Materials Used For MEMS

System requirements vary with application. To best succeed, one should let the requirements govern which of the many complementing MEMS materials to choose.

Silicon detail The most common material for micromachining is, just as for semiconductors, silicon. Although not always the best material, it is a general and flexible material that can be used for a large number of components. This has made silicon a natural choice for some large companies that focus on a broad range of MEMS-based products, which, in its turn, has influenced universities and financiers to focus less on alternative materials. The investment in know-how and production equipment creates an inertia that further strengthens silicon's domination.

Both single and poly-crystalline silicon are used. Crystalline silicon has anisotropic etch properties which makes it possible to create geometries with features such as vertical walls and thin membranes using wet etching. Examples of sensors that successfully use the silicon technology are pressure sensors for medical applications and accelerometers for airbag systems. Excitation is often done electrostatically, thermally or magnetically. Vibration amplitudes are often detected capacitively or via the stress-dependence of piezoresistors (silicon is not piezoelectric).

GaAs is another micromachinable semiconductor material that is of interest for niche applications. It is mainly its optical properties that attract attention.

Tuning fork wafer Nevertheless, the most successful micromachined product, the watch crystal tuning fork, is based on quartz (crystalline SiO2). Quartz's piezoelectric effect, inertness, and excellent temperature stability and aging properties makes it the unsurpassed choice for this application. Micromachined watch crystals are manufactured in six-digit quantities each hour at a production cost of less than $0.20 a piece, including package and individual laser trimming. High performance MHz resonators, accelerometers and chemical sensors are other niche applications for which micromachined quartz is used successfully. Its high-yield process steps have been developed mainly by the watch crystal industry. Only a few universities include quartz in their research programs.

Polymeric microstructures offer an interesting alternative that can lead to cost savings on the order of 100 to 1000 times for the finished parts. The structures are produced by a microreplication technique similar to that used to create the optical tracks on CD records. The masters are created using conventional micromachining and MEMS-materials. Several replicated MEMS products are available on the market today, particularly for microfluidic platforms and in consumer and low-cost disposable applications.

Additional promising materials are being explored for MEMS, such as metals and diamond. Here, we have only seen the tip of the iceberg.

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