ICMCTF2007 Session F3/E1: Nanotribology Instrumentation and Diagnostics

Microelectromechanical system (MEMS) surfaces that have impacting and sliding contacts frequently necessitate modification by coatings to improve the tribological or electrical performance. The principal challenges associated with surface modification for microsystems are 1) limited ability to insert surface modification steps during manufacturing, requiring post-fabrication surface treatment; 2) small gaps between structures (<1 micron) that limit coating thickness and tolerance for variation; and 3) contacting interfaces that require modification may be deeply buried in complex structures. Although contacting surfaces of polycrystalline silicon structures have arithmetic roughness of less than 50 nm, the contact forces applied (typically below 1 mN) prescribe that real contact occurs at only a few asperities. Therefore, relating composition and structure of the contacting surfaces to tribological behavior is quite challenging due to the limited size and number of real contact spots. MEMS ohmic switches present similar challenges in terms of coating application and minimization of adhesion and friction, with the added requirement that surface modification must also enable low-loss electrical performance.

A variety of surface treatment approaches have been investigated for microsystems, including chemisorbed monolayers, atomic layer deposited films, and physical vapor deposited materials. An understanding of the mechanical and chemical degradation mechanisms of tribological coatings is of primary importance for the reliable operation of microsystems. Micromachined diagnostic structures such as tribometers and electrical contact switches have been employed to investigate changes in adhesive and friction forces due to coating degradation. Relationships between tribological behavior, processing, and degradation of surface treatments for dynamic MEMS interfaces will be discussed.

While nanotribological simulations are generally performed for opposing parallel surfaces, the AFM experiments to which they are often compared measurethe interactions between a curved probe tip and a sample. These simulations cannot replicate effects seen in experiments, including load-dependent contact areas and transfer of material from the substrate to the tip. We present the results of true dynamical simulations of alkylsilane self-assembled monolayers (SAMs) with realistic tip/substrate geometries. The adhesion and friction of tips matching experimental dimensions in contact with SAM-coated amorphous silica substrates were studied with massively parallel molecular dynamics simulations. We discuss the contact mechanics of the tip/substrate geometry and compare to previous simulations and AFM measurments. The effects of SAM morphology will also be discussed.

Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.