Micropyramidal hillocks on KOH etched {100} silicon surfaces: formation, prevention and removal

The formation, prevention and removal of micropyramids at the {100} bottom of anisotropically etched cavities are investigated and discussed. In the case of pure KOH solutions the base of micropyramids has been found to be always rectangular or octagonal shaped. The formation is independent of the KOH supplier and the etchmask opening process. The arrangement of the rectangular based micropyramids on the {100} etch bottom depends on the etching time, etching position of the chip (vertical or horizontal) and on the oxygen content in connection with the thermal history of the wafer material. For both types of micropyramid the surface density and size increase with decreasing KOH concentration and etching temperature. Moreover the proportion of octagonal micropyramids rises in those conditions. The origin is discussed in terms of micromasking by H₂-bubbles in connection with plateau generation and layer by layer peeling of {111} planes. Other mechanisms which are suggested in the literature are discussed. Furthermore it was exhibited that already arisen micropyramids can be removed by a short re-etching in the same etch bath in the same conditions.