2013 Volume 8 Pages 1306114
The laser Thomson scattering (LTS) method has been applied to measure the electron density ne and electron temperature Te of very-high-frequency (VHF) argon plasmas. When the probing laser wavelength was 532 nm and the laser power density was ∼1015 W/m2, the Thomson scattering spectrum was obviously deformed by the effect of the photo-ionization of metastable argon atoms. The threshold laser power density at which the scattered light intensity from electrons in the plasma and that from electrons produced by photo-ionization are equivalent was found to be unexpectedly low (4×1013 W/m2). To avoid the photo-ionization of metastable argon atoms, the laser power density was decreased to around 1×1013 W/m2 by using a cylindrical lens as the focusing lens. Then, the ne and Te values measured by LTS and the probe method were compared for a VHF plasma using argon gas at a pressure of 100 mTorr. This comparison confirmed that the LTS method gave reasonable ne and Te values.