In the rapid telecom growth period of the late 1990's and into the 2000's, optical MEMS devices gained considerable attention in the telecommunications industry, particularly in the optical networking and switching arenas. Since optical MEMS are micro-systems, which rely on high precision optics, electronics, and mechanics working in close concert, these emerging devices pose some unique packaging challenges yet to be addressed by the general packaging industry.
Optical MEMS packaging differs from traditional semiconductor/ microelectronics packaging in several ways. Optical MEMS packages often are required to provide optical and electrical access, hermeticity, mechanical strength, dimensional stability and long-term reliability. Hermetic optical access necessitates the use of metalized or glass sealed and anti-reflection coated windows. In addition, the ever-increasing electrical I/O count has prompted the use of higher density substrate/package technologies . The reason for hermetic packaging is to ensure the long-term reliability of the expensive, state-of-the-art optical systems. Traditional FR-4 laminate-based technology is eliminated and packaging choices are restricted to mostly ceramic-based technologies such as high temperature co-fired ceramic (HTCC), low temperature co-fired ceramic (LTCC), and thin-film ceramic technologies. Since frames and windows are first sealed together, their mechanical properties, especially their coefficient of thermal expansion (CTE), have to be well matched to those of mating package materials. These requirements limit the material choices; frames are often made of KovarTM, and windows are often made of sapphire. Some windows are wedge-shaped to avoid Fabry-Perot effects. In general, optical MEMS packages are mounted to metal housings to maintain critical optical alignments. They have to be rigid and dimensionally stable over all operating temperatures to have optimized and repeatable optical performance. Any misalignments between the optics and the MEMS devices can easily translate into losses that may result in out of tolerance conditions that degrade system performance.
Date: 2015-02-28; view: 659