Fabrication of two-gimbal Ni–Fe torsional micro-gyroscope by SU-8 based UV-LIGA process

Abstract

This paper reports fabrication of two-gimbal Ni---Fe torsional micro-gyroscope by SU-8 based UV-LIGA process. The device structure is designed to overcome the limitations of current surface micromachining process, allowing it to be excited with practical voltage levels and increasing its sensitivity to angular rate. The design equations of electrostatic actuation, moment of inertia, and suspension system are presented. The design is then optimized to be compatible with SU-8 based UV-LIGA process having 8 µm thick nickel as the key structural layer. However, due to the higher residual stress in the electroformed nickel layer, the released structure is found to be buckled. Because of this buckling, the structural layer touches the electrodes underneath and shortens the two plates of drive capacitor as confirmed by I---V testing. To overcome this problem, the structural layer material is chosen as Ni---Fe alloy instead of pure nickel at the end of an exercise to reduce residual stress in the released structure. Scanning electron microscope (SEM) images of the Ni---Fe micro-gyroscope confirms negligible buckling in the released structure. The compositional analysis of electroformed Ni---Fe alloy is carried out using energy dispersive X-ray spectroscopy (EDS) and is found to contain 70.02 % nickel (Ni), 15.91 % ferrous (Fe), 10.3 % carbon (C), and 3.77 % oxygen (O). The prototype device is also characterized for amplitude and phase spectral responses using Polytec MSA-500 Micro System Analyzer. The resonance frequencies of drive and sense oscillators are observed at about 8.24 and 7.76 kHz, respectively against the mode-matched design frequency of 8 kHz.