Doruk Senkal

Demonstration of 1 Million

In this paper, we report Q-factor over 1 million on both n = 2 wineglass modes, and high-frequency symmetry (Af/f ) of 132 ppm on wafer-level microglassblown 3-D fused silica wineglass resonators at a compact size of 7-mm diameter and center frequency of 105 kHz. In addition, we demonstrate for the first time, out-of-plane capacitive transduction on microelectromechanical systems wineglass resonators. High Q-factor is enabled by a high aspect ratio, self-aligned glassblown stem structure, careful surface treatment of the perimeter area, and low internal loss fused silica material. Electrostatic transduction is enabled by detecting the spatial deformation of the 3-D wineglass structure using a new out-of-plane electrode architecture. Out-of-plane electrode architecture enables the use of sacrificial layers to define the capacitive gaps and 10 μm capacitive gaps have been demonstrated on a 7-mm shell, resulting in over 9 pF of active capacitance within the device. Microglassblowing may enable batch-fabrication of high-performance fused silica wineglass gyroscopes at a significantly lower cost than their precision-machined macroscale counterparts.

Q

We demonstrate, for the first time, sub-1 Hz frequency symmetry in micro-glassblown wineglass resonators with integrated electrode structures. A new fabrication process based on deep glass dry etching was developed to fabricate micro-wineglasses with self-aligned stem structures and integrated electrodes. The wineglass modes were identified by electrostatic excitation and mapping the velocity of motion along the perimeter using laser Doppler interferometry. A frequency split (Δf) of 0.15 and 0.2 Hz was demonstrated for n=2 and n=3 wineglass modes, respectively. To verify the repeatability of the results, a total of five devices were tested, three out of five devices showed . Frequency split stayed below 1 Hz for dc bias voltages up to 100 V, confirming that the low frequency split is attributed to high structural symmetry and not to capacitive tuning. High structural symmetry and atomically smooth surfaces (0.23 nm Sa) of the resonators may enable new classes of high performance 3-D MEMS devices, such as rate-integrating MEMS gyroscopes.

-Factor on Microglassblown Wineglass Resonators With Out-of-Plane Electrostatic Transduction

This paper reports a new type of degenerate mode gyroscope with measured Q-factor of > 100,000 on both modes at a compact size of 1760 μm diameter. The toroidal ring gyroscope consists of an outer anchor ring, concentric rings nested inside the anchor ring and an electrode assembly at the inner core. Current implementation uses n = 3 wineglass mode, which is inherently robust to fabrication asymmetries. Devices were fabricated using high-temperature, ultra-clean epitaxial silicon encapsulation (EpiSeal) process. Over the 4 devices tested, lowest as fabricated frequency split was found to be 8.5 Hz (122 ppm) with a mean of 21 Hz (Δf/f = 300 ppm). Further electrostatic tuning brought the frequency split below 100 mHz (<; 2 ppm). Whole angle mechanization and pattern angle was demonstrated using a high speed DSP control system. Characterization of the gyro performance using force-rebalance mechanization revealed ARW of 0.047°/√hr and an in-run bias stability of 0.65 deg/hr. Due to the high Q-factor and robust support structure, the device can potentially be instrumented in whole angle mechanization for applications which require high rate sensitivity and robustness to g-forces.

Achieving Sub-Hz Frequency Symmetry in Micro-Glassblown Wineglass Resonators

In this paper, we report development of deep plasma etching process for Fused Silica (FS) and Borosilicate Glass (BSG) using magnetic Neutral Loop Discharge (NLD) plasma, achieving a depth of 100 μm, a high aspect ratio of 8:1, nearly vertical walls, and etch rate as high as 0.75 μm/min. The plasma conditions, such as gas flow, power, pressure, and masking materials were experimentally analyzed and optimized for improved aspect ratio, selectivity, sidewall angle, etch rate, and etch quality. The design of experiment with etching parameters and variation in masking materials provides a systematic approach to the fabrication of sensors, resonators and microsystems using FS and BSG.

100K Q-factor toroidal ring gyroscope implemented in wafer-level epitaxial silicon encapsulation process

This paper reports the experimental demonstration of Quality factor (Q-factor) improvement of fused quartz hemispherical resonators after post-fabrication annealing. Several identical fused quartz 3D hemispherical resonators, thickness 1 mm and diameter 10 mm were fabricated using micro-glassblowing process and tested before and after annealing. Results showed that annealing at 800 °C for 6 hours followed by slow overnight cooling significantly improved Q-factor, from 666×103 to 932×103 in our experiments. The annealing dependence was investigated by varying temperature from 500 °C to 800 °C. Change in Q-factor was observed to be minimal (<;10%), when annealed at 500 °C, for both n=2 and n=3 wineglass modes. Q-factor changes were ~40% when annealed at 800 °C. The experimental result on Q-factor enhancement is valuable for the development of very high Q fused quartz vibratory MEMS resonators for use in precision inertial sensing.

Deep NLD plasma etching of Fused Silica and Borosilicate Glass

We present an adaptable test-bed for characterization of 3-D micro-wineglass resonators. The test-bed provides two interchangeable modes of excitation: (1) Mechanical pinging using a piezo actuated probe assembly, (2) electrostatic excitation using assembled electrode structures with <; 20 μm capacitive gaps. Two modes of detection is also available: Optical pick-up using laser Doppler vibrometry and capacitive detection. 3-D micro-glassblown wineglass resonators were used to demonstrate the capabilities of the test-bed. Electrostatic excitation and capacitive detection was used to obtain the frequency response of a glass resonator showing a Q-factor of 40,000 at 14.8 kHz. Piezo-pinging was used to obtain the time domain response of a resonator, showing a 28 Hz frequency split (0.1% relative split) between the two degenerate wineglass modes (f1 = 22036 Hz, f2 = 22064 Hz).

Effect of annealing on mechanical quality factor of fused quartz hemispherical resonator

In this paper, we report parametric drive of a MEMS rate integrating gyroscope for reduction of drifts induced by drive electronics. A Toroidal Ring Gyroscope (TRG) with 100k Q-factor at 1760 μm diameter was fabricated in epitaxial silicon encapsulation (EpiSeal) process. Device was operated in rate integrating mode using a FPGA based control system. In contrast to the conventional amplitude control architecture, the central star electrode of the gyroscope, connected to a single drive channel was utilized for amplitude control of both modes. Due to the parametric pumping effect, energy added to each (x and y) mode is proportional to the existing amplitude of the respective mode. As a result, errors associated with finding the orientation of the standing wave and x-y drive gain drift are bypassed. Compared to conventional x-y drive architecture, as high as 14× improvement in scale factor stability was observed with parametric pumping, resulting in better than 20 ppm scale factor stability without any compensation or temperature stabilization.

Adaptable test-bed for characterization of micro-wineglass resonators

We report a new type of MEMS degenerate mode lumped mass gyroscope architecture. The Dual Foucault Pendulum (DFP) gyroscope consists of two dynamically equivalent, mechanically coupled proof masses, oscillating in anti-phase motion. This dual axis tuning fork behavior creates a dynamically balanced resonator with x-y symmetry in frequency and damping. Phase synchronization is established by mechanical coupling of the two proof masses, whereas quadrature suppression is achieved by four differential shuttle pairs placed in-between. Dual axis tuning fork behavior provides vibration immunity and anchor loss mitigation, resulting in a Q-factor over 300,000 on both modes at a center frequency of 3.2 kHz. Due to high-Q degenerate mode operation, open-loop performance of 0.003 °/√h ARW and 0.27 °/h in-run bias stability were experimentally demonstrated. We believe Dual Foucault Pendulum is the minimal realization of a dynamically balanced lumped mass degenerate mode gyroscope.

Parametric drive of a toroidal MEMS rate integrating gyroscope demonstrating < 20 PPM scale factor stability

This paper reports analytical and finite element models for predicting the final 3-D geometry of micro-glassblown inverted-wineglass and hemi-toroidal structures from a set of initial conditions. Using these methods critical geometric param-eters for wineglass resonator and gyroscope operation, such as shell thickness, shell height, stem type (hollow/solid), and stem diameter can be estimated from initial parameters. Analytical models for first order calculation of shell height and minor radius, as well as finite element models based on Arbitrary Lagrangian-Eulerian (ALE) methods for calculating the thickness of the shell and diameter of the stem structure are presented. Developed models were validated against fabricated micro-glassblown structures and showed better than 10 % match to experimental results. Methods presented in this paper can be used to design micro-glassblown wineglass resonators with specific dimensions and resonance frequencies, essentially taking the guesswork out of the design process and significantly lowering the development time.

Dual Foucault Pendulum gyroscope

In this paper we report an optimized post-fabrication annealing process for improvement of sidewall roughness in deep glass etched resonant MEMS devices. The method utilizes thermal-reflow behavior of glass and does not require chemical or mechanical treatment. Experiments were conducted to explore the trade-off space between roughness improvement and structural deformation by varying temperature from 300 °C to 900 °c and duration from 2 to 240 minutes. The optimal results were obtained at 700 °C for 30 minutes, resulting in an 10x improvement in roughness from 900 nm Ra to 85 nm Ra and low feature deformation of <;10%. The method was successfully utilized in deep glass etching of micro-glassblown wineglass resonators, which resulted in very low side wall roughness and sub-hz frequency symmetry.