Investigation of the Deposition and Integration of Hard Coatings for Moving MEMS Applications
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While coating planar silicon substrates is easily possible, coating MEMS devices is a more complex problem and there are a number of relevant concerns. A major aspect is that MEMS devices upon release are fully assembled. Hence, in order to coat components of fully fabricated devices that are non-line-of-sight, one has to face the complications of patterning and structuring the film, or using the alternative method of selective area deposition. In addition, conventional deposition techniques are not well suited to coat surfaces that are concealed by apertures of less than 1 ýtm due to highly undesirable problems that can arise, such as sealing or shadowing. In this paper, we investigate two strategies, both of which rely on the use of a hard coating for protecting the Si surfaces from wear. The parallel approaches adopted by us aim at answering two equally important questions, (a) can we successfully coat 3D-microstructures with good sidewall coverage? and (b) how can we address non-line-of-sight contacting surfaces with very small gaps? The hard coating material that we employ as the basis for our investigations is titanium carbide (TiC). TiC has been used as a wear-resistant coating on ball bearings in spacecraft missions [13,14], and has drawn widespread interest due to its potential applications as a hard coating. We have previously reported on the room temperature pulsed laser deposition (PLD) of nanocrystalline TiC coatings on ball bearing steels with excellent coating properties [15-18]. Our present choice of a TiC coating for application to Si MEMS is additionally motivated by the fact that TiC is not soluble in hydrofluoric acid (HF), the most aggressive chemical encountered in Si MEMS processing [19]. Our first approach was to fabricate simple 3D-Si microstructures that could be coated with thin films of TiC and also investigate some basic film properties. We selected microstructures that were well suited for sliding wear-tests and could be easily fabricated using anisotropic etching of Si. The resulting structures had sidewalls angled at 54.70 rather than 900. However, due to the practical limitations of coating MEMS structures with conventional deposition techniques, we have also addressed the more desirable solution, namely, the integration of wearresistant coatings into currently employed MEMS processing sequences. Hence our second approach was to design sandwich structures on Si substrates to investigate the possibility of introducing the TiC coating on either side of the sacrificial layer used in the MEMS fabrication. Dissolution of the sacrificial layer in such structures would potentially leave the wear-resistant TiC layers as the layers in direct sliding contact, rather than Si layers. Using this scheme we have addressed the specific question of whether the TiC coating is compatible with, and resilient to, the entire sequence of standard processing steps used for subsequent MEMS fabrication. EXPERIMENT The modified PLD system used for these experiments has been described in detail elsewhere [16-1
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