Implementation of micromirror arrays as optical binary switches and amplitude modulators

Abstract

Five types of micromirror arrays were designed and fabricated using a three-level, polysilicon, surface micromachined, microelectromechanical systems (MEMS) process. The electrostatically deflectable micromirror designs included arrays of simple cantilever beams, torsion beams, tethered (piston-style) beams, circular membranes and oval membranes. The smallest micromirror element was the simple cantilever beam, measuring 50 μm square. The largest micromirror element was the oval membrane; it possessed an active optical surface that was 320 μm by 920 μm. Each of the remaining micromirror designs have gold-coated polysilicon optical surfaces with geometries between these two limits. Electrostatically induced vertical deflections on the order of 2.75 μm were achieved. The torsion beam micromirror design exhibits both in-plane and out-of-plane deflection. The other micromirror designs only manifest in-plane deflections. The modeling phase focused on the microdynamical behavior of the torsion beam micromirror. The IntelliCAD® finite element analysis program was used to generate a plot of the micromirror’s deflection (d) versus applied direct current voltage (V). The data was least-squares fitted to the well-established V∝d3/2 relationship. A resonant frequency analysis predicted an approximate switching speed of 6 μsec. The reliability (number of operational cycles) of each micromirror design, when operated with a rectified 60 Hz alternating current (ac) signal, was measured to exceed more than 1 million flexure events. Experimental evidence supporting the potential for using micromirrors as binary optical switches and amplitude modulators was also addressed.