Surface quality of tribochemically polished silicon nitride

The surface quality of tribochemically polished silicon nitride was explored. Since a previous study has shown that chromic(VI) acid produces the best combination of polishing rate and surface quality, silicon nitride surfaces polished in 3 wt % chromic acid have been examined in detail. These surfaces are ultrasmooth and defect-free. No worn particles are observed on the surface nor in the liquid. The surface roughness measured by atomic force microscopy is 0.5 nm at 50 mu m cutoff length and 2 nm at 150 mu m. On a large scale, the surface roughness is 6 nm at 8 mm cutoff; the latter corresponds to a slight macroscopic curvature with a radius of 94 m. Secondary ion and photoelectron emission measurements showed that the polished surfaces are free of chromium. When stored in air, they are covered with the same oxide layer as a fracture surface. Surface residual stress measurements by X-ray diffraction indicate the presence of a very small compressive stress of 50 MPa. No degradation of the fracture strength is found. The average fracture strength measured by the biaxial stress method is about 770 MPa. (C) 1999 The Electrochemical Society. S0013-4651(97)12-072-9. All rights reserved.