MEASUREMENTS OF MOLECULAR DENSITIES IN LOW-PRESSURE DISCHARGE PLASMASUSING LASER RAMAN-SCATTERING

The possibility of using Raman scattering as a method of molecular den sity measurements in low pressure discharge plasmas has been investiga ted. Firstly, the detection limit of Raman signal was examined in low pressure air. It was shown that the Raman scattering from nitrogen mol ecules was detectable down to 0.06 Pa (particle density of 1.4 x 10(19 ) m(-3)) when a photon counting detection system was used. The method was then applied to detect methane and its reaction products in an ele ctron cyclotron resonance plasma operated at a pressure of 50 Pa. Dens ities of CH4, C2H2, C2H6 and H-2 were absolutely determined using the calibrated data of the optical system. Also, it was clearly demonstrat ed that the gas composition in the discharge was dependent on gas flow rate. These experimental results showed the usefulness of the method for quantitative measurements of molecular densities in processing pla smas.