Monitoring of electron energy distribution change from optical emission for nonmagnetic ultrahigh-frequency plasma

Fractional electron density, which is partial electron density in energy distribution, has been measured in a high-density non-magnetic ultrahigh-frequency (UHF) plasma from optical emission of rare gases (Xe, Ar, He). The technique was applied to rare gas mixed plasma, as well as fluorocarbon gas containing plasma. In the calculation procedure, the fractional electron density was assumed to be constant between the two threshold energies of the different emissions. In the experiment, total electron density was changed by changing the UHF source power without changing electron temperature measured by a single probe. However, in UHF plasma, the fractional electron density between the threshold energy of ArI and HeI emissions increased more than total electron density increase. This result was obtained both for Ar/He plasma and Ar/C₄F₈/He/Xe plasma. On the other hand, fractional electron density over the threshold energy of He increased at about the same rate of or less than the total electron density increase. In addition, the fractional electron density between the threshold energy of XeI and ArI in Ar/C₄F₈/He/Xe plasma increased less than total electron density increase. These results indicate that the electron temperature, which is commonly used as a typical index value of electron energy of the plasma, does not reflect the line structure of EEDF. The optical technique can supplement this point, especially at the high energy tail of EEDF.