Tribology on the Macro- to Nanoscales of MEMS Materials: A Review

Abstract

Abstract Silicon-based MEMS devices are made from single-crystal silicon, LPCVD polysilicon films and other ceramic films (Bhushan, 1998). For high temperature applications, SiC films are being developed to replace polysilicon films. Tribology in the MEMS devices requiring relative motion is of importance. Atomic force/friction force microscopy (AFM/FFM) and nanoindentation techniques (Bhushan et al., 1995; Bhushan, 1995, 1998) have been used for tribological studies on micro- to nanoscale on materials of interest. These techniques have been used to study surface roughness, friction, scratching/wear, indentation and boundary lubrication of bulk and treated silicon, polysilicon films and SiC films (Bhushan, 1996a, b; Bhushan et al., 1994, 1997a, b, 1998; Koinkar and Bhushan, 1997; Li and Bhushan, 1998; Sundararajan and Bhushan, 1998; Zhao and Bhushan, 1998). Macroscale friction and wear tests have also been conducted using the ball-on-flat tribometer. (Bhushan and Venkatesan, 1993; Gupta et al., 1993, 1994a, b; Venkatesan and Bhushan, 1993, 1994). Measurements of microscale and macroscale friction forces show that friction values on both scales of all the silicon samples are about the same among different silicon materials and higher than that of SiC. The microscale values are lower than the macroscale values as there is less ploughing contribution in the microscale measurements. Surface roughness has an effect on friction. In microscale and macroscale tests, C+-implanted, oxidized and PECVD oxide-coated single-crystal silicon samples exhibit much larger scratching and wear resistance as compared to untreated samples. Polysilicon films and undoped single-crystal silicon show similar friction and wear characteristics. Doping of polysilicon film does not affect its tribological properties. Microscratching, microwear and nanoindentation, and macroscale friction and wear studies indicate that SiC films are superior when compared to the other materials currently used in MEMS devices. Higher hardness and fracture toughness of the SiC film is believed to be responsible for its superior mechanical integrity and lower friction. Chemically grafted self-assembled monolayers and chemically-bonded liquid lubricants show promising performance for boundary lubrication in MEMS devices.