Optical interference induced by deposited wall-film in inductively coupled plasmas

Temporal periodic variation and long-time decrease of optical emission intensity collected by a spectrometer in an inductively coupled CH4/N-2 plasma, which deposits a film of growing thickness on the chamber wall, are measured. The periodic variation is interpreted as a result of the interference between the light reflected from the film surface and that which re-emerged from the surface after being reflected by the chamber wall. The re-emerged light is attenuated due to absorption by the film and thus leads to the long-time decrease of the intensity. The periodic variation of the line intensities, however, can be used for in situ film thickness monitoring.