By detecting vibrational frequencies for N–H structures, the chemical states of N atoms at silicon nitride on silicon, SiN/Si, and silicon oxide with nitrogen on silicon, SiO(N)/Si, systems were deduced after being chemically decollated with H atoms in dilute HF solution. Using the fact that the decollation was associated with the dissolution of these films, we obtained their depth profile. The frequency was increased to an extent of 10 cm-1 by the existence of a thin oxide layer between the SiN layer and the Si substrate. The N–H structure bound to the Si substrate was observed at 3390 cm-1 just before the completely removal of the surface layer. We also observed the NH structure of crystal SiN form, whose frequency was unchanged to be 3325 cm-1. The same technique was applied to detect the chemical structure of the N atoms in an ultra-thin SiO(N) with 5% of the N atoms. The frequency peak of the bulk N structure was observed with that of OH structures. The peak was shifted from 3375 cm-1 to 3405 cm-1 at the interface region, suggesting densification of the film in the interface region. Prior to complete removal, we observed the surface state of the N atoms and the H atoms. Then the frequency of the NH was unaffected by the film formed with the various methods. No SiH structure was observed on the resultant film surface during etching.