Absolute Measurement of Partial Pressures in the Medium Vacuum Region Using Raman Scattering of Laser Light

The possibility of using Raman scattering as a method of partial pressure m6asurements for Raman-active molecular gases has been investigated in the medium vacuum region (10-1―10-2 Pa). The second harmonic beam from a YAG laser having an output energy of 0.5 J/pulse and a repetition rate of 10Hz was used as the light source． Raman scattering experiments using nitrogen (N2) and oxygen (O2) gases in air showed that signals from N2 were linear against the N2 partial pressure in the range of 2-103 Pa with an error of less than 10%, and that the intensity ratio between the Raman scattering signals from N2 and that from O2 agreed well (within 5%) with the value expected from the air composition and Raman scattering cross sections. By using a photon counting technique, the lower limit of the detection was further improved and Raman scattering signals were found to be linear against the N2 partial pressure down to 6×10-2 Pa. These results were used to estimate signal intensities from various processing gases, and it was found that the technique could be useful for partial pressure measurements in plasma processing conditions