Tribological Coatings for LIGA MEMS

Many techniques are suitable for depositing tribological coatings on conventional machine elements, but the miniature nature of micro-electromechanical system (MEMS) elements presents a tough challenge to the coatings community. Handling of individual parts is impractical, and applying coatings by such techniques as resin bonding and burnishing is not possible. Physical vapor deposition is a line-of-sight process, making it difficult to deposit material on the sidewalk where it is needed. Chemical vapor deposition of tribological coatings such as diamond, though conformal in nature, requires a high-temperature process that could potentially alter the microstructure of the base nickel alloy. To circumvent these problems, a novel coating strategy has recently been developed at Sandia. Coatings are applied by the plasma-enhanced chemical vapor deposition (PECVD) technique, in which ionized species are deposited onto biased substrates, thereby limiting the substrate temperatures to below 150 deg C (300 deg F). Furthermore, instead of coating the individual parts, the entire wafer is coated before releasing the parts, but after electroplating, planarizing, and dissolving the mold material. LIGA is a microfabrication process in which individual mechanical elements are created by deposition of material into lithographically formed molds, followed by assembling the micromachine elements into a micro-electromechanical device.