ABSTRACT
Silicon is one of the most commonly used micromachining materials not only because of its excellent electronic and mechanical properties, the incredible degree of control one can exert over doping this semiconductor material, and the ease of insulating its surface with SiO2 but also because it has been the dominant material in the integrated circuit (IC) industry ever since the early 1950s. However, silicon is not perfect for all applications, nor can it function on its own: the IC arsenal features a small group of additional materials, such as the metals Al, Cu, W, B, and P and the insulators SiO2 and Si3N4. There are plenty of new demands that are challenging the continued dominance of silicon and will require the introduction of new materials that work in conjunction with silicon or necessitate the switch to alternative semiconductors altogether: higher speeds; better heat dissipation; merging of sensors and actuators with active electronics, optoelectronics, and quantum devices; lower power consumption; more environmentally friendly manufacturing processes, etc. In microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS), and in some application-specific ICs, alternative semiconductor materials already are being used, including gallium arsenide (GaAs), AlxGa1-xAs compounds, SiGe, CdSe, SiC, TiO2, ZnO, and carbon nanotubes. A wide variety of polymers, ceramics and metals, foreign to the standard IC process also have been adopted, especially in bio-MEMS. In this chapter, we review some selected
materials and processes that have affected and will continue to affect the direction MEMS and NEMS take on next. The chapter is headed by a description of possible substrate materials for MEMS and NEMS: quartz, GaAs, diamond, SiC, ceramics (with emphasis on Al2O3), polyimides, silicon nitride, and amorphous and hydrogenated amorphous silicon. For exciting new MEMS/NEMS techniques, we survey carbon-based MEMS (C-MEMS), bulk metallic glasses (BMGs), fast microfluidic compact disc (CD) prototyping, and micro glass blowing. Finally, we dedicate a section to biocompatibility and the various chemistries for surface derivation of solids to make them biocompatible. Studies of in vivo MEMS devices are regaining popularity, making a deeper understanding of biocompatibility a necessity.
