comparable levels of manufacturability, performance, cost, and reliability to those of modern VLSI circuits.

Recommendation. Efforts to stimulate solutions to the challenges of producing MEMS should capitalize on the families of relatively well understood and well documented IC materials and processes. These solutions may be found in current IC practices but may also result from creatively re-establishing older IC technologies. This recommendation calls for continuing strategic investment.

ENLARGING THE SUITE OF MATERIALS SUITABLE FOR INTEGRATED-CIRCUIT-LIKE PROCESSING

Although there may be commercial advantages to leveraging the present suite of IC-process materials, they will not be able to meet all of the demands that a growing number of users and applications will place on MEMS. Easily foreseen requirements (e.g., higher forces, stability in harsh and high-temperature environments, and robust high-aspect-ratio structures) will compel the application of new materials and extend the MEMS field beyond the boundaries of the IC world.

Materials that are not usually used in IC processes include magnetic, piezoelectric, ferroelectric, and shape-memory materials. Actuating-force requirements for valve closures and motor drives, for example, are already drawing attention to the advantages these materials would bring to MEMS. Other developments, such as MEMS for optics, biological purposes, chemical-process controls, high-temperature applications, and other hostile environments, will inevitably draw attention to the need for an even broader range of materials.

In the IC world, new materials are typically incorporated as thin-films and are produced by a limited number of techniques (e.g., low-pressure chemical-vapor deposition or sputtering). Many of these materials either do not show optimal mechanical properties in thin-film form or are difficult to deposit by typical IC-fabrication methods or are incompatible with the microelectronic IC process. For some MEMS designs, it is possible to apply these specialized materials either by incorporating them in a step prior to more-conventional processing or by adding them as a final step. Either option raises the possibility that the technology will be substantially different from better known processing techniques. Materials that are incompatible with the IC-processes might have to be handled by a specialized foundry.

Conclusion. Extending the list of materials that have useful MEMS properties and can be processed using lithography-based, IC-compatible techniques will be beneficial to MEMS development.

Recommendation. Research and development should be encouraged to develop new materials that extend the capabilities of MEMS. The new materials should be integrable, at some level, with conventional IC-based processing. This recommendation calls for continuing strategic investment.

Recommendation. Research should be encouraged to develop techniques to produce repeatable, high-quality, batch processed thin-films of specialized materials and to determine the dependence of their properties on film-preparation techniques. For some materials, it may be advisable to establish ''foundries" that are available to the entire MEMS community and can serve as repositories for equipment and know-how. This recommendation calls for new strategic investment.

CHARACTERIZING MEMS MATERIALS

The IC industry has been built on an extensive, constantly expanding body of knowledge about the behavior of silicon and related materials as they are scaled down in size. No comparable resource has been established for MEMS, however. For example, although a great deal is known about the electrical properties of polysilicon thin-films, not much is known about their micromechanical properties or about specific details of the long-term reliability of mechanically stressed polysilicon or the surface mechanics related to friction, wear, and stress-related failure. There is a similar lack of fundamental knowledge about other thin-film materials borrowed from the electrical domain that are now exercised mechanically (e.g., silicon nitride, silicon dioxide, and thin-film metals). Many thin-film materials that are used in the IC industry (e.g., aluminum, silicon dioxide, amorphous silicon, porous silicon, various other deposited and plated metals, and polyimide) have still not been extensively studied and evaluated for their applicability to MEMS.

Conclusion. A thorough understanding of the micromechanical properties of the materials to be used in MEMS at appropriate scales is not available.

Recommendation. The characterization and testing of MEMS materials should be an area of major emphasis. Studies that address fundamental mechanical properties (e.g., Young's modulus, fatigue strength, residual stress, internal friction) and the engineering physics of long-term reliability, friction, and wear are vitally needed. It is important that these studies take into account fabrication processes, scaling, temperature, operational environment (i.e., vacuum, gaseous, or liquid), and size dependencies. Studies of the size effects of physical elements, on a scale comparable to the crystallite regions in a polycrystalline material, are required. This recommendation calls for continuing strategic investment.



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