Papers

AVS1996 Session VT-WeM: Developments in Vacuum Science and Technology

Wednesday, October 16, 1996 8:20 AM in Room 104A/B

Wednesday Morning

Start	Invited?	Item
8:20 AM		VT-WeM-1 The Application of a High Critical-Temperature Superconductor Bearing for High and Ultra-High Vacuum Measurement A. Chew (Edwards High Vacuum International, United Kingdom); A. Chambers (University of York, United Kingdom); A. Troup (Edwards High Vacuum International, United Kingdom) A novel ultra-high vacuum-compatible high critical-temperature superconductor suspension is described in its application to a rotating disc gauge. The non-contacting and robust suspension comprises a permanent magnet (and attached disc) suspended freely in vacuum below a liquid nitrogen-cooled YBaCuO pellet which is held outside the vacuum. This configuration supersedes an earlier fibre suspension. Molecular torque developed from a high speed disc accelerates the suspended disc and facilitates a continuous and vibration-insensitive measurement of total pressure in high and ultra-high vacuum. The measurement technique is described and an analogy is made with the spinning rotor gauge in relation to an equivalent offset term; the offset of the superconductor suspension is shown to be highly stable over several days. Good agreement with spinning rotor gauge measurements will be reported and improvements in the pellet structure to reduce internal losses in the bearing (and in the offset term) discussed.
8:40 AM		VT-WeM-2 Calculation of Gas Flow using Accurate Combination of Component Models D. Moore (MKS Instruments, Inc.) A new technique for the calculation of flow in gas systems (to be published) has been encapsulated in a computer code applicable to molecular flow. In the technique, flow across a flat interface is represented by an expansion in orthogonal moments, the lowest of which is the total flow. This technique is accurate (no ad hoc adjustments required), and can potentially be used in all flow regimes, but it is ill-suited to manual computation. Use of the code for the modeling of molecular flow will be illustrated, including comparison to analytical, numerical, and experimental data for some simple cases. The main function of the computer code is to combine the results of models of individual components accurately. Management of Monte Carlo simulations or analytical representations of individual components is also incorporated. Because the code retains statistical information at each step, termination of the many infinite sums present in the calculations can be performed intelligently, and the accuracy of the final results estimated.
9:00 AM		VT-WeM-3 Quantitative Pressure Measurements using Cavity Ring-down Spectroscopy J. Hodges, J. Looney, R. van Zee (National Institute of Standards & Technology) We have investigated the use of cavity ring-down spectroscopy (CRDS) to measure absolute pressures in the mid- through ultrahigh-vacuum ranges. The effects of cavity design and laser bandwidth on quantitative CRDS have been investigated using the \super r\R transitions of the A band of O\sub 2\. It is found that failure to account for laser bandwidth leads to systematic errors in the number densities determined from measured ring-down signals. By fitting the frequency integrated expression for the ring-down signal and using the measured laser lineshapes, excellent agreement between the measured and predicted number densities is found. The quantitative measurement of gas densitites over a wide dynamic range will be presented. The extension of CRDS to other gases, such as CO\sub 2\, H\sub 2\O, and CH\sub 4\, will also be discussed.
9:20 AM		VT-WeM-4 Initial Studies of Field Emission Cathodes for Cold Cathode Ionization Gauges D. Baker, R. Outlaw (Teledyne Brown Engineering-Hastings Instruments) Field emission cathode arrays have been used as an electron source for cold cathodic type ionization gauges. These sources allow the production of electron currents \<= 1 mA\ without the usual contaminating gases such as CO, CO\sub 2\, CH\sub 4\, associated with standard hot filament ionization gauges and do not require strong magnetic fields used in commercially available cold cathode type gauges. A comparative study of n-TaSi\sub 2\ / Si\sub x\Ta\sub y\ matrix, Mo/Si substrate and W/YZrO\sub 2\ matrix emitters with regard to their use in an ionization gauge is presented. Data on the current-voltage (IV) characteristics, emission stability, and temperature stability are presented. The cathode reliability in various gas environments and pressures is also given. The IV behavior is compared with the Fowler-Nordheim equation to quantify array performance. Various electrode designs and ionization geometries employing these cathodes are analyzed with computer simulation of electron and ion trajectories (SIMION). The sensitivities are measured for different gasses and various electrode potentials.
9:40 AM		VT-WeM-5 The Invention of the Conflat Flange W. Wheeler (Tencor Instruments) The space research and physical science communities in the late 1950's generated a demand for ultra high vacuum systems of unprecedented size, quality and complexity. Varian Associates had in 1959, established a new Vacuum Division to address this demand. An ambitious group of physicists and engineers set about to develop new generations of UHV apparatus. A major problem was encountered in the operation of large systems containing as many as 50 metal gasket flange seals when too much time spent dealing with the vacuum leaks resulting from even a mild bakeout. Consequently, a study of a bake performance of contemporary flange seal designs was begun with the aim of selecting the most reliable one. But, of course, the urge to invent could not be resisted, and the Conflat seal was born. The circumstances of existing technology, technical motivation and enabling factors which affected the invention are discussed. Some observations on the general process of invention are also attempted. The analysis of the Conflat performance is described and some fallout from the analysis is also included.
10:00 AM		VT-WeM-6 Gas Permeation and Leakage through Reusable Seals D. Manos, M. Johnson (The College of William and Mary) This paper describes a study of outgassing and permeation of high vacuum seal materials for use in multi-chamber deposition tools and other production tools to make electronic materials and devices. The study compares the performance between all-metal seals, single-use seals (copper knife-edge and aluminum spring loaded seals), elastomer o-rings, and a "compound seal" taking into account both cost and performance. The "compound seal" consists of an outer baked elastomer o-ring surrounding an inner compressible aluminum seal with rough pumping (<40 mtorr) on the interspace. Conceptually, this seal performs like an elastomer from the standpoint of repeatable sealing against the outside atmosphere, yet it performs like aluminum from the standpoint of its material outgassing into the chamber. The study focuses on measurements of quantitative gas influx, including mass-resolved measurements of permeation and outgassing, for various o-ring materials and conditioning sequences. The study compares the quantitative outgassing, leakage, and permeation of baked and unbaked elastomer seals in single ring configurations to baked and unbaked configurations of double o-rings with either an elastomer or aluminum used for the inner seal. To evaluate performance after multiple breaks of a seal, we observe double o-ring configurations as they are cycled open and closed.
10:20 AM		VT-WeM-7 Outgassing Rate Characteristic of a Stainless Steel Extreme High Vacuum System I. Chun, B. Cho (Pohang University of Science and Technology); S. Chung (Pohang University of Science and Technology, Korea) Using an extremely dry nitrogen venting system, a fast return to ultrahigh vacuum conditions was achieved. The system pumps to 10\sup -11\ Torr range in 5 h, after start of pumpdown and without bakeout. The system pressure is determined by surface outgassing which decays algebraically as t\sup -\alpha\\, where the slope \alpha\ changes from 2.13 through 1.24 to 0.82 down to 10\sup -11\ Torr. However, \alpha\ = 0.5, which would be expected for bulk diffusion, was not observed. Curve fitting yields desorption energies of 19, 20, 21, 22, and 23 kcal/mol for five types of adsorption sites on the surface for water. Analysis suggests that the elimination of water with 20 and 21 kcal/mol is essential for rapid pumpdown fo the 10\sup -10\ Torr decade. A desorption energy of 10.4 kcal/mol for hydrogen was obtained by outgassing rate measurement. This relatively small value is suggestive of a smooth oxide layer formed on the chamber wall.
10:40 AM		VT-WeM-8 A Method of Evaluating Outgassing of a TMP Itself K. Odaka (Hitachi, Ltd., Japan) We investigated the influence of outgassing from a turbomolecular pump (TMP) on pumping speed using both theoretical and experimental methods. The theoretical investigation was based on the transmission probabilities of the gas through the blade rows, and took into account outgassing from every part of the TMP. This served as the foundation for deriving a new formulation for evaluating pressure using the concept of intrinsic pumping speed and the TMP's own outgassing rate. This formulation also clarifies the actual physical meanings of the conventionally measured pumping speed. Several of the principal quantities used in conventional methods of evaluating pumping speed can be used with the new formulation, as well. A method for using the formulation to conduct actual measurments of a TMP's own outgassing rate is also presented. Experiments showed that a TMP's own outgassing rate is much greater than that of the vessel, and showed that the ratio of effective pumping speeds to nominal ones is about 0.2 before baking, and 0.03 after baking. The evaluation method presented makes possible quantitative evaluations of the influences of TMP materials, surface treatments, and configurations on pumping speeds. The high accuracy of the new method's pressure evaluations makes it easier to select the appropriate pump size for vacuum systems.
11:00 AM		VT-WeM-9 High Sensitivity Helium Leak Detection-A Review M. Rao (CEBAF); G. Rooks (Varian Vacuum Products) The presently available helium leak detectors have a designed leak detection sensitivity to 10\super -10\ atm cc s\super -1\ and lower. However in real practice, it may be difficult to achieve this designed sensitivity because of the trapping of helium in elastomers and/or oils of the pumping system and also back streaming of atmospheric helium (4 x 10\super -3\ torr partial pressure) through the pumping system. Many applications require use of the leak detectors in high helium background environments, which induces constraints even to less sensitive applications. In recent years several advances have been made in improving the dynamic leak detection sensitivity to 10\super -13\ atm cc s\super -1\ or better. These advances include the adsorption - desorption helium leak detection techniques\super 1,2\ and use of several simple and effective solutions in limiting the effects of helium trapping and back streaming in the vacuum systems.\super 3\ In this paper, advances in high sensitivity and high helium background tolerance leak detection techniques are reviewed. 1. M. G. Rao., J. Vac. Sci. Technol. A 11, 1598 (1993) 2. Ganapati R. Myneni, United States Patent No 5, 343, 740 Sept. 6, 1994 3. Ganapati R. Myneni, United States Patent Pending *Work supported by the Department of Energy, Contract DE-AC05-84ER40150.