The nature of Si(100) surfaces during immersion in dilute hydrofluoric acids (DHF), HF/H 2 O 2 /H 2 O mixture and buffered hydrofluoric acids (BHF) has been comparatively investigated using confocal Raman spectroscopy. In DHF solution, silicon surfaces are covered mainly with silicon trihydrides (Si H 3 ) at the beginning of etching. As the etching goes on, silicon dihydrides (Si H 2 ) become main surface bonds, and silicon monohydride (Si H) signal appears clearly. In HF/H 2 O 2 /H 2 O solution, silicon surfaces are terminated with hydrides, oxides and hydrogen-associated silicon fluorides. In BHF solution, silicon surfaces are covered with hydrides and hydrogen-associated silicon fluorides.