KATRIN NEG pumping concept investigation

The Karlsruhe Tritium Neutrino experiment (KATRIN) is going to use a very large electro-static tandem spectrometer to measure the electron spectrum from the tritium beta decay, where several kilometers of non-evaporable getter (NEG) strips (type ST707) are to be used to achieve the UHV requirements. Different geometrical configurations of the NEG strips have been studied by Monte Carlo simulations. It is shown that the resulting pumping speed of the getter pumps will allow to get a pressure below 10⁻¹¹ mbar in the huge vessel with a volume of 1400 m³. By systematic assessment of the statistics of the pumping surfaces, it could be demonstrated that the design is sound.     

Conceptual design of the vacuum system of the Spanish synchrotron light source (ALBA) storage ring

ALBA will be a third generation synchrotron light facility to be built near Barcelona (Spain). The design phase of ALBA is almost completed and the first components are ready to be ordered. Commissioning of the storage ring is foreseen to start at the end of 2008. The circumference of the storage ring of ALBA is 268.8 m and it will be divided into 16 vacuum sections by ultra high vacuum (UHV) gate valves. The vacuum chamber will be made of stainless steel with vertical aperture of 28 mm and 72 mm width. The vacuum chamber will be connected to an antechamber with a slot of 10 mm height. The antechamber will have the crotch absorbers which will absorb the unwanted synchrotron radiation. The pumping will be by sputter ion pumps (SIP), NEG pumps and titanium sublimation pumps (TSP), with an overall pumping speed from SIP of 57,400 l/s. This will maintain an average dynamic pressure of around 1 × 10⁻⁹ mbar to achieve a beam lifetime >15 h at the designed current.     

Design of the vacuum system for Diamond, the UK third generation light source

The Diamond synchrotron, which is due to come into operation with beam for users in 2007, is being constructed at the Rutherford Appleton Laboratory, Oxford. The design status of the vacuum system of the storage ring as at 31st December 2002 will be reported.Diamond is based on a 24 cell 3 GeV electron storage ring of 561.6 m circumference. As is the case for most such machines, the operational pressure has been specified as 10⁻⁹ mbar to give a beam lifetime >10 h at the design current of 300 mA. The storage ring vacuum system will use conventional technology and most of the vacuum vessels will be constructed of stainless steel. With the exception of the insertion device (ID) vacuum chambers, the ring has not been designed to be baked in situ, but all components will be vacuum baked as sub assemblies before installation. The vacuum system is designed to achieve the required pressure after 100 A h of beam conditioning using the pumping scheme, which will be described.Twenty-one ID straights are available for ID, of which seven will be installed at the start of operations. Three types of vacuum vessel will be used in these straights, a stainless-steel make-up pipe, a NEG coated narrow-gap vacuum chamber for conventional IDs and a wider vacuum chamber to house in vacua IDs. The ID straights have an isolation valve installed at each end, and in situ baking can be used in these restricted locations, for example to activate the NEG coatings.The vacuum system for the beam line front ends has been designed to provide good vacuum isolation between the storage ring and the experimental beam lines, whether or not an interposing window is fitted.Pumping schemes, pressure measurement and calculated pressure profiles will be described.     

Outgassing measurement of an LHC collimator and estimation for the NEG performances

The outgassing rate of the collimators in the Large Hadron Collider (LHC) at CERN has an important role for the life-time of the Non-Evaporable Getter (NEG), and an accurate analysis allows the definition of future activities, like NEG vacuum activation. For these reasons, both, total outgassing rate and gas composition of a secondary collimator have been measured in the laboratory. The outgassing rate decreases by about two orders of magnitude by after bake-out and moreover, repeated bake-out further reduced the outgassing rate. The gas transmission through the NEG coated beam pipes and the resulting pressure distributions near the collimator were also measured in a dedicated setup. It is found that the main gas component after just 2 m of NEG coated beam pipe is CH₄ due to the extreme pumping speed of NEG for the other gases. Large amount of outgassing for H₂ and carbon related molecules are released when moving the collimator jaws. It is found that the NEG is very effective even in such case with large gas load.     

Effect of coating morphology on the electron stimulated desorption from Ti-Zr-Hf-V nonevaporable-getter-coated stainless steel

The electron stimulated desorption (ESD) was studied for quaternary Ti-Zr-Hf-V alloy coated stainless steel samples with different surface structures: dense on one sample and columnar on another. The ESD yields were measured as a function of electron accumulated dose up to ∼10²³ e⁻/m² or greater and three different NEG coating activation temperatures: 150, 180 and 250 °C. After each ESD experiment the samples were saturated with a mixture of H₂, CO and CO₂. Both samples depicted lower ESD yields for all desorbed species compared to a ternary Ti-Zr-V alloy. It was also shown that although the columnar NEG coating demonstrated better pumping properties and, for NEG activated at 150 °C, lower initial ESD yields, the higher activation temperature may result in a significant H₂ yield increase with dose for the columnar NEG coating. This effect was demonstrated for the first time and should be considered for application in particle accelerators.     

Synthesis and characterization of 5,7-dimethyl-8-hydroxyquinoline and 2-(2-pyridyl)benzimidazole complexes of zinc(II) for optoelectronic application

Bis(5,7-dimethyl-8-hydroxyquinolinato)zinc(II) (Me₂q)₂Zn and 5,7-dimethyl-8-hydroxyquinolinato(2-(2-pyridyl)benzimidazole) zinc(II) Me₂q(pbi)Zn have been synthesized and characterized by various techniques. These metal complexes have high thermal stability (>300 °C) and high glass transition temperatures (>150 °C). The vacuum deposited films of these materials show good film forming property and are suitable for opto-electronic applications. Multilayered organic electroluminescent (EL) devices have been fabricated having structure ITO/α-NPD/zinc complex/BCP/Alq₃/LiF/Al, which produce emission with chromaticity having Commission Internationale d’Eclairage (CIE) coordinates x = 0.506 and y = 0.484 for (Me₂q)₂Zn; x = 0.47 and y = 0.52 for (Me₂q)(pbi)Zn complex. The electroluminescence spectra show peak emission centered at 572 and 561 nm respectively for these materials.     

Outgassing from stainless steel and the effects of the gauges

The true outgassing from surfaces is often masked by the production of gases and pumping within the measuring gauge. Quadrupole mass analysers have been used to measure the outgassing from carefully prepared samples of stainless steel. By using an isolation pressure rise technique coupled to non-evaporable getter pumping, the problem of production of methane by the gauge is overcome and it has been possible to measure the outgassing of methane to very low levels. The specific outgassing rate of methane from 316 L stainless steel is found to be ?5×10⁻²² mbar l s⁻¹ cm⁻². The outgassing of the other gases has been measured using several gauges. The specific outgassing of hydrogen is found to be 4.5×10⁻¹⁵ mbar l s⁻¹ cm⁻² and the total of all other gasses amounted to no more than 10% of this figure. Although the measurements of the other gases are more ambiguous than those for methane, it is concluded that, in all probability, only hydrogen is outgassed from stainless steel.     

Thin film getter coatings for the GSI heavy-ion synchrotron upgrade

For the future project the Facility for Antiproton and Ion Research (FAIR) at GSI, an accelerator system with a base pressure of 10⁻¹⁰ Pa is required. The low pressure is needed to reduce the charge exchange rate between the accelerated ions and the residual gas molecules and therefore to increase the ion beam lifetime. Among the different measures undertaken to upgrade the existing UHV system, the installation of non-evaporable getter (NEG)-coated dipole and quadrupole chambers is foreseen. For this purpose a licence agreement for the non-evaporable thin film getters was signed between GSI and CERN in the end of June 2005.A new dedicated magnetron sputtering facility was designed and commissioned at GSI to perform the Ti-Zr-V coating on the dipole chambers of the heavy-ion synchrotron (SIS 18).Those pipes, made from stainless steel, have an elliptical cross section, are 3 m long, and are characterised by a wall thickness of 0.3 mm and a 15° bending angle.The characterisation of the thin films produced has been carried out by Rutherford backscattering spectroscopy (RBS), energy dispersive X-ray spectroscopy (EDX) and elastic recoil detection analysis (ERDA) for the chemical composition, scanning electron microscopy (SEM) for the morphology, and X-ray photoelectron spectroscopy (XPS) for the activation behaviour.The coating facility, its operating mode, and the first results obtained on the NEG characterisation by means of the different techniques will be described.     

非蒸散型吸气剂泵在真空计量中的应用研究

本文简要阐述了非蒸散型吸气剂(NEG)泵的吸气机理,着重介绍了NEG泵在超高/极高真空标准、静态膨胀法真空标准等真空计量标准中的最新应用结果。结果表明:利用NEG泵在室温下获得了10-10Pa的XHV;利用NEG泵将超高/极高真空标准校准下限延伸到了10-10Pa量级;利用NEG泵将静态膨胀法真空标准的校准下限延伸到了10-7Pa量级。最后还对用NEG泵延伸固定流导法微流量标准测量下限的可行性进行了分析。     

Structure and optical properties of Zr1−xSixN thin films on sapphire

A series of zirconium silicon nitride (Zr_1−xSi_xN) thin films were grown on r-plane sapphire substrates using reactive RF magnetron co-sputtering of Zr and Si targets in a N₂/Ar plasma. X-ray diffraction pole figure analysis, X-ray reflectivity, X-ray photoelectron spectroscopy (XPS), optical microscopy, and optical absorption spectroscopy were used to characterize the film stoichiometries and structures after growth at 200 °C and post-deposition annealing up to 1000 °C in ultra-high vacuum. The atomically clean r-plane sapphire substrates induce high quality (100) heteroepitaxy of ZrN films rather than the (111) orientation observed on steel and silicon substrates, but the addition of Si yields amorphous films at the 200 °C growth temperature. After the annealing treatment, films with Si content x < 0.15 have compressive stress and crystallize into a polycrystalline structure with (100) fiber texture. For x > 0.15, the films are amorphous and remain so even after ultra-high vacuum annealing at 1000 °C. XPS spectra indicate that the bonding changes from covalent to more ionic in character as Si―N bonds form instead of Zr―N bonds. X-ray reflectivity, atomic force microscopy (AFM) and optical microscopy data reveal that after post-deposition annealing the 100 nm thick films have an average roughness < 2 nm, except for Si content near x = 0.15 corresponding to where the film becomes amorphous rather than being polycrystalline. At this stoichiometry, evidence was found for regions of film delamination and hillock formation, which is presumably driven by strain at the interface between the film and sapphire substrate. UV-visible absorption spectra also were found to depend on the film stoichiometry. For the amorphous Si-rich films (x > 0.15), the optical band gap increases with Si content, whereas for Zr-rich films (x < 0.15), there is no band gap and the films are highly conductive.     

Influence of incidence angle and distance on the structure of aluminium nitride films prepared by reactive magnetron sputtering

Aluminum nitride (AlN) films were reactively deposited on (100) oriented silicon substrates by reactive radio frequency (RF) magnetron sputtering for different incidence angles and distances between substrate and target.X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to consider the influence of process parameters such as reactive gas flow rate, grazing incidence angle (α), and distance (d) between substrate and target surface on the property of AlN films. XRD results showed that AlN film prepared at a constant distance (d) of 3 cm and an incidence angle of 45° revealed a mixture of AlN (002), (100), and (101) planes, while the film prepared at α = 0° revealed a strong AlN (002) orientation which has a perpendicular growth direction to the substrate surface. AFM results showed that AlN film prepared at α = 0° exhibited more flat surface morphology than that of film prepared at α = 45°.     

Current transport mechanism in CdS thin films prepared by vacuum evaporation method at substrate temperatures below room temperature

CdS thin films were deposited by vacuum deposition method at low substrate temperatures instead of the commonly used vacuum deposition at high substrate temperatures (T_S > 300 K). The effect of low substrate temperature on the current transport mechanisms in polycrystalline CdS thin films has been studied as a function of temperature over the temperature range 100-300 K. Both thermally assisted tunneling of carriers through and thermionic emission over the grain boundary potential have contributions to the conduction in the range 250-300 K for the sample prepared at 300 K substrate temperature. The dominant conduction mechanism of the samples prepared at 200 K and 100 K is determined as thermionic emission over 200 K and Mott's hopping process below 200 K. The Mott's hopping process is not applicable for the sample prepared at 300 K.     

Annealing influence on electrical transport mechanism of electroless deposited very thin Ag(W) films

Morphology and conductivity (σ) of the non-tarnishing electroless Ag(W) films for interconnect were studied as a function of thickness (d) by Atomic Force Microscopy (AFM) and Tunneling Atomic Force Microscopy methods. For d ≤ 100 nm the conductivity dependence on thickness can be modeled as percolation of the electrical transport while for thicker d > 100 nm layers it was independent on d (σ = σ₀). A simple electrical circuit model that described the experimental dependence σ(d) for both thin and thick layers was proposed. The AFM study has shown that the small network changes in the film morphology, due to vacuum annealing cause the significant (few orders of magnitude) improvements in electrical conductivity. Although, near bulk conductivity was achieved for the thicker sample, vacuum annealing was not sufficient to achieve such conductivity for very thin Ag(W) films.     

Growth and characterization of tris(8-hydroxyquinoline)-aluminum molecular films

Various tris(8-hydroxyquinoline)-aluminum (Alq3) molecular solid films were grown on top of indium-tin-oxide (ITO) glass substrates using physical vapor deposition. The effect of changing the growth conditions on the properties of the films was studied. From scanning electron microscopy, an Alq3 planar layer over an ITO-substrate was observed at the initial period, and an Alq3 tubular structure (which becomes dominant at substrate temperature T_sub ≧ 90 °C) was found to nucleate from this layer. From X-ray diffraction, the Alq3 planar layer possesses an amorphous character while the Alq3 tubular layer has a triclinic α-phase structure. Based on an Arrhenius plot of the growth rate versus 1/T_sub, the growth behaviors in various T_sub-regions were discussed to be dominated by adhesion (for T_sub < 90 °C), steric effect (90 °C < T_sub < 150 °C), and re-evaporation (T_sub > 150 °C). Then, from optical transmission and photoluminescence spectra performed on the high crystalline Alq3 films, two signals associated with the optical-bandgap E_g absorption and the gap-state absorption were determined and discussed in terms of the optical properties of the constituent Alq3 molecules. Finally, from a fit of E_g(T) by an effective electron-phonon interaction model, the physical significance of these fitting parameters for the Alq3 molecular solid was investigated.     

LHC: The world's largest vacuum systems being operated at CERN

With the successful circulation of beams in the Large Hadron Collider (LHC), its vacuum system becomes the world's largest vacuum system under operation. This system is composed of 54 km of ultra high vacuum (UHV) for the two circulating beams and about 50 km of insulation vacuum around the cryogenic magnets and the liquid helium transfer lines (QRL). The LHC complex is completed by 7 km of high vacuum transfer lines for the injection of beams from the SPS and their dumping.Over the 54 km of UHV beam vacuum, 48 km are at cryogenic temperature (1.9 K), the remaining 6 km are at ambient temperature and use extensively non-evaporable getter (NEG) coatings, a technology that was born and industrialised at CERN.The cryogenic insulation vacuum systems, less demanding technically, impress by their size and volume: 50 km and 15,000 m³. Once cooled at 1.9 K, the cryopumping allows pressure in the 10⁻⁴ Pa range to be attained.     

Measuring volume ratios of vacuum vessels using non-evaporable getters

Several methods have been used in vacuum metrology for volume determination. Volume ratios can be determined by different gas expansion methods, which were primarily developed for the precise determination of expansion ratios in static expansion systems. Measurements of the volume ratios using the gas expansion methods below 10⁻² Pa are influenced by outgassing from chamber walls. To reduce outgassing in the expansion chambers during measurements at low pressures, we have installed non-evaporable getter (NEG) pumps, which pump hydrogen and other active gases but have a negligible pumping speed for inert gases. The volume ratio of two chambers of an experimental static expansion system has thus been measured using the inert gases argon, helium and krypton. The obtained results were compared with the measurements without the use of an NEG pump. Measurements of the pressure ratios were performed with a spinning rotor gauge (SRG).     

Electronic structure and formation mechanism of complex Ti-Nb oxide

Two-layer thin film specimens of Nb₂O₅ and TiO₂ were deposited on optical-grade quartz and n-type single crystalline silicon substrates with (100) crystallographic orientation by a magnetron deposition source under high vacuum. All samples were subjected to 1-5 h of resistive heating at ultra high vacuum, and in situ X-ray diffraction measurements (XRD) were made in the temperature range of 300-1373 K. Analysis of the XRD data confirmed the growth of TiNbO₄ during cooling of the two-layered specimens which had been previously heated to 1373 K. Optical measurements revealed a band gap value of 3.78 eV for the direct transition and 3.29 eV for the indirect one. The samples had a transmittance of 85% in the visible range. Electrophysical measurements in high vacuum established the electroresistivity vs. temperature dependence in the range of 300-773 K, from 7.3 ? 10^− 1 to 3.9 ? 10^− 2 Ω cm, respectively. X-ray photoelectron spectroscopy measurements were used to examine the chemical shift for Nb 3d, with a value of − 1.1 eV in comparison with Nb⁵⁺ and matched to Nb⁴⁺[1], while the Ti lines correspond to Ti⁴⁺[2].     

Investigations of failure mechanisms at Ta and TaO diffusion barriers by secondary neutral mass spectrometry

One of the most important processes in Cu metallization for highly integrated circuits is to fabricate reliable diffusion barriers. Recently, thin films made of refractory metals and their compounds have been widely used in solid-state electronics as barriers because of their good electric properties, favourable thermal properties and chemical stability. Thermal stability of Tantalum (Ta) and Tantalum-oxide (TaO_x) layers as a diffusion barrier in Si/Ta/Cu, Si/TaO_x/Cu and Si/Ta-TaO_x/Cu systems have been investigated. Si/Ta (10 nm)/Cu (25 nm)/W (10 nm), Si/TaO_x (10 nm)/Cu (25 nm)/W (10 nm) and Si/Ta (5 nm)TaO_x (5 nm)/Cu (25 nm)/W (10 nm) thin layers were prepared by DC magnetron sputtering. A tungsten cap layer was applied to prevent the oxidation of the samples during the annealing process. The samples were annealed at various temperatures (473 K-973 K) in vacuum. Transmission Electron Microscopy, X-ray diffraction, X-Ray Photoelectron Spectroscopy and Secondary Neutral Mass Spectrometry were used to characterize the microstructure and diffusion properties of the thin films. Our results show that at the beginning phase of the degradation of the Si/Ta/Cu system Ta atoms migrate through the copper film to the W/Cu interface. In the Si/TaO_x/Cu system the crystallization of TaO and the diffusion of Si through the barrier determine the thermal stability. The Ta-TaO bilayer proved to be an excellent barrier layer between the Si and Cu films up to 1023 K. The observed outstanding performance of the combined film is explained by the continuous oxidation of Ta film in the TaO_x-Ta bilayer.     

In-situ magnetic deposition field effect on sheet resistance of Permalloy films

We have measured the real-time in-situ sheet resistance (R_□) of a Permalloy (Py) film during the film-growth period in a vacuum under various magnetic deposition-field (H) conditions: a longitudinal direct-current (dc) field (H = H_x), a transverse dc field (H = H_y), and a rotating field (H = H_r) with the frequency (f) at 0.6, 1.4, 2.5, 10, and 100 Hz, respectively. The results show that R_□(H_y) > R_□(H_x) ? R_□(H = 0) ? R_□(H = H_r), when Py film is just before or near the coalescence stage of the film-growth process. The reason for these phenomena is that a dc field (H = 0, H_x, or H_y) tends to make the structure of the film more anisotropic, while a rotating field (H = H_r) tends to make the film more isotropic. Moreover, based on the quantum tunneling mechanism, the fact that there will be many more tunneling events through which an electron can be transported down a piece-by-piece connected film with an anisotropic than with an isotropic structure should naturally lead to the observed results above. Finally, we also explain why in the R_□ versus f plot (under the condition of film thickness t_f = 1 nm), there exists a minimum R_□ at f = f_min = 2.5 Hz.     

Thermal stability of amorphous molybdenum trioxide films prepared at different oxygen partial pressures by reactive DC magnetron sputtering

Thin films of MoO₃ were prepared on quartz and Si (1 0 0) substrates by reactive dc magnetron sputtering of a Mo target in an oxygen and argon atmosphere. The structural and optical changes induced in the films due to post-growth annealing have been systematically studied by Rutherford backscattering (RBS), X-ray diffraction (XRD), X-ray reflectivity (XRR) and by optical methods. RBS studies reveal no change in composition of the films upon annealing at high temperatures. Grazing angle XRD studies show that the as-deposited films are amorphous and crystallize to β-MoO₃ phase with small contribution of α-MoO₃ upon annealing at 300 °C. The film prepared at 0.40 Pa transforms to α-MoO₃ upon annealing at 650 °C, while the film deposited at 0.19 Pa still has some β-MoO₃ phase contribution. XRR measurements reveal that the film thickness decreases upon annealing with simultaneous increase of film density. The surface roughness of the films strongly increases after crystallization. The contraction of the film deposited at 0.40 Pa is much greater than the contraction of the film prepared at 0.19 Pa. The mass variation of the film deposited at 0.19 Pa and that deposited at 0.40 Pa are completely different. The optical properties of MoO₃ films deposited at 0.19 and 0.40 Pa are changed strongly by annealing.