Detection of Trace Water Vapor in High-Purity Phosphine Using Cavity Ring-Down Spectroscopy

Abstract

The presence of trace water vapor in process gases such as phosphine, used for compound semiconductor epitaxial growth, can negatively affect the optical and electrical properties of the final device. Therefore, sensitive H₂O measurement techniques are required to monitor precursor purity and detect unacceptable contamination levels. A commercial cavity ring-down spectrometer that monitors an H₂O absorption line at a wavelength of 1392.53 nm was investigated for service in high purity PH₃. Spectral parameters such as the line shape of water vapor in the presence of PH₃ as well as background features due to PH₃ were measured at different pressures and incorporated into the data analysis software for accurate moisture readings. Test concentrations generated with a diffusion vial-based H₂O source and dilution manifold were used to verify instrument accuracy, sensitivity, linearity, and response time. H₂O readings at 13.2 kPa corresponded well to added concentrations (slope = 0.990 ± 0.01) and were linear in the tested range (0–52.7 nmol mol⁻¹). The analyzer was sensitive to changes in H₂O concentration of 1.3 nmol mol⁻¹ based on 3σ of the calibration curve intercept for a weighted linear fit. Local PH₃ absorption features that could not be distinguished from the H₂O line were present in the purified PH₃ spectra and resulted in an additional systematic uncertainty of 9.0 nmol mol⁻¹. Equilibration to changing H₂O levels at a flow rate of 80 std cm³ min⁻¹ PH₃ occurred in 10–30 minutes. The results indicate that cavity ring-down spectroscopy (CRDS) at 1392.53 nm may be useful for applications such as on-line monitoring (and dry-down) of phosphine gas delivery lines or the quality control of cylinder sources.