Ionization gauge errors at low pressures

Abstract

Factors affecting the accuracy of ionization gauge measurements at low pressures are reviewed. In hot-cathode gauges these include electron-stimulated desorption at the electron collector, forward and reverse x-ray effects, the potential of the gauge envelope, outgassing, and various controller-related errors. In cold-cathode gauges they include nonlinearities below the “magnetron knee,” plasma instabilities, and background currents. Case studies are given to illustrate many of these sources of error and their elimination. The case studies were gathered in the course of long-term stability measurements on over 30 ionization gauges at pressures ranging from the 10−8 to 10−11 Torr ranges. The investigation included Bayard–Alpert (both conventional and modulated), extractor, inverted magnetron, double inverted magnetron, and magnetron gauges. Errors caused by outgassing from virtual leaks in all-metal seals are also considered. By far the largest and most unpredictable source of error proved to be electron-stimulated desorption in hot-cathode gauges, avoidable by operating the grid at a sufficiently high temperature. It is concluded that, with proper precautions, better then 10% reproducibility in the 10−10 Torr range is easily achievable over periods of many months with either hot-cathode or cold-cathode gauges. Over shorter periods, reproducibility of better than 1% is obtainable with well-designed hot-cathode gauges.

Primary vacuometer based on an ultracold gas in a shallow optical dipole trap

“…For lithium λ ∆ = 0.015 fs nm, which may be compared to λ ∆ = 42 fs nm for caesium. Fermionic isotope 6 Li is particularly attractive due to easy suppression of losses related to intrap collisions. Atoms in hyperfine level 2S 1/2 (F = 1/2), unlike in 2S 1/2 (F = 3/2), do not undergo inelastic binary collisions because this level is the lowest.…”

“…At temperatures / U k 0 B , the flux into untrapped momentum sates has additional exponential suppression. Therefore, in 6 Li gas, the evaporative losses can be nulled even without polar izing fermions into a single magnetic state.…”

“…4 10 12 Pa [4]. Systematics of the ionization gauges include the dependance on the gas composition [5] as well as on the electric and magnetic fields, intermittent desorption from the electrodes [6], aging and contamination of the elec trodes, and absence of calibration in the whole operation range.…”

“…Typically the systematic related to unknown gas composi tion is significantly lower than that of the hotcathode ioniz ation gauges. For these gauges, the coefficient between the pressure and the current depends not only on the gas but also on the gauge design because the ionization probability is a function of the electron velocity [6,36]. Also there are no commonly accepted data for the relative sensitivity (correc tion factors) to different gases [36][37][38].…”

Band-to-Band Tunneling-Dominated Thermo-Enhanced Field Electron Emission from p-Si/ZnO Nanoemitters

“…For example, the dose of X-ray in a portable X-ray tube should be modified by cathode temperature variation to meet the tolerance/requirement of different applications (i.e., imaging or sterilization) without changing the voltage. 10 For ionization vacuum gauges, 11 high emission current is required for sufficient signal intensity, while low electron energy is expected to reduce the X-ray-induced residual photocurrent and the electron-stimulated desorption effect. In low energy electron beam lithography, ideally, it is desired to attain high exposure efficiency by increasing the amount of injection electrons per unit time.…”