Spatial distribution of the neutral gas temperature in rf and dc magnetron sputter discharges determined by fitting rotational spectra of the N2+ using a two-temperature fit

The spatial distribution of the neutral gas temperature in a dc and rf magnetron sputter discharge (boron target) with pure nitrogen was estimated by fitting the rotational spectra of the first negative system of the nitrogen molecule ion. The rotational levels of the excited nitrogen molecule ion are populated mainly by two mechanisms. Only one of them leads to a rotational level distribution corresponding to the neutral gas temperature. Therefore, fitting of the spectra assuming a single Boltzmann distribution of the rotational levels often leads to unreliable and too high rotational temperatures and the spectra have to be fitted using a two-temperature model. Beside the neutral gas temperature the value of the second temperature as well as the contribution of the higher rotationally excited molecules to the spectra are studied in more detail. The dependence of the spatial distribution of the neutral gas temperature on pressure and discharge power was investigated. Additionally, the results are compared to rotational temperatures obtained by fitting the spectra assuming a single Boltzmann distribution of the rotational levels. These rotational temperatures often do not reflect the neutral gas temperature and wrong dependences would be concluded.