The original model of axial-symmetric transmission gauge (ATG) was designed for the pressure measurement of extreme high vacuum. The basic characteristics of the gauge has been reported in the previous papers1) 2). The ATG consists of a hot cathode type ionizer, a Bessel-box type energy analyzer, and an ion detector. The analyzer is effective to reduce the effects of the soft x-rays and the electron stimulated desorption (ESD) ions generated in the ionizer on the pressure measurement. The ion detection of the original ATG was carried out by the pulse counting method with a secondary electron multiplier. In the latest model of ATG (AxTRAN, ULVAC Inc.), the method of ion detection has been changed to the direct current measurement with a plate type ion collector. The pressure range from 10-11Pa to 10-3Pa is possible to measure by this model.
This paper reports the pressure measurement by the ATG of the vacuum system in which the principal component is H2O. The effect of ESD ions due to H2O adsorbed on the grid surface of the ATG was examined by comparison between the ion currents and the pressure readings which are obtained by the ATG, quadrupole mass spectrometer with a Bessel-box type energy analyzer (BB-QMS)3), and the extractor gauge (EXG). The ESD ion current observed by BB-QMS were m/z=1(H+), 16(O+), 17(OH+) and 19(F+ or (H2O)H+) 4). The H2O pressure dependence of the ESD ion currents of the ATG and of the BB-QMS for various m/z peaks are consistent each other. The difference of the H2O pressure readings observed between ATG and EXG is explained by the certain extent of residual ESD ion current in the EXG. In contrast with the EXG, the ESD ion effect in the ATG is reduced by the energy analyzer.
The recent results on the characteristics and the long-term stability of the sensitivity factor of ATG are also presented.
1) H.Akimichi,et al.,JVST A15,753(1997).
2) N.Takahashi,et al.,JVST A23,554(2005).
3) N.Takahashi,et al.,JVST A19,1688(2001).
4) N.Takahashi,et al.,Vacuum 81,66(2006). |