Anosh Daruwalla

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

This paper presents the design, fabrication, and characterization of a novel high quality factor (Q) resonant pitch/roll gyroscope implemented in a 40 μm (100) silicon-on-insulator (SOI) substrate without using the deep reactive-ion etching (DRIE) process. The featured silicon gyroscope has a mode-matched operating frequency of 200 kHz and is the first out-of-plane pitch/roll gyroscope with electrostatic quadrature tuning capability to fully compensate for fabrication non-idealities and variation in SOI thickness. The quadrature tuning is enabled by slanted electrodes with sub-micron capacitive gaps along the (111) plane created by an anisotropic wet etching. The quadrature cancellation enables a 20-fold improvement in the scale factor for a typical fabricated device. Noise measurement of quadrature-cancelled mode-matched devices shows an angle random walk (ARW) of 0.63° √h−1 and a bias instability of 37.7° h−1, partially limited by the noise of the interface electronics. The elimination of silicon DRIE in the anisotropically wet-etched gyroscope improves the gyroscope robustness against the process variation and reduces the fabrication costs. The use of a slanted electrode for quadrature tuning demonstrates an effective path to reach high-performance in future pitch and roll gyroscope designs for the implementation of single-chip high-precision inertial measurement units (IMUs).

Stiffness trimming of high Q MEMS resonators by excimer laser annealing of germanium thin film on silicon

This paper reports on a new method to optically trim the resonance frequency of a low-loss micromechanical silicon (Si) resonator coated with a thin-film germanium (Ge) layer. A focused ultra-violet (UV) laser beam (349nm wavelength) locally heats Ge to high temperatures, enabling selective micro-crystallization of SiGe. The stiffness of the optically crystallized SiGe regions decreases with increasing concentration of Ge. These stiffness variations lead to an overall downward shift of the resonance frequency. Large trimming range (~5000ppm) is demonstrated for low-frequency out-of-plane resonant modes. In addition, fine frequency trimming of in-plane Lame mode resonators is repeatedly achieved (~ 4 ppm per scan) without introducing any damping throughout the trimming process (Q~300,000), enabling fine frequency control of high-Q microresonators. The resonance frequency of a trimmed germanium-coated silicon resonator remains stable after being heated to 450°C for 30 minutes in a Rapid Thermal Annealing (RTA) chamber.

A dual-axis single-proof-mass angular accelerometer for a vestibular prosthesis

A dual-axis single-proof-mass angular accelerometer has been developed for a vestibular prosthesis. Designed to sense head rotations both in the yaw and the pitch planes, the output of the inertial sensor may be coded as amplitude or rate modulated biphasic current pulses to stimulate vestibular nerves. Fabricated with a high aspect ratio commercial process, a sensor with small form factor (1.4 mm × 0.8 mm) is achieved with a scale factor of 95.5 μV/rad/sec2 and 145.8 μV/rad/sec2 in the yaw and the pitch planes, respectively. Superior linear acceleration rejection was demonstrated for both rotating axis, and an overall power consumption of 296 μW was estimated including sensor and interface circuit.A dual-axis single-proof-mass angular accelerometer has been developed for a vestibular prosthesis. Designed to sense head rotations both in the yaw and the pitch planes, the output of the inertial sensor may be coded as amplitude or rate modulated biphasic current pulses to stimulate vestibular nerves. Fabricated with a high aspect ratio commercial process, a sensor with small form factor (1.4 mm × 0.8 mm) is achieved with a scale factor of 95.5 μV/rad/sec2 and 145.8 μV/rad/sec2 in the yaw and the pitch planes, respectively. Superior linear acceleration rejection was demonstrated for both rotating axis, and an overall power consumption of 296 μW was estimated including sensor and interface circuit.

A 0.5MHz mode-matched pitch or roll annulus gyroscope with nano-gap slanted electrodes for quadrature cancellation

This paper presents, for the first time, the effective quadrature cancellation in a high-frequency pitch or roll bulk acoustic wave (BAW) gyroscope to enable perfect mode-matching for sensitivity improvement and noise reduction. The presented gyroscope has a high operation frequency of 0.5MHz and high translational stiffness for shock and vibration robustness. Quadrature cancellation, which is the biggest challenge in high-performance pitch or roll gyroscopes, is achieved by incorporating nano-gap slanted electrodes through the new HARPSS+ fabrication process, which combines anisotropic wet-etching of silicon with conventional DRIE-based HARPSS process. The HARPSS+ process enables precision capacitive transduction and tuning in all desired orientations, providing tuning coverage over typical process corners and significantly improving the yield of pitch or roll gyroscopes. Quadrature cancellation is verified on fabricated devices, allowing a perfectly mode-matched operation with a scale factor of 95.2pA/(°/s) and an open-loop bandwidth of 15Hz. The HARPS S+ process also enables simultaneous fabrication of single-chip tri-axial BAW gyroscopes: high-frequency (0.5–5MHz) gyroscopic modes with high quality factors (10,000s–100,000s) are measured on both yaw and pitch/roll BAW gyroscopes on the same wafer.