Neutral Gas Desorption Research Articles

Origin of gas impurities in sputtering plasmas during thin film deposition

The incorporation of alien atoms in thin metal coatings produced by sputtering processes determine the properties of these films. The incorporation depends on the amount of reactive gases in the sputtering gas. The main impurities in the sputtering gas are H 2, H 2O, CO/N 2 and CO 2. We determined the partial pressure of these gases in a sputtering plasma for a typical high vacuum system with a planar magnetron sputtering source. The measurements show that electron stimulated neutral gas desorption, beside thermal desorption of water from the vacuum chamber walls, is one main source for the alien gases. Depending on the deposited metal (Ag, Mo) the above mentioned gases can be gettered in large quantities, whereby the ratio between incorporated reactive gas atoms and deposited metal atoms reaches up to 5%.

Neutral gas desorption and photoelectric emission from aluminum alloy vacuum chambers exposed to synchrotron radiation

In an aluminum alloy vacuum chamber exposed to synchrotron radiation, the photoelectron currents produced were measured with the photons incident at low angles on the side wall and compared with normal incidence. The calculated photocurrents for normal incidence, using published values of the photoyield for oxidized Al, agree to within 15% with the measured values. Differences in the photocurrent dependence on photon spectrum at normal and glancing incidence were attributed to low-energy photons being totally reflected and hence producing no photoelectrons. It was established that, at glancing angles of incidence down to 11 mrad, a substantial—more than 20%—fraction of the synchrotron radiation is scattered around the vacuum chamber from the initial point of impact. During exposure to synchrotron radiation, the gases desorbed were H2, CO, CO2, and CH4. The similar shapes of the dependence of the gas desorption and the photoelectron currents on the photon spectrum suggested that it is mainly the photoelectrons that are contributing to the desorption. It was estimated that electrons of 60 eV would produce the same gas desorption as synchrotron radiation with a critical energy of 3 keV.

Damage of oxide layers on an Al-alloy by electron bombardment

Electron and photon stimulated neutral gas desorption from technical metal surfaces after heat treatment in an UHV environment is in general determined by chemical reactions at the oxide layer which is always present on these materials rather than by molecules simply physisorbed on the surface. The desorption rate of the neutral gases H 2, CH 4, H 2O, CO and CO 2 from the surface of an Al-alloy and its decrease during continued 300eV electron bombardment has been studied. In parallel the changing surface composition was measured by Auger electron and X-ray photoelectron spectroscopy. During continuous electron bombardment the original aluminium hydroxide on the surface is reduced and carbides are formed. These processes are frequently called radiation damage. In addition the readsorption and the recontamination of a cleaned Al-alloy surface by the above mentioned gases during exposure to atmosphere was measured. Chemical reactions between the surface and water vapour were shown to play an important role in the recontamination of the surface.

Comparison of chemical cleaning methods of aluminum alloy vacuum chambers for electron storage rings

Three chemical precleaning methods suitable for aluminum alloy vacuum chambers for electron storage rings have been compared. The methods ranged from vapor degreasing with no chemical attack to light alkaline etching and strong alkaline etching in NaOH. The techniques used to evaluate and compare the three treatments included Auger analysis, thermal outgassing, electron and x-ray induced neutral gas desorption, argon glow discharge cleaning, and scanning electron microscope examination of the surface. In general, the NaOH treatment was best followed closely by the light alkaline etch and then the vapor degreasing. A variant of the light alkaline etch method was used to clean the 2000 Al vacuum chambers of the CERN large electron positron storage ring project. Two chambers cleaned by this method were exposed to synchrotron radiation in a dedicated beam line of a synchrotron radiation source. Compared to a reference chamber cleaned by NaOH these two chambers were initially worse, but after 1 A h of machine operation the difference practically vanished.

Synchrotron radiation induced neutral gas desorption from samples of vacuum chambers

Synchrotron radiation induced neutral gas desorption from samples of different vacuum chamber materials has been measured at the DCI storage ring at LURE, Orsay. In order to study the effect of a surface oxide layer on the desorption rate and its decrease during continued photon exposure (frequently called beam cleaning), a comparison has been made between lightly and heavily oxidized samples of aluminium and steel (Nimonic). The dependence of the desorption rate for hydrogen on the photon dose and on time for the different types of samples supports a model of gas diffusion from near surface layers and from the bulk to the surface. The experimental set-up also permitted the measurement of the photoelectron current from the different samples and hence comparison with electron stimulated neutral gas desorption measurements in the laboratory on identically prepared samples.

Studies of photon induced gas desorption using synchrotron radiation

In view of finalizing the design of the vacuum system of the Large Electron and Positron Storage Ring (LEP) we have studied synchrotron radiation induced neutral gas desorption. A 3 m section of an aluminum vacuum chamber has been exposed to the photon beam emerging from the electron storage ring DCI in Orsay, under conditions closely simulating the environment in a particle acceletor. In order of importance the gases desorbed were H2, CO2, CO and CH4 with H2O practically absent. Under the experimental conditions of an unbaked chamber and 11 mrad glancing incidence of the photons, the initial molecular desorption yields for these gases were typically 0.5, 8 × 10−2, 2 × 10−2 and 8 × 10−3 molecules per photon respectively. These values could be reduced by about 1 to 2 orders of magnitude during continued photon exposure and most cases without evidence that this ‘beam cleaning action’ would be limited. After exposure to air and pumpdown of the previously cleaned chamber, we observe a significant memory effect. The dependence of the photon desorption on the angle of incidence has been studied down to a glancing angle of 11 mrad showing a definite deviation from the previously assumed 1/sin ø scaling. The implications of the results in terms of the expected beam-gas lifetime in LEP are discussed.

Influence of oxygen and water vapor on the ion induced desorption from stainless steel

Ion induced neutral gas desorption, residual gas composition, and pumpdown times were measured after 1 h exposures of a baked stainless steel tube (SS 304 L) to the following atmospheres (atmospheric pressure): (a) pure N2, (b) normal air, (c) pure 80% N2, 20% O2 mixture, (d) pure N2 with the system open during the exposure. It is shown that exposure to oxygen-containing atmospheres increases the subsequent ion induced desorption of CO, CH4, and CO2 while the H2 desorption is reduced. Exposure to water-containing atmospheres, on the other hand, stimulates the ion induced desorption of all the four commonly desorbing species (CO, H2, CH4, and CO2). The effects of exposure to ordinary air on the ion induced desorption and residual gas composition is attributed mainly to its water vapor content. It is shown that if filaments (ion gauges, etc.) are on during low pressure oxygen exposures, the ratio between the ion induced desorption of CO and CO2 will be affected, probably due to the formation of excited oxygen molecules at the filaments. Backfilling the vacuum system with pure N2 before opening to air is shown to give a considerable decrease in surface contamination, which is seen from the ion induced desorption response as well as the reduced evacuation time.

The effect of bakeout temperature on the electron and ion induced gas desorption coefficients of some technological materials

The electron and ion induced neutral gas desorption coefficients at 1.4 keV have been measured for stainless steel, Inconel 600, Inconel 718, a titanium alloy, OFHC copper and an aluminium alloy after bakeout at temperatures ranging from 150 to 600°C. The surface composition was also determined by Auger spectroscopy after heating at various temperatures up to 1000°C. The similarity of the results from all the materials investigated was explained in terms of a porous surface oxide layer which provided a reservoir for desorbable gas.