Materials Aspects of Micro- and Nanoelectromechanical Systems
Two of the more significant technological achievements during the last 20 years have been the development of MEMS and its new offshoot, NEMS. These developments were made possible in large measure by significant advancements in the materials and processes
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Materials Asp Two of the more significant technological achievements during the last 20 years have been the development of MEMS and its new offshoot, NEMS. These developments were made possible in large measure by significant advancements in the materials and processes used in the fabrication of MEMS and NEMS devices. And while initial developments capitalized on a mature Si infrastructure, recent advances have used materials and processes not associated with IC fabrication, a trend that is likely to continue as new application areas are identified. A well-rounded understanding of MEMS and NEMS requires a basic knowledge of the materials used to construct the devices, since material properties often govern device performance. An understanding of the materials used in MEMS and NEMS is really an understanding of material systems. Devices are rarely constructed of a single material, but rather a collection of materials, each providing a critical function and often working in conjunction with each other. It is from this perspective that the following chapter is constructed. A preview of the materials selected for inclusion is presented in Table 7.1. From this table it is easy to see that this chapter is not a summary of all materials used in MEMS and NEMS, as such a work would itself constitute a text of significant size. It does,
7.1
Silicon ................................................. 7.1.1 Single Crystal Silicon ..................... 7.1.2 Polysilicon................................... 7.1.3 Porous Silicon .............................. 7.1.4 Silicon Dioxide ............................. 7.1.5 Silicon Nitride ..............................
203 204 205 208 208 209
7.2
Germanium-Based Materials ................. 210 7.2.1 Polycrystalline Ge ......................... 210 7.2.2 Polycrystalline SiGe ...................... 210
7.3
Metals ................................................. 211
7.4
Harsh Environment Semiconductors ....... 212 7.4.1 Silicon Carbide ............................. 212 7.4.2 Diamond ..................................... 215
7.5
GaAs, InP, and Related III-V Materials .... 217
7.6
Ferroelectric Materials........................... 218
7.7
Polymer Materials................................. 7.7.1 Polyimide.................................... 7.7.2 SU-8........................................... 7.7.3 Parylene .....................................
7.8
Future Trends....................................... 220
219 219 220 220
References .................................................. 221
however, present a selection of some of the more important material systems, including examples that illustrate the importance of viewing MEMS and NEMS in terms of material systems.
Part A 7
7.1 Silicon Use of silicon (Si) as a material for microfabricated sensors dates back to the 1950s when C. S. Smith published a paper describing the piezoresistive effect in germanium (Ge) and Si [7.1]. Smith found that the piezoresistive coefficients of Si were significantly higher than those associated w
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