To meet the increasing demands for ensure the tightness reliability of vacuum or pressure devices, especially for the long-term reliability of the sealed vacuum devices with small volume, an ultra sensitive MSLD system with detection limit less than 10-15 Pa•m3/s has been developed [1], which is desirable for MEMS and the further developments of UHV/XHV as well.
The ultra sensitive MSLD system consists of a quadrupole mass spectrometer (QMS) in an ultra high vacuum system under accumulation mode [1]. A variable reference leak rate system is developed to do real time calibration of ultra sensitive leak detection as well as to study the long term stability of the ultra sensitive MSLD system. It is based on the molecular flow characteristics of the platinum wire-glass leak element combined with an inlet helium pressure adjustable device. Helium reference leak rate range from 10-12 to 10-10 Pa•m3/s is provided by using pure He as the inlet gas. The lower range of the helium reference leak rates can be realized by reducing the He concentration. For example, if reducing the helium concentration of the inlet gas by the mixture of 1% He-99% N2 in stead of pure helium for the above system, 10-14 ~ 10-12 Pa•m3/s helium reference leak rates have been obtained and has been used for real time calibration of ultra sensitive MSLD system[2]. Furthermore, the mixture of 100 ppm He-N2 inlet gas has been tried to get much lower helium reference leak rates.
The most important characteristic for leak detection is the smallest leak rate that an instrument, method, or system is capable of detecting under specified conditions. The long term stability of the minimum detectable leak rate is extremely important for the ultra sensitive MSLD system. The incessant experimental studies have been completed during the past 4 years, which proven that 10-15 Pa•m3/s minimum leak detection limit is still maintained. The experimental, results and the discussion on the major influence factors of the minimum detectable leak limit for the ultra sensitive MSLD system will be presented and discussed in this paper.
[1] Chen P, Chen X,Jin Q, Cha L, Vacuum (China), 2003(3):38-42
[2] Xu Chen et al, Vacuum 2006; 81: 257-259
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