Fundamental requirements on MEMS packaging and reliability

Packaging of MEMS has been identified as one of the most significant areas of research for enabling MEMS usage in product applications by among others USA's NSF. In order to make MEMS a real-life opportunity, it is vital to explore and develop an understanding of the possibilities and limitations of MEMS packaging and reliability. Today's MEMS packaging can be divided into two very different approaches, protection at wafer level and protection at package level. To protect MEMS functions at package level places the responsibility and package selection in the hands of traditional military style, robust and preferably hermetic technologies. This is a branch of packaging industry in rapid decrease and the main trend in military and other high reliability electronics is to turn to packaging with less performance requirements, so called Commercial Off The Shelf, COTS, electronics. Wafer level packaging and protection of MEMS structures at wafer fabrication opens up the possibility of using either very low cost packaging that are not hermetic, such as plastic packages, or tailored RF packaging with optimized performance from an electromagnetic standpoint. Furthermore there is a potential for significant cost savings at the packaging subcontractor if the wafer level protected chip to be packaged is kept as similar to microelectronics chips as possible. It is obvious that quite a few MEMS functions are realized without any consideration for the packaging of the product. There is one so called one-stop shop for MEMS which proudly presents suggestions using packages that have been outdated in commercial products since the mid 1980s, claiming to target the mobile phone market. Another example is the large semiconductor company which has developed relays that would be ready for market, but are delayed for about one year due to lack of appropriate packaging. The prevailing opinion that MEMS packaging is expensive perhaps holds MEMS developers back from approaching packaging at earlier stages in product development. It is vital to make sure that the proper demands of the application are made clear, especially when hermetic packaging is claimed to be needed. Maybe the correct requirement is really a dry package or a constant atmosphere.