Residual Gas Composition Research Articles

Low temperature and cold emission of scandate cathodes

The unprecedented emission capability of top-layer scandate cathodes prepared by laser ablation deposition (LAD) was already reported [1,2] yielding 460 A cm −2 at 1030 °C true temperature, which is a factor of about 20 higher than for the best impregnated (I) cathodes. Therefore their emission behaviour has been investigated down to room temperature, yielding 0.5 A cm −2 at 500 °C and 0.001 A cm −2 at 300 °C. The measurements underline, that the work function (WF) remains about constant over a wide temperature range. The effective work function with the theoretical thermionic constant of 120.4 A (cm K) −2 amounted to 1.42 eV, a Richardson plot determination resulted in a Richardson WF of 1.16 eV and a Richardson constant of 8.5 A (cmK) −2. Field emission was observed at room temperature and at about 300 °C for LAD-top-layer scandate cathodes (‘Sc’/Re–I) and was compared with other Ba dispenser cathodes. The threshold field strengths were 3.2 V μm −1 for ‘Sc’/Re–I, 12.4 V μm −1 for Re–I and 15.7 V μm −1 for W–‘I. The emission stability at low operating temperatures is dependent on residual gas pressure and composition.

Change in the Composition of Residual Gases in a Vacuum Chamber during Ti Film Deposition

Changes in the spectrum of chemically active residual gases in a vacuum chamber during the deposition of Ti films on its inside walls using magnetron sputter guns are investigated. It was revealed that the decomposition of hydrocarbons in the plasma of operating magnetrons and the interaction of the products of their decay with oxygen influence the gas composition.

Physical vapor transport of lead telluride

Mass transport properties of physical vapor transport of PbTe are investigated. Thermochemical analysis of the system and its implications for the growth conditions are discussed. The effect of the material preparation and pre-processing on the stoichiometry and residual gas pressure and composition, and on related mass flux is shown. A procedure leading to high mass transport rates is presented.

Contactless growth of ZnSe single crystals by physical vapor transport

ZnSe crystals were grown by self-seeded physical vapor transport (PVT) technique in the horizontal configuration. The source materials were heat treated by H 2 reduction to remove the oxides followed by baking under dynamic vacuum to adjust the source composition toward that of congruent sublimation. Contactless growth of ZnSe single crystals have been performed using three different source materials. The crystals grew away from the ampoule wall with large (1 1 0) facets tending to align parallel to the gravity direction. The SEM micrographs and the AFM images showed that large (1 1 0) terraces and steps dominate the as-grown facets. The measured residual gas pressure and composition (mostly CO and H 2) in the processed ampoules were similar for all source materials used. One-dimensional diffusion model on the mass flux of a multi-species PVT system was employed to analyze the conditions for contactless growth. The calculated thermal profile for supersaturation is very close to the thermal profile measured inside the empty furnace bore in the region of contactless growth. The effects of convective flows in the vapor phase inside the ampoule on the growth processes are discussed.

Comparison of a-C:H films deposited from methane–argon and acetylene–argon mixtures by electron cyclotron resonance–chemical vapor deposition discharges

Abstract Hydrogenated amorphous carbon (a-C:H) films are deposited from methane–argon and acetylene–argon gas mixtures in a microwave electron cyclotron resonance plasma reactor. The films deposited with the two different gas mixtures under similar input parameter conditions have substantially different properties, including deposition rate, mass density, optical absorption coefficient, refractive index, optical bandgap and hydrogen content. The deposition parameters varied include rf-induced dc substrate bias voltage (0 to −60 V), pressure (1–5 mTorr) and argon/hydrocarbon gas flow ratio (0–1.0). The discharge properties of the two different gas mixtures, including electron temperature, ion saturation current, and residual gas composition of the exit gas flow, are measured to help explain the different deposition results from the two different gas mixtures. The use of lower pressures is found to be critical for obtaining denser, lower hydrogen content films from acetylene. For the methane-deposited films the addition of argon to the discharge increased the film's mass density and lowered the hydrogen content. In both methane- and acetylene-based deposition processes the rf-induced bias is also a critical determining factor of film properties.

In-Situ Real-Time Depth Profiling by Elastic Recoil Detection and Its Application to Ion Nitriding of Stainless Steel

ABSTRACTA dual beam experiment has been set up combining real-time depth profiling and low energy ion implantation in order to study the kinetics of ion nitriding. The choice of low energy ion implantation allows precise and independent control of the important ion nitriding parameters ion energy, ion current density, temperature and residual gas composition. Real-time depth profiling is achieved by Elastic Recoil Detection (ERD) using an angle resolving ionization chamber telescope detector. A large solid angle (5.4 msr) allows a fast data acquisition and hence real-time depth profiling with a time resolution of about 30 s combined with a depth resolution of about 10 nm. The capabilities of the setup and its limitations will be discussed. Measurements revealing the role of mechanisms relevant for the nitrogen transport during ion nitriding of stainless steel i.e. diffusion, sputtering, adsorption of oxygen from the residual gas will be presented.

非ベーク真空系の圧力ビルドアップ特性

The outgassing rates of an unbaked vacuum chamber were measured by the orifice pumping and the pressre rise methods. It was found that the measured outgassing rates were dependent on pumping speed. The change of residual gas composition in the test chamber during measurement of outgassing rate was observed by a quadrupole mass analyzer. Before cut-off pumping, water vapor was a predominant desorbing gas, but after cut off pumping, hydrogen became a predominant desorbing gas instead of water.

The influence of residual gas pressures on the degradation of ZnS powder phosphors

Phosphor powder of ZnS:Cu,Al,Au has been subjected to electron bombardment (2 keV, 2 mA/cm2) in residual gas pressures ranging from 0.6×10−8 to 7.0×10−8 Torr. Auger electron spectroscopy and cathodoluminescence (CL), both excited by the same electron beam, were used to monitor changes in the surface chemistry and cathodoluminescent brightness versus vacuum conditions during electron bombardment. A direct correlation between the surface reactions and the degradation of CL brightness was observed. The formation of a nonluminescent ZnO layer on the surface of the phosphor was largely responsible for the degradation of the ZnS. The aging of the phosphor was not only a function of the charge per unit area (Coulomb dose) bombarding the surface, but also a function of residual gas pressure and composition. In particular, H2O had the greatest effect on the rate of degradation. The results are interpreted in terms of an electron-beam stimulated surface chemical reaction.

Temperature and vacuum measurements on the commercial assembly of a large-volume Ge detector

This work reports on tests which have been extensively made on the assembly of a “dummy” large-volume germanium detector with the aim of measuring temperature gradients along the crystal, as well as in different parts of the cryostat, during the steady cold, room temperature and annealing phases and in the transitions between them. Measurements of the vacuum level and trends of the residual gas composition in the cryostat in the different phases of operation are also reported.

Hrtem and Eels Studies of Reacted Materials From Caf2 by Electron Beam Irradiation

ABSTRACTThe change of calcium fluoride (CaF2) by electron beam irradiation has been investigated in TEMs operated at 200 kV. By irradiation fluorine is desorbed rapidly from CaF2 crystal. Although plasmon peaks in EELS spectra suggest the formation of Ca metal, no evidence for the existence of Ca metal is found in electron diffraction pattern or HRTEM images. This is due to perfect topotactic formation of Ca metal from CaF2 with a similar crystal structure and closely similar lattice parameters. Irradiation also forms amorphous material near the edge of Ca/CaF2 and randomly oriented CaO crystallites grow from the amorphous material. This amorphous material is regarded as hydroxide formed through the reaction of Ca and water, considering the residual gas composition and the formation rate of the material.

The effects of a quadrupole mass analyser on measurement of UHV residual gas composition

The effect of both the quadrupole mass analyser (QMA) and the ion-getter-pump (IGP) on the residual gas composition and on the composition during gas dosing were investigated. At the background pressure the measured height of mass 16 was more than 10–14 times the sum of contributions of methane and oxygen ions from other molecules and a significant amount of inert gases were found at this pressure level. Pure oxygen, carbon monoxide, hydrogen and a mixture of carbon monoxide and hydrogen were introduced into the vacuum system while the IGP was running. The final concentration of the dosed gas depended on the ratio of its pumping speed to that of the inert gases. The pumping speeds followed the sequence: oxygen ⪢ carbon monoxide ⪢ hydrogen. Exceptional behaviour of mass 16 was observed during introduction of carbon monoxide and hydrogen.

Parameters of Vacuum Process as the Factors Changing the Growth Kinetics of Chromized Layers Produced on Low-Carbon Iron Alloys by Means of CVD

Chemical vapour deposition of chromium on the surface of low-carbon iron alloys has been investigated using a novel CVD method that combines the low cost of pack metallisation with the advantages of vacuum technique. Chromizing processes have been carried out in a hot-wall vacuum oven with a mass spectrometer to determine the chemical composition of residual gases. The processes have been performed in chromium chlorides atmosphere at a low pressure range from 1 to 800 hPa, the treatment temperature 800 to 950°C. The effect of the vacuum level during the process and the process parameters such as time and temperature on the growth kinetics of diffusion layers has been determined. Studies of layer thickness, its morphology, its phase composition, Cr and Fe depth profiles in the diffusion zone of chromized layers have been conducted. The investigations have proved that the chromizing process under low pressure, with static vacuum instead of dynamic vacuum conditions during the holding is the most effective as far as the growth kinetics of diffusion layers is concerned. It has been shown that the kinetics of the process is controlled by the diffusion of chromium in the steel.

The effect of heat treatment on the magnitude and composition of residual gas in sealed silica glass ampoules

The residual gas pressure and composition in sealed silica glass ampoules as a function of different treatment procedures has been investigated. The dependence of the residual gas on the outgassing and annealing parameters has been determined. The effects of the fused silica brand, of the ampoule fabrication, and of post-outgassing procedures have been evaluated.

Experimental results on the space charge compensation of ion beams in a magnetic dipole field

The space charge potential of a positively charged ion beam traversing a magnetic dipole field has been measured with a modified Langmuir probe. The measurements were carried out in the homogeneous field region as well as in the fringe field using dc ion beams of the elements He, Ne, Ar and Kr in the energy range between 30 and 45 keV. The magnetic field strength was varied from 0.05 up to 0.35 T. The ion beam potential depends on the energy and the mass of the ions, the current and the diameter of the ion beam and on the pressure and the composition of residual gas inside the beam transport system. The results are related to that obtained without magnetic field. The ion beam potential shifts to larger values with increasing magnetic field strength.

Use of quadrupole mass spectrometers in ultrahigh vacuum systems

In ultrahigh vacuum systems quadrupole mass spectrometers are most widely used for the determination of the residual gas composition as well as for process control, e.g. at higher pressure. Quadrupoles here have several advantages over other types of mass spectrometers such as ease of installation and operation, small size, high dynamic range, fast measuring speed and no magnetic fields. For residual gas measurements in the extreme UHV specially adapted quadrupoles must be used. They have to show adequately low outgassing rates for keeping the original gas composition undisturbed. This is obtained by the choice of special materials and by a high temperature treatment of all components. In addition, very low detection limits are needed. This requires a special detector design for the suppression of possible background signals. An overview on the detection capabilities, background effects, possible artifacts, operation software as well as multiplexing opportunities will be given. Another application for quadrupoles in uhv systems is process control. Examples here are (a) parallel determination and on-line closed loop control of the evaporation rates of several sources during, e.g. molecular beam epitaxy (MBE) processes utilizing the fast measuring speed ; and (b) process gas monitoring during, e.g. sputter processes utilizing the high dynamic range.

Characterization and modeling of the complete volcanic gas phase.

During the survey on the eruption of the volcano “La Soufriere de la Guadeloupe” (French West Indies), the behaviour of sulfur in volcanic gases was examined. Matsuo (1962) studied chemical equilibrium in volcanic gases which led us to develop the “in situ” gas analysis. A field gas chromatograph allowed direct injection of hot gases, before water and sulfur condensation occurred. A silica tube equipped with thermocouples was used for sublimates sampling and for measuring the condensation temperature. Conventional condensors and caustic soda bottles were used for sampling and later complementary analyses in the laboratory. A free energy minimisation computational method modelled the physical and chemical changes that occurred during cooling of volcanic gases. The high temperature composition of the .gas mixture was recalculated from the concentrations of the gaseous and solid components obtained during sampling. The equilibrium composition was first calculated at the collection temperature for 22 elements. The model then calculated the equilibrium compositions at 50°C intervals using the residual gas composition after condensation at the previous temperature. The depositional sequence observed in the silica tube depend on the temperature and the concentration of elements in the initial mixture. The computational method was applied to gases sampled from Mt. St Helens. The calculated results agree with observed sublimates. A new method for volcano monitoring is proposed which allow to determine the magmatic origin of volcanic gases and their emission temperature from remote plume analysis. The model is also applicable to estimate the temperature and the composition of the gases entering hydrothermal systems or participating in ore deposits in the basement of the volcano. The model predicts the behaviour of the main and minor species emitted in the volcanic gases. This approach is not only restricted to the volcanic gas studies but can be applied to studies of high temperature reactive gas reactor to simulate cooling reactions.

Impurity and recycling control with gettering in the Advanced Toroidal Facility

The vacuum vessel of the Advanced Toroidal Facility (ATF) is Ti gettered with a surface coverage of 70%. The major effects of gettering are (1) reduction of the oxygen, carbon, and nitrogen content in the plasma and (2) improved density control due to wall pumping of the working gas hydrogen. The overall leak rate in ATF is 2×10−4 Torr l/s which is too high for successful plasma operation. Ti gettering is routinely employed every morning prior to operation and compensates for this shortcoming by reducing the partial pressure of nitrogen and other residual gas components to the low 10−9 Torr range which is close to the residual gas analyzer background pressure. Rate‐of‐rise measurements at this stage show only argon and some methane. The argon is used to monitor the leak rate. In addition to impurity reduction, gettering leads to low recycling of the working gas which appears to be crucial for density control in ATF. The capacity of the gettered surface is large enough to show a strong effect even after 24 h. An extensive data base on the short‐term and long‐term effects of gettering on the residual gas composition and its effects on plasma performance has been established over the past three years and will be discussed in this article.

Investigations of partial reactions during magnetron sputtering of TiN coatings by in situ Auger electron spectroscopy analysis and plasma diagnostics

Abstract Reactive magnetron sputtering of hard coatings is a complex process and the interrelations between the process parameters such as partial pressure or bias voltage and chemical and physical reactions occuring in the plasma and on the substrate surface are not well understood. For the study of such problems a three-chamber ultrahigh vacuum system has been designed and tested. It consists of a magnetron sputtering chamber supplied with a quadrupole plasma diagnostic system and with an optical multichannel analyser, a sample charging and transfer chamber, and an Auger electron spectroscopy analysis chamber. Thus, the residual gas composition, the composition of the plasma-gas atmosphere and the composition of the substrate surface can be controlled at any stage of the deposition treatment. First results on the sputter cleaning process are presented for various values of the residual O 2 partial pressure and the bias voltage. It is demonstrated that impurity gas reactions depend strongly on ionization and dissociation processes in the plasma atmosphere which are a function of the bias voltage. The system is also well suited for investigations on the interrelation between the oxygen impurity content in the sputter gas atmosphere and the composition of TiNO coatings.

On the origin of a lattice expansion in palladium and PdAu vapour deposits on various substrates

In order to gain more insight into the possible lattice expansion mechanism in vapour deposits of palladium, vapour deposits of palladium and of PdAu alloy particles and films were produced under well-controlled conditions, either in ultrahigh vacuum or in high vacuum, with and without the admission of H 2O, on amorphous films of carbon and SiO 2 as well as on single-crystal surfaces of NaCl and KBr. In all cases, lattice expansion values of up to 2.6% were measured, depending on the condition of degassing of the substrate and on the residual gas composition during deposition, especially with respect to water vapour. It was found that the lattice expansion was correlated with the presence of hydrogen on or in the substrate materials, as detected by secondary-ion mass spectroscopy (SIMS). X-ray photoelectron spectroscopy (XPS) analyses of a satellite of the main palladium peaks indicated a correlation with the concentration of hydrogen on the samples. Other impurities such as carbon or oxygen were excluded as being responsible for the lattice expansion by SIMS and XPS data. Furthermore, as no systematic difference in the lattice expansions was found for epitaxially grown deposits on alkali substrates and for non-oriented deposits on other substrate materials, the effect of pseudomorphism was shown not to be significant, too. Therefore the only remaining mechanism for lattice expansion is believed to be the formation of hydride, although the stabilization of this phase still needs to be clarified.

Properties of evaporated target foils studied by computer simulation of ballistic aggregation

Target foils often show defects during evaporation, floating, or mounting. These defects depend in subtle ways on the evaporation conditions including: angle(s) of incidence of evaporant on substrate, residual gas pressure and composition, rate of evaporation, and temperature of substrate. Some progress has been made in understanding these effects using a computer program that simulates the buildup of such foils during evaporation. The program runs on a microcomputer and models simple ballistic aggregation of sticky atoms on a square lattice. Although real atoms move in three dimensions and presumably relax after collision to positions of minimum potential energy, two-dimensional simulations illustrate many of the properties of real aggregations. A striking behavior of such simulations is the columnar structure in the deposited film and the dependence of that structure upon the distribution of angles of incidence of the evaporant atoms. Atoms incident from a single direction produce foils having narrow fibrous structures, while atoms incident from several directions produce treelike patterns having lower average density and greater horizontal extent. Simulated deposited films show variations in density in the direction perpendicular to the substrate; these variations may be responsible for the tendency of some foils to curl or crumple while being floated.