Developing a New Material for MEMS: Amorphous Diamond
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Developing a New Material for MEMS: Amorphous Diamond J. P. Sullivan, T. A. Friedmann, M. P. de Boer, D. A. LaVan*, R. J. Hohlfelder, C. I. H. Ashby, M. T. Dugger, M. Mitchell, R. G. Dunn, and A. J. Magerkurth† Sandia National Laboratories, Albuquerque, NM 87185, U.S.A. *Langer Research Lab, Massachusetts Inst. of Tech., Cambridge, MA 02139, U.S.A. † Dept. of Physics, Cornell Univ., Ithaca, NY 14853, U.S.A. ABSTRACT Amorphous diamond is a new material for surface-micromachined microelectromechanical systems (MEMS) that offers promise for reducing wear and stiction of MEMS components. The material is an amorphous mixture of 4-fold and 3-fold coordinated carbon with mechanical properties close to that of crystalline diamond. A unique form of structural relaxation permits the residual stress in the material to be reduced from an as-deposited value of 8 GPa compressive down to zero stress or even to slightly tensile values. Irreversible plastic deformation, achieved by heat treating elastically strained structures, is also possible in this material. Several types of amorphous diamond MEMS devices have been fabricated, including electrostatically-actuated comb drives, micro-tensile test structures, and cantilever beams. Measurements using these structures indicate the material has an elastic modulus close to 800 GPa, fracture toughness of 8 MPa·m1/2, an advancing H2O contact angle of 84° to 94°, and a surface roughness of 0.1 to 0.9 nm R.M.S. on Si and SiO2, respectively.
INTRODUCTION In the quest to improve the mechanical performance and reliability of MEMS, diamond and hard amorphous carbon have recently emerged as one promising class of materials. Diamond has the highest hardness (~ 100 GPa) and elastic modulus (~ 1100 GPa) of all materials. Amorphous forms of carbon, specifically the hard carbons, amorphous diamond (aD), tetrahedral amorphous carbon (ta-C), and diamond-like carbon (DLC), can also 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 with their extreme wear resistance (up to 10,000 times greater wear resistance than Si)[1], their hydrophobic surfaces that offer inherent stiction resistance (parts don’t stick together due to capillary forces from entrapped water)[2], and their chemical inertness (allows use in aggressive chemical environments). Crystalline diamond also possesses the highest thermal conductivity and the largest range of optical transparency (from far IR to UV) of any material, making it useful for thermal heat sink structures or micro-optics[3]. Recently, researchers have made considerable progress in the fabrication of MEMS structures fabricated from polycrystalline and nanocrystalline diamond, both in the area of surface micromachining and in mold-based processes[3-13]. A variety of diamond microstructures have been demonstrated, including substrates with integrated channels for active-cooling[3], micromachined fresnel optics[3], optical fiber alignment structures[4], free-standing cap
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