Tribology on the macroscale to nanoscale of microelectromechanical system materials: A review

Silicon-based microelectromechanical system (MEMS) devices are made from single-crystal silicon, polycrystalline silicon (polysilicon) films obtained by low-pressure chemical vapour deposition and certain ceramic films. 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 microscopy/friction force microscopy (AFM/FFM) and nanoindentation techniques have been used for tribological studies on a microscale to nanoscale on materials of interest. These techniques have been used to study surface roughness, friction, scratching and wear, indentation and boundary lubrication of bulk and treated silicon, polysilicon films and SiC films. Macroscale friction and wear tests have also been conducted using the ball-on-flat tribometer. Measurements of microscale and macroscale frictional forces show that friction values on both scales of all the silicon samples are about the same for 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 plasma-enhanced chemically vapour-deposited oxide-coated single-crystal silicon samples exhibit much larger scratching and wear resistance than untreated samples. Polysilicon films and undoped single-crystal silicon show similar friction and wear characteristics. The doping of polysilicon film does not significantly affect its tribological properties. Microscratching, microwear and nanoindentation, and macroscale friction and wear studies indicate that SiC films are superior 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.