Design for reliability of drift-free MEMS micromirrors

The rapid commercialization and long-term reliability of optical MEMS is greatly facilitated by a Design-for-Reliability mindset, relying on an interdependent development framework simultaneously optimizing design, materials choices, processing, reliability, subsystem design, and packaging. Even with the best mechanical design, the electrical design and packaging choices of these devices has a large impact both on performance (e.g., speed and stability) and on reliability (e.g., corrosion and dielectric or gas breakdown). In this paper we discuss the reliability and performance of two-axis MEMS micromirrors and present several design, processing and packaging steps that were needed to achieve open-loop drift-free operation and mean-time-to-failure in excess of 2000 years. In particular the relationship between leakage currents and the accumulation of quasi-static charge in dielectrics are discussed, along with several techniques to mitigate charging and the associated drift in electrostatically actuated or sensed MEMS devices. Two key parameters are shown to be the electrode geometry and the conductivity of the dielectric. Electrical breakdown in sub-micron gaps is presented as a function of packaging gas and electrode spacing. We discuss the trade-offs involved in choosing gap geometries, dielectric properties, and packaging solutions. Finally galvanic corrosion of poly-silicon in HF release etch baths is discussed along with techniques to minimize this corrosion.