Non-Evaporable Getter Thin Film Coatings for Vacuum Applications

Getters are solid materials capable of chemisorbing gas molecules on their surface: getters are chemical pumps. They are widely used for a variety of applications such as in particle accelerators, vacuum tubes, field-emission display (FED), inert gas purification systems, H2 plasma purification, hydrogen species recycling as in the Tokamak Fusion Test Reactor. Among the different Non-Evaporable Getter (NEG) materials tested, the TiZrV alloys have the lowest activation temperature. For this reason, the TiZrV coatings were the object of this work. In particular, the aim of this investigation was to understand how to optimise three important properties of TiZrV coatings: to achieve the lowest possible activation temperature (Ta), and to obtain the highest pumping speed and surface pumping capacity. This objective is important in the context of the Large Hadron Collider (LHC) accelerator, since, before this work, the understanding and the knowledge of the TiZrV coatings properties were insufficient to adopt it for this machine. In the present investigation, TiZrV coatings (250 samples) of various compositions have been deposited by DC (Direct-Current) diode magnetron sputtering. The influence of the substrate materials, substrate roughness, substrate temperature (T&#1109;), film composition, on the activation temperature and pumping properties have been investigated in order to optimise the deposition parameters for vacuum applications. The characterization of the films has been carried out by Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-ray spectroscopy (EDX) for the surface and bulk chemical composition, respectively, and by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Scanning Tunnelling Microscopy (STM) for the morphology and crystallinity. The performances of the best coatings selected through the above mentioned methods have been investigated by pumping speed measurements of H2, CO and N 2. The main results of this investigation are the following. XRD measurements prove that depending on their composition, the TiZrV coatings can be grouped into two families which are sharply separated with respect to their grain size: polycrystalline films, with grain size above 100 nm and nanocrystalline films with grain size below 5 nm. TEM images clearly reveal grains with sizes in the nanometer range for the tested samples of the second family. The surface of the NEG TiZrV coatings was analysed by AES, in order to monitor their activation behaviour. It resulted that TiZrV coatings of different compositions can be divided into two groups: one group of high activation temperature (Ta > 200 °C for 1 hour heating) and one of low activation temperature (Ta < 200 °C for 1 hour heating). For the first time, a clear relation between morphology and activation has been found: without exception, the TiZrV NEG films with low activation temperature are among the coatings crystallizing with small grain size (below 5 nm). For TiZrV coatings sputtered in identical conditions, different substrate materials were found to induce different morphologies, but identical crystallinity (nanocrystallinity) and activation temperature. The increased surface roughness, obtained by deposition on rough copper, does not accelerate the activation process, but clearly increases pumping speed and surface pumping capacity. The substrate temperature during sputter deposition (T&#1109;) also increases the roughness of a coating on copper without changing its crystallinity up to 250 °C. The ideal copper substrate temperature is T&#1109; = 250 °C both in terms of sticking probabilities and surface pumping capacity. By combining this coating temperature with a rough substrate, the surface pumping capacity results in the same level of pumping capacity for a 2 &#956;m thick coating as for the available commercial NEGs more than 100 &#956;m thick. The maximum pumping speed is obtained by heating the coating at 250 °C for activation. For higher heating temperatures, the pumping speed decreases. This feature had already been observed, although in a different temperature range, for the other NEGs (namely the St707). Thermal instability of the crystalline structure, pollution of the NEG coating due to the substrate, pollution by the gas, do not appear to be relevant for the deterioration of the NEG properties. The possibility of other types of surface modifications (formation of unsuitable surface phases by segregation, removal of lattice defects and/or nanoprotusions), which could not be detected by means of the available investigation methods, remains open. In conclusion, the present study has improved the knowledge of the properties of TiZrV getter films. Coatings of optimised characteristics may be reliably produced on vacuum chambers made of different materials. The acquired knowledge is sufficient to adopt this solution for the LHC accelerator at CERN, where about 5 km of this machine will be coated as well as the intersection vacuum chambers for the physics experiments. Meanwhile, coated chambers have already been successfully used on other accelerator, namely at ESRF (Grenoble, France) and Elettra (Trieste, Italy).