Absolute radical densities in etching plasmas determined by broad-band UV absorption spectroscopy

Broad-band UV absorption spectroscopy was used to determine radical densities in reactive gas plasmas generated in a 13.56 MHz capacitively coupled parallel plate reactor. Five radical species were detected: CF₂, CF, AlF, SiF₂ and S₂. Absolute (line-integrated) CF₂ densities were determined in CF₄ and C₂F₆ plasmas, as were the CF₂ vibrational and rotational temperatures in the latter case. In CF₄ plasmas the CF radical was also detected, along with the etch products AlF (from the Al powered electrode) and SiF₂ (when an Si substrate was present). The fraction that SiF₂ comprises of the total etch products was estimated. Finally, the S₂ dimer was detected in an SF₆ plasma in the presence of an Si substrate. This simple technique allows absolute concentrations of many key reactive species to be determined in reactive plasmas, without the need to analyse the complex rotational spectra of these polyatomic molecules.