Fabrication of a polymer-based torsional vertical comb drive using a double-side partial exposure method

A novel approach which uses a double-side partial exposure method to fabricate a polymer-based torsional vertical comb drive (VCD) with thick photoresist AZ9260 as the structural material is proposed in this paper. Front-side partial exposure defines the height of the fixed lower fingers, and back-side partial exposure creates the suspending space of the upper fingers, where the overlap and self-alignment between fingers can easily be achieved in this way. It does not need any sacrificial layer and etching process. A metal layer is finally deposited on the structural surface by a sputtering system for suitable electric conductivity to activate the polymer torsional VCD. The finite element method is used here to simulate the capacitance at different finger positions, and twelfth-order polynomial curve fitting is performed to obtain capacitance as a function of finger displacement. Then the rotation angle can be calculated analytically with the capacitance function. Also, the static deflection and dynamic response of polymer torsional VCDs are characterized experimentally, where the dimensions of the torsion plate are 300 νm wide and 360 νm long with movable fingers of length 100 νm; the torsion spring is 60 νm long and 4 νm wide. Both have a thickness of 31 νm, and the initial overlap is 11 νm in depth and 80 νm in length between the lower and upper fingers. By comparing the simulated and experimental results, the feasibility of the proposed fabrication method of polymer torsional VCDs is verified with a measured rotation angle of 2.31° under a driving voltage of 158.3 V.