Optical Techniques in Plasma Diagnostics

Recent developments in optical techniques for plasma diagnostics are critically reviewed. The primary emphasis is on plasma-induced emission and laser-induced fluorescence probes of radical and ionic concentrations and temperatures in low pressure discharges like those used in microelectronic fabrication. Other techniques such as optogalvanic, infrared, spontaneous and stimulated Raman, and multiphoton spectroscopy are also discussed briefly. Generally, emission techniques are seen to be useful primarily for qualitative analysis and characterization of the electron energy distribution. However, in some systems, emission actinometry, where an inert gas is used to account for changes in electron density, appears to be a valid means of determining relative changes in concentration. Laser-induced fluorescence spectroscopy has only recently been utilized for in situ measurements of ground state ions and radicals but has already yielded insight into mechanisms for formation, destruction, energy transfer, and maintenance of the discharge. By and large the other techniques show great promise but remain to be exploited as diagnostic tools for plasmas.