Circular etch pits in ion-implanted amorphous silicon films

Using a dilute HF solution as the etchant, we found very different etching behaviors between amorphous silicon films prepared by vapor deposition and by ion implantation. The latter are etched preferentially forming circular pits on the surface, but not the former. However, when the evaporated films were subjected to high dosages of ion bombardment and followed by etching, the same kind of circular pits were observed. We conclude that the bombardment of ions can cause weak spots on the amorphous Si surface which possess low resistance to the etching.