Non-evaporable Getter Thin Films Research Articles

Surface characterization and secondary electron yield measurement of the Ti-Zr-V films deposited on laser-etched copper

The electron multipacting effect and distributed vacuum profile, induced by the interaction of the particles in vacuum chamber with the surrounding walls, are the significant issues in modern accelerators. Laser-etching the inner surface of the vacuum chambers can effectively eliminate the electron multipacting effect, and depositing the non-evaporable getter (NEG) film on the vacuum chamber walls can provide distributed pumping properties. In the present study, Ti-Zr-V NEG films were prepared by magnetron sputtering technique on laser-etched oxygen-free copper (OFC) with different deposition times. The secondary electron yields (SEY), activation kinetics, and resistance of the samples were evaluated. The results show that although the SEYs of the coated laser-etched samples are higher than those of the uncoated laser-etched sample, the maximum SEY values are still below the threshold of the operation requirements. The Ti-Zr-V film deposited on laser-etched surface exhibits obvious nucleation phenomenon with columnar microstructure and can be activated at 180^∘C. Moreover, the coated laser-etched samples present lower resistance compared with the uncoated laser-etched sample. The combination of the laser-etching technique and the NEG thin film is aim to provide a practical treatment for the vacuum chambers in modern accelerators.

On the synergy manipulation between the activation temperature, surface resistance and secondary electron yield of NEG thin films

The operation of the modern accelerators has long been challenged with the problems associated with distributed vacuum profile and electron multipacting. Applying a Ti-Zr-V based non-evaporable getter (NEG) film provides an ideal solution to these problems, except that TiZrV thin film may deteriorate the wakefield effect because of the increased wall resistivity. To deal with the trade-offs between those effects, an innovative quaternary TiZrVCu NEG film was prepared using magnetron sputtering technique in the present study. Through comparative study on the activation behavior, secondary electron yields and the surface resistivity of the novel TiZrVCu getter film with that of the conventional TiZrV film, it is found that alloying Cu into the TiZrV film would encouragingly eliminate the secondary electron yield and the d.c. resistivity of the NEG films, with a minor deterioration on the activation kinetics. The present study provides insights for optimizing the composition of NEG films from the aspect of realizing synergy manipulation of the activation kinetics, SEY and d.c resistivity.

Analysis of a Non-Evaporable Getter of Ti-Al-Zr Thin Film with Enhanced Performance Used for Ultra-High Vacuum Generation

Non-evaporable getter (NEG) thin films of Ti-Al-Zr and Ti-Zr were prepared on stainless-steel substrate by using the cathodic arc deposition technique. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and in-situ X-ray photo-electron spectroscopy (XPS) were used to analyze the getter behavior of the non-evaporable getter thin films. The SEM images displayed the porous and rough surfaces of the thin films, which is an indication of high adsorption and sticking probabilities for the residual gases present in the chamber. The volume elemental composition of the thin films was measured by using EDX. The XRD patterns of the synthesized thin films indicated that the films had an amorphous nature and high structural stability. The in-situ X-ray photoelectron spectroscopy (XPS) studies revealed that the superficial layers disappeared when the non-evaporable getter of Ti-Al-Zr thin films was heated at a temperature below 250 °C for 30 minutes, there by activating the surface. However, for Ti-Zr thin films, no reasonable activation was observed at 250 °C. This outcome reveals that thin films of Ti-Al-Zr are good for reducing the outgassing and producing an ultra-high vacuum.

Electron cloud buildup and impedance effects on beam dynamics in the Future Circular e+e− Collider and experimental characterization of thin TiZrV vacuum chamber coatings

The Future Circular Collider FCC-ee is a study toward a high luminosity electron-positron collider with a centre-of-mass energy from 91 GeV to 365 GeV. Due to the beam parameters and pipe dimensions, collective effects and electron cloud can be very critical aspects for the machine and can represent the main limitations to its performance. An estimation of the electron cloud build up in the main machine components and an impedance model are required to analyze the induced instabilities and to find solutions for their mitigation. Special attention has been given to the resistive wall impedance associated with a layer of nonevaporable getter (NEG) coating on the vacuum chamber required for electron cloud mitigation. The studies presented in this paper will show that minimizing the thickness of this coating layer is mandatory to increase the single bunch instability thresholds in the proposed lepton collider at 45.6 GeV. For this reason, NEG thin films with thicknesses below 250 nm have been investigated by means of numerical simulations to minimize the resistive wall impedance. In parallel, an extensive measurement campaign was performed at CERN to characterize these thin films, with the purpose of finding the minimum effective thickness satisfying vacuum and electron cloud requirements.

Development and Characterization of Non-Evaporable Getter Thin Films with Ru Seeding Layer for MEMS Applications.

Mastering non-evaporable getter (NEG) thin films by elucidating their activation mechanisms and predicting their sorption performances will contribute to facilitating their integration into micro-electro-mechanical systems (MEMS). For this aim, thin film based getters structured in single and multi-metallic layered configurations deposited on silicon substrates such as Ti/Si, Ti/Ru/Si, and Zr/Ti/Ru/Si were investigated. Multilayered NEGs with an inserted Ru seed sub-layer exhibited a lower temperature in priming the activation process and a higher sorption performance compared to the unseeded single Ti/Si NEG. To reveal the gettering processes and mechanisms in the investigated getter structures, thermal activation effect on the getter surface chemical state change was analyzed with in-situ temperature XPS measurements, getter sorption behavior was measured by static pressure method, and getter dynamic sorption performance characteristics was measured by standard conductance (ASTM F798–97) method. The correlation between these measurements allowed elucidating residual gas trapping mechanism and prediction of sorption efficiency based on the getter surface poisoning. The gettering properties were found to be directly dependent on the different changes of the getter surface chemical state generated by the activation process. Thus, it was demonstrated that the improved sorption properties, obtained with Ru sub-layer based multi-layered NEGs, were related to a gettering process mechanism controlled simultaneously by gas adsorption and diffusion effects, contrarily to the single layer Ti/Si NEG structure in which the gettering behavior was controlled sequentially by surface gas adsorption until reaching saturation followed then by bulk diffusion controlled gas sorption process.

The Effect of Pulsed Current and Organic Additives on the Hydrogen Incorporation in Electroformed Copper Used in High Vacuum Applications

The trend in electron accelerators design consists in approaching the poles of the steering magnets close to the electron beam. This implies reducing the bore hosting the vacuum chamber and using very small diameter vacuum pipes [1]. The application of functional thin films by physical vapour deposition in such small diameter chambers becomes then very difficult. A new approach relies on copper electroforming to build the vacuum chamber onto a pre-deposited thin film coating [2]. In this method, unlike the usual process, the pipe is built from the inside out. An aluminium mandrel is coated with a functional thin film first, then by a copper film and finally followed by a thick electroplated copper (1-3mm) structural layer. The replacement of high-purity copper parts by electroformed ones may nevertheless lead to the incorporation of various impurities among which hydrogen, which is generated by solvent reduction and thus trapped into the coating. The aim of the present study is the evaluation of the effect of two main triggers, namely polarization wave modulation and copper electrolyte composition by the addition of organic additives. For this purpose, transient curves in response to pulsed currents have been studied for various cathodic current densities. Two main patterns have been observed as shown in figure 1a in the absence of additive. At low current densities, i.e. below 50 A/dm2, the potential increases rapidly and reaches an average constant value. For higher current densities, two characteristic plateaux can be observed: the first one corresponds to the faradic reduction of copper below the diffusion limit, and the second one, at higher potential, is attributed to the diffusion-limited range that is well known for promoting the hydrogen generation. When adding 33 mg/L of d-xylose, it is possible to reach higher current densities before the occurrence of the transition between the faradic reduction of copper, near the electrode, and the diffusion-limited mechanism. The d-xylose acts clearly by inhibiting the hydrogen discharge. Thank to this, four singular situations have been identified on the curves (marked by the stars on figure 1). They correspond to four pulse sequences (40A/dm2, Ton=20 ms), (40A/dm2, Ton=80 ms), (70A/dm2, Ton=20 ms), (70A/dm2, Ton=80 ms), which were used to prepare samples for characterization in terms of hydrogen outgassing by Thermal Desorption Spectroscopy (TDS) and X-Ray Diffraction (XRD) measurements. TDS experiments show two peaks (figure 2), corresponding to the release of hydrogen trapped into the copper under different forms. The temperature of desorption of the first peak (around 420°C for a ramp of 10K/min) corresponds well to hydrogen interstitial diffusion in the copper deposit [3]. The nature of the second peak (around 600°C) is not yet understood. [1] C. Steier, A. Anders, D. Arbelaez, J.M. Byrd, K. Chow, S. De Santis, R.M. Duarte, J.-Y. Jung, T.H. Luo, A. Madur, H. Nishimura, J.R. Osborn, G.C. Pappas, L.R. Reginato, D. Robin, F. Sannibale, D. Schlueter, C. Sun, C.A. Swenson, W.L. Waldron, E.J. Wallen and W. Wan, "Progress of the R&D towards a diffraction limited upgrade of the Advanced Light Source", in Proceedings of IPAC, Richmond, USA (2015), p. 1840. [2] L. Lain Amador, P. Chiggiato, L. M.A Ferreira, V. Nistor, A. T. Perez Fontenla, M. Taborelli, W. Vollenberg, M-L Doche, J-Y Hihn “Development of copper electroformed vacuum chambers with integrated non-evaporable getter thin film coatings” Journal of Vacuum Science and Technology A, accepted. [3] Y. Fukai “Formation of superabundant vacancies in M–H alloys and some of its consequences: a review” Journal of Alloys and Compounds 356–357 (2003) 270–273. Figure 1

Development of copper electroformed vacuum chambers with integrated non-evaporable getter thin film coatings

First results concerning a new approach of TiZrV non-evaporable getter (NEG) thin films coating on very small diameter vacuum chambers are presented. This new process is based on the electroforming of a vacuum chamber around a sacrificial mandrel, which is precoated with a NEG thin film. Aluminum was selected as the material of the mandrel and magnetron sputtering deposition for the coating. To improve the quality of the NEG coating, different coating layer sequences were applied and tested. The NEG activation was characterized by x-ray photoelectron spectroscopy (XPS). Data from flat samples prepared with the new technique were compared with those produced by the common magnetron sputtering method. Afterward, vacuum chambers equipped with flanges were produced. In addition to the XPS characterization, pumping speed measurements were performed. The results show CO pumping speeds comparable with the ones measured on a standard NEG coated vacuum chamber. However, H2 pumping speed exhibits ten times lower values than expected for standard NEG.

Research on deposition rate of TiZrV/Pd film by DC magnetron sputtering method

An accelerator storage ring needs clean ultra-high vacuum. A TiZrV non-evaporable getter (NEG) film deposited on interior walls of the chamber can realize distributed pumping, effective vacuum improvement and reduced longitudinal pressure gradient. But accumulation of pollutants such as N2 and O2 will decrease the adsorption ability of the NEG, leading to a reduction of NEG lifetime. Therefore, an NEG thin film coated with a layer of Pd, which has high diffusion rate and absorption ability for H2, can extend the service life of NEG and improve the pumping rate of H2 as well. In this paper, with argon as discharge gas, a magnetron sputtering method is adopted to prepare TiZrV-Pd films in a long straight pipe. By SEM measurement, deposition rates of TiZrV-Pd films are analyzed under different deposition parameters, such as magnetic field strength, gas flow rate, discharge current, discharge voltage and working pressure. By comparing the experimental results with the simulation results based on Sigmund’s theory, the Pd deposition rate C can be estimated by the sputtered depth.

Synthesis of Ti-Zr-V Non-Evaporable Getter Thin Films Grown on Al Alloy and CuCrZr Alloy

The Ti-Zr-V non-evaporable getter (NEG) films were grown on Aluminum (Al) alloy and CuCrZr alloy, which can be used to fabricate the vacuum chambers in the ultra-high vacuum status. The Al alloy and CuCrZr alloy samples with different surface roughness were prepared by the different manufacturing methods. We studied whether the behavior and the microstructure of the Ti-Zr-V getter films are influence by the surface roughness of the substrate. The surface morphologies of Ti-Zr-V NEG films appear distinct and the growth of the films follows the nature of the substrate surface. The Ti-Zr-V films have nanocrystalline structures and the grain sizes of the films become slightly larger with increasing the surface smoothness. In addition, it was found that the reduction of the Ti-Zr-V NEG films to the metallic state was affected by presence of surface defects on the films. The surface defects should result from the existence of micro-pores, pockmarks, and micro-cracks on the original substrate, which produced from the manufacturing process.

Low Activation Temperature Au/Ti Getter Films for Wafer-Level Vacuum Packaging

Vacuum packaging is required for mechanical microresonators and microbolometers to obtain a high quality factor and a low heat loss respectively. Because the internal volume of wafer-level packages is small, integration of getter films in the cavity is needed to maintain a low internal pressure (<10-2 mbar) for a long period of time (several years or more) even if a highly hermetic wafer bonding process is used [1,2]. Many Non-Evaporable Getter (NEG) films based on sputtered highly reactive transition metal have been investigated for this purpose. In this work we investigate thermal diffusion of titanium through an ultrathin capping Au layer as an alternative way to produce a getter film activated at low temperature. Both films were evaporated to avoid rare gas contamination found in sputtered films which can outgas afterwards. [3]The films have been annealed in N2 and Ti out-diffusion from Au/Ti thin films was studied as a function of the annealing temperature using four probe resistivity measurements, spectroscopic ellipsometry, XPS analysis, residual stress variations, SEM cross section observations and Energy Dispersive X-Ray Spectrometry (EDX) measurements. These measurements were completed by AFM and SEM surface analyses. All results are consistent and support the occurrence of significant Ti out-diffusion followed by oxidation, even at low temperature (<300°C). The electrical resistivity already increases by 10% after 1 hour annealing at 200°C. XPS measurements confirm the presence of a TiOx layer after annealing which contribution increases with the temperature and with a composition close to x » 1.8. SEM and AFM images of the as-deposited films surface show a smooth surface of gold, with RMS roughness below 1.5 nm. After annealing at 300°C, the titanium oxide is visible at the surface, in a shape of flakes and after annealing at 350°C, this layer covers completely the gold. Analysis of the sample cross-section shows that after annealing the gold layer is buried under the TiOx layer, that this layer is continuous and that its depth, corresponding to the TiOx thickness, increases with the annealing temperature (see fig., left-SE and right-BSE). Finally, TiOx films of several nm can be obtained by diffusion of the Ti through the Au at low temperature, showing that Au/Ti film can be used as a low temperature getter film for wafer-level vacuum packaging.[1]. M. Deniel, A. Naumann, D. Sorger, S. Tenholte, J. Voigt, O. Mehner, The influence of vacuum on the design and characterization of MEMS, Vacuum, 2012, 86 pp. 536-546.[2]. S. García-Blanco, P. Topart, Y. Desroches, J. Caron, F. Williamson, C. Alain and H. Jerominek, Low temperature hermetic wafer-level package for uncooled microbolometer FPAs, Proceedings of SPIE, 2008, 68840P-1.[3]. L. Tenchine, X. Baillin, C. Faure, P. Nicolas, E. Martinez, NEG thin films for under controlled atmosphere MEMS packaging, Sensors and Actuators A, 2011, 172 pp. 233-239.

Thin films of Ti–Nb–Zr as non-evaporable getter films

Thin films of Non-evaporable getter (NEG) Ti–Nb–Zr, have been deposited using a DC magnetron sputtering technique. The NEG films were produced from a Ti–Nb–Zr target in the form twisted wires of pure individual metals combined to give the appropriate stoichiometry. The performance and morphology dependence on deposition pressure, sputtering conditions and substrate surface roughness are investigated. Energy dispersive X-ray and X-ray photoelectron spectroscopic techniques are utilized to characterize and study the activation behaviour of the NEG thin films. After characterizing with surface techniques the activation temperature of this alloy is found to be in the range 250 – 300°C for 2 h heating with well defined composition. It is seen that film topography and density depend largely on the substrate surface roughness, while the activation temperature depends almost exclusively on the final stoichiometry of the thin film.

NEG thin films for under controlled atmosphere MEMS packaging

Non-evaporable getter (NEG) thin films for residual gases control in micro-electro-mechanical systems (MEMS) cavities after sealing are considered. For this study, several getters are deposited and analyzed in terms of sorption performances, microstructure and activation mechanism. Compared to single layer evaporated titanium reference, the addition of a chromium sub-layer is shown to decrease the activation temperature of the NEG. Moreover, when activated under 0.2mbar of N2, nitrogen diffusion in the titanium layer is enhanced by sub-layer addition. Effective pumping of nitrogen is monitored by residual gas analysis (RGA). Transmission electron microscopy investigation of as-deposited thin film getters reveals Ti layer microstructure modification due to platinum sub-layer addition. Furthermore, activation under nitrogen atmosphere is shown to induce microstructure modifications for both getters. X-ray photoelectron spectroscopy (XPS) and depth profiling monitoring of NEG activation under high vacuum conditions (10−8mbar) shows the dissolution of the NEG surface oxide in the getter bulk, along with the formation of carbide and nitride compounds. These results are in good agreement with already published results on getter alloys.

UHV-ERDA investigation of NEG coatings

Abstract Non-evaporable getter (NEG) coatings are widely used in large vacuum systems such as particle accelerators. Since NEG coatings produced at the GSI Helmholtzzentrum fur Schwerionenforschung have a finite thickness of typically 1 μm, ion beam analysis techniques such as ERDA and RBS are the methods of choice to study element and depth-resolved evolution of the getter film during gas pumping. The film thickness and stoichiometry can be determined by RBS while ERDA is sensitive to lighter elements such as carbon, nitrogen and oxygen, the main residual gas components of a vacuum environment. In this work we first present exsitu and insitu measurements of NEG thin films using ERDA. For the investigation of the pumping speed and capability of the getter films, we have used a vacuum test chamber where getter coated samples can be activated and saturated in a very controlled way. Afterwards, depth distributions of the pumped gas components have been measured using ERDA. To study the gas components in NEG coatings online, i.e., during activation or pumping, we have set up an ERDA system working at vacuum pressures down to 10 - 11 mbar . A special target heater system allows to activate the getter coatings insitu at typical temperatures between 180 and 300 °C in a UHV environment with tolerable influence to the total and partial gas pressure. With this UHV-ERDA setup, a temperature dependent diffusion process of gas components into the getter material could be analyzed for the first time.

NEG thin films for under controlled atmosphere MEMS packaging

Non-evaporable getter (NEG) thin films for residual gases control in micro-electro-mechanical systems (MEMS) cavities after sealing are considered. For this study, several getters are deposited and analyzed in terms of sorption performances, microstructure and activation mechanism. Compared to single layer evaporated titanium reference, the addition of a sub-layer is shown to decrease the activation temperature of the NEG. Effective pumping of nitrogen is monitored by Residual Gas Analysis (RGA). Microstructure investigation by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) reveals no apparent modification in NEG morphology due to sub-layer addition. X-ray Photoelectron Spectroscopy (XPS) and depth profiling monitoring of NEG activation under high vacuum conditions (10−8 mbar) shows the dissolution of the NEG surface oxide in the getter bulk, along with the formation of carbide and nitride compounds. These results are in good agreement with already published results on getter alloys.

Electron stimulated carbon adsorption in ultrahigh vacuum monitored by Auger electron spectroscopy

Electron stimulated carbon adsorption at room temperature has been studied in the context of radiation induced surface modifications in the vacuum system of particle accelerators. The stimulated carbon adsorption was monitored by Auger electron spectroscopy (AES) during continuous irradiation by 2.5 keV electrons and simultaneous exposure of the sample surface to CO, CO2, or CH4. The amount of adsorbed carbon was estimated by measuring the carbon Auger peak intensity as a function of the electron irradiation time. Investigated substrate materials are technical oxygen-free electrolytic copper and TiZrV nonevaporable getter thin film coatings, which are saturated either in air or by CO exposure inside the Auger electron spectrometer. On the copper substrate electron induced carbon adsorption from gas phase CO and CO2 is below the detection limit of AES. During electron irradiation of the nonactivated TiZrV getter thin films, electron stimulated carbon adsorption from gas phase molecules is detected when either CO or CO2 is injected, whereas the CH4 partial pressure has no influence on the carbon Auger peak intensity evolution.

The secondary electron yield of TiZr and TiZrV non-evaporable getter thin film coatings

The secondary electron yield (SEY) of two different non-evaporable getter (NEG) samples has been measured ‘as received’ and after thermal treatment. The investigated NEGs are TiZr and TiZrV thin film coatings of 1 μm thickness, which are sputter deposited onto copper substrates. The maximum SEY δ max of the air exposed TiZr and TiZrV coating decreases from above 2.0 to below 1.1 during a 2 h heat treatment at 250 and 200°C, respectively. Saturating an activated TiZrV surface under vacuum with the gases typically present in ultra-high vacuum systems increases δ max by about 0.1. Changes in elemental surface composition during the applied heat treatments were monitored by Auger electron spectroscopy (AES). After activation carbon, oxygen and chlorine were detected on the NEG surfaces. The potential of AES for detecting the surface modifications which cause the reduction of SE emission during the applied heat treatments is critically discussed.

XPS analysis of the activation process in non-evaporable getter thin films

The surface activation process of sputter-coated non-evaporable getter (NEG) thin films based on Ti–Zr and Ti–Zr–V alloys has been studied in situ by means of x-ray photoelectron spectroscopy. After exposure of the NEG thin films to ambient air they become reactivated after a thermal treatment in an ultrahigh vacuum. In our case the films are heated up to ∼250 °C for 2 h in a base pressure of ∼10−9 Torr. Copyright © 2000 John Wiley & Sons, Ltd.

A novel route to extreme vacua: the non-evaporable getter thin film coatings

Vacuum chambers sputter-coated with a thin film (∼1.5 μm) of a getter material, and subsequently exposed to ambient air, have been found to recover a pumping action after “in situ” bakeout. Coatings of Ti, Hf, Zr have been initially produced and studied, followed by their binary quasi-equiatomic alloys TiHf, HfZr, TiZr. All these coatings display activation at temperatures lower than 400°C, as observed by means of X-ray photoelectron spectroscopy, electron stimulated desorption (ESD), pumping speed and ultimate pressure measurements. The lowest activation onset temperature, about 200°C, has been measured for the quasi-equiatomic TiZr alloy. A further activation temperature decrease has been achieved for TiZr based ternary alloys/compounds, which are currently being investigated.