GaN stack.

This letter reports the first results on a new generation of anti-symmetric lamb-wave devices using a simple solidly mounted platform of AlN/Mo/Si and operating at very-high-to-super-high frequency range. AlN/Mo/Si-based solidly mounted resonators (SMRs) with sub-micrometer interdigitated transducers were fabricated using the electron beam lithography and liftoff process. Plasma etching was used to etch vias in ground pads to get access to the bottom electrode for grounding. By comparing the floating, grounded, and no bottom electrode device performances, it has been observed that the presented SMR design with grounded bottom electrode results in enhancement of the coupling efficiency by 500% compared with conventional SAW devices. In addition, these devices are more mechanically robust and can be monolithically integrated with a CMOS platform without requiring complex packaging processes compared with conventional released lamb-wave resonators.

This letter reports on first-time demonstration of non-reciprocal acoustic transmission in a gallium nitride (GaN) delay line structure. The split of forward (S21) and backward (S12) transmissions is observed by applying a dc electric field through the active area with a two-dimensional electron gas (2DEG) sheet. The non-reciprocity (~20.7 dB/mm max. in this letter) varies by tuning the gate voltage to deplete the 2DEG sheet carrier density, which agrees with model prediction for the interaction between 2DEG and surface acoustic waves. Our preliminary results prove the feasibility of implementing chip-scale non-reciprocal acoustic devices in a GaN platform through acoustoelectric amplification.

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”).

There is an increasingly high demand for compact, low-loss reconfigurable filters in various advanced wireless applications: cognitive radios, modern transceivers, and antijamming communication systems, to name a few [1]. With reconfigurable filters, wireless communication systems that require multiple operating frequency bands can be implemented in a smaller size and at a reduced cost compared to conventional systems using multiple single-frequency filters. Bandpass filters provide a better RF performance compared to low-pass or high-pass filters, resulting in improved signal-to-noise ratio and out-of-band performance [2].

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]

With advances in telecommunications, an increasing number of services rely on high data rate spectrum access. These critical services include banking, telemedicine, and exchange of technical information. As a result, spectrum resources are in ever-greater demand and the radio spectrum has become overly crowded. For efficient usage of spectrum, smart or cognitive radios are sought after. However, current wireless phones can only select a few specific bands. In this paper, we discuss the advantages of reconfigurable radios in not only increasing the efficiency of spectrum usage but also in potentially reducing the cost of wireless handsets and the barriers for new wireless service providers to enter the market. We review available technologies that make the implementation of reconfigurable radios possible and discuss technical challenges that need to be overcome before multistandard reconfigurable radios are put into practice. We also evaluate the ability of reconfigurable radios in reducing entry costs for new competitors in wireless service.

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.

A GaN bulk acoustic wave resonator is presented in this letter, showing fundamental thickness-mode resonance at 2.18 GHz, with a quality factor ( Q ) of 655 and a coupling coefficient ( \({k_{t}}^{2}\) ) of 1%. The resonator is integrated with an AlGaN/GaN high electron mobility transistor (HEMT); the integrated resonator/HEMT structure is coated with a silicon dioxide (SiO2) passivation layer. It is shown that a 400-nm-thick SiO2 layer reduces the temperature coefficient of frequency (TCF) of the GaN-based resonator by >50%, while improving Q and \({k_{t}}^{2}\) of the fundamental thickness-mode resonance. The effect of SiO2 passivation layer is studied on \({k_{t}}^{2}\) , Q , and TCF of the device. Furthermore, the effects of temperature and input RF power on the resonator performance are characterized.

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.

In this letter, we report on the design, fabrication, and measured results of a directly heated phase change RF switch (or via) using germanium telluride in a four-terminal configuration. The switch is heated using a separate heater path combining the advantages of directly heated vias, such as low power dissipation for phase transition, and indirectly heated vias, such as high power handling capability. The phase change switch shows an insertion loss of less than 0.6 dB and an isolation of higher than 20 dB at frequencies up to 20 GHz, indicating a cutoff frequency of more than 3.7 THz. The switch area is only 4 μm× 6 μm, which is smaller than RF MEMS switches with similar insertion loss performance. To the best of our knowledge, this is the first report on a four-terminal, directly heated, RF phase change switch.

We report on a thermoelectric model that can be used to analyze the nonlinearity and power handling capability of RF switches based on phase change (PC) materials. To analyze the nonlinear behavior of the PC switches, the Poole-Frenkel effect, the variation of thermal and electrical conductance with temperature, and the effect of Joule heating with increased RF signal power are all taken into account. The simulated input third-order intercept point and the 1-dB compression point (P1dB) are compared with measured results of directly heated PC switches, showing a good agreement. Using this model, we provide insights into the effect of electrical resistance, and thermal properties, as well as the operation frequency on the switch nonlinearity and power handling capability.

Diverse RF passive devices and microelectro- mechanical systems (MEMS) can be monolithically integrated on a high-resistivity silicon (HR-Si) substrate. However, parasitic surface conduction (PSC) at the interface of HR-Si and a silicon dioxide (SiO2) passivation layer reduces the effective substrate resistivity, which in turn results in deterioration of the device quality factor (Q) and non-linearity. Trap-rich HR-Si has been proposed as a low substrate loss alternative, eliminating the problems associated with PSC. However, the full potential of trap-rich HR-Si as a common platform for implementing MEMS passives is not fully explored. In this letter, we evaluate the effectiveness of the trap-rich layer by comparing the frequency response of a number of RF passive devices fabricated on standard and trap-rich HR-Si substrates. In addition, we suggest an electromagnetic (EM) simulation setup that can be used to efficiently and accurately simulate the device performance.

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 reports on a new implementation of high-quality factor copper inductors on CMOS-grade silicon substrates (p = 10-20 Omega ldr cm) using a CMOS-compatible process. A low-temperature fabrication sequence (

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.