Clearly the achievement of many of the characteristics planned for MEMS-based systems will depend primarily on the physical performance of internal elements. However, especially with this new class of microbased subsystems, additional problems related to packaging, assembly, and testing (PAT) must be overcome before success is achieved with commercial products. In fact, packaging, assembly, and testing issues now loom as substantial barriers to the practical integration, and subsequent commercialization, of a large body of developments being generated in MEMS-related R&D and business communities.
Packaging of MEMS devices is more complicated than in the case of electronic or mechanical ancestors of microdevices. Since new sensors and actuators must physically interact with the environment, packages require pass-throughs for transmission of fields, photons, fluids, moving shafts, chemicals, and so forth. The integration of subsystems and components into complete products will require the development of new housings, conduits, connectors, seals, manufacturing approaches, structures, and central controllers.
It is probably true that PAT processes will dominate the economics of final products as well as substantially determine factors related to ruggedness, reliability, and maintainability. Fortunately, many new processes (or old processes modified for use in smaller applications) such as those listed in Table 6.1 are being developed and will be available for use in PAT. Also, such centralized fabrication facilities as MCNC are emerging, and express interest not only in fundamental fabrication processes, but also in providing packaging services.
Acquiring information about PAT activities during the JTEC trip was a difficult task for a number of reasons. (It should be mentioned here that obtaining similar information in the United States or Europe would have been equally difficult.) First, PAT methods are not long-term developments, but relate to the important three-year window that renders them critical to the commercial performance of a product in the marketplace. Since patents in the PAT area are less fundamental, their legal status is less secure, and secrecy is the mechanism for maintaining competitive advantage. Second, PAT is application specific. MEMS devices include small, embedded, interconnected, fragile elements, the assembly and evaluation of which is unique to the specific device being produced. Therefore, it is difficult to discuss MEMS-PAT in a brief fashion based on fundamental first principles. Third, with respect to emerging MEMS-based systems, PAT technology is so new that in many cases there is nothing to discuss because the technology has simply not developed yet. Fourth, since PAT processes are evolving rapidly, the language and structure necessary to discuss issues completely is just emerging.
In the following sections, PAT methods will be discussed in terms of four systems levels and in terms of levels of packaging that range from closed-rigid to open-exposed.