Failure Analysis of Polysilicon Micromirror Arrays

Abstract Surface micromachined micromirror technologies are being employed for various commercial and government applications. One application of micromirror technologies in the commercial sector can be found in Digital Light Projection (DLP™) systems used for theater and home entertainment centers. DLP™ systems developed by Texas Instruments uses DMD™ technology (Digital Mirror Device), an array of micromirrors, to project light onto a screen [1]. This technology is also used by Infocus™ projection systems and widescreen tabletop televisions [2]. Here, the micromirrors act as individual pixels, reflecting light onto the screen with high ¡§digital¡¨ resolution. The most recent application of surface micromachined micromirror technology is optical switching [3], which uses micromirrors to switch optical signals from fiber to fiber for lightwave telecommunications [4]. Companies such as Lucent have fabricated entire optical micromirror switching systems based on their Microstar™ technology [5]. For government applications, surface micromachined micromirror arrays have been developed for potential use in a spectrometer system planned for NASA's Next Generation Space Telescope (NGST) [6]. Various processing technologies are used to fabricate surface micromachined micromirrors. The micromirror arrays developed by TI and Lucent [1,4] uses metal for their structural and reflective components. Micromirrors fabricated at Sandia National Laboratories use the SUMMiT™ (Sandia's Ultra-planar MEMS Multi-level Technology) process with metal deposited on the surface of mechanical polysilicon components to reflect light. Optical micromirror arrays designed and fabricated at Sandia for potential use in the NGST have undergone reliability testing and failure analysis. This paper will discuss the failure modes found in these micromirrors after reliability testing. Suggestions and corrective actions for improvements in device performance will also be discussed.