Thermometry of gas layers in thermal plasma CVD by pulsed-laser Rayleigh scattering

The application of Pulsed-Laser Rayleigh Scattering Thermometry (PLRST) to Chemical Vapor Deposition (CVD) processing of diamond films in a thermal induction plasma is demonstrated. Temperature profiles were measured in the reactive thermal boundary layer, characteristic features of which are a steep temperature gradient and strong chemical non-equilibrium. The temperature in the layer varies between 5000 and 1000 degrees C. The stray light rejection problem typical for Rayleigh scattering measurements was solved successfully. In the plasma core, where atomic emission spectroscopy can be applied to temperature measurement, agreement has been obtained between plasma temperatures measured by emission spectroscopy and PLRST. In the boundary layer both point-by-point measurements and laser sheet measurements were made and adequate agreement was obtained. Technical reasons for observed variations were identified. It is concluded that PLRS is a viable non-invasive thermometer for thin reactive gas layers typical or various CVD deposition processes and technical improvements are suggested.