Design and fabrication of a MEMS micromirror with integrated charge sensor for feedback control

Abstract

The "snap-through" or "pull-in" phenomenon limits the operating region of an electrostatic MEMS device controlled using a constant bias voltage to only one third of its physical deflection range. It has been shown that by controlling the charge on the actuator any static deflection inside the full gap range can theoretically be stabilized [18, 21, 22, 23]. Some existing approaches to this problem use passive charge feedback, implemented, for example, through a series capacitor. Others use open-loop charge control, involving the application of charge to a fixed capacitor, and the transfer of that charge to the MEMS. Electrostatically-actuated MEMS structures are fabricated at Texas Tech University using surface micromachining methods. The devices are intended to test voltage control laws now under development. The test structure includes the auxiliary electrode located directly on top of the lower drive electrode, to establish an integrated capacitor in order to make the desired charge measurement. This thesis concentrates on the manufacturability of such a device. Several fabrication processes were developed specifically for the previously mentioned devices. These processes include an isotropic plasma etch for deep undercut of MEMS structures, a low stress metal deposition process, and a photoresist profile modification procedure to ensure conformal step coverage in an electron beam metal deposition system. The development of several mirror designs are discussed, leading to the successful fabrication of a MEMS micromirror with integrated charge sensing capabilities.