Recent Developments in Polymer MEMS

The microelectromechanical systems (MEMS) area has been developed extensively during the past three decades. In the 1970s, with the advancement of semiconductor microelectronics processing, researchers investigated wet anisotropic chemical etching processes for forming three dimensional silicon geometries. In the 1980s, researchers adopted the Metal-Oxide-Semiconductor (MOS) process to realize polycrystalline silicon micromachining with silicon dioxide as a sacrificial layer material. This process leads to so-called surface micromachined devices, including electrostatically actuated motors and interdigitated finger capacitive sensors and resonators. In the 1990 s, many commercially successful MEMS products began to appear in the market, including digital light processor (DLP) from Texas Instruments, ink jet printer nozzles from Hewlett Packard, and solid-state accelerometers for air bag deployment from Analog Devices and Motorola (now Freescale). Government funding for MEMS research in the US and in many other parts of the world leads to an explosive growth of MEMS processes and devices. Many substantially large sub-domains of MEMS emerged, including optical MEMS (e.g., mirrors, attenuators, phase shifters), RF MEMS (e.g., switches, resonators, capacitors, inductors), bioMEMS (e.g., microfluidics, medical devices, prosthesis implants), nanoelectromechanical systems—NEMS (e.g., resonators and biosensors), MEMS for harsh environment, and power MEMS (actuators, power generators, and micro chemical systems). The MEMS field evolved from the semiconductor industry and is strongly tied to it in many ways. The predominant substrate material is silicon. The predominant surface thin film is polycrystalline silicon and silicon dioxide, widely used as the gate and insulation material in the MOS process. Often, MEMS devices are made in cleanrooms, similar to microelectronics devices. Photolithography is used to define both transistors and MEMS devices. The MEMS field also contributes new knowledge and practice to the general area of microfabrication, stemming from its unique needs, such as the needs for functional materials (including piezoelectric and piezoresistive materials) and for three-dimensional features (such as cavities, throughwafer holes, flaps, and suspended beams and membranes). The MEMS field provided incentives for the conceptualization and sustained development of such processes as deep reactive ion etching, photodefinable SU-8 epoxy, and self-assembly. Over the years, the MEMS field has steadily expanded its material base to include compound semiconductors, diamond, and ceramics. R EV EW