IEEE Standard on Piezoelectricity

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.

Gallium Nitride as an Electromechanical Material

This paper is a review of the remarkable progress that has been made during the past few decades in design, modeling, and fabrication of micromachined resonators. Although micro-resonators have come a long way since their early days of development, they are yet to fulfill the rightful vision of their pervasive use across a wide variety of applications. This is partially due to the complexities associated with the physics that limit their performance, the intricacies involved in the processes that are used in their manufacturing, and the trade-offs in using different transduction mechanisms for their implementation. This work is intended to offer a brief introduction to all such details with references to the most influential contributions in the field for those interested in a deeper understanding of the material.

Micromachined Resonators: A Review

Over the last couple of decades, the advancement in Microelectromechanical System (MEMS) devices is highly demanded for integrating the economically miniaturized sensors with fabricating technology. A sensor is a system that detects and responds to multiple physical inputs and converting them into analogue or digital forms. The sensor transforms these variations into a form which can be utilized as a marker to monitor the device variable. MEMS exhibits excellent feasibility in miniaturization sensors due to its small dimension, low power consumption, superior performance, and, batch-fabrication. This article presents the recent developments in standard actuation and sensing mechanisms that can serve MEMS-based devices, which is expected to revolutionize almost many product categories in the current era. The featured principles of actuating, sensing mechanisms and real-life applications have also been discussed. Proper understanding of the actuating and sensing mechanisms for the MEMS-based devices can play a vital role in effective selection for novel and complex application design.

/pdf/a-review-of-actuation-and-sensing-mechanisms-in-mems-based-450ftrxe90.pdf

A Review of Actuation and Sensing Mechanisms in MEMS-Based Sensor Devices

Advances in silicon technology continue to revolutionize micro-/nano-electronics. However, Si cannot do everything, and devices/components based on other materials systems are required. What is the best way to integrate these dissimilar materials and to enhance the capabilities of Si, thereby continuing the micro-/nano-electronics revolution? In this paper, I review different approaches to heterogeneously integrate dissimilar materials with Si complementary metal oxide semiconductor (CMOS) technology. In particular, I summarize results on the successful integration of III–V electronic devices (InP heterojunction bipolar transistors (HBTs) and GaN high-electron-mobility transistors (HEMTs)) with Si CMOS on a common silicon-based wafer using an integration/fabrication process similar to a SiGe BiCMOS process (BiCMOS integrates bipolar junction and CMOS transistors). Our III–V BiCMOS process has been scaled to 200 mm diameter wafers for integration with scaled CMOS and used to fabricate radio-frequency (RF) and mixed signals circuits with on-chip digital control/calibration. I also show that RF microelectromechanical systems (MEMS) can be integrated onto this platform to create tunable or reconfigurable circuits. Thus, heterogeneous integration of III–V devices, MEMS and other dissimilar materials with Si CMOS enables a new class of high-performance integrated circuits that enhance the capabilities of existing systems, enable new circuit architectures and facilitate the continued proliferation of low-cost micro-/nano-electronics for a wide range of applications.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.2013.0105

Beyond CMOS: heterogeneous integration of III–V devices, RF MEMS and other dissimilar materials/devices with Si CMOS to create intelligent microsystems

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.

/pdf/gallium-nitride-on-silicon-micromechanical-overtone-4jjfyq260v.pdf

Gallium nitride-on-silicon micromechanical overtone resonators and filters

In this paper, we explore the piezoelectric transduction of in-plane flexural-mode silicon resonators with a center frequency in the range of 13-16 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 Temperature-Compensated Flexural-Mode Silicon Resonators

In this paper, we study the fundamental cause of anchor dissipation in Lame- 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 Lame-mode frequency of the resonating plate. This has a significant bearing on the design of Lame-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 Lame mode at 41.57 MHz. The fxQ for this device is 1.7×1013, one of the highest values reported in silicon.

/pdf/optimization-of-tether-geometry-to-achieve-low-anchor-loss-1b1m88abhr.pdf

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

/pdf/piezoelectrically-transduced-high-q-silica-micro-resonators-13vqhrkdp7.pdf

Piezoelectrically transduced high-Q silica micro resonators

Electrostatically actuated Lame-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 Lame-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.

/pdf/temperature-compensated-piezoelectrically-actuated-lame-mode-4yvfyz4s55.pdf

V. A. Thakar

Papers

Gallium nitride-on-silicon micromechanical overtone resonators and filters

Piezoelectrically Transduced Temperature-Compensated Flexural-Mode Silicon Resonators

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

Piezoelectrically transduced high-Q silica micro resonators

Temperature-compensated piezoelectrically actuated Lamé-mode resonators