When a hydrogen-terminated Si (100) surface is irradiated with a focused ion beam, the irradiated areas are not affected by a subsequent KOH etch. We discuss the influence of irradiation-induced doping, amorphization, defect doping, and surface oxidation on this effect. Point defects and amorphization were measured by thermal wave analysis. X-ray photoelectron spectroscopy was used for surface analysis. We show that both damage and oxidation can lead to the observed etch stop in KOH, but can be controlled independently from each other.