Investigations of a method for determining pumping speed and sorption capacity of nonevaporable getters based on in situ calibrated throughput

Abstract

This paper presents a gas-sorption method, based on an in situ calibrated throughput, for characterizing nonevaporable getters (NEGs) of Ti- and Zr-based alloys. The main characteristics of the method are the adjustment of a constant injected gas flow, the prompt calibration of a throughput, and the measurement of an increasing sorption pressure. It is essential that inert vacuum gauges be selected for measurements of total pressure to prevent contamination of a test gas and poisoning of an NEG under test. A capacitance diaphragm gauge is used for preparation of the injected gas flow by filling the gas reservoir with a test gas and adjusting the conductance of a variable leak valve. A spinning rotor gauge (SRG) is used for in situ calibration of the throughput by means of an integrated gas-flow calibration facility and the rate-of-pressure-rise method, and by continuous measurement of the sorption pressure in a test chamber. A getter pumping speed and a sorbed quantity are calculated while considering the throughput, the sorption pressure, and the sorption time. The throughput must be selected in accordance with both the sorption characteristics of a particular NEG type and the measurement capabilities of the SRG. At room temperature, porous thick-film NEGs and structured thin-film NEGs exhibit an initial getter pumping speed on the order of 1 and 0.1 l s−1 cm−2, respectively. The corresponding sorption capacity of a few times 10−4 mbar l cm−2 is comparable for all chemically active gases except for H2, which amounts to a few times 100 × 10−3 mbar l cm−2 in the case of porous NEGs and ∼1 × 10−3 mbar l cm−2 in the case of structured NEGs. Detailed analysis of uncertainties of SRG pressure measurements below 1 × 10−3 mbar gives an expanded uncertainty of the getter pumping speed and the sorbed quantity of 1.9% and 3.5%, respectively. While temporally interrupting the throughput, the setup enables discrete measurements of the background pressure, which increases because of the accumulation of nongetterable gases. After completion of the gas-sorption test, a qualitative determination of the accumulated inert gas composition is performed by the gas-burst method using a quadrupole mass spectrometer (QMS) mounted on a vacuum system. The quantitative analysis is enabled by the use of a QMS calibrated in situ by known amounts of a certain gas such as H2, CH4, CO, Ar and Kr.