Diamond and Amorphous Carbon MEMS
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Diamond and
Amorphous Carbon MEMS
J.P. Sullivan, T.A. Friedmann, and K. Hjort Introduction The designer of microelectromechanical systems (MEMS) can increase MEMS performance either by improved mechanical design or by the selection of a MEMS material with improved mechanical performance. In the quest to identify highperformance MEMS materials, diamond and amorphous carbon have recently emerged as a promising class of materials. These materials offer excellent tribological properties, low-stiction (hydrophobic) surfaces, chemical inertness, and high elastic moduli. The primary challenge with these materials lies not in improving the materials’ performance, but rather in integrating their relatively new deposition processes with the well-established processes of the silicon microelectronics industry. Diamond has the highest hardness (100 GPa) and elastic modulus (1100 GPa) of all materials (see Figure 1). Amorphous forms of carbon, specifically the hard carbons, amorphous diamond (a-D), tetrahedral amorphous carbon (ta-C), and diamond-like carbon (DLC), can also
Figure 1. Hardness and elastic modulus of a variety of hard materials.
MRS BULLETIN/APRIL 2001
approach crystalline diamond in hardness (up to 90 GPa) and modulus (800 GPa). The main appeal of these materials for the MEMS designer, however, lies in their extreme wear resistance (up to 10,000 times greater wear resistance than Si),1 their hydrophobic surfaces with inherent stiction resistance (parts do not stick together due to capillary forces from entrapped water),2 and their chemical inertness (which allows their use in aggressive chemical environments). Recently, researchers have made considerable progress in the fabrication of MEMS structures from these materials, both in the area of surface micromachining and in mold-based processes.3–10
Materials Requirements for Surface-Micromachined MEMS and Amorphous Diamond MEMS On the most highly disordered (i.e., amorphous) end of the range of carbon materials (diamond being the most highly ordered), one new class of carbon MEMS materials is a stress-relieved form of hard amorphous carbon, colloquially referred to as stress-free amorphous diamond.3 Technically, the material is best described as a mixture of amorphous nanophases of tetrahedrally coordinated carbon, comprising about 70% of the total, with threefold-coordinated carbon comprising the remaining 30%. The threefold-coordinated carbon is not randomly distributed but is instead clustered as conjugated chain-like or, perhaps, sheetlike structures.11 The material is deposited at room temperature using an energetic pure-carbon beam that contains a significant fraction of carbon ions with energies peaked near 100 eV.12 Because of subsurface implantation of the energetic carbon species, these films typically exhibit extremely high levels of compressive stress (8 GPa); surprisingly, with suitable control of film deposition, 100% stress relief (down to 0 10 MPa) can be achieved. Such dra-
matic stress relief is accomplished by thermal annealing for a
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