Studies of photon induced gas desorption using synchrotron radiation

In view of finalizing the design of the vacuum system of the Large Electron and Positron Storage Ring (LEP) we have studied synchrotron radiation induced neutral gas desorption. A 3 m section of an aluminum vacuum chamber has been exposed to the photon beam emerging from the electron storage ring DCI in Orsay, under conditions closely simulating the environment in a particle acceletor. In order of importance the gases desorbed were H₂, CO₂, CO and CH₄ with H₂O practically absent. Under the experimental conditions of an unbaked chamber and 11 mrad glancing incidence of the photons, the initial molecular desorption yields for these gases were typically 0.5, 8 × 10^?2, 2 × 10^?2 and 8 × 10^?3 molecules per photon respectively. These values could be reduced by about 1 to 2 orders of magnitude during continued photon exposure and most cases without evidence that this ‘beam cleaning action’ would be limited. After exposure to air and pumpdown of the previously cleaned chamber, we observe a significant memory effect. The dependence of the photon desorption on the angle of incidence has been studied down to a glancing angle of 11 mrad showing a definite deviation from the previously assumed 1/sin ø scaling. The implications of the results in terms of the expected beam-gas lifetime in LEP are discussed.     

Bombardon: an apparatus for investigating the influence of 5–20 keV ions on metal targets

An apparatus for ion bombardment in the energy range of 5–20 keV is described. The ion beam, produced by a duoplasmatron ion source, is purified by a magnetic sector field and directed into a target chamber 60 cm in diameter. The chamber is evacuated by sputter ion and titanium sublimation pumps with a total pumping speed of 20000 ls^?1 for H₂. The energy and angular distributions of particles backscattered from metal targets can be measured with an electrostatic spherical condenser which can be pivoted around the target. The whole apparatus is bakeable up to 250°C. With a 100 μA proton beam a pressure of 10^?9 torr is maintained in the target chamber. The black off pressure is a few 10^?11 torr.     

Stress tuning in AIN thin films

Aluminium nitride films were deposited on fused silica by reactive dc magnetron sputtering from an Al-target in an Ar/N₂ atmosphere. In-situ measurements during deposition provided data concerning mechanical stresses inherent to the growing thin films. By variation of both the gas composition (Ar, N₂) and the total gas flow in the vacuum chamber, the occuring intrinsic stresses could be shifted in magnitude and direction. Stress values of the AIN films ranged from ?0.9 GPa (compressive) to +1.2 GPa (tensile) when the Ar/N₂ ratio was varied between 3:1 and 1:3 for the different total gas flows of 50 sccm, 100 sccm, and 200 sccm (corresponding to total gas pressures of approximately 2 × 10^?1 Pa, 4 × 10^?1 Pa, and 8 × 10^?1 Pa respectively). Investigations of optical and structural film properties were carried out and the results were related to the observed film stress.     

Investigation by techniques of electron stimulated desorption of the merits of grow discharge cleaning of the surfaces of vacuum chambers at the CERN intersecting storage rings

To provide surface cleaning of the CERN intersecting storage rings, in which the pressure is below 10^?10 torr over a 2 km length of mostly elliptical pipe (160 X 52 mm), ion bombardment during a glow discharge has been utilized. The discharge in argon at a pressure of about 3 × 10^?2 torr was carried out intermittently in the course of a 15 h bake-out at 300°C. Subsequent electron scrubbing enabled the electron desorption efficiency of the surface of the ISR vacuum chambers to be reduced by two to three orders of magnitude. Information is given on the apparatus used and the procedure adopted, together with a discussion of the diagnostic technique and a consideration of the action of the discharge cleaning.     

Studies of photon induced desorption of surface gas within an aluminium vacuum chamber using an X-ray source

The role of photon induced desorption of gases arising from hard X-rays has been investigated within a vacuum chamber of aluminium alloy (Extrudal) as proposed for the new European LEP accelerator. The radiation was derived from a commercial 100 kV X-ray unit, and was introduced into the test chamber of length around 1 m by means of an end window formed of aluminium foil 25 ωm thick. In this manner a photon flux density of up to some 10⁹ photons cm² s^?1 irradiated a total surface area of around 4 × 10³ cm². The mean energy of the radiation was continuously variable in the range 9–30 keV. Measurements of desorption efficiency appropriate to each species of desorbed gas have been carried out with chamber pressures at around 1.5 × 10^?8 and 5 × 10^?11 torr attained, respectively, with evacuation at room temperature and following bakeout at 150°C. Studies have further been extended to assess the contribution of the pretreatment of the chamber by glow discharge cleaning in argon gas. In all cases the desorbed gas species under irradiation are predominantly H₂ and CO₂ which contribute in excess of 80% of the total desorption. The relative preponderance of these species is influenced by the treatments of bakeout and glow discharge cleaning, although the total desorption efficiency at around 10^?1 mol photon^?1 is not strongly affected by these actions. The desorption efficiencies of all species and at all stages of preparation of the chamber are found to decrease with increasing mean energy of the radiation, a factor of reduction of one third being observed over the range of energies available in the test.     

The mechanism of N-H bond decomposition in silicon nitride films

Thin dielectric films produced by the ammonolysis of monosilane in the temperature range 750–900 °C have 5–30% of their nitrogen atoms and 0.5–5% of their silicon atoms chemically bonded with hydrogen.We investigated the process of N-H bond decomposition under heat treatment at 900–1100 °C in vacuum and in different ambient gases by means of multiple internal reflection spectroscopy in the IR. It was found that in vacuum or in an inert gas ambient the activation energy of the process is 65 ± 2 kcal mol^?1, which is close to the N-H bond dissociation energy. When hydrogen or ammonia are added to the inert gas the activation energy of the process increases; pureammonia (P_NH3 = 1.01 × 10³ hPa) or hydrogen (P_h2 = 1.01 × 10³ hPa) suppress the decomposition entirely. The mechanism of the process and some features of the degradation of MNOS memory devices are discussed.     

8. A specimen-exchange device for an ultra-high vacuum atom-probe field-ion microscope

A specimen-exchange device is described for an ultra-high vacuum field-ion microscope (FIM). This device completely eliminates the long pump-down period that is required if the FIM chamber is brought back to atmospheric pressure. The pressure in an air-lock is reduced to 10^?6 torr before the exchange takes place and the pressure in the FIM chamber remains below 10^?7 torr during the exchange and it drops to less than 3 × 10^?9 torr within 15 min after the exchange.     

Growth of polycrystalline Cd3As2 films on room temperature substrates by a pulsed-laser evaporation technique

Cd₃As₂ films were prepared by a pulsed-laser evaporation technique. The deposition onto fused quartz substrates was carried out in a vacuum chamber under a background pressure of about 0.6 × 10^?4 Pa. Energy-dispersive X-ray analysis, scanning electron microscopy, transmission electron microscopy and Hall measurements were used to characterize the films. The properties of the film as a function of their thickness and of the deposition parameters are discussed. It is shown that polycrystalline layers of Cd₃As₂ grow on substrates held at temperatures as low as 295 K. The room temperature electron concentrations and mobilities for such layers were (3.5–5.3) × 10¹⁸ cm^?3 and (0.6–1.06) × 10³ cm² V^?1 s^?1 respectively.     

Study of the velocity distribution in an intense pulsed hydrogen beam

We present the results of measurements of the velocity distributions of particles in a pulsed hydrogen beam obtained from a dissociator with a radio frequency discharge (duration 1.0 ms, repetition rate 1 Hz). It is shown that the hydrogen inside the dissociator is heated up to ～2800 K, so the thermal dissociation of hydrogen molecules is essential. In order to cool the atoms, the gas was let through a pyrex channel 5 mm in diameter. The cooling channel walls being at room temperature and the channel having a length of 50 mm, we have obtained a supersonic beam of hydrogen atoms with a Mach number M_| = 2.7±0.25. When the channel walls were cooled by the flowing liquid nitrogen and the channel was 70 mm long we obtained a beam of cooled atoms with a Mach number M_| = 4.14±0.35. The velocity distribution of atoms depends on the power of the rf discharge inside the dissociator and on the gas consumption per pulse, and varies during the discharge pulse. For a temperature of the cooling channel walls T_ch = 77 K, a gas consumption N = 3.3×10¹⁷ molecules per pulse and a discharge power of 0.23 kW cm^?3, we have obtained an atomic beam with intensity I(0) = (2.8±0.8)×10²⁰ atoms sr^?1 s^?1 and a most probable velocity ν_MP = (1.97±0.07)×10⁵ cm s^?1.     

Ion beam nitriding of titanium and titanium alloy

Titanium and Ti8A/1Mo1V alloy have been nitrided with an ion beam source of nitrogen or agon and nitrogen, at a total pressure of 2?10×10^?4 torr. The treated surface has been characterized by surface profilometry, X-ray diffractometry, Auger Electron Spectroscopy (AES), and microhardness measurements. It was found that tetragonal Ti₂N phase forms in pure titanium and Ti8A/2Mo1V alloy with traces of AIN in the alloy. Two opposite processes were found to compete during the ion beam nitriding: (a) formation of nitrides in the surface layers; (b) sputtering of the nitrided layers by the ion beam. The highest surface hardness, of about 500 kg mm^?2 in titanium and 800 kg mm^?2 in Ti8A/1Mo1V, was obtained by nitriding with an ion beam of pure nitrogen at 4.2×10^?4 torr, at beam voltage of 1000 V.     

How to use electrodeless drift chambers in experiments at accelerators

Several types of wide-gap electrodeless drift chambers, including those with dimensions 1×1 m², have been tested at accelerator beams. The chambers work with high efficiency (>99%), good spatial resolution (σ = 0.2−0.4 mm) and good linearity at flux rates up to 2×10⁵ particles⁻¹ per wire in spill, which corresponds to 3×10⁶ particles/s m² on colliders.It is shown that at the proper mode of operation beam intensity oscillations within the range of up to 4×10⁵ particles/spill per wire do not affect the chamber efficiency.     

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.     

Getter pumping to allow economical sputtering with xenon

Increases in sputter rate, under equivalent commercial operating conditions, can often be achieved by using inert gases heavier than Ar. High atomic weight targets and high sputter voltages result in Kr and Xe having higher sputter yields than Ar. The larger ionization cross sections of Kr and Xe also result in a plasma enhancement. The high consumption rate of sputter gas has previously prevented the economical utilization of Kr and Xe. A d.c.-powered Ti sputter target was added to an r.f. planar diode sputtering system. The sputtered titanium was deposited directly on the vacuum chamber walls to form a large room temperature getter pumping surface. Calculations showed an increase of two orders of magnitude in the pumping speeds of O₂, N₂, H₂O and H₂ over that of a conventional throttled 10 cm diffusion pump. This pumping speed allowed improved sputter cleaning of substrates and the deposition of higher purity films. Operating the Ti getter pump with the diffusion pump closed allowed sputtering with very little inert gas consumption. R.f. sputtering of a Ti target with 750 W at 8 mTorr gas pressure resulted in a sputter gas consumption rate of 3 × 10^-3 cm³ min^-1 for both Ar and Xe. This consumption rate corresponds to a Xe cost of less than 1 ¢ h^-1.     

Modifications induced by silicon and nickel ion beams in the electrical conductivity of zinc nanowires

This paper presents the modification in electrical conductivity of Zn nanowires under swift heavy ions irradiation at different fluences. The polycrystalline Zn nanowires were synthesized within polymeric templates, using electrochemical deposition technique and were irradiated with 80 MeV Si⁷⁺ and 110 MeV Ni⁸⁺ ion beams with fluence varying from 1 × 10¹² to 3 × 10¹³ ions/cm². I–V characteristics of exposed nanowires revealed a decrease in electrical conductivity with increase in ion fluence which was found to be independent of applied potential difference. But in the case of high fluence of Ni ion beam (3 × 10¹³ ions/cm²), electrical conductivity was found to increase with potential difference. The analysis found a significant contribution from grain boundaries scattering of conduction electrons and defects produced by ion beam during irradiation on flow of charge carriers in nanowires.     

Method for measuring a vacuum in an environment at liquid helium temperatures

The experimental results of a new method to measure a vacuum in an environment at liquid helium temperatures are presented. The method uses a thin superconducting wire suspended in the vacuum vessel, the wire is heated by a current pulse > l_c. The cool down time, which depends on the heat transfer into the rest gas and on the axial heat conductance of the wire, is a measure of the vacuum and it is detected by the recovery of the wire to superconductivity. The results exhibit good resolution in the range of pressures from 5 × 10^?3 to 5 Pa (5 × 10^?5 to 5 × 10^?2 mbar).     

Characteristics of a mini Drift chamber

The characteristics of a drift chamber with 3 mm drift gap were investigated at a particle beam momentum of 10 GeV/c. The chamber has good linearity of the drift characteristics. It was tested and could reliably work in a particle flux of ∼ 3 × 10⁵ s⁻¹ cm⁻². A spatial resolution of ∼ 45 μm in the centre of the drift gap has been achieved.     

Ion beam reactively sputtered silicon nitride coatings

An ion beam source was used to deposit silicon nitride films by reactive sputtering a silicon target with an Ar+N₂ beam. The nitrogen fraction in the sputtering gas was 0.05 to 0.80 at a total pressure of 6 to 20×10^?5 torr. The ion beam current was 50 mA at 500 V. A rate of deposition of about 2 nm min^?1 (0.12 μm h^?1) was found, and the spectra indicated that Si₃N₄ was obtained for a fraction of nitrogen higher than 0.50. However, the AES spectra also indicated that the sputtered silicon nitride films were contaminated with oxygen and carbon and contained significant amounts of iron, nickel, and chromium, most probably sputtered from the holder of the substrate and target.     

Copper nanoparticles synthesized in sapphire by ion implantation

Sapphire-based composite layers implanted with 40-keV Cu⁺ ions to a total dose of 1.0×10¹⁷ cm⁻² at an ion beam current density varied from 2.5 to 10 μA/cm² were studied using Rutherford backscattering and optical reflectance methods. The appearance of optical plasma resonance lines in the reflectance spectra indicates that ion implantation allows copper nanoparticles to be synthesized in the subsurface region of the dielectric crystal studied.     

Synthesis of zirconium silicide in Zr thin film on Si and study of its surface morphology

Zirconium silicide was synthesized on Si (100)/zirconium interface by means of swiftly moving 150 MeV Au ion beam. Thin films of zirconium (~60 nm) were deposited on Si (100) substrates in ultra high vacuum conditions using the electron-beam evaporation technique. The system was exposed to different ion fluencies ranging from 3 × 10¹³ to 1 × 10¹⁴ ions/cm² at room temperature. Synthesized zirconium silicide thin film reasonably affects the resistivity of the irradiated system and for highest fluence of 1 × 10¹⁴ ions/cm² resistivity value reduces from 84.3 to 36 μΩ cm. A low resistivity silicide phase, C-49 ZrSi₂ was confirmed by X-ray analysis. Schottky barrier height was calculated from I–V measurements and the values drops down to 0.58 eV after irradiation at 1 × 10¹⁴ ions/cm². The surface and interface morphologies of zirconium silicide were examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM shows a considerable change in the surface structure and SEM shows the ZrSi₂ agglomeration and formation of Si-rich silicide islands.     

Vapor‐Phase‐Gating‐Induced Ultrasensitive Ion Detection in Graphene and Single‐Walled Carbon Nanotube Networks

Designing ultrasensitive detectors often requires complex architectures, high‐voltage operations, and sophisticated low‐noise measurements. In this work, it is shown that simple low‐bias two‐terminal DC‐conductance values of graphene and single‐walled carbon nanotubes are extremely sensitive to ionized gas molecules. Incident ions form an electrode‐free, dielectric‐ or electrolyte‐free, bias‐free vapor‐phase top‐gate that can efficiently modulate carrier densities up to ≈0.6 × 10¹³ cm^?2. Surprisingly, the resulting current changes are several orders of magnitude larger than that expected from conventional electrostatic gating, suggesting the possible role of a current‐gain inducing mechanism similar to those seen in photodetectors. These miniature detectors demonstrate charge–current amplification factor values exceeding 10⁸ A C^?1 in vacuum with undiminished responses in open air, and clearly distinguish between positive and negative ions sources. At extremely low rates of ion incidence, detector currents show stepwise changes with time, and calculations suggest that these stepwise changes can result from arrival of individual ions. These sensitive ion detectors are used to demonstrate a proof‐of‐concept low‐cost, amplifier‐free, light‐emitting‐diode‐based low‐power ion‐indicator.