CVD diamond deposition processes investigation: CARS diagnostics/modeling

Abstract

We have applied Coherent Anti-Stokes Raman Spectroscopy (CARS), using a narrowband, scanned colinear configuration, to measure temperatures, relative concentrations, and detect species in low pressure CVD of polycrystalline diamond. CARS measurements were obtained for methane, hydrogen, and acetylene in either or both a rf plasma reactor and a hot filament reactor. In the rf PACVD experiments a mixture of 1% CH₄ in H₂ was used at a total pressure of 5 torr. The rf power input to the plasma was 300 watts and the H₂ and CH₄ flow rates were 99 and 1 sccm, respectively. As acetylene (C₂H₂) has been proposed as an intermediate in diamond growth, it was selected for the initial series of measurements. In the absence of rf power, a sensitivity of 5 mtorr was observed; in the plasma downstream of the rf coils, no observable signal attributable to C₂H₂ was evident. This places an upper limit to conversion of methane to acetylene at 20%, a figure representing the observed sensitivity to C₂H₂. In the hot filament reactor, the gas flow was 200 sccm of 1% CH₄ in H₂ at a total pressure of 150 torr. Under these conditions, C₂H₂ was detectable. Absolute concentrations were not calculated, but the observed spectra are within an order of magnitude of our sensitivity limit. This allows estimation of the C₂H₂ partial pressure near the substrate as 5–50 mtorr or from 0.66 to 6.6% conversion from methane. In view of this low conversion percentage, the absence of a signal in the rf experiments must be taken as inconclusive. CARS spectra of methane were also obtained in both reactors. In the rf reactor, under similar conditions to those described previously, the methane relative concentration decreased to 25% as the rf power was increased from zero to 400 watts. In the hot filament reactor, CH₄ CARS signal profiles were obtained as a function of axial distance from the hot filament, and parametrically as a function of filament temperature. Comparison of these profiles, in which the observed signal decayed monotonically as the filament was approached and increased monotonically downstream of the filament, was made with theoretical calculations. This comparison showed that the fluctuations were attributable to temperature/pressure effects and not to chemistry. To determine if the observed depletion in the rf plasma was similarly attributable, the CARS signal of hydrogen was observed as a function of axial distance downstream of the rf coil centerline and parametrically as a function of rf power. In contrast to expected behavior in the thermal hot filament reactor, little rotational excitation was observed in the plasma. Rotational temperatures were assigned to hydrogen based upon comparison with theoretically derived spectra. At 450 watts of rf power, rotational temperatures of 340 K were observed 4 to 6 cm downstream of the coil, the region where the 25% decrease in CH₄ was observed. Little or no density fluctuations accrue due to these temperatures, indicating that the observed depletion in methane signal is attributable to decomposition or chemical reaction in the plasma. In summary, CARS is applicable to reactant species (CH₄) axial profiling in both reactors, but can be limited by sensitivity in the detection of intermediate or product species (C₂H₂). In addition, CARS thermometry can be utilized to profile the rotational temperatures of selected species.