Abstract

Temperature and radical concentration measurements are important for validation of theoretical models of the complex interplay between flow dynamics, gas phase and surface chemistry in chemical vapour deposition plasmas. Furthermore it is useful to have rapid measurements of temperature and other parameters to aid in monitoring and control of the growth environment for deposition of such materials as diamond and diamond-like carbon. Multiplex CARS of N₂ is the well established technique for remote, nonintrusive thermometry of flames and hostile environments [1]. Unfortunately, in several industrially important plasma systems used for synthesis of diamond, N₂ is absent, and required to be so to avoid corruption of the growing diamond. An alternative to N₂ is molecular hydrogen which is present in large quantities and is a suitable Raman active medium for CARS thermometry. However, owing to the open structure of the H₂ molecular spectrum, only a very few rotational lines may usually be sampled by multiplex CARS in single shot spectra. When conventional lasers are used to provide the broadband Stokes beam mode noise can render the CARS spectrum difficult to interpret quantitatively. For this reason H₂ CARS is usually done by using a narrow linewidth Stokes laser, scanned across the spectrum leading to only a time averaged temperature measurement. For turbulent flames, unstable plasmas or for rapid temperature monitoring for process control, multiplex H₂ CARS would be very useful. We report here the use of a modeless laser [2] as the Stokes source for H₂ CARS thermometry of a low pressure microwave excited diamond CVD plasma.

© 1996 Optical Society of America