Si(100) surfaces treated by ultrapure water with 5 ppb dissolved oxygen concentration after dipping in HF solution were examined by attenuated total reflection (ATR)–Fourier‐transform infrared (FT‐IR) spectroscopy, transmittance FT‐IR, and reflection high energy electron diffraction (RHEED). FT‐IR spectra and RHEED patterns depended on the rinsing time in the ultrapure water. The (111) and (110) facets appeared after rinsing for a long period of time (20–45 h) in 5 ppb dissolved oxygen concentration ultrapure water. It was suggested that the surface morphology depended on not onlypH value but also the amount of etching. ©1995 American Institute of Physics.