Paper VT-MoA3
Comparison of Cavity Ring-Down Spectroscopy, Oscillating Quartz Crystal and Electrical Impedance Technologies for Trace Water Vapor Detection below 100 ppb

Monday, October 31, 2011, 2:40 pm, Room 111

Control of trace water vapor in high purity process gases at low ppbv levels is critical to the performance of many micro-electronic and photonic devices [1]. Consequently a variety of measurement technologies, have been developed to detect water below 100 ppb. However, the performance characteristics of each technology can vary and this is not always well understood by users. In this presentation three different approaches are considered: Oscillating quartz crystal microbalance (QCM), Al₂O₃ based electrical impedance sensor and laser induced cavity ring-down spectroscopy (CRDS). QCM technology, developed in the early 1960’s, is still widely applied today. It is based on adsorption of water vapor on the hygroscopic coating of the QCM, which causes an increase in the mass of the crystal, and in turn, decreases its oscillation frequency. CRDS is a laser absorption technique based on the light decay in a high finesse optical cavity. The high resolution laser ~1-2 MHz (~10^-5 cm^-1), high reflectivity mirrors (~ 0.99998) results in a long effective path-length which enables high selectivity and sensitivity for H₂O detection. Impedance-based sensors for trace water vapor detection have typically suffered from drift and equilibration issues. However, recently an impedance-based Al₂O₃ sensor chip with integrated heater for cycling the temperature within 60ºC to 200ºC has been developed. Water vapor is measured dynamically as impedance changes during wet-up of the sensor resulting in rapid response. In this work, we present and discuss data showing the performance of the above detection technologies with respect to sensitivity, speed of response and measurement stability in the <100 ppbv range.

[1] H.H. Funke et al., Rev. Sci. Instrum., 74 (9) 2003, 3909-3933.