Calibration of reactive process gases for the characterization of semiconductor processes by FTIR

Quantitative characterization of process gases by infrared spectroscopy relies on the availability of suitable reference spectra. However, reference spectra of semiconductor process gases are sparse in commercial libraries, and certified standard reference gases are often not available. To overcome these problems we dynamically prepared diluted gas mixtures based on pure liquid tungsten hexafluoride and pure gaseous carbonyl fluoride. Oxygen difluoride, which was not commercially available at any concentration, was synthesized on-line in our laboratory. Infrared spectra of appropriate concentrations were collected and their concentration determined by absorption of the calibration gas in aqueous solution and subsequent ion chromatography. Integrated cross-sections were calculated for the most prominent absorption bands and compared to commercial spectral libraries as well as literature data. For tungsten hexafluoride our spectra agreed well with spectral libraries. In contrast, integrated cross-sections of carbonyl fluoride and oxygen difluoride were significantly higher in our study as compared to some spectral libraries. Our results indicate that calibration gas preparation is a common source of error, especially for reactive process gases. This underlines the importance of an alternative method for concentration measurement during calibration. The concepts that are described for the preparation of adequate calibration gases can be applied to a variety of other substances.