The influence of the substrate and the substrate temperature on the crystallinity of tin films

Thin films of tin were prepared on various substrates (NaCl, KCl, KBr and KI) and at various substrate temperatures (28–90°C) and were studied using transmission electron microscopy and selected area electron diffraction techniques. To determine the dependence on cleanness, tin was deposited on single-crystal surfaces prepared by vacuum cleaving. Films were prepared on substrate surfaces to which a parallel d.c. electric field (0–1000 V cm⁻¹) was applied. It was observed that the crystallites of tin films deposited on KBr were more preferentially oriented than those deposited on NaCl, KCl or KI. The effects of vacuum cleaving and of d.c. electric fields are discussed.     

Running experience with the vacuum system of the superconducting linac of the TESLA Test Facility

The superconducting linear accelerator of the TESLA Test Facility (TTF) at DESY/Hamburg is running successfully since several years. The main focus was so far on machine developments as well as on a proof of principle experiment for a self amplifying spontaneous emission free electron laser (SASE FEL) for the proposed 500 GeV e⁺e⁻ linear collider TESLA with integrated X-ray free electron laser (XFEL) laboratory. Presently, the machine is substantially modified to become a VUV-FEL user facility with tunable wavelengths in the nm range within 2004.The beam pipe of the accelerator contains sections operated at room temperature as well as at 2 K in the areas of the superconducting structures used for acceleration of the beam. Three cryogenic modules, each containing 8 solid niobium cavities have been tested. In addition to standard UHV requirements, the vacuum system for this machine needs to preserve the cleanliness of the superconducting cavity surfaces. Thus the preparation of all vacuum components includes cleaning steps to remove particle contaminations, installation of components into the machine under local clean rooms and special procedures for pump down and venting.     

Soft vacuum processing of materials with electron beams

The use of soft vacuum (10^–3 to 1 Torr) rather than high vacuum (10^–5 to 10^–4 Torr) in connection with electron beam processing of materials is discussed. It is argued that practical, technical and economic advantages can be realized in a wide range of applications from heat-treatment, through welding and melting processes, to vapour deposition and gas processing. The new technology of glow discharge electron beam sources is especially suited to soft vacuum processes. While mainly developed so far for welding applications, it can be applied to this much wider field which presently employs thermionic guns working in high vacuum conditions.     

Ion induced gas desorption problems in the ISR

Not least among the many stringent conditions which must be satisfied to achieve the proper operation of the Intersecting Storage Rings at CERN are those pertaining to its 2 km of ultrahigh vacuum. Apart from the direct influence of the residual gas molecules through nuclear and Coulomb scattering on the stored beam lifetimes (of the order of days during which the stored protons should travel the astronomical distance of some light days!) there are also indirect processes which additionally involve adsorption/desorption phenomena on the vacuum chamber wall. The beam of high energy protons creates low energy secondary ions in the residual gas. These ions, generated in the space charge potential of 1 kV (10 A)^?1 of proton beam, are accelerated by this potential onto the stainless steel vacuum chamber wall where a variety of adsorption/desorption phenomena may be stimulated. Desorbed molecules are fed back into the residual gas resulting in the possibility of creating an unstable run-away pressure increase which can finally destroy the stored beam. This danger depends on the local parameters of the vacuum system, and most critically, on the state of the vacuum chamber surface.The paper summarises recent observations in this field and describes the results of investigations aimed at producing cleaner surfaces in vacuum chambers. These investigations, which have included high temperature bakeouts, ion bombardment scrubbing by glow discharge, oxidation, etc, will show that under favourable conditions, net negative desorption coefficients can be obtained which turn the Intersecting Storage Rings into a large ion pump. Results, supported and compared with measurements using Auger and Sims surface analysis, are presented mainly from the empirical point of view with the emphasis on practical implications for machines where the surfaces of the vacuum system are subjected to ion bombardment.     

Vacuum stability and residual gas density estimation for the vacuum chamber upgrade of the ATLAS interaction region of the Large Hadron Collider

The CERN Large Hadron Collider (LHC) has 54 km of ultra-high vacuum (UHV) beam chambers out of which about 90% are at cryogenic temperature (1.9 K) and the rest at room temperature. During operation, the residual gas density in the beam pipes is dominated by beam induced effect such ion, electron and photon-stimulated gas desorption. Therefore, the computation of gas density profile is of great importance to confirm the vacuum stability, and to estimate the beam lifetime. Moreover, the gas density profiles are essential to determine the machine induced background in the experimental areas, and to define the pressure profile in the cryogenic sectors where there is no vacuum instrumentation available.In this paper, the vacuum stability is studied for a newly proposed upgrade of the vacuum chamber at the ATLAS interaction point, using the vacuum stability code called VASCO. The residual gas density profile along the ATLAS vacuum chambers and the effects of photon and electron flux hitting the vacuum chamber walls are presented and analysed.     

The electrical properties of polycrystalline tin films evaporated onto a cooled substrate

The electrical properties of polycrystalline tin films evaporated in vacuum (p≈10^-6 Torr) onto fused quartz substrates at temperatures T_e of 157 and 142 K were examined in situ. The influence of annealing temperature on the film resistivity and structure was studied. It was found that the size effect in polycrystalline tin films is consistent with the Wissmann model. On the basis of this model it was shown that grain boundary scattering is considerable and depends on the annealing temperature. The electron mean free path for the temperature range 100–370 K was found to vary from 17.9 to 5 nm.     

Epitaxial growth and thermal stability of silicon layers on crystalline gadolinium oxide

In this work, an unconventional approach for epitaxial growth of Si on single-crystalline rare-earth oxide is presented using molecular beam epitaxy under ultra-high vacuum. Surface and bulk crystalline structures as well as chemical content were examined. Silicon-on-insulator layers were fabricated by encapsulated solid phase epitaxy on Si(111) substrate. The gadolinium oxide capping layer was removed by wet-chemical etching. The remaining silicon layer is single crystalline without any impurities and exhibits 7 × 7 reconstructed surface after annealing in very low silicon flux in the growth chamber. The thermal stability of the fabricated silicon-on-insulator structure was studied by step-wise heating under ultra-high vacuum conditions. The fabricated ultra-thin (10-15 nm) silicon-on-oxide layers remain structurally and chemically stable up to 900 °C.     

A study of the infrared absorption spectra of thin amorphous films of molybdenum trioxide

The spectra of thin amorphous films of MoO₃ deposited by vacuum evaporation have been studied within the spectral range 4000–400 cm^–1 by the Fourier transform infrared technique. Some samples in the thickness range 100 to 400 nm were investigated at room temperature. Four samples were investigated in the substrate deposition temperature range 293 to 543 K. Some samples were annealed in vacuum in the temperature range 473 to 673 K and their IR spectra were recorded as soon as the samples were cooled to room temperature. It has been observed that annealing of the samples at 473 K or above and also heating the substrates in the temperature range 473 K or above, results in the formation of molybdenum species of lower oxidation state than the normal Mo(VI), i.e. Mo(V). This new oxidation state is formed by electron transfer from the oxygen 2p to the molybdenum 4d level.     

Fabrication of W target on carbon backing

¹⁸⁴W enriched isotopic target of 210 μg/cm² thickness on carbon backing of 100 μg/cm² thickness has been made in ultra-high vacuum environment by evaporation method using 6 kW electron beam at Inter University Accelerator Centre (IUAC). Hundred and thirty milligrams of enriched ¹⁸⁴W powder was used in this target preparation process. This target has been successfully used in two nuclear reaction experiments performed at IUAC. Methods adopted to convert the tungsten powder to a special pellet form in order to minimize the consumption of the expensive material, preparation of stress relieved carbon-backing foil, steps taken to make the carbon withstand the heat generated during the tungsten evaporation, the method of tungsten fabrication and details of ultra-high vacuum evaporator facility of IUAC are discussed.     

Optical properties of sub-surface silver nanoparticulate films on 8 MeV electron beam irradiated polymer blends

Results of the investigations carried out on the optical properties of silver nanoparticulate films deposited on the electron beam irradiated blends of polystyrene (PS) and poly(4-vinylpyridine) (P4VP) are reported. Sub-surface silver particulate films are deposited at a constant deposition rate of 0.4 nm/s on irradiated blends held at 455 K in a vacuum of 8 × 10^?6 torr. The optical absorption studies indicate that the morphology of the nanoparticulate films can be tuned by electron beam irradiation of polymer blends. The change observed due to irradiation is attributed to the formation of free radicals, thereby altering the polymer–metal interaction. A shift in the plasmon resonance peak towards longer wavelengths is observed for 50 nm thick silver particulate films deposited on softened 8 MeV electron irradiated P4VP as compared to unirradiated P4VP. The shift is more in the case of P4VP than for PS, possibly due to the combined effect of free radicals produced by irradiation and the lone pair of electrons present in P4VP in enhancing the metal–polymer interaction. The saturation of free radicals takes place in P4VP at doses higher than 25 kGy. Further, the results also indicate that the free radical creation due to irradiation is more in PS than in P4VP, as the saturation takes place at lower doses in a PS/P4VP blend as compared to P4VP and in a blend with higher PS content. X-ray diffraction studies on silver films deposited on unirradiated and irradiated, softened polystyrene substrates do not show any significant change due to irradiation.     

Vacuum sealing using surface activation bonding of Si wafer

Bonding technology of Si wafer for vacuum seal is important in MEMS. We have tried the vacuum seal using surface activation bonding without any binder. It is the ultimate bonding technique and gives the precise dimension due to the direct contact. The technique is, however, not easy. We have investigated the surface conditions in order to achieve the bonding. The surfaces cleaned by Ar ion beam bombardment were measured by XPS and AFM. The natural oxide on the Si surface was removed by Ar ion bombardment. The surface roughness depended on the condition and the irradiation time of the Ar ion beam. The surface bonding at room temperature was achieved for the clean surface of the surface roughness less than Ra = 1 nm, but it was not done with the rough surfaces more than Ra > 2 nm. The vacuum sealing was checked using the cavities made in the Si wafer. The cavity part sealed in vacuum was depressed in the atmosphere, which was measured using a needle-contact profiler and a 3D laser profiler. The gas in the cavity was measured with a mass spectrometer by clashing the seal in vacuum. Any other gas except Ar gas closed in the cavity was not detected. We concluded that the vacuum sealing using surface activation bonding of Si wafer was achieved. The sealing condition has not changed even after one year.     

Interfacial reactions in bimetallic Ag-Sn thin film couples

Interfacial reactions in bimetallic Ag-Sn thin film couples have been investigated by measurement of electrical resistance and contact resistance as a function of time and temperature in order to understand kinetic behaviour in the above system where the intermetallic phase γ-Ag₃Sn is formed. Since the reaction is found to start at room temperature, the conventional vacuum coating unit has been modified for preparing such films and conducting subsequent measurements without breaking the vacuum. The results from the above different methods of resistance measurement indicate that interfacial reactions are characterized by a mean diffusion coefficient of 10^-13 cm² s^-1 at room temperature. X-ray diffraction indicates growth of the γ-Ag₃Sn phase immediately after deposition. Scanning electron microscopy confirms the diffusion of tin into silver by grain boundary diffusion rather than by bulk diffusion. The results from transmission electron microscopy confirm the presence of a γ-Ag₃Sn phase.     

The characterization of magnesium oxide surfaces prepared by electron beam cleavage

The surface of freshly electron beam cleaved MgO was analyzed by in situ gold decoration experiments. The depositions were performed under controlled vacuum, temperature and supersaturation conditions. Particular attention was directed to the influence of ion and electron irradiation of the substrate surface prior to the depositions. Several significant results were obtained. The decoration behavior on electron beam cleaved (001)MgO is substantially different from published data obtained on mechanically cleaved (001)MgO surfaces. The differences are believed to be due to the extreme temperature conditions during the electron beam cleavage process. No such unusual behavior was found for the (111)-oriented MgO surfaces which are also generated during the electron beam cleavage process. Furthermore, the MgO surface is noticeably susceptible to electron irradiation damage under normal transmission electron microscope imaging conditions, and very small doses of noble gas ion bombardment induce strong structural surface damage which annihilates the differences in nucleation behavior between (100) and (111) substrate areas.     

Substrate-diffusion-controlled film growth: Silver and copper on lead (111)

Silver and copper were vapour deposited onto atomically clean lead (111) surfaces under ultrahigh vacuum conditions. Low energy electron diffraction, reflection high energy electron diffraction and Auger electron spectroscopy measurements in the same chamber showed that, for a 0.1 nm min^-1 impingement of silver or copper, the deposit completely re-evaporated. At 10 nm min^-1 both silver and copper condensed onto a lead (111) surface at room temperature. The silver deposit was a continuous-layer single crystal which had an atomically flat upper surface covered by an equilibrium monolayer of substrate atoms epitaxed to it. The results for the copper experiments were similar to those for silver, except for the presence of small amounts of misoriented copper (111) and polycrystalline copper.It is proposed that substrate diffusion during the condensation of silver or copper onto lead (111) is crucial in determining the mode of film growth. It is thought that two-dimensional nucleation results from a mixed mobile layer of adsorbate and substrate atoms.     

An ultra-high vacuum reflection scanning electron diffraction system

A reflection-mode scanning electron diffraction system which operates with ultra-high vacuum is described. The instrument was developed for the study of surfaces and their reactions with admitted gases, and specimen processing facilities are incorporated. An electron diffraction pattern formed using electrons in the energy range 20–50 kV is measured by magnetically scanning it across a defining aperture. A retarding-field energy filter is incorporated to remove inelastically scattered electrons. The intensities are measured using a scintillator and a photomultiplier tube. The accuracy of the measurement is ±2 per cent at a filter potential of 5 V, and a resolution of 1 × 10⁻⁴ radians is achieved. Typical results are presented.     

Fabrication of layered nanostructures by successive electron beam induced deposition with two precursors: protective capping of metallic iron structures

We report on the stepwise generation of layered nanostructures via electron beam induced deposition (EBID) using organometallic precursor molecules in ultra-high vacuum (UHV). In a first step a metallic iron line structure was produced using iron pentacarbonyl; in a second step this nanostructure was then locally capped with a 2-3 nm thin titanium oxide-containing film fabricated from titanium tetraisopropoxide. The chemical composition of the deposited layers was analyzed by spatially resolved Auger electron spectroscopy. With spatially resolved x-ray absorption spectroscopy at the Fe L? edge, it was demonstrated that the thin capping layer prevents the iron structure from oxidation upon exposure to air.     

A continuous-flow helium cryostat and sample holder with unrestricted manipulation for ion-scattering experiments in uhv

A cryostat plus a sample holder is described for use in ultra-high vacuum in the temperature range 29–600 K. The base temperature can be reached within 10 min starting from room temperature. The steady-state consumption rate of liquid helium is 500 ml h⁻¹.     

Chemistry and structure of GaAs surfaces cleaned by sulfur annealing

A new method for cleaning and passivating GaAs surfaces, sulfur annealing, is proposed. The GaAs surfaces are exposed to sulfur atoms generated by an Ag/AgI/Ag₂S/Pt electrochemical cell at elevated substrate temperatures without arsenic beam irradiation. The chemistry and structure of the GaAs surfaces cleaned by sulfur annealing are studied by synchrotron radiation photoemission spectroscopy and low energy electron diffraction (LEED). The S 2p and Ga 3d spectra indicate that Ga---S bonds are formed on the GaAs surfaces where no oxides remain. 4 × 1 and 2 × 1 LEED patterns were observed for the surfaces with an average sulfur layer thickness of 0.15 nm and 0.24 nm respectively. The surface band bending is found to be reduced by 0.2–0.3 eV for the sulfur-annealed surfaces, but slightly increased for the conventionally cleaned surfaces with arsenic beam irradiation. We conclude that the surface cleaning and the submonolayer sulfur passivation can be attained simultaneously by sulfur annealing without arsenic beam irradiation. The method is promising for pre-cleaning the GaAs surfaces before semiconductor crystal growth and thin film deposition.     

Growth of oriented Ag nanocrystals on air-oxidized Si surfaces: An in-situ reflection high energy electron diffraction study

Growth of Ag nanoislands on air-oxidized Si(001), (111) and (110) surfaces has been investigated by reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and cross-sectional transmission electron microscopy. We have shown that the oriented nanocrystalline Ag, similar to the epitaxial growth of Ag on clean Si surfaces, can be grown on oxide-covered Si surfaces. A thin oxide layer (~ 2-3 nm thick) is formed on ultra-high vacuum (UHV)-cleaned Si surfaces via exposure of the clean reconstructed surface to air. Deposition of Ag was carried out under UHV at different substrate temperatures and monitored by RHEED. RHEED results reveal that Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while, in spite of the presence of the oxide layer between Ag islands and Si, preferred orientations with an epitaxial relationship with the substrate evolve when Ag is deposited at higher substrate temperatures. STM images of the oxidized surfaces, prior to Ag deposition, apparently do not show any order. However, Fourier transforms of STM images show the presence of a short range order on the oxidized surface following the unit cells of the underlying reconstructed Si surface. It is intriguing that Ag nanoislands follow an epitaxial orientational relationship with the substrate in spite of the presence of a 2-3 nm thick oxide layer between Ag and Si. Apparently, the short range order existing on the oxide surface influences the orientation of the Ag nanoislands.