Papers

AVS2001 Session VST-ThA: Total & Partial Pressure Gauges & Their Calibration

Thursday, November 1, 2001 2:00 PM in Room 125

Thursday Afternoon

Start	Invited?	Item
2:00 PM		VST-ThA-1 Partial Pressure Measurements at milliTorr Pressure using a Miniature RGA with an Electron Multiplier R.E. Ellefson, L.C. Frees, T.L. Karandy (Inficon, Inc.) Operation of an extended pressure range (XPR) miniature residual gas analyzer at millitorr pressures allows direct measurement of process gases involved in physical vapor deposition without the need for pressure reduction and associated pumps. A new electron multiplier (EM) capable of sustained operation at all XPR operating pressures (up to 20 mTorr) extends the measurement speed at process pressure while maintaining nanoTorr detection limits at base pressure. Design features that allow the EM to operate continuously at high pressures are presented together with data that shows that a gain of 100 is optimum for practical measurements of gas species from 10^-9 to 10^-2 Torr. A comparison of the operation of the XPR with a standard RGA shows each instrument has a similar dynamic range of ion currents but the pressures producing the currents are shifted two decades of pressure higher for the XPR. The physical phenomena of ion-molecule reactions do occur at millitorr pressures but do not interfere with useful measurements of the impurities in Ar sputtering gases or the detection of contaminants (like photoresist) in degas chambers. Examples are given of ion molecule reactions, e.g. N₃⁺ and Ar₂⁺ produced at pressures of 10 mTorr for N₂ and Ar, respectively.
2:20 PM		VST-ThA-2 The Use of a Quadrupole Residual Gas Analyser to Automatically Verify the Purity of Tokamak Fuelling Gases R.J.H. Pearce, A. Henshaw, J. Bruce, S. Bryan (EURATOM/UKAEA Fusion Association, UK) It is essential that the gases injected into the JET experimental fusion tokamak be as requested and free from contamination. To give maximum flexibility a matrix architecture is used to handle the many different gases which can be introduced. In addition to problems of human error and contaminated gas bottles, the matrix architecture provides the potential for gases to become cross-contaminated either through valve leakage or due to control problems. A gas species verification system has been designed and commissioned to automatically confirm on-line the conformity of the gas being supplied to the torus. The system uses a quadrupole channeltron residual gas analyser (RGA) within a chamber, which is pumped by a turbomolecular drag pump. Under the control of the main control programme, samples of the module gases are leaked at regular intervals, through the controllable leak valve into the analysis chamber. Using the RGA the gas composition is computed in parts per million (ppm) of the 64 most relevant mass numbers. The analysis is corrected for offsets, background, and the mass positions. The mass spectrum is then compared with a reference gas spectrum. An acceptable tolerance on each mass number is defined for each reference spectrum. The system allows contamination of <1ppm to be detected. If a mass number falls out of the specified tolerance, an alarm message is communicated. The control of the gas checking is performed by the main matrix control program. The gas analysis is performed by a dedicated code written using a bespoke programming language designed for mass-spectrometry applications. Communication between these codes is performed through a SCADA database using dynamic data exchange (DDE). The system has been used to optimise the method of pumping and purging the matrix when changing gases. The results of tests, which have allowed the development of fast gas change cycles with little contamination or gas wastage, are presented.
3:20 PM		VST-ThA-5 Calibration Stability of Hot Cathode Ionization Gauges: A Discussion of the Importance of Electron Path Length and Gauge Constant R.N. Peacock (Retired) The ion current, I, in an ionization gauge is given by the equation I = K i P where K is the gauge constant, i, the electron current, and P the pressure. Values of K for gauges designed for use at UHV and XHV range from 10/Torr to 10⁶/Torr. It is important to know whether calibration stability is sacrificed when K, and the electron path length, are large. Using a simple model, the electron path length is estimated as a function of the probability, β, that an electron will make another pass through the ionizing region. An equation is obtained for K as a function of β. The fractional change in K, ΔK/K, is calculated for a 1% reduction in the probability that an electron will make another pass through the ionizing region. The fractional change is zero for those gauges where the electrons make a single pass, 0.015 for a B-A gauge with K = 25, and 0.91 for a gauge with K = 10⁴.
3:40 PM		VST-ThA-6 Calibrating Cold-Cathode Gauges at Very Low Pressures B.R.F. Kendall (Elvac Laboratories); E. Drubetsky (Televac Division of the Fredericks Company) Cold-cathode ion gauges are now used in a wide range of high vacuum applications. Because of their freedom from many of the errors associated with hot-cathode gauges, they are becoming increasingly popular for use at ultra-high vacuum. Their calibration at low pressures, especially below about 10^-9 Torr, presents special challenges because of the rather complex (yet stable) logarithmic nature of their current-pressure response. These parameters must be established by tests at known pressures extending to the 10^-11 Torr range or lower. This in turn requires the use of special hot-cathode reference gauges such as the extractor, modulated Bayard-Alpert or X-ray-neutralized Bayard-Alpert types. We describe calibration procedures and results for a number of cold-cathode gauges at pressures in the 10^-8 to 10^-11 Torr ranges. Some aspects of conventional calibration techniques may be inappropriate or counterproductive at very low pressures because of X-ray, outgassing and ion desorption errors in the hot-cathode reference gauges. Preliminary results with a newly-developed cold-cathode reference gauge are discussed.
4:00 PM		VST-ThA-7 Residual Gas Analysis using Microengineered Systems S. Taylor (University of Liverpool, UK) There has been an increasing trend in recent years towards miniaturisation in mass spectrometry. Miniature versions of time of flight (TOF), magnetic sector and quadrupole ion trap have all been demonstrated. The use of silicon integrated circuit fabrication techniques (MEMS) has led to microengineered (submillimetre) versions of many of the more popular mass spectrometers including crossed field, travelling wave and quadrupole (QMS) instruments. In this presentation a brief survey of the various miniature systems which have been or are being developed will be given and their potential for use in residual gas analysis will be assessed. Recent results arising from the UK microengineered QMS project will also be presented and discussed. The performance and reliability of microengineered single-element quadrupole devices have both been raised considerably by improved construction methods. The mass range was raised from 50 to 150 amu, and the mass resolution was increased to 70 (measured at 10% peak height). Sensitivity was enhanced by an order of magnitude using a miniature multiplying detector. The useable pressure range has been established and modelled. Ion coupling was optimised by using software simulations (SIMION), and several different miniature ion sources have been investigated. Array-type devices containing either nine parallel quadrupoles or five independent mass filters have been developed, together with a self-aligning ion detector array. Theoretical simulations of the microengineered QMS have been undertaken by computing the trajectories of large numbers (>10,000) of ions injected into the mass filter and show that the experimental performance for a range of operating conditions may be successfully modelled.