Controlled Capacitive Gaps for Electrostatic Actuation and Tuning of 3D Fused Quartz Micro Wineglass Resonator Gyroscope Mohammad H. Asadian, Yusheng Wang, Sina Askari and Andrei Shkel MicroSystems Laboratory, University of California, Irvine, CA, USA Email: {asadianm, yushengw, sina.askari, andrei.shkel} @ uci.edu Abstract—In this paper, we present the assembly process and an electrostatic actuation, detection, and tuning method of micro-glassblown wineglass resonators using an out-of-plane electrode architecture with controlled uniform capacitive gaps. This process is developed based on wafer-level deposition of a sacrificial layer on planar electrodes wafer to achieve a uniform electrostatic gap (<5μm). The materials, bonding type, etch selectivity, temperature and vacuum compatibility of the process are considered in the assembly process of the micro- glassblown fused quartz shell resonators. Tuning the frequency mismatch between two orthogonal (n=2) in-plane wineglass modes is demonstrated experimentally on a 7 mm micro-glassblown wineglass resonator using the out-of-plane electrodes. An open- loop rate response is demonstrated using out-of-plane capacitive actuation and detection. The proposed process enables a low-cost assembly with a high transduction efficiency toward wafer-scale fabrication of micro 3D wineglass gyroscopes. Keywords—fused quartz; electrostatic tuning; assembly; micro- glassblowing; 3D wineglass resonator; capacitive gap I. I NTRODUCTION 3D MEMS is an area of interest for the development of micro-scale high-performance sensors for inertial navigation applications. The micro-glassblowing process was proposed in [1] to fabricate 3D inverted micro-wineglass resonators out of low internal loss materials, such as Fused Quartz and Ultra Low Expansion Titania Silicate Glass (ULE TSG). The initial characterization results of a micro-glassblown 3D fused quartz resonator were reported at JMEMS 2015, [2]. Blow torch- molding was proposed as an alternative approach to fabricate 3D fused quartz shell resonators, [3]. An energy decay time of 130 seconds was reported in [4], demonstrating the potential of the micro-scale 3D shell to achieve the high-performance gyroscope operation. Unlike conventional Silicon MEMS fabrication, in which the electrodes are co-fabricated with the sensing elements and capacitive gaps are defined by the high aspect ratio DRIE process, our current approach for 3D micro shell device fabrication requires an additional assembly step to integrate the 3D shells with actuation and detection electrodes. One of the challenges in the assembly of micro shell devices is achieving a uniform gap with efficient transduction. On macro-scale, spherical electrodes are machined and precisely assembled on HRGs to make 3D radial capacitive gaps [5]. An alternative approach is proposed by SAGEM [6], where a planar electrode configuration was utilized to makes capacitive gaps to actuate This material is based on work supported by the Defense Advanced Research Projects Agency under Grant W31P4Q-11-1-0006. Device layer Substrate layer Device layer Substrate layer Fig. 1. Micro-wineglass fabrication process; (left) device layer bonded to the pre-etched cavities, (right) pre-etched device layer bonded to a flat substrate (this work). and detect the axial displacement of the rim of the shell. The planar electrode configuration was adapted for electrostatic actuation and detection of micro-glassblown wineglass shells and a wafer-level assembly process was proposed in [7]. Utilization of the planar electrodes make the assembly process less complicated, cost-effective, and accommodating to the small off-center error as compared to the radial electrode arrangement. In this paper, a complete assembly process is presented to reduce the capacitive gaps to smaller than 5μm and obtain a higher efficiency for the actuation, detection, and the frequency mismatch tuning. The proposed process also considers the temperature and high-vacuum compatibility of the materials being used in the assembly of micro shell resonators. II. FUSED SILICA MICRO- GLASSBLOWN WINEGLASS RESONATOR In the micro-glassblowing process, a fused quartz substrate wafer is initially coated with a thin film of LPCVD PolySilicon (2μm) as a hard mask for etching. Cavities are etched in the wafer using 48% Hydrofluoric acid (HF) wet etching. After removing the hard mask, a blank fused quartz wafer is bonded to the pre-etched wafer using plasma-assisted fusion bonding. This creates encapsulated and hermetically sealed cavities. Heating up the bonded pairs of wafers above the softening point of the fused quartz (>1700 °C) causes (1) expansion of the trapped air inside the enclosed cavities and (2) viscous flow of the device layer due to combination of high temperature and high-pressure effects, creates a 3D axisymmetric geometry with self-aligned stem. The fabrication process of fused quartz shell resonators is shown in Figure 1. 978-1-5090-3234-1/17/$31.00 ©2017 IEEE 19