Trace Moisture Analysis in Specialty Gases

Abstract

Even though we have been successful in reducing impurities, particularly moisture in bulk gases such as , Ar, and to below the 1 ppb level for semiconductor industry, the same performance has not been achieved in specialty gas delivery systems. This is due to the fact that these specialty gases are very active and corrosive, and they contain trace impurities including moisture. These gases react readily with gas delivery system surfaces and deteriorate the process equipment and performance. In order to reduce impurities in specialty gases, it is essential to improve our methods for measuring the impurity level in these gases, in particular, the residual moisture. The current techniques such as impedance‐type or crystal oscillation moisture detection systems are inadequate because these gases not only attack the system but also dissolve in adsorbed moisture on the sensor, causing the measuring conditions to change drastically, necessitating frequent cleaning and recalibration of the detector. To improve the reliability and the sensitivity of measuring the moisture level in specialty gases, we have developed a method to overcome these difficulties. The concept of the new method is quite simple. First, we pass a specialty gas of interest through tubing to allow the moisture to completely adsorb onto the tubing surface and reach the equilibrium between the gas and the solid surface. Then, we switch the gas and pass a carrier gas such as Ar through the tubing to desorb the moisture. The stripped moisture in the Ar carrier gas is then measured. We have demonstrated this method experimentally to measure the moisture level in and . We heated a piece of stainless steel tubing to 400–450°C and passed ultrahigh purity Ar containing 50 ppt moisture to completely strip off the adhered moisture from the inner surface. Next, the and gas with an unknown moisture level was introduced to the tubing system for a sufficient period of time for all the moisture to adsorb onto the surface and reach the gas and solid equilibrium. Then we switched the gas and passed an ultrahigh purity Ar carrier gas through the system to strip off the adsorbed moisture and subsequently the moisture in the carrier Ar was measured by atmospheric pressure ionization mass spectrometry by using the flow ratio of the Ar carrier gas and the gas. This new method was found to be useful to detect impurities in all specialty gases to ppb level.