Measurements of Molecular Densities in Low Pressure Discharge Plasmas Using Laser Raman Scattering

The possibility of using Raman scattering as a method to measure molecular in low pressure discharge plasmas has been investigated. This technique was applied to detect methane and its reaction products in an ECR plasma operated at a pressure of 50 Pa and microwave power of 100 W. Raman signals from CH_4, C_2H_2, C_2H_6, and H₂ were clearly detected and the densities of these species were absolutely determined using a calibration of the optical system (Y. B. Song et al., Proc. 7th LAPD Symposium, Fukuoka, 1995). Similar measurements were performed for different gas flow rates in the range of 10 - 75 sccm. The initial methane gas pressure was kept constant at 50 Pa and the microwave input power was fixed at 100 W. The results clearly indicated that the gas flow rate has a strong influence on the reaction processes in the discharge These experimental results showed the usefulness of the method for quantitative measurements of molecular! densities in processing plasmas.