Crystallization of K2SiF6 has been observed during Secco etching of polycrystalline silicon wafer and thin films deposited on rough silica substrates. The crystals grew along grain boundaries and rough edges of the substrates, and were analyzed quantitatively by secondary ion mass spectrometry, electronic microprobe, and x-ray diffraction. It is suggested that a low nucleation barrier of the rough surface stimulates the crystal growth.