Ultrathick and High-Aspect-Ratio Nickel Microgyroscope Using EFAB Multilayer Additive Electroforming

This paper presents a new approach for the development of a microgyroscope that has a 240-mum-thick multilayer electroformed-nickel structural mass and a lateral aspect ratio greater than 100. The gyroscope is fabricated using commercial multilayer additive electroforming process EFAB of Microfabrica, Inc., which allows defining the thickness of different structural regions, such as suspensions, proof mass, and capacitive electrodes, unlike many classical surface-micromachining technologies that require a uniform thickness for the structural features. The capacitive gaps of the gyroscope are designed to be laying parallel to the substrate plane and can be set as small as 4 mum, and therefore, the lateral aspect ratio, which is defined as the ratio of the overlap length of capacitor plates to the gap spacing, can easily exceed 100. The high capacitive aspect ratio, multilayer capacitors, and thick proof mass altogether yield a highly sensitive gyroscope for in-plane angular-rate measurements. Characterization of the fabricated gyroscope using a very simple capacitive interface circuit constructed from standard IC components yields a measured mechanical sensitivity of 65 muV/(deg/s) and a noise equivalent rate of 0.086deg/s at atmospheric pressure in a bandwidth of 1 Hz. The response bandwidth is limited to 100 Hz when the resonance frequencies of the drive and sense modes are matched by electrostatic tuning. The measured stability of the drive-mode resonance frequency is better than 0.1% within a 40-h period, demonstrating reliability of electroformed nickel of EFAB process. In addition, the mechanical quality factor of the sense mode reaches 2000 at 20-mtorr vacuum, verifying the quality of structural nickel. The resonance-frequency variations of the drive and sense modes with respect to increasing temperature are characterized to be smaller than 1 and -2 Hz/degC, respectively, in the -40-degC to +85-degC measurement range. The gyroscope performance can be further increased by reducing the minimum capacitive gaps, by increasing the number of stacked layers, by using a nickel-alloy structural layer with better mechanical properties, and by using a dedicated CMOS-application-specified integrated circuit which are under consideration.