Sublimation Pump Research Articles

Abstract: The Production of Vacuum Via Ion and Sublimation Pumping

First Page

Production and measurement of ultra-high vacuum below 10 −10 torr

Experience on operating two experimental UHV systems down into the 10 −12 torr region are briefly reviewed. Results of tests are reported on the attainment of a given level of vacuum with a particular concentration distribution when using hot and cold cathode devices, demonstrating the perturbing influence of hot cathode devices at these vacuum levels. Experimental results on system response to the operation of a triode ion pump leads to gassing rates, for baked stainless steel surfaces, of less than 10−16 torr I. s −1 cm −2. A magnitude, which the volume oriented kinetic model is unable to explain, but is in good agreement with a surface oriented model involving permeation and consideration of the state of the surfaces. This approach predicts the attainment of lower pressures, in larger systems, provided the correct processing techniques are adapted. Confirmation is obtained with a 50 l. working volume system, as tested with pump combinations involving sorption, turbo-molecular, magnetic ion, and sublimation pumps. Problems due to measurement procedure, and due to insufficient sensitivity of mass spectrometers are emphasized.

Vacuum and cryotechniques in space research

There are three types of installation in space research laboratories in which vacuum and low temperature are concerned. These are: 1. 1. Space simulation chambers used to investigate the thermal balance of the satellite (solar simulation testing). 2. 2. Vacuum-temperature chambers in which the satellite or its component parts can be tested in the working condition for reliability under extreme temperatures (thermal-vacuum testing). 3. 3. Special installations for the study of particular phenomena or for the development of components. Only the first two types of installation are dealt with in this article. The two installation types can also be combined. In these installations, the test object is situated under vacuum in a chamber which is closed by a thermal wall. For the solar simulation testing, the thermal wall is cooled to a temperature of between 80 K and 100 K by means of LN 2, in order to simulate the cold ambient conditions applying in space. A temperature range of from 100 to 370 K is covered during thermal-vacuum testing. The construction of a thermal wall is explained, together with the various systems of obtaining and maintaining definite temperatures. using LN 2 and GN 2. In the first generation of installations, simulation of the vacuum applying in space was obtained in the USA by a combination of 20 K cryopumps and diffusion pumps, whereas in Europe only diffusion pumps were used. During the past few years, new projects in space research have shown the provision of an ultra-clean vacuum in space simulation chambers to be an urgent necessity. Thus the original conception of a vacuum pump system has been changed in favour of the following combination of pumps: 1. 1. Titanium sublimation pump with LN 2-cooled getter surface, 2. 2. Sputter-ion pump and/or turbomolecular pump, 3. 3. 20 K cryopump. Installations built in the USA during recent years have used 20 K cryopumps as the main element providing the pumping action, whereas in the new installations in Europe, this function sublimation pump. The developments described are illustrated by means of examples, in particular by the IABG 4 m infrared space simulation and thermal-vacuum chamber in Ottobrunn, near Munich. This plant was completed a short time ago for use in connection with the research work on the Helios sun probe.

An Analytical System for Secondary Ion Mass Spectrometry in Ultra High Vacuum

A system for routine secondary ion mass spectrometry (sputter ion mass spectrometry) under uhv conditions has been developed. Positive as well as negative ions released from metals, semiconductors, and insulators can be measured. The system can be used for monolayer analysis (with low primary intensities) and for profile measurements in thin films. Electron induced desorption investigations are possible with the same arrangement. Means are provided to combine the secondary ion analysis with Auger spectrometry. A multitarget array allows up to 12 samples to be measured. The analytical system consists of a primary ion gun, a charge neutralizing electron source and a specially modified quadrupole mass spectrometer. Fast pulse counting technique is used for the ion detection. The uhv system is pumped by a turbomolecular pump, a liquid nitrogen cooled trap and a titanium sublimation pump. Pressures in the 10−11 Torr range can be obtained. In a number of typical examples the versatility of the instrument is demonstrated.

A Radiant Heated Titanium Sublimator

A new radiant heated sublimator is described which employs an approximately spherical shell of titanium enclosing a tungsten filament heater. The sublimator is relatively simple in construction and can dispense ∼37 g of titanium at sublimation rates up to 0.5 g/h. The sublimation rate at a given power input remains relatively constant throughout the life of the sublimator, making it very simple and reliable to operate. Comparative experiments are described with both spherical and cylindrical sublimators which illustrate the effect of various designs on the sublimation rate versus power and time characteristics. The operating characteristics of this device are discussed relative to other sublimation sources in common use in the field of titanium sublimation pumping.

Vacuum System for Irradiation Studies

An oil-free vacuum system capable of achieving vacuum levels of 10−6 Torr for in-pile experiments has been constructed at the pool-type Battelle Research Reactor (BRR). The pumping system which is mounted on a moveable bridge above the pool surface, consists of a 7500 liter/sec titanium sublimation pump and two 350-liter/sec triode-type ion pumps; system roughing is accomplished with a roots-type blower with a double-walled liquid nitrogen trap. Nine meters of 20.3-cm (8-in.) diam vacuum piping connect the pumping system to a test chamber at the core level. Test chamber geometry can be varied to fit either a 7.62×7.62-cm test cell or the core face. Neutron fluxes of 2×1013 neutron/cm2⋅sec (thermal) and 1×1013 neutron/cm2⋅sec (fast) are available and the maximum gamma rate is 108 R/h. Adjustments of desired gamma/neutron ratio are made by employing appropriate shielding at the core face. Irradiation effects on the performance of electro-mechanical components for space nuclear power systems have been studied at elevated temperatures (620–820 K) and a full-scale nuclear fueled thermionic diode is currently being prepared for irradiation in the system.

Gas incorporation in sputtered and evaporated gold films

An all steel sputtering system has been built to operate at argon pressures as low as 2 × 10 −4 torr. Background gas pressures are held below 4 × 10 −8 torr using a liquid nitrogen cooled sublimation pump during all experiments. A second ultra high vacuum system is connected to the main sputtering chamber by means of a gate valve so that film samples, of various materials on various substrates, can be withdrawn for study using a rack and pinion mechanism under continuously maintained vacuum conditions. Test films are analysed by thorough outgassing in the second system using an inductor coil heater over a glass section of the system so that desorption from other parts of the system is kept to a minimum. Gas analysis is achieved using a continuously operated quadrupole mass spectrometer and the results are displayed on an XY recorder. Film thicknesses are monitored using a previously calibrated quartz crystal oscillator and the dimensions of samples and systems are known from careful measurement. Using this equipment it is possible to measure the concentrations of argon and nitrogen in both sputtered and evaporated films under various conditions of gas pressure, film thickness, target voltage, substrate voltage, target and substrate material etc., etc. Films may be examined after experimentation using an electron microscope, and it has been seen that there is a significant change in the surface appearance of sputtered films after degassing which is not apparent on degassed evaporated films. This effect is being studied more closely in addition to the gas content and distribution in films. Preliminary results for gold on tungsten, and electron microscope studies of nickel on glass, will be presented and discussed.

A Field Emission Electron Microscope

We have developed a field emission electron microscope. Although field emission gun requires ultra high vacuum and skillful technique, it brings about the favorable characteristics of high brightness and small energy spread. This characteristics will enable a significant progress in coherent electron optics and high resolution electron microscopy, especially in electron beam holography.Its column is Hitachi HU-11C Electron Microscope modified for ultra high vacuum operation, and it is evacuated with five ion pumps. Field emission gun is divided into two parts and is evacuated differentially with two ion pumps and a sublimation pump. The final pressures in these rooms are 5x10-10 Torr and 5x10-8 Torr respectively.

A direct comparison of the visible and ultraviolet reflectance of aluminum films evaporated in conventional and ultra-high vacuum systems

Aluminum was evaporated in a conventional evaporator using an oil diffusion pump ( p=3 × 10 -6 torr) and in an ultra-high vacuum system evacuated by ion and titanium sublimation pumps ( p=5 × 10 -9 torr) and deposited at rates ranging from 2.5 Å/sec to 500 Å/sec. For aluminum deposited at rates of 300 Å /sec and greater there is no measurable difference in the visible and ultraviolet reflectance of the two types of films. However, if lower deposition rates are used the reflectance of aluminum films deposited at 5 x 10 -9 torr is much less dependent on deposition rate than that of films condensed at 3 x 10 -6 torr. Aluminum films deposited at high rates, both in conventional and ultra-high vacuum systems, show less degradation of reflectance during exposure to air than the films deposited at low rates.

Automatic Apparatus for the Determination of Helium Diffusion in Solids

An apparatus has been designed and built to determine automatically the rate of release of helium from various solids as a function of temperature. The diffusion of He in single crystals of neutron irradiated LiH is used to describe the operation of the apparatus. Samples can be maintained in a vacuum or in an automatically maintained gas (other than noble gas) atmosphere and held at temperatures from 200 to 1400°C. Helium evolved from the sample is periodically removed. A quantitative determination and standardization for He are automatically performed. If a gas atmosphere is used, this is removed from the helium by use of a titanium sublimation pump. The He which remains is then measured with a low resolution high sensitivity residual gas analyzer (partial pressure gauge). The system is capable of measuring at least 10−10 cm3 STP helium.

Application of Dry-Pumped Systems Operating above 10−5 Torr

Dry-pumped systems using various combinations of ion, titanium sublimation, and cryogenic pumping are now widely accepted in vacuum processes utilizing pressures below 5×10−5 Torr. High-capacity, dry-pumped systems have now been proven which operate at much higher pressures. This paper discusses the design of these high-capacity, dry-pumped systems for evaporation and melting and refining processes. A variety of titanium evaporation pumping systems—using wire, rod, and ingot-feed systems, with large titanium inventories for continuous operation—are discussed. A high-throughput titanium sublimation pumping system is described, along with its high-pressure operating characteristics, which offers more economical pumping for large gas loads. Conventional inert gas pumping systems are described for use with these high-throughput titanium sublimation systems, to meet particular requirements.

Deposition of High-Purity Nb Films by Tetrode Sputtering

Niobium thin films with extremly low resistivity (14.7 μΩ cm) have been deposited by tetrode sputtering in Ar gas purified by the getter action of titanium sublimation pump. The effect of gaseous impurities on the resistivity, superconducting transition temperature, and crystal structure were investigated. The superconducting transition temperature of such films was enhanced to 9.8±0.1 K and the temperature coefficient of resistance was increased to +3.0×103 ppm/deg.

Design and Performance Characteristics of the Zero Gradient Synchrotron (ZGS) H- Ion Source and Related Systems

An expression for the maximum H- current obtainable from an H+ beam by charge exchange in hydrogen has served as a guide in the design and development of a source of H- ions for booster injection into the ZGS. Here it is used as a criterion in the evaluation of testbench results obtained with the present test source. Space and power limitations in the 750-keV terminal of the preinjector have led to the use of titanium sublimation pumping to handle large instantaneous gas loads. Power limitations and the need for high pumping conductance have led to the substitution of electrostatic deflection for magnetic deflection in the beam separator.

Conductivity of CdSe, CdSe-SiO x thin films : I Szczurek et al, Prace PIE, 10 (2/3), 1969, 77–84 ( in Polish)

The experimental apparatus for measurments of sputtering yield of graphite by hydrogen and helium ions was constructed. At present day the operational parformance of the system with a torsion microbalance (sensitivity: 1.5 × 10−7 g/div) has been established.Experimental result is not yet obtained.Modified torsion microbalance is under construction.We have designed the apparatus with vacuum microbalance system for measurements of sputtering yields of graphite sample. As incident ions, H+ and He+ are used, but because B-A gauge type ion source is adopted, ion beam intensity is about order of 10 100 μA/cm2. It can be considered sputtering yield of graphite by light ion with keV energy is not so large, therefore the vacuum microbalance having maximum sensitivity of 5 × 10−9 g has been designed, which is made of quartz bar (diameter of the bar is 0.5 mmϕ and the mean length is 10 cm, the balancer is about 4 g). The balance is installed in UHV chamber. The UHV system could be pumped down to pressure of 10−9 Torr, in order to keep impurity gas pressure as low as possible during sputtering. During sputtering experiment admitted sample gas is pumped with a turbo-molecular pump (70 1/sec), and after sputtering when gas filling is stopped and operation of the balance is started, the UHV system is pumped by an orbitron pump and a Ti sublimation pump to avoid gas adsorption on the sample. We have prepared two kinds of graphite samples, one is a pyloritic graphite plate 2 (5 × 5 mm2, 1 mm in thickness) and the other is a graphite thin film (500 A) sample deposited on gold coated mica plate. These sample plates could be outgassed by electron bombardment.For the graphite film low energy He+ ion scattering technique (ISS) can be applied as well as vacuum microbalance technique. As the thin film is gradually sputtereched by impact ions, the scattering signal intensity ratio I(c)/I(Au) will show a change, i.e. shadowing of gold atom by carbon atom could be measured by as a function of film thickness.

Selective Pumping of an 800 000 Liter Chamber

A new 38.5-ft-diam spherical vacuum chamber with a selective pumping system has just been completed. The pumping system for this 800 000 liter chamber consists of mechanical roughing, 80 K liquid-nitrogen-cooled shroud, 20 K cryopanels, and ion and titanium sublimation pumps. The chamber and pumping system are described. Extensive acceptance tests were run on the chamber. These tests included pumping speed measurements and ultimate pressure measurements with various combinations of the pumping units operating. Data on the outgassing of the 5 000 000-cm2 stainless-steel shell is presented. From residual gas analysis, approximate pumping speeds for helium and neon were determined. At ultimate pressure of the chamber, water vapor outgassing from the stainless-steel shell was found to be the limiting factor.

Some problems associated with the attainment of ultra high vacuum

The behaviour of pumps used in the attainment of ultra high vacuum has been studied. Sorption pumps remove gas selectively and as a result leave high partial pressures of neon and helium which are likewise poorly sorbed by getter-ion pumps. Oil filled rotary pumps overcome this problem but are a source of organic contamination unless carefully trapped. Recent improvements in Penning pump design in which the phenomena of argon instability can now be overcome are discussed. A mass spectrum is shown for a system exhausted by a combination of Penning and sublimation pumps, exhausted by an alumina trapped rotary pump during the prebaking period.

New Developments in Getter—Ion Pumps in the U.S.S.R.

A series of sublimation titanium getter-ion pumps with directly heated titanium evaporators has been described by the authors in earlier publications. The pumps have pumping speed capability of 500–5000 liter/sec. The pumps find an application in thin-film technology, in the manufacture of vacuum electron tubes, in the metallurgy of extra-pure metals, and in particle accelerators. The present work reports results of the development of sublimation pumps which offer higher capacity and can operate under more rigid temperature conditions.

Effect of Ultrahigh Vacuum on the Friction between Metals and Granular Soils

An apparatus has been developed to measure the friction between metals and granular materials in ultrahigh vacuum. Pressures below p=10−9 Torr have been routinely reached with the vacuum system consisting of a turbomolecular, a getter-ion, and a liquid-N2-cooled titanium sublimation pump. The frictional resistance between steel and ground basalt in two particle size ranges was measured under various normal loads. The experiments showed that the ultrahigh vacuum did not affect the coefficient of friction significantly, but increased the frictional resistance by an adhesion ranging from 6 to 24 g/cm2 for the various materials and conditions.

Performance of a Large Vacuum Chamber During a Continuous 10 000-h, 2200 °F Materials Test

A 4-ft-diam by 12-ft vertical vacuum chamber was operated at 1–5×10−8 Torr to contain a 10 000-h, 2200 °F boiling-potassium refractory metal test. This test required what is believed to be a record 11 000 h of continuous operation of a large size sublimation and getter-ion pumped chamber containing high-temperature test equipment. Sublimation pumping capacity of 20 000 liters/sec and ion pumping capacity of 2400 liters/sec were provided. An unexpected exponential increase in ion pump current from 0.35 to 35.0 mA in 8500 h was experienced before “hi-potting” and alternate operation of the two ion pumps was used to reduce the current increase. This test unexpectedly demonstrated that during long term operation, excessive ion pump currents may force early pump maintenance or cause premature test termination. Argon, nitrogen, and hydrogen remained the principal gases throughout the test period. Lesser amounts of methane and carbon dioxide were present. Other organic compounds and water remained at, or decreased to, very low concentrations.

1243. Construction and performance of a liquid nitrogen cooled, titanium sublimation pump, the “Sublimvide 8000”: P Beaufils and R Geller, Le Vide, 23 (134), March–April 1968, 67–74 ( in French)

1243. Construction and performance of a liquid nitrogen cooled, titanium sublimation pump, the “Sublimvide 8000”: P Beaufils and R Geller, Le Vide, 23 (134), March–April 1968, 67–74 ( in French)