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Abstract

Measurement of ultrahigh vacuum (UHV) and extremely high vacuum (XHV) is important in many advanced research and technology fields such as: equipment for surface analysis, particle accelerators, simulation of interstellar space, processing of advanced semiconductors, electron tubes, FE displays, etc. For traceable calibrations of UHV and XHV gauges we need adequate primary standards which deduce the pressure unit to corresponding base SI units. Primary standards for high and ultrahigh vacuum are normally pressure generators, which are realized and maintained at national metrological laboratories of the highest level. Basically two methods are used: static expansion and dynamic (continuous) expansion.
The aim of this work was to explore the use of nonevaporable getter pumps (NEG pumps) for extending the low range of vacuum primary standards. Since the main residual gas in UHV and XHV systems (after a suitable bake-out to remove adsorbed water) is hydrogen, the ultimate pressure and lowest limit of pressures which can be generated with high precision in primary static and dynamic expansion systems is determined by the hydrogen outgassing of vacuum calibration chambers. We used NEG pumps because they have zero pumping speed for noble gases and large pumping speed for active gases, especially for hydrogen. NEG pump can be used to mitigate hydrogen outgassing and achieve lower ultimate pressure.
Besides using pumps with large pumping speed for hydrogen in XHV systems it is also important to reduce outgassing rate of chamber walls. A study of reduction of hydrogen outgassing rate from stainless steel was performed. The outgassing rate of q =2.86x10-13 mbar l s-1 cm-2 was achieved in stainless steel 304L vacuum chamber (wall thickness of 2.62 mm) at room temperature after baking the system at 250 °C for 380 h, while this outgassing rate was reduced even further to q = 5.7x10-14 mbar l s-1 cm-2 using T = 350 °C for 140 h.
NEG pump must be activated by heating to appropriate temperature (depending on getter material) to remove passivating surface oxide layer. The diffusion of oxygen in St 707 NEG material during heat treatment was investigated. NEG characterization has been carried out by techniques such as EDS and AES for determination of bulk and surface chemical composition, respectively, and by XRD for determination of morphology and crystallinity.
We have demonstrated that the use of NEG pump in static expansion calibration system can diminish the influence of outgassing and offers the possibility to reliably generate lower calibration pressures. Contamination of the inert calibration gas by outgassing was found to be the main reason for the pressure dependence of the calculated volume ratio when the gas expansion was done at lower pressures.
We have constructed a model XHV dynamic expansion calibration system with a built-in NEG pump. The ultimate pressure in the calibration chamber of the order of 2x10-12 mbar was achieved. System is equipped with a gas flow meter for very low flows of inert gases. Flow is measured by pressure rise method. The use of another NEG pump in the flow meter practically diminishes the effect of hydrogen outgassing on measured flow. Lower limit of pressure rise method is 5x10-10 mbar l s-1 and can be extended by constant conductance method down to 3x10-12 mbar l s-1.
With this flow meter we can generate calibration pressures of argon in the range from 8x10-13 mbar up to 2x10-7 mbar. Based on our knowledge, this is the lowest calibration pressure reached in any National Laboratory so far. Operation of XHV calibration system has been demonstrated by calibration of two extractor ionization gauges having low pressure limit 1x10-12 mbar.