Investigation and reduction of spurious peaks caused by electron-stimulated desorption and outgassing by means of a grid heating method in a hot-cathode quadrupole residual gas analyzer

Abstract

In the year 2001, two issues remain for extreme high vacuum (XHV) pressure measurements with hot-cathode ionization gauges: measurement limits caused by electron-stimulated desorption (ESD) and outgassing. The limit caused by ESD is significantly clarified and at least partially solved by means of a grid heating method in a residual gas analyzer. After system bakeout and at 10−9 Pa, when the surface of a platinum alloy grid remains contaminated with F and Cl atoms and CO molecules, the mass spectrum is dominated by spurious peaks (which set the measurement limit for a total pressure gauge) caused by ESD species of F, Cl, and O, with a yield of 10−11 (species/electron, with neutral/ion ≅0.3) and by CO molecule ESD, with a yield of 10−11 (species/electron, with CO neutral/O ion ≅18). In this condition, a spurious H peak caused by hydrogen ESD is still low. However, once the grid surface is thoroughly cleaned at over 900 °C by ohmic heating or electron bombardment, the major ESD species observed for both neutrals and ions dramatically changes to mainly a spurious H peak with a yield of 10−11 (species/electron, with neutral/ion ≅2). When the operational grid temperature is then increased to 500 °C, the spurious H peak is decreased by over 3 orders of magnitude and the spectrum becomes mainly H2 molecules, reflecting the true pressure of the system; i.e., the relative error for the pressure is reduced to 8% from 614%. The lower limit caused by outgassing in these instruments is due to the grid bulk content of hydrogen. Finding a lower-hydrogen solubility material for the grid may become the last work required for lower-limit XHV gauge development. In order to decrease the limits caused by ESD and outgassing in a hot-cathode gauge, employing ion energy filters and cold-cathode field emitters is unnecessary.