Influence of vacuum gauges on outgassing rate measurements

Abstract

When the outgassing rates from eighteen chamber samples with different surface treatments for ultrahigh vacuum were measured by the throughput method, the minimum specific outgassing rate measured was limited to about 10−13 Torr ℓ/s/cm2 due to the outgassing of a Bayard–Alpert gauge (BAG), about 10−9 Torr ℓ/s. A measurement of outgassing rates with a BAG and an extractor gauge (EG) showed that the outgassing rate of the EG is lower than that of the BAG and that outgassing due to thermal desorption from the wall surrounding of the gauges cannot be neglected in seeking to obtain correct outgassing rates for the gauges, especially for the case after baking. A new type of quadrupole mass spectrometer (QMS) and an extractor gauge (EG) showed an outgassing rate of 6–7×10−11 Torr ℓ/s. Using the QMS and EG, a minimum specific outgassing rate from the sample chambers was measured on the order of 10−14 Torr ℓ/s/cm2 from the total pressure. The measurement showed that the main residual gas species are H2O, before baking, and H2, after baking. Therefore, the outgassing rate (q) must be described using the partial pressures of the residual gases and was 2.0×10−10 Torr ℓ/s/cm2 for water (qH2O) and 1.4×10−13 Torr ℓ/s/cm2 for hydrogen (qH2) from an A6063 EX mirror-finished sample.