Analysis of mechanically induced subsurface damage and its removal by chemical mechanical polishing for gallium nitride substrate

Abstract

A quantitative analysis of the surface and subsurface damage to gallium nitride (GaN) substrate from precise mechanical polishing with diamond abrasives and characterizations of their removal by the chemical mechanical polishing (CMP) process with colloidal silica slurry was performed. The cross-sectional transmission electron microscopy showed that the highly-defected regions were approximately 200 nm from the surface. The cathodoluminescence (CL) imaging showed that the subsurface damage layers were a cluster of extremely high-density networks of line-shaped defects. The depth-resolved and CMP-process-time-resolved CL imaging sensitively detected that while the high defect density subsurface damage layers were approximately 200 nm from the surface some of the subsurface damage reached over 1 μm in depth. Using the high-resolution x-ray diffraction, the CMP process was shown to decrease the dislocation densities, enlarge the correlation length, and release the compressive stress in the crystals induced by the mechanical polishing process. Improvement in these crystal properties was found to occur by removing the highly-defected surface regions.