Thomson Scattering Measurement of Electron Density and Temperature of a Microwave Plasma Produced in a Hydrogen Gas at a Moderate Pressure

Laser Thomson scattering was applied successfully for the first time to a microwave discharge plasma suitable for deposition of diamond thin film. The plasma was produced in pure hydrogen gas, but the conditions were otherwise identical to those used for diamond deposition. The local electron density and temperature at the plasma center were (3.0±0.2)×1017 m-3 and (1.7±0.2) eV. Although the degree of ionization of the plasma was very low (<10-6), Thomson scattering signals could be detected clearly by using a double-monochromator which has a stray light rejection of 10-6 at the differential wavelength of Δλ=1 nm from the laser wavelength. Various checks were made to exclude the possibility of the probing laser from affecting the measured values, confirming the reliability of the measurement.