Vikram A. Thakar

Gallium nitride-on-silicon micromechanical overtone resonators and filters

In this paper, for the first time, we report on high-performance GaN-on-silicon micromechanical resonators and filters. A GaN-on-silicon resonator is reported which exhibits a quality factor of 1850 at 802.5 MHz, resulting in an f×Q value twice the highest reported for GaN-based resonators to date. The effective coupling coefficient for the GaN resonator is extracted to be 1.7%, which is among the best reported in the literature.

Piezoelectrically Transduced Temperature-Compensated Flexural-Mode Silicon Resonators

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 <; 2 ppm/°C and a turnover temperature of around 90 °C, ideally suited for use in an ovenized platform. Using electrostatic tuning, the temperature sensitivity of the resonator is compensated across a temperature range of +50 °C to +85 °C, demonstrating a frequency instability of less than 1 ppm. Using proportional feedback control on the applied electrostatic potential, the resonator frequency drift is reduced to less than 110 ppb during 1 h of continuous operation, indicating the ultimate stability that can be achieved for the resonator as a timing reference. The resonators show no visible distortion up to -1 dBm of input power, indicating their power handling capability.

Piezoelectrically transduced high-Q silica micro resonators

In this paper, we report on high-performance piezoelectric-on-silica micromechanical resonators for integrated timing applications. Fused silica is used as the resonator structural material for its excellent material properties, and thin film aluminum nitride is used as the piezoelectric transduction layer. A silica resonator is demonstrated with a high quality factor (QU~25,841), low motional impedance (Rm ~350 Ω), and good power handling capability. The measured f×Q product of this resonator is the highest amongst reported micromachined silica/fused quartz resonators.

Acoustically coupled thickness-mode AIN-on-Si band-pass filters-Part II: simulation and analysis

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 <;5 dB) with percentage bandwidth up to about 1% and ripple less than 1 dB.

Temperature-compensated piezoelectrically actuated Lamé-mode resonators

Electrostatically actuated Lamé-mode resonators are known to offer high quality factors (Q) in the low MHz frequency range but require large bias voltages and suffer from low power handling. In this work, we utilize piezoelectric transduction to circumvent the limitations of electrostatic actuation. Silicon dioxide refilled islands, used to achieve temperature compensation, are shown to provide a 20× improvement in the total charge pick-up, enabling piezoelectric actuation of Lamé-mode resonators. By optimizing the placement of the oxide-refilled islands and without changing the total oxide volume, the turnover temperature (TOT) can be designed to occur across a wide range from -40°C to +120°C without any significant Q degradation. Using such an approach multiple piezoelectric resonators with different TOTs can be fabricated on a single wafer, enabling multi-resonator systems stable across a wide temperature range.

Optimization of tether geometry to achieve low anchor loss in Lamé-mode resonators

In this paper, we study the fundamental cause of anchor dissipation in Lamé- or wineglass-mode resonators and show that by carefully optimizing the resonator tether geometry low anchor losses can be achieved, making it possible to reach the intrinsic fxQ limit of the resonating material. Through analytical and finite element investigation, we demonstrate that the anchor loss is most significant when the flexural-mode resonance frequency of the tether is in sync with the Lamé-mode frequency of the resonating plate. This has a significant bearing on the design of Lamé-mode resonators with release holes or temperature-compensation trenches, where the modification of the resonator structure requires modification to the tether geometry to maintain a high Q. We verify the predicted results through experimental measurements and present an optimized design that exhibits a mechanical Q of 408,270 for the fundamental Lamé mode at 41.57 MHz. The fxQ for this device is 1.7×1013, one of the highest values reported in silicon.

Acoustically coupled thickness-mode AIN-on-Si band-pass filters-part I: principle and devices

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.

A temperature-stable clock using multiple temperature-compensated micro-resonators

In this work, we propose a multi-resonator system capable of achieving a temperature-stable frequency output across a wide temperature range. This system utilizes three temperature-compensated MEMS oscillators whose frequency output undergoes two stages of multiplication and mixing to generate a stable clock signal. In contrast with other circuit compensation techniques, this implementation obviates the need for accurate temperature sensing of the resonator and power hungry ovenization circuits and makes the output stable across the entire calibration range. A system consisting of three temperature-compensated MEMS Pierce oscillators based on AlN-on-silicon ring resonators is implemented and serves as a proof of concept verification for the presented algorithm. A total frequency shift of +/- 8 ppm is seen across the temperature range of -40 °C to +60 °C.

Temperature compensated silicon resonators for space applications

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.

Monolithic implementation of AlN-on-silicon bandpass filters with a high-Q notch within the passband

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.