Alkyltrichlorosilane-based self-assembled monolayer films for stiction reduction in silicon micromachines

• Published in: Journal of microelectromechanical systems Vol. 7; no. 2; pp. 252 - 260
• Main Authors: Srinivasan, U., Houston, M.R., Howe, R.T., Maboudian, R.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01-06-1998; Institute of Electrical and Electronics Engineers
• Subjects: Adhesives; Applied sciences; Coatings; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Fluid surfaces and fluid-fluid interfaces; Heat treatment; Mechanical and acoustical properties; adhesion; Mechanical engineering. Machine design; Microelectromechanical systems; Micromechanical devices; Microstructure; Physics; Precision engineering, watch making; Silicon; Solid surfaces and solid-solid interfaces; Structural beams; Substrates; Surface energy (surface tension, interface tension, angle of contact, etc.); Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Thermal stability; Self assembly; Monolayer; Stiction; Precision engineering; Hydrophobic compound; Organic silane; Cantilever; Experimental study; Micromachine; Coatings; Adhesion; Organic chlorine compounds; Cantilever beam; Silicon; Miniaturization
• ISSN: 1057-7157; 1941-0158
• DOI: 10.1109/84.679393

We have investigated the potential of self-assembled monolayer (SAM) coatings for the purpose of adhesion reduction in microelectromechanical systems (MEMS). Two types of SAM coatings, derived from the precursor molecules octadecyltrichlorosilane [CH/sub 3/(CH/sub 2/)/sub 17/SiCl/sub 3/, OTS] and 1H,1H,2H,2H-perfluorodecyltrichlorosilane [CF/sub 3/(CF/sub 2/)/sub 7/(CH/sub 2/)/sub 2/SiCl/sub 3/, FDTS], were applied to polycrystalline silicon microstructures in a liquid-based process. Due to the hydrophobicity of these coatings, the water capillary forces responsible for the phenomenon known as release-related stiction are eliminated, and SAM-coated cantilever beams 2 /spl mu/m thick, 2 /spl mu/m above the substrate, and up to 2 mm in length emerge dry and free standing when removed from the final water rinse. The effects of SAM coating on adhesion encountered during device operation, termed in-use stiction, were characterized using arrays of cantilever beams of varying lengths. Structures made with a polycrystalline silicon of 3-nm rms roughness gave apparent works of adhesion of 30 and 8 /spl mu/J/m/sup 2/ for the OTS and FDTS SAM coatings, respectively, in comparison to 56 mJ/m/sup 2/ for standard oxide-coated structures. These results demonstrate that OTS coating reduces adhesion by more than three orders of magnitude over the conventional process and that the fluorinated SAM can lessen it further by four times. With regard to thermal stability, both SAM coatings can withstand heat treatment for 5 min at 450/spl deg/C in an N/sub 2/ ambient. In air, the OTS film begins to degrade at 150/spl deg/C while the fluorinated coating remains intact up to 400/spl deg/C. Therefore, both types of SAM coatings are compatible with several MEMS packaging techniques, with the FDTS monolayers exhibiting superior stiction and thermal stability properties to those derived from OTS. Furthermore, the FDTS formation does not require any chlorinated solvents such as carbon tetrachloride, which has been banned from industrial use, making the latter coating an industrially viable antistiction treatment.