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TDLAS for a diagnostic of dilute molecular species in a H(2)O/CH(3)OH diamond RF CVD system
Abstract
Second-harmonic tunable diode laser absorption spectroscopy (TDLAS) is one of the most sensitive techniques for the detection of trace gases. TDLAS was employed to measure the number densities of the dilute molecular species, CH$\sb3$ and $\rm C\sb2H\sb2,$ in a $\rm H\sb2O/CH\sb3OH$ RF CVD diamond system. The data were used to understand diamond growth mechanisms. Low pressure ($\sim$1 torr), high density plasmas were used for the deposition of diamond or diamond like carbon (DLC) films with various molecular mixing fractions. The second harmonic technique was studied theoretically and experimentally to determine the optimum modulation amplitude, frequency, and calibration coefficients. It was found that about 73% of the water was dissociated with 1500 W of RF power while more than 98% of the methanol was dissociated with only 500 W of power. The CH$\sb3$ and $\rm C\sb2H\sb2$ concentrations gradually increased with RF power up to 900 W. The ranges of the number densities were $\rm 0.56\times 10\sp{12}/cm\sp3$ to $\rm 4.34\times 10\sp{12}/cm\sp3$ for CH$\sb3$ and $\rm 0.50\times 10\sp{13}/cm\sp3$ to $\rm 3.18\times 10\sp{13}/cm\sp3$ for $\rm C\sb2H\sb2.$ The number density of CH$\sb3$ showed an abrupt decrease at 1000 W, while a sudden increase of $\rm C\sb2H\sb2$ occurred at the same power. These sudden changes are due to a transition of the plasma from a low density state plasma to a high density, more localized state, caused by a change in the plasma coupling mechanism. This transition also occurs in the dissociation fraction of water. The films were deposited in a pure methanol plasma for the first two hours of each growth run to enhance the nucleation, then a water/methanol plasma was used for the remainder. H$\sb{\alpha}$ and CH emission lines were observed for all deposition plasmas. The total pressure and RF power were fixed at 1 torr and 1500 W respectively, and the molecular mixing fractions were varied. For the film characterization, SEM and Raman backscattering were used. The molecular fractions of H$\sb2$O and CH$\sb3$OH, the molecular number density ratio of CH$\sb3$ and $\rm C\sb2H\sb2,$ and the normalized emission intensities of H and CH were analyzed for trends. The plasmas with the higher number density ratios of CH$\sb3$ to $\rm C\sb2H\sb2$ and H to CH$\sb3$ made films which have comparably more diamond phase composition while the films made in the high $\rm C\sb2H\sb2$ density plasmas have a higher sp$\sp2$ composition. The deposition rate was studied under various conditions, but did not show any noticeable trend.
Subject Area
Optics|Materials science|Fluid dynamics|Gases
Recommended Citation
Kim, Suchan, "TDLAS for a diagnostic of dilute molecular species in a H(2)O/CH(3)OH diamond RF CVD system" (1997). ETD collection for University of Nebraska-Lincoln. AAI9804327.
https://digitalcommons.unl.edu/dissertations/AAI9804327