Microtribological studies of doped single-crystal silicon and polysilicon films for MEMS devices

Abstract

Microelectromechanical systems (MEMS) devices are made of doped single-crystal silicon, LPCVD polysilicon films and other ceramic films. Very little is understood about tribology and mechanical characterization of these materials on micro- to nanoscales. Atomic force microscopy/friction force microscopy (AFM/FFM) have been used to study surface roughness, friction, scratching/wear and indentation hardness, and Young's modulus of elasticity of p-type (lightly boron-doped) single-crystal silicon (referred to as ‘undoped’), p +-type (boron-doped) single-crystal silicon, polysilicon, and n +-type (phosphorus-doped) LPCVD polysilicon films. Undoped silicon samples are very smooth (r.m.s. = 0.08 nm). Surface-roughness values for p +-type silicon and lapped polysilicon bulk samples are comparable. As-deposited polysilicon films are rougher than undoped and doped single-crystal silicon, unlapped and lapped polysilicon bulk samples. Microscale friction measurements show that the friction values for undoped silicon, p +-type silicon and lapped polysilicon bulk samples are comparable. Polysilicon films exhibit the highest coefficient of friction as compared to other samples, due to the roughness effect. Macroscale friction values for undoped silicon and polysilicon bulk are comparable with those of p +-type silicon sample having the highest value and polysilicon films having the lowest values. The microscale friction values for all samples are lower than the macroscale friction values as there is less ploughing contribution in microscale friction measurements. Local variation in microscale friction is found generally to correspond to the local surface slope. Directionality in the friction is generally observed on the microscale which results from the surface preparation and anisotropy in surface roughness. Microscratching/wear studies indicate that scratch and wear resistance of p +-type silicon is poor as compared to other samples used in this study. Nanoindentation studies indicate that the p +-type silicon sample exhibits lowest hardness and Young's modulus as compared to the other samples, which is responsible for high macrofriction and poor microscratch and microwear behavior of the p +-type silicon sample. Microtribological behavior and nanohardness and Young's modulus values for polysilicon bulk and n +-type polysilicon film are comparable.