Jae Yoong Cho

Fused-Silica Micro Birdbath Resonator Gyroscope (

We present a 3-D fused-silica micro-scale shell gyroscope, called the birdbath resonator gyroscope (BRG). The BRGs axisymmetric geometry leads to a good frequency and Q symmetry. The birdbath resonator can be fabricated with a good structural symmetry because its anchor is self-aligned to the rest of the structure. The BRG has n=2 wine-glass modes at 10.5 kHz and has a large frequency separation between the n=2 wine-glass modes and the closest parasitic mode (|fparasitic-fn=2|/fn=2=0.3), which will potentially lead to a low vibration sensitivity. The equations of motion for 3-D shell gyroscopes are derived and the effective mass and angular gain of the BRG is estimated using finite element method (FEM). The BRG is fabricated using a 3-D micrometer-blow-torching process and assembly on an electrode substrate made with the silicon-on-glass process. The BRG is operated in the force-rebalance mode at vacuum at room temperature and has a scale factor of 27.9 mV/(deg/s), a full-scale range , an angle random walk of 0.106 deg/√h, and a bias stability of 1 deg/h. A large angular gain (0.317) is measured, which is close to the estimated value of 0.25 obtained via FEM.

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This paper presents a novel 3-D fabrication process of microstructures using high-Q materials. The key feature of this process is the use of a blow torch that can provide intensive localized heat up to 2500°C in a very short time (<;10 sec), above the melting temperatures of many high-Q materials such as fused silica. Surface roughness of 5.3 Å is realized in fused silica, which is crucial for high optical and mechanical quality factors (Q). We demonstrate the fabrication of micro hemisphere and half-toroid (birdbath) geometries from 100μm-thick fused silica substrates. We also create micro birdbath resonators by batch-level releasing the birdbath shells and demonstrate one of the best mechanical Q and low stiffness and damping anisotropies among existing micro mechanical resonators. The birdbath resonator is promising for emerging applications such as micro rate-integrating gyroscope (μ-RIG).

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This paper presents a generic vacuum packaging technology for environment-resistant MEMS devices. This packaging approach simultaneously provides low-power oven-controlled thermal environment and vibration isolation using an isolation platform. The oven-controlled structure is thermally isolated from the environment by crab-leg suspensions made out of a 100 ¿m-thick glass wafer, an anti-radiation shield, and vacuum encapsulation. Performance is evaluated by packaging Pirani gauges and mode-matched tuning fork gyroscopes (M2-TFGs). The package has maintained vacuum pressure of ~6 mTorr for ~1 year. A packaged M2-TFG shows a high-Q mode-matched operation (Q~65,000) at a constant temperature of -5 °C. Allan variance analysis displays an estimated angle random walk (ARW) of 0.012 °/¿hr and a bias instability value of 0.55 °/hr at a constant -5 °C. Drive frequency stability of 0.22 ppm/°C is obtained using a compensated oven-control approach. Low power consumption of 33 mW for oven-control at 80 °C is demonstrated when the environment temperature is -30 °C.

High-Q fused silica birdbath and hemispherical 3-D resonators made by blow torch molding

This paper presents a new and simple microfabrication process for creating various 3-D microstructures with high-aspect ratios . The key feature of this process is the use of a blow torch, which provides very intense localized heat for a short amount of time . The flame temperature can be up to 2500 °C for a propane-oxygen torch, above the melting temperatures of many high- Q materials. We demonstrate the fabrication of hemispherical and half-toroid (birdbath) shells from 100- μm-thick fused silica substrates. The structures have an rms surface roughness of 5.3 Å, which is crucial for achieving both high mechanical and optical Q. We create microbirdbath resonators by batch-level releasing the birdbath shells. We verify the resonance mode shapes of the degenerate n=2 wineglass modes using laser vibrometry and measure the frequency and Q of these modes using laser vibrometry and a capacitive measurement method. We demonstrate one of the best mechanical Q and smallest frequency split between the n=2 wineglass modes among existing micromechanical resonators. The birdbath resonator is promising for emerging applications such as the microrate-integrating gyroscope.

A Low-Power Oven-Controlled Vacuum Package Technology for High-Performance MEMS

This paper describes the design, control system and test results of the Single-Crystal-Si (SCS) Cylindrical Rate-Integrating Gyroscope (CING), operating at 3kHz. The CING is a MEMS gyroscope to demonstrate continuous rate-integrating operation, operating over several hours at a time. The sensor is fabricated with the Silicon-On-Glass (SOG) process, calibrated for deep (>;300μm), high aspect-ratio (>;1:15), and narrow-gap (2μm) Si structures. The sensor has an original Δf of 7Hz, and a nominal damping time (τ0) of 8.7 seconds when modes are matched within 20mHz. It was evaluated in rate- and rate-integrating modes using a hybrid FPGA and software control. The rate-mode angle random walk was 0.09°/√Hr with Allan Variance of 129°/hr and an angular gain of 0.011.

3-Dimensional Blow Torch-Molding of Fused Silica Microstructures

We report a new fabrication technology for making fused silica (FS) wineglass resonators with arbitrarily sized FS solid stems through a simultaneous process of micro blow-torching and microwelding. The process allows the welding of multiple FS structures at controlled locations during blow torching. We demonstrate a new micro FS wineglass resonator with high quality factor (Q) and long ring down time (τ). The resonator is formed by blow torching and flowing a thin FS substrate using vacuum to form the resonator shell, and by welding the shell to a solid post at a controlled location. The flowing of the shell and welding to the rod is performed in one step and in a single mold. This solid-stem resonator offers low anchor loss due to the large stem length/stiffness, and small shell rim thickness. The device has a shell radius/height of ~2.8 mm, and a stem radius of 0.5 mm. At <;10 μTorr vacuum, the n=2 wineglass mode, located at a frequency (f) of 22.6 kHz, has σ = 35.9 s and Q = 2.55 million.

High-Q, 3kHz Single-Crystal-Silicon Cylindrical Rate-Integrating Gyro (CING)

We propose two new controls to improve the performance of rate-integrating MEMS gyroscopes (RIGs). The first control loop dynamically determines and compensates for damping mismatch by creating a force on the gyroscope to oppose the apparent force due to damping mismatch. The second control loop extends this concept to frequency mismatch to eliminate residual quadrature which would otherwise accumulate due to lag in the control system. The proposed control loops have been investigated using simulation results from a non-ideal gyroscope including frequency and damping mismatch as well as control delay and capacitive feed-through. Experimental results of the controls being applied to a MEMS rate-integrating gyroscope are also presented. In simulation, the controls can reduce the drift in a gyroscope by better than two orders of magnitude. In a physical gyroscope, the mismatch parameters do not match the normal first order model, however RMS drift is reduced by 25%.

A high-q all-fused silica solid-stem wineglass hemispherical resonator formed using micro blow torching and welding

We present the fused-silica micromachined birdbath resonator gyroscope (μ-BRG) operating in the whole-angle (WA) mode. The key advantages of the whole angle mode operation is rotation angle measurement, large bandwidth, and full-scale range which is needed in detecting the motion of fast-moving objects. The μ-BRG is made with fused silica using a micro blow-torching process and has n = 2 wineglass modes at 10.46 kHz with a small frequency mismatch (Δf = 10 Hz) and a decay time (τ) of 2.2s. The WA-BRG achieves a stable angular gain (Ag) and a large full scale range (700°/s).

Novel mismatch compensation methods for rate-integrating gyroscopes

Maximizing quality (Q) factor is key to enhancing the performance of micro mechanical resonators, which are used in a wide range of applications such as gyroscopes, filters, and clocks. There are several energy loss mechanisms commonly associated with micro resonators including anchor loss through the substrate, squeeze film damping, thermoelastic dissipation (TED), and surface loss. This work focuses on the thermoelastic loss as one of the major energy dissipation mechanisms of micro shell resonators.In this article, the effects of material properties, thickness, conductive coating and operating temperature on the Q-factor of micro shell resonators are investigated. Numerical simulation shows shell resonators have higher Q-factors when they are operating at lower temperatures. Although, the magnitude of the simulated Q-factors of an uncoated bare resonator made from fused silica is more than 70 million and so it is too high to have a remarkable effect on the total Q-factor, our study shows that even a thin layer of some conductive coatings like gold on the surface of a bare shell reduces Q-factor significantly. The sensitivity of the coated shell resonator design to the TED phenomenon provides useful information for the development of new micro shell resonators with improved performance and Q-factors.Copyright

Whole-angle-mode micromachined fused-silica birdbath resonator gyroscope (WA-BRG)

This paper reports the architecture and operation of a single-crystal cylindrical rate-integrating gyroscope (CING). The attractive features of the CING include mode stability due to the separation of the wineglass modes from the in-phase-parasitic modes, self-alignment of sensor components, and fabrication. The CING is built using a silicon-on-glass (SOG) process using a (111) Si wafer. It operates at 17.9kHz, has an average Q of 21,800 and a damping-time mismatch of Δτ of 0.7ms along the measured axes, corresponding to a damping-constant mismatch of (Δ1/τ) of 0.0047Hz at a pressure of &#60;5mTorr. The CING is operated in the whole angle mode with first-generation interface circuitry consisting of software-defined radio.