CVD diamond deposition processes investigation: CARS diagnostics/modeling
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I. INTRODUCTION A variety of techniques1"10 has been developed and demonstrated to be viable for the deposition of thin coatings of diamond. As an example, nonequilibrium reactive plasmas are recognized as a novel approach for J. Mater. Res., Vol. 5, No. 11, Nov 1990
a w i d e v a r i e t y o f m a t e rial
coating applications.1 This type of plasma provides a unique environment wherein deposition of thin, hard face, conformal coatings such as diamond can occur at much lower temperatures (critical for inhibiting stresses) and within more complex © 1990 Materials Research Society
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S.O. Hay, W.C. Roman, and M.B. Colket, III: CVD diamond deposition processes investigation
chemical environments than are possible by conventional vapor deposition techniques. A complete understanding of the process requires information on a large number of physical and chemical processes involving gas phase and gas-surface interactions and the associated synergistic effects. Knowledge of plasma species concentrations and temperature is required for correlation with the corresponding physical and chemical properties of the coatings. Measurements of gas-phase properties (temperature, velocity, concentrations of species, etc.) are extremely difficult, especially for the low pressure conditions often found to be favorable for diamond deposition. Physical probes inserted into the reactor tend to change the local conditions due to flow perturbations, cooling effects, and radical terminations. The first two of these processes may be very significant for diffusive environments typical of these reactors. The temperature of thermocouples may be dramatically enhanced by radiation from a nearby filament heated to 2500 K, especially when convective heat transfer rates are low. Consequently, experimental research in this laboratory has focused on in situ, nonintrusive diagnostics for temperature and species measurements. Fourier transform infrared (FTIR) spectroscopy has been shown to be a useful technique for identification and measurement of species near heated filaments; however, since this is a line-of-sight technique, the results must be interpreted carefully due to steep temperature and concentration gradients. Emission spectroscopy is often used for the measurement of atomic and molecular species, but the emission from excited atomic levels is often much larger than for excited molecular levels; thus the atomic species dominate the spectra and molecular signatures are difficult to extract. Species concentrations are difficult to infer from plasma emission spectra because they involve excited electronic levels that are not generally in thermodynamic equilibrium with the ground state. Thus, in the case of plasma-assisted CVD (PACVD), an alternative diagnostic technique, sensitive to the molecular ground state, is Coherent Anti-Stokes Raman Spectroscopy (CARS). This nonlinear optical technique is useful for measuring the concentration of molecular constituents since all molecules have at least one Raman active vibrational mode.1112 In add
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