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be introduced as thin-films and could be used with processes and equipment familiar to the IC world (e.g., low-pressure chemical-vapor deposition [LPCVD] or, less favorably, sputtering). Similarly, CVD processes in standard CVD equipment could be used with temperature and flow changes to make familiar materials with new properties. Low-stress silicon nitride is a material that could fall into this classification. It is generally deposited in the same LPCVD tubes that historically have produced stoichiometric silicon nitride but with significantly different gas flows and pressures. Efforts are also under way to incorporate materials with useful properties for sensing and actuation, such as ferroelectrics, piezoelectrics, and magnetic films, into MEMS processes (see Chapter 3).
The selective deposition of materials on patterned substrates is common in ICs and will increase as new materials are introduced. The selective deposition techniques for silicon and metals (e.g., tungsten) used in IC processes could find their way into MEMS processing over time. The ways, means, and materials suitable for this whole family of techniques require significantly more fundamental research, however.
MEMS with New Materials and New Tools
The combination of new materials and new tools presents formidable challenges, and progress will probably be slowest in this category. This should not, however, rule out the consideration of this class of MEMS research, but the benefits should be compelling (see Chapter 3). The "newness" of either materials or tools can vary considerably because some materials and tools previously used for special purposes may provide sufficient basic knowledge for them to be transferred easily to the MEMS area. For example, electroplated magnetic materials and processes are familiar from their use in the magnetic memory storage area. If the manufacturing issues specific to micromechanical materials can be successfully addressed, these materials and tool sets may move from being the most difficult to the least difficult to incorporate. Nevertheless, the application of electroplating will require improved facilities and extensive characterization before the full potential of this technique can be realized.
The enthusiasm for and promise of MEMS has, to a large extent, arisen from the demonstrated ability to produce three-dimensional fixed or moving mechanical structures using lithography-based processing techniques derived from the established IC field. Conventional IC materials can be used innovatively in MEMS, and much of the needed MEMS-specific hardware can still be leveraged from IC-technology. These MEMS developments are most likely to be accepted in traditional IC fabrication facilities and are, therefore, most likely to succeed commercially.
There are many opportunities for creative work in MEMS based on what is already known about IC processing, particularly in re-evaluating the range of knowledge compiled during the history of IC development. MEMS products that rely on conventional IC tools, materials, processes, and fabrication techniques have the highest probability of achieving the same manufacturability, performance, low cost, and high reliability as in the production of modern VLSI circuits.
At the heart of MEMS development is the ability to construct extremely small mechanical devices, preferably using batch processing. Wet etching has historically dominated the MEMS field because (1) three-dimensional structures can be micromachined from substrate silicon and (2) chemical-etch equipment is well established, simple, and inexpensive. The disadvantages of wet-chemical processing are its inability to achieve vertical-sidewalls and non-orthogonal linear geometries in <100> silicon and its reaction with films on the wafer surface. Although dry etching is a mainstay of IC processing and gas-phase "dry" etching techniques are currently being investigated for MEMS production, the film thicknesses or substrate-etch depths for MEMS are often significantly greater than for IC fabrication. Therefore, MEMS etching will typically present additional challenges. If only IC-based techniques are used, it will limit the number of applications that can be pursued. As will be seen in the next chapter, flexibility may open broad new areas for MEMS, although problems with manufacturability and reliability should be anticipated in the early stages.
Conclusion. The expertise and advanced state of the current microelectronics industry provides an enormous advantage for the development of MEMS. Leveraging and extending existing IC tools, materials, processes, and fabrication techniques are excellent strategies for producing MEMS with comparable levels of manufacturability, performance, cost, and reliability to those of modern VLSI circuits. Because controlled etching is so important to the fabrication of three-dimensional structures and the progress of MEMS, improving etching methods, including those that tailor isotropic or anisotropic etch-rates of various materials, will be important.
Recommendation. Efforts to identify solutions to the challenges of producing MEMS should capitalize on relatively well understood and well documented IC materials and processes. Solutions may be found in current IC practices but may also result from creatively re-establishing older IC technologies.
Recommendation. Further research and development should be undertaken to improve etches, etching, and etching controls for MEMS. This work should take into account the realities and limitations of manufacturing process equipment.