AVS2014 Session VT-TuA: Vacuum Quality Analysis, Outgassing, and Control

Outgassing means basically the diffusion of atoms usually hydrogen through the bulk material, entering the surface and desorbing from it. The important consequence is it limits the lowest achievable pressure in a vacuum chamber and is a central issue in vacuum science with respect to ultra high (UHV) and extreme high vacuum (XHV). Stainless steel (SS) is one of the most commonly used constructional materials for vacuum chambers and components. A considerable body of work is documented on the hydrogen outgassing behaviour of SS. For the description of the outgassing rate basically two models common as diffusion limited model (DLM) and recombination limited model (RLM) have been discussed so far. Experimental studies in the last decade show that the real situation on the complex SS surface cannot be fully described by DLM or RLM. Hydrogen atoms approaching the surface from the bulk are desorbing in a second-order process. The rate of recombination depends strongly on the atomic structure of the surface and is e.g. generally higher on stepped surfaces than on flat close packed planes. A new insight was gained by atomic level studies on the real morphology of SS with atomic force microscopy (AFM) and the scanning tunnelling microscopy (STM).

Beside surface morphology surface composition additionally controls the desorption kinetics. Auger electron spectroscopy (AES) gives reason for a composition change. Since the information depth of AES covers several atomic layers complementary atom probe analysis were performed, measuring the chemical composition on the surface atomic layer by layer. Energy calculations using the ASED method (Atom superposition and Electron Delocalization) result in lower energy levels in Fe vacancies. It supports the picture that surface and subsurface defects form traps with different energetic levels. They may control the recombinative desorption process and give explanation for the observed outgassing behaviour of stainless steel. From this results a more complete description of the outgassing process may be given by a more or less dynamic equilibrium between diffusion, sojourn in different level traps and recombinative desorption.

This work was supported by the Austrian „Fonds zur Förderung der wissenschaftlichen Forschung“ P 12099 and “Zukunftsfonds Steiermark” P 119.

Mild steel, i.e. low carbon steel, is a soft magnetic material and widely used for shielding sensitive experimental apparatuses from stray magnetic field because of its relatively low price and high magnetic permeability. Mild steel vacuum chambers are usually nickel-plated in order to prevent corrosion and improve the vacuum. For example, electron microscopes employ nickel plated mild steel for constructing their specimen vacuum chambers in which the electron beam propagates and interacts with specimens; presence of stray magnetic field deteriorates proper propagation of the electron beam, degrading the resolution of the electron microscope.

The mild steel has not been employed, to the best of authors’ knowledge, for ultra-high vacuum (UHV) use because its outgassing rate has been known to be too high; reported values were on the order of 10^-8~10^-9 mbar l s^-1 cm^-2 or higher [1]. Ishimori et al. [2] reported that the outgassing rates of a mild steel (carbon ~0.15%), a chromium-plated mild steel and a stainless steel were 2~3×10^-11 mbar l s^-1 cm^-2 , 7~9×10^-11 mbar l s^-1 cm^-2 and 2~3×10^-12 mbar l s^-1 cm^-2, respectively, after baking at 300 °C for 3 hours. The outgassing rate of UHV chambers are normally on the order of 10^-12 mbar l s^-1 cm^-2 or less after baking at 100 ~ 200 °C.

The outgassing rates of a mild steel and a stainless steel 304 chamber were measured by using the so-called rate-of-rise (RoR) method [3]. We present that the outgassing rate of the mild steel purchasable on the market is much smaller than that of a stainless steel type 304L which is most widely used as a UHV vacuum chamber material. The ultimate pressure of a vacuum chamber made of the mild steel was 2.7x10^-11 mbar, and its outgassing rate was of < 3×10^-14 mbar l s^-1 cm^-2, which indicates the mild steel is even appropriate for extreme high vacuum use. Vacuum annealing of the mild steel at 850 °C reduced the outgassing rate further.

[1] B. B. Dayton : 6th Nat. Symp. Vac. Tech. 101 (1959).

[2] Yoshio Ishimori, Nagamitsu Yoshimura, Shuzo Hasegawa, and Hisashi Oikawa, SHINKU 14(8), 295(1971).

[3] C. D. Park, S. M. Chung, Xianghong Liu and Yulin Li, J. Vac. Sci. Technol. A 26(5), 1166(2008).

Miniaturization of modern sealed vacuum devices and higher demands for their stable operation on the long-term scale require accurate determination of the gas composition in the early stage of their operation, as well as after a long operating period. Since particular gases may have detrimental effect on the device performance even at low concentrations, accurate quantification of the gas mixture is an important as well as a challenging task. Among a few highly gas-sensitive methods capable to detect quantities below 10^-4 mbar L, the quadrupole mass spectrometry seems to be the most appropriate one for this task.

A two-step procedure, consisting of sample puncture inside an expanding chamber, followed by opening the leak valve to the quadrupole mass spectrometer, kept in the analytical chamber at ~3x10^-11 mbar, is proposed. A limited number of ion current readings are used for the reconstruction of the original total pressure and gas composition. Calibration of such instruments at particular partial pressure is regularly achieved at stable gas influx and constant pumping speed. Several discrete points have to be recorded to get the sensitivity of the instrument expressed in A/mbar.

In this presentation, a systematic approach for preparing the instrument for routine quantification of small gas amounts is described. In the first stage, the instrument was calibrated as the precise partial gas flow meter by an innovative in-situ calibration procedure by three different gases, hydrogen, argon and nitrogen. Each gas was admitted into the expanding chamber, having a precisely determined volume of 0.312 L and equipped by a capacitance manometer. By opening the leak valve, ion currents versus gas flux were recorded over three orders of magnitude, expressing the partial flux sensitivity in As/(mbar L). In the second stage, known gas quantities ~10^-4 mbar L of pure gas were admitted at different leak valve conductance to determine the instrument’s response. This data enabled minimizing the error by searching for a compromise between the number of the readings and the level of recorded ion currents. In the third stage, gas mixtures with various contents of three gases were prepared and analyzed. This evaluation enabled a much better prediction of the ultimate limits in reconstructing of the unknown gas mixture in a real device. Anyhow, uncertainty in evaluation increases by lowering the gas amounts as ion currents become indistinguishable from the background readings of the instrument.

This talk will highlight some important aspects of gas-surface interaction in vacuum chambers by focusing on the dynamical behavior of gas species competing for adsorption sites. We will show how the adsorption energies and concentrations influence the equilibrium that is reached, but we will also show how the adsorption energies and the concentrations determine the kinetic behavior before equilibrium is reached and how they influence surface coverage by different species during non-equilibrium. A simple model solving the coupled kinetic equations is able to predict the time dependent behavior, which can be used to determine for instance outgassing times and sampling times and the relationship between measured gas concentrations and surface coverage during non-equilibrium.

Residual gas analyzers (RGAs) are commonly used in ultra-high vacuum applications to measure vacuum quality. The RGA fragments and ionizes the molecules that are present in the gas phase in the vacuum system. These fragments of all the molecules make up the RGA spectrum. The RGA spectrum has to be interpreted to identify the contaminants that are present in the vacuum system. This is complicated and often impossible in case of complex mixtures of organics in the vacuum atmosphere.

The goal of this project is to develop a simple-to-use diagnostic tool that is able to identify the contaminant molecules in vacuum directly. After a trade-off of various options, we selected a removable cold trap in combination with an off-line gas chromatography–mass spectrometry ( GC-MS) system for analysis of the samples.

The cold trap is installed on a vacuum flange. A removable sample tube is positioned inside the cold trap in connection with the vacuum system. The cooling of the cold trap is achieved with Peltier elements, which makes it simple to operate as well as independent of supply of coolants such as liquid nitrogen. After sampling, the sample tube can be removed without venting the vacuum system and a new sample tube can be installed to continue measurements if required.

After sampling of the vacuum vessel, the sample tube is connected to a GC-MS system for analysis of the sample and identifying and partly quantify the organic molecules present.

The first results are promising and we continue to improve the system. In this presentation we will present our sampling method and the results of vacuum quality measurements using the new diagnostic tool.

In our studies of static gas release properties of various solid materials at low temperatures ranging from 25 to 80 °C, we find the amounts of gas collected from the experimental samples in sealed vacuum vessels over extended times (weeks to months) are only a few to less than 100 torrs in a free volume of 10 - 50 cc. In order to analyze these small batch gas samples, a quantitative method was developed using a quadrupole mass spectrometer. This method involves introducing a small pulse of the gas with a custom-designed sample manifold into a quadrupole mass spectrometer and analyzing the gas component quantity in a few seconds. The method is relatively quick and is particularly suitable for gas components that have low sticking coefficients to stainless steel surfaces. This method was evaluated for hydrogen, methane, and argon. In this presentation, the setup, the calibration and measurement procedures, and the performance of the method are presented and discussed.