Novel MEMS CSP to bridge the gap between development and manufacturing

Micro Electro Mechanical Systems (MEMS) are today commonplace in the automotive field, in the chemical, medical as well as in the IT industry and have found their way even into toys. While hybrid integration paved the way for the electronic into many of today's systems, MEMS/hybrids will continue to play a more and more dominant role in the future. Packaging innovations here are crucial to fulfill the stringent requirements for the application's demands: Especial the cost aspect may be the crucial point between commercial success and failure. E.g. for RF switches using MEMS technology, packaging alone accounts forsimilar to70%-9 0% of the total device cost /1/ The challenges associated with bringing down the costs and therewith the devices to market are justified by the tremendous market growth expected for these systems. MEMS packaging plays an important role within the function of the MEMS device. Besides the electrical interconnection, the package provides mechanical protection, media separation or coupling (e.g. pressure), signal conditioning, etc., it must enable the resulting package to be manufactured and assembled at low cost. Also, high reliability requirements even under harsh conditions (e.g. within the gearbox: continued measuring of the oil pressure in the oil sink) must be fulfilled in order to be accepted in the market. Capping and hermetic sealing of MEMS devices is a key factor in enabling them to be processed with further packaging steps like TO can, ceramic package or molded plastic. Aside from mounting a hermetic cap, the assembly of a backside structured cap wafer providing electrical routing allows to realize a chip scale package type of device protecting the internal MEMS structures and providing direct electrical interconnect structures for subsequent surface mount assembly, omitting additional packaging steps. Instead of developing for each device and sensor an adapted packaging process, it is required to provide a technology platform that enables a fast, low cost processing of protected devices. Concepts with electrical feedthroughs have been presented but lack the infrastructure for immediate use. Wirebonding as optional approach can be used to overcome the 3D wiring issues and provide a fast and mature process with a well established infrastructure to realize packaged MEMS devices with minimal dimensions. Solder balls terminate the electrical interconnects, allowing a seamless integration into a SMD compatible process. The paper presents the concept, the technical realization of this approach and the extension to a full wafer level chip scale MEMS package.