High precision chemical mechanical polishing of highly-boron-doped Si wafer used for epitaxial substrate

The surface waviness with concentric circular pattern is generated on highly-boron-doped Si wafer by chemical-mechanical polishing (CMP) with amine system polishing slurry. To investigate the generation mechanism of the waviness, the mechanical and chemical characteristics were clarified using the silicon crystal samples with various boron concentration level ranging from 2.9 × 10¹⁷ cm^-3 to 1.3 × 10²⁰ cm ^-3. The conventional silicon substrate used as epitaxial wafer has boron concentration of about 2.5 × 10¹⁸ cm^-3, a region at which the radical change of etching rate is induced with amine system chemical reagent. The mechanical micro-hardness of highly-boron-doped Si is 30% higher than that of lightly-doped Si. It is found that SiB bond in crystal lattice is firmed up and stabilized for mechanical stress and chemical reaction. To cancel the difference in CMP rate based on boron concentration deviation, increasing the mechanical action in CMP was proposed and performed. The precision CMP was performed using the harder polishing pad and a smooth surface without waviness was obtained.