Improved geometry of double-sided polished parallel wafers prepared for direct wafer bonding

Optical technological applications have upgraded polishing, including flat-surface polishing, to an extremely high level of geometrical precision. We deal with the application of this type of precision technology for the preparation of, e.g., silicon or fused-silica wafers that are thin compared to their diameter. To this end a standard optical polishing process using a double-sided polishing machine was modified by giving the polishing pad holder an adaptable curvature. By carefully choosing the process conditions 10-cm-diameter silicon and fused-silica wafers (500-µm thickness) were obtained with a very small deviation from parallelism in the 0.01-µm range. The level of smoothness, surface and subsurface damage, was identical with that required for integrated-circuit processing.