P Chiggiato

Vacuum properties of TiZrV non-evaporable getter films

Sputter-deposited thin films of TiZrV are fully activated after 24 h “in situ” heating at 180°C. This activation temperature is the lowest of some 18 different getter coatings studied so far, and it allows the use of the getter thin film technology with aluminium alloy vacuum chambers, which cannot be baked at temperatures higher than 200°C. An updated review is given of the most recent results obtained on TiZrV coatings, covering the following topics: influence of the elemental composition and crystal structure on activation temperature, discharge gas trapping and degassing, dependence of pumping speed and surface saturation capacity on film morphology, ageing consequent to activation–air-venting cycles and ultimate pressures. Furthermore, the results obtained when exposing a coated particle beam chamber to synchrotron radiation in a real accelerator environment (ESRF Grenoble) are presented and discussed.

Decreasing surface outgassing by thin film getter coatings

Abstract The UHV behaviour of stainless steel vacuum chambers, coated ex situ by sputtering with a thin film of a getter material, has been investigated. The purpose of this study was to ascertain if the getter film could be activated after air exposure by in situ baking, so as to transform the vacuum chamber from a gas source into a pump. Many elements and alloys have been tested, all of which could be activated by baking at temperatures acceptable for stainless steel components, i.e. lower than 400 °C. In one case (equiatomic TiZr alloy) an activation temperature of 200–250 °C has been measured. This investigation has been carried out using Electron Stimulated Desorption, pumping speed and ultimate pressure measurements.

Nonevaporable getter films for ultrahigh vacuum applications

The vacuum behavior of stainless steel vacuum chambers, ex situ sputter coated with a thin film (∼1 μm) of getter material, has been studied to determine if after air exposure the getter film could be activated by a bakeout so as to transform the coated vacuum chamber into a pump. The materials studied so far are Ti, Zr, Hf, and some of their binary alloys. They all display an activation temperature lower than 400 °C, i.e., within the reach of the baking temperature of stainless steel vacuum chambers. The lowest activation temperature of 200–250 °C, measured for an equiatomic alloy of Ti and Zr, allows extension of this method to chambers made of copper and aluminum alloys. The experimental results, described here in detail, indicate that the values of the activation temperature obtained using electron stimulated desorption, pumping speed, and Auger spectroscopy measurements are self-consistent.

Pumping characteristics of the St707 nonevaporable getter (Zr 70 V 24.6‐Fe 5.4 wt %)

The room temperature pumping speeds of the St707 nonevaporable getter (NEG) have been measured both for individual gases and for gas mixtures as a function of the quantities of gas pumped. The interesting feature of this NEG consists in its moderately low activation temperature. Therefore particular attention has been devoted to defining the optimum temperature and duration of the activation process to obtain the highest possible pumping speed in a given practical situation. It has been found that heating at 400 °C for about 1 h, or at 350 °C for one day, results in pumping speeds of about 1000 l s−1 m−1 for H2, 2000 l s−1 m−1 for CO, and 450 l s−1 m−1 for N2, values very close to those obtained after activation at the higher temperature of 740 °C. The St707 NEG is therefore particularly suitable for passive activation during bakeout of stainless steel vacuum systems, avoiding the need for electrical insulation and feedthroughs which are mandatory when activation is carried out by resistive heating.

Synchrotron Radiation-Induced Desorption from a NEG-Coated Vacuum Chamber

Abstract When the whole inner surface of a vacuum chamber is coated with a non-evaporable getter film, very low static and dynamic pressures are expected after activation. In an accelerator environment this could result in a longer beam lifetime, in a lower risk of pressure bumps, and in a lower level of bremsstrahlung radiation due to the beam–gas interactions. To substantiate these favourable characteristics a Ti–Zr–V coated stainless-steel chamber has been tested on a dedicated beamline at the ESRF. It is shown that a large reduction of the synchrotron radiation-induced desorption occurs after activation.

Influence of the substrate coating temperature on the vacuum properties of Ti–Zr–V non-evaporable getter films

Abstract Non-evaporable thin film getters of various compositions have been produced by sputtering. Among about 20 materials which have been studied, the lowest activation temperature (about 180°C) has been displayed by a Ti–Zr–V coating obtained from a cathode made of intertwisted elemental wires. In order to optimize the vacuum properties of this film various production parameters, including the substrate temperature during coating, have been varied. The films have been characterized by pumping speed measurement, secondary electron microscopy, and X-ray diffraction. It has been found that the substrate coating temperature affects significantly the activation temperature, the pumping speed and the gas surface capacity. The highest pumping speed values, obtained for substrate coating temperatures of 250°C and 300°C, are clearly correlated with the increased surface roughness and porosity of the Ti–Zr–V film.

Influence of the elemental composition and crystal structure on the vacuum properties of Ti–Zr–V nonevaporable getter films

Nonevaporable thin film getters based on the elements of the fourth and fifth columns of the periodic table were deposited by sputtering. Among the some 20 alloys studied to date, the lowest activation temperature (about 180 °C for a 24 h heating) was found in the Ti–Zr–V system with a well-defined composition range. Characterization of the activation behavior of such Ti–Zr–V films is presented. The evolution of the surface chemical composition during activation is monitored by Auger electron spectroscopy and the functional properties are evaluated by pumping speed measurements. The pumping speed characteristics are quite similar to those already measured for commercially available nonevaporable getter materials, except for the much lower saturation coverage for CO. This inconvenience, which is due to the smooth surface structure of these films, can be counteracted by increasing the roughness of the substrate.

Obtention of pressures in the 10−14 torr range by means of a ZrVFe non evaporable getter

Abstract Pressures in the low 10 −14 torr range have been reproducibly achieved in a 3 m long vacuum chamber. Pumping was provided by a 400 I s −1 sputter-ion pump, a titanium sublimation pump and 43.5 m of a ZrVFe Non Evaporable Getter (NEG) strip. Activation of NEG has been passively achieved during the initial bakeout of the vacuum system. The pressure has been measured by means of a modified version of the Helmer gauge, designed and manufactured at CERN.

SURFACE CLEANING EFFICIENCY MEASUREMENTS FOR UHV APPLICATIONS

Abstract High-energy particle accelerator UHV systems subjected to energetic charged particle or photon bombardment are particularly sensitive to surface contamination. Well-established cleaning procedures employing solvents such as Freon and perchloroethylene to prepare vacuum system components have been used at CERN for many years. The recently adopted legislation which prescribes or strongly limits the use of some chemicals has led CERN to undertake a qualification programme using electron stimulated desorption, Auger spectroscopy and radioactive tracers to compare the cleaning efficiency of a wide range of alternative cleaning agents. It is found that detergents are preferable to solvents for cleaning UHV components. Some solvents have been found to be as good, if not better, than Freon when detergents cannot be used.

Study of the discharge gas trapping during thin-film growth

Discharge gas trapping in thin films produced by sputtering is known to be due to high-energy neutrals bouncing back from the cathode. Qualitatively, the phenomenon is enhanced by raising the discharge voltage and is strongly dependent on the atomic masses of the discharge gas and of the cathode material. In addition to these known effects it is shown that, for a given gas, the trapped amount decreases with increasing melting temperature of the deposited material. The results obtained both by sample melting and laser ablation are presented and discussed.