Calculation of Gas Spectra for Quantitative Fourier Transform Infrared Spectroscopy of Chemical Vapor Deposition

A method has been developed to calculate quantitatively the infrared spectrum of a gas as measured by Fourier transform infrared (FTIR) spectroscopy. Using spectroscopic parameters found in the HITRAN database, a high resolution spectrum (<0.01 cm −1 ) of the gas at a given temperature, pressure, and partial pressure is calculated first, while accounting for the major line broadening mechanisms. Then, the high resolution spectrum is smoothed to the typical spectrometer resolution (>1 cm −1 ) by mimicking the mathematical processes occurring in the FTIR. The smoothing consists of applying an inverse Fourier transform to generate an interferogram, truncating and apodizing the interferogram, and then Fourier transforming back to yield the finished spectrum. Comparisons to experimental measurements of CH 4 in air and H 2 at various concentrations, pressures, and resolutions have confirmed the accuracy of the method. The major advantages of this technique are that an entire gas band is calculated (not just peak values), the method works on overlapping and closely spaced peaks, and quantitative calculation of spectra at any temperature and pressure is possible. These features are particularly useful for making quantitative FTIR measurements in chemical vapor deposition reactors where there are substantial variations in temperature and partial pressure.