Two- and Three-Dimensional Ultrananocrystalline Diamond (UNCD) Structures for a High Resolution Diamond-Based MEMS Techn
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73 Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
class 2. Only high temperature alloys or ceramics (e.g., Si 3N 4 and SiC) that develop protective, chemically stable oxygen diffusion barriers (oxide layers) may be reasonable candidates for class 1 materials 6,7. Class 2 materials include those suitable for MEMS components that are exposed to tribo-thermo-oxidative degradation. Conventional MEMS materials such as Si and metals (e.g. Ni) used in LIGA processes are especially vulnerable to this degradation mode. Class 2 tribomaterials must exhibit high hardness and fracture toughness at relatively high temperatures, and since it is not possible to use separate liquid lubricants on the MEMS scale, the sliding/rolling interfaces must somehow be rendered self-lubricating. Diamond exhibits self-lubrication as a result of either hydrogen or oxygen termination layers, depending on the temperature and gas environment, and it appears to perform better than SiC in low oxygen partial pressure (up to several Torr) environments' at temperatures up to -900 'C. In addition, diamond is a superhard material of high mechanical strength, low friction coefficient, and outstanding thermal stability. The projected wear life of diamond MEMS components is 10,000 times greater than that of Si9 . In spite of all the desirable properties of diamond, being the hardest-known material, it is notoriously difficult to fabricate in bulk form. Therefore, various groups are exploring the use of diamond coatings to produce Si-MEMS devices capable of performing useful work. A large number of MEMS structures have been reported' 1°7, using conventional CH 4/(98-99%)H 2 plasma chemistry18 to grow diamond films by chemical vapor deposition (CVD) on Si and Si0 2 patterned structures. The main limitations of the conventional CVD diamond coatings are roughness (> 0.5 Vm) and large grain size (> 1 pm), which limits the feature resolution. Most problems related to the integration of diamond with MEMS can be overcome using a new synthesis technique developed at Argonne National Laboratory (ANL) that produces phase-pure ultrananocrystalline diamond (UNCD) with morphological, tribological and mechanical properties that are ideally suited for MEMS applications. UNCD films are distinguished from other micro and nanocrystalline diamond and diamond-like carbon films by their grain size, sp 3 / sp 2 bond content and surface roughness (see Table 1). We demonstrate here that the UNCD coatings can be used in the development of a new diamond-based MEMS technology. Table I: Distinguishing Characteristics of Diamond and Diamond-Like Carbon Films MicroNano-crystalline UltrananoDiamond-Like Carbon crystalline diamond crystalline (DLC) diamond (NCD) diamond ta-C ta-H:C (MCD) (UNCD) Growth 3 3 C7 C Species (HO) (H°) Crystallinity Columnar Mixed diamond Equiaxed Mixed diamond Amorphous (100) or (111) & non-diamond diamond amorphous Diamond Phases phases NA variable 2-5 nm 50-100 nm -0.5-10 prm Grain Size Surface Roughness
400 nm-1 ptm
Electronic Bonding
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