Substrate-decoupled, bulk-acoustic wave gyroscopes: Design and evaluation of next-generation environmentally robust devices

Abstract

This paper reports on a new type of high-frequency mode-matched gyroscope with significantly reduced dependencies on environmental stimuli such as temperature, vibration, and shock. A novel stress-isolation system is used to effectively decouple an axis-symmetric bulk-acoustic wave (BAW) vibratory gyro from its substrate, minimizing the effect that external sources of error have on the offset and scale factor of the device. Substrate-decoupled (SD) BAW gyros with a resonance frequency of 4.3 MHz and Q values near 60 000 were implemented using the high aspect ratio poly and single-crystal silicon (HARPSS) process to achieve ultra-narrow capacitive gaps. Wafer-level packaged sensors were interfaced with a customized application-specific integrated circuit (ASIC) to achieve low variations in the offset across temperature (±26°s − 1 from − 40 to 85°C), supreme random-vibration immunity (0.012°s − 1 g RMS − 1 ) and excellent shock rejection. With a scale factor of 800 μV (°s, the SD-BAW gyro system attains a large full-scale range (±1250°s) with a non-linearity of less than 0.07%. A measured angle-random walk (ARW) of 0.39°/√h and a bias instability of 10.5°h − 1 are dominated by the thermal and flicker noise of the integrated circuit (IC), respectively. Additional measurements using external electronics show bias-instability values as low as 3.5°h INTRODUCTIONMicromachined gyroscopes have enabled a myriad of applications that range from basic motion detection for gaming to safety control systems in automobiles 1 . More recently, an increased interest in the use of microelectromechanical system inertial sensors for dead reckoning and pedestrian navigation in handheld electronics has placed stringent requirements on the die size, power consumption, and overall performance of this type of device. To date, most commercially available rate sensors have been designed as low-frequency flexural tuning-fork gyroscopes (TFGs), which are typically sensitive to random vibrations and prone to linear accelerations, such as those experienced under shock. These limitations complicate the use of TFG technology in large-volume, high-end applications, particularly in personal navigation, for which dependencies on fluctuations in the environment translate into long-term drift in the output of the system. Additionally, in recent months, concerns about the high sensitivity of consumer-grade gyroscopes in response to lowfrequency pressure signals that can be used to recover audio have increased as a potential threat for eavesdropping 2 , justifying the need for more environmentally robust rotation sensors.Acceleration suppression mechanisms can be implemented in TFGs to alleviate part of this problem using redundant proof masses that reject shock and vibration as common-mode signals 3 .

Taxonomy and Challenges of Out-of-Band Signal Injection Attacks and Defenses

“…As discussing all possible EMI-resistant amplifier designs is out-of-scope, the interested reader should refer to various comparative works [166]- [168] as a starting point. Similarly, one should refer to advances in gyroscopic technologies [124], [169], [170] which do not use MEMS constructions, or reduce sensitivity to random vibrations: as Khazaaleh et al noted [129], removing the "misalignment between the sensing and driving axes" will make systems more secure against outof-band acoustic attacks.…”

Resonant pitch and roll silicon gyroscopes with sub-micron-gap slanted electrodes: Breaking the barrier toward high-performance monolithic inertial measurement units

“…Therefore, a large electrode area (W and L) and small gap size (g) are favorable. In typical MEMS gyroscopes, an L/g ratio~100 is necessary to obtain sufficient tuning with reasonable voltages 12,17 .…”

“…However, because different designs are involved, it is difficult to obtain consistent high performance for all three sensing axes. Different types of high quality factor (Q) resonant yaw gyroscopes (z axis) have demonstrated promising performance for navigation applications [10][11][12] . However, development of high-performance resonant pitch/roll gyroscopes (x/y axis) with OOP degrees of freedom is known to be challenging due to fabrication non-idealities.…”

A mode-matched force-rebalance control for a MEMS vibratory gyroscope

“…Degenerate modes exist in cyclically symmetric structures. Gyroscope design utilising disks, rings, and hemispheric shells [1] [2] [3][4] [5] or common due to the inherent mode matching caused by degeneracy. For rate mode CVGs, the primary mode is excited into resonance with a constant amplitude of vibration controlled by a phase locked loop (PLL) and an automatic gain control (AGC).…”