Characterization of anisotropic wet etching properties of single crystal silicon: Effects of ppb-level of Cu and Pb in KOH solution

We investigated the effect of ppb impurity level of Pb and Cu in KOH solution on the etching characteristics of Si for a number of crystallographic orientations using a hemispherical specimen. With the ppb-level of Cu, the location of the maximum etch rate shifted from the vicinity of {2 1 1} to {1 1 0}, i.e. the anisotropy changed, while the etched surface became rough showing fine textures regardless of the orientations. With the addition of the ppb-level of Pb, the etch rate decreased at almost all orientations, while the morphology of all etched surfaces did not show any substantial change. Because the ppb-level of Pb in KOH solution reduced the etching reaction uniformly for all orientations, the anisotropy in the etch rate did not change. These results support our hypothesis that: (i) Cu in the solution deposited onto the Si surface as fine particles and acted as etching masks, and (ii) the reduction–ionization of Pb is in competition with the etching reaction of Si and leads to a change in the etch rate.