Electrostatic MEMS Actuators using Gray-scale Technology

Abstract

The majority of fabrication techniques used in micro-electro-mechanical systems (MEMS) are planar technologies, which severely limits the structures available during device design. In contrast, the emerging gray-scale technology is an attractive option for batch fabricating 3-D structures in silicon using a single lithography and etching step. While gray-scale technology is extremely versatile, limited research has been done regarding the integration of this technology with other MEMS processes and devices.

This work begins with the development of a fundamental empirical model for predicting and designing complex 3-D photoresist structures using a pixilated gray-scale technique. A characterization of the subsequent transfer of such 3-D structures into silicon using deep reactive ion etching (DRIE) is also provided. Two advanced gray-scale techniques are then introduced: First, a double exposure technique was developed to exponentially increase the number of available gray-levels; improving the vertical resolution in photoresist. Second, a design method dubbed compensated aspect ratio dependent etching (CARDE) was created to anticipate feature dependent etch rates observed during gray-scale pattern transfer using deep reactive ion etching (DRIE).

The developed gray-scale techniques were used to integrate variable-height components into the actuation mechanism of electrostatic MEMS devices for the first time. In static comb-drives, devices with 3-D comb-fingers were able to demonstrate >34% improvement in displacement resolution by tailoring their force-engagement characteristics. Lower driving voltages were achieved by reducing suspension heights to decrease spring constants (from 7.7N/m to 2.3N/m) without effecting comb-drive force. Variable-height comb-fingers also enabled the development of compact, voltage-controlled electrostatic springs for tuning MEMS resonators. Devices in the low-kHz range demonstrated resonant frequency tuning >17.1% and electrostatic spring constants up to 1.19 N/m (@70V).

This experience of integrating 3-D structures within electrostatic actuators culminated in the development of a novel 2-axis optical fiber alignment system using 3-D actuators. Coupled in-plane motion of electrostatic actuators with integrated 3-D wedges was used to deflect an optical fiber both horizontally and vertically. Devices demonstrated switching speeds <1ms, actuation ranges >35μm (in both directions), and alignment resolution <1.25μm. Auto-alignment to fixed indium-phosphide waveguides with <1.6μm resolution in <10 seconds was achieved by optimizing search algorithms.