NAN/PHY/MSE 546 (Venables) Sect 2.3

Notes for NAN/PHY/MSE 546 Sect 2.3 (Venables)

© Arizona Board of Regents for Arizona State University and John A. Venables

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Lecture notes by John A. Venables. Notes revised Spring 2005. Latest version 26 August 2012, reformatted..

2.3 UHV Hardware: Pumps, Tubes, Materials and P-Measurement

  • Types of Pumps
  • There are many types of pumps, but the ones used to create UHV conditions are typically one or more of the following: Turbomolecular (T), Diffusion (D), Ion or Sputter Ion (I), Sublimation (S) or Cryopumps (C). In choosing a pump for a system, you need to know:

    (i) General pump charactersitics. For example, T is good for high throughput, it works on a pressure ratio, and is poor for low mass molecules, especially hydrogen. It may have an oil limit, and it may suffer from vibration. But, there are new versions with magnetic levitation which may overcome the oil limit to the ultimate pressure. I and S are poor for rare gases, and also they trap the gas inside the system; thus they are useless if there is a heavy gas load, but can be good for a static vacuum under clean conditions. C has very high speed, but it may suffer from vibration, due to closed cycle refrigeration, and may cost a lot. We can try to outline specific characteristics of each if you feel it is worthwhile in class. Some comparisons are set out in diagram 44. We will point these out on the laboratory visit.

    (ii) Pumping speed and flange size. These are the design requirements, and affect the

    (iii) Size, weight and cost.

  • Chambers, Tube and Flange Sizes
  • Tubes and Flanges are standard, as can be seen from the manufacurers’ catalogues, or Luth, page 8. I have listed the standard sizes on a handout (Appendix F in the book), and given their conductance per meter length, and with a 10 cm end into the chamber. The conductance calculations are then sufficient to make estimates (they are rarely better than this) which will enable you to sketch a reasonable design. Then, one typically needs to discuss it with someone who has done such a design previously; it may be the most important factor in your experiment, and should not be done 'blind'.

    Useful formulae for conductances, as used for the handout, are the following:

    The flanges are typically made of stainless steel, and are sealed with copper gaskets. They are loosely referred to as Conflat flanges, though this is in fact a trade mark of one manufacturer. These tubes/flanges are referred to as ports on the central chamber. One can get your workshop to make these chambers, but in fact this is not cost-effective except for very special needs: there are several specialist firms who make such tubes, flanges and chambers on a routine basis.

    What we then have to do is to ‘pick and mix’ accessories for our needs, typically around a special chamber which has been designed for the job in hand, and made by one of these firms. Many of the accessories relate to particular measurement or sample handling techniques, which are the subject of section 3.

    We need to match pump speeds to pipe dimensions and conductances, as set out in the second table on the handout. These values are approximate, and are subject to small improvement by the manufacturers. For example, there was a typical commercial 'stir' several years ago now, when a new small turbopump on a 64 mm flange was announced with a specification of 60, rather than 50, liters/s. This was in fact a real improvement, and many sales were made: the data in this table are not that accurate: you would need to consult the latest sales brochures. On the other hand if the brochure claimed 200 liters/s for the same size, you could reasonably be skeptical.

    A manufacturer's rule of thumb, that you need 1 liter/s of pump speed for every 100 cm2 of wall area, is given in diagram 45. This is derived by taking a value for q (see section 2.2(a)) = 2x10-12 mbar.liter.s-1.cm-2 or 2x10-8 mbar.liter.s-1.m-2, which is a reasonably conservative design figure. It is reasonable, in that a) they want to sell pumps, and b) they don't want you complaining that your system only works when the wind is favorable. We have to respect both these reasons!

    Both sublimation and cryopump designs can trap a large fraction of the gas which enters the throat of the pump, in partice certainly greater than a quarter. This means that S > C/4, where C is the aperture conductance, which can be high, e.g. for 4-8 inch diamter pipes in the range 400-3000 liter/s, depending on the precise flanging arrangements (see diagram 46). A (titanium) sublimation pump (TSP) chamber can be designed relatively easily for your needs. For example, a tube 20 cm long and 20 cm (8") in diameter, has an internal area of about 1200 cm2.

    Table: Manufacturer's quoted information for TSP pumping speeds (liter.s-1.cm-2)

    Temperature H2 N2 O2 CO CO2 H2O CH4/Inert
    20oC
    3
    4
    2
    9
    8
    3
    0
    Liquid N2 (77.3 K)
    10
    10
    6
    11
    9
    14
    0

    But we should note other points too. A TSP outgasses when it is being 'fired', and the pressure therefore goes up before coming down; if the walls are too close to the hot filament, this problem is worse, i.e. what goes up does not have to come down as far as vacuum is concerned. Second, these pumps don't pump unreactive gases at all well. Cooling the walls with liquid nitrogen helps; even water cooling is quite effective in improving the performance, but it still doesn't pump unreactive gases. The result of such concerns means that you should not economise on the wall area, and you should use a somewhat larger diameter tube than you might calculate on the simplest basis.

  • Choice of Materials
  • UHV experiments require the use of materials with low vapor pressures, and it is helpful to have your own diagrams which give you easy access to such information (handout). Since the outgas leak rate Qo = qdA, we should use low qd materials, and minimize the area A of the design. As materials and accessories have improved there is a tendency to put more and more equipment into the vacuum system. This may make life more difficult in the long run: to try to do everything means that you may achieve nothing: there must be a Chinese proverb along these lines.

    There are lists of qd values for different materials and treatments, see diagram 47, but these give such a wide range as to be almost useless. The main materials, stainless steel, copper, aluminum, ceramics, all produce values below 2x10-12 mbar.liter.s-1cm-2 after a modest bakeout at around 200oC. These values are quite satisfactory for most purposes, and the trend is to avoid more stringent bakeouts at higher T, for longer time, which has been done for research into the limits of vacuum achievement and measurement (diagram 48). Some complex equipment contains materials, particularly high temperature plastics, e.g. for insulating electrical wires, which is very sensitive to the exact bakeout T, say between 150 and 220oC.

    It is imperative that you know what is in your system before you bakeout, or this important stage in your experiment will cause irreversible damage. Expensive. Despite this caution, the availablity of such plastics, coated wires, and even electric motors which work under UHV, has made surface science techniques much more widespread and routine. This one development has been a major factor in the acceptance of such techniques.

  • Pressure Measurement and Gas Composition
  • As with pumps, you need to know what the different types of gauge can do, and what principles they are based on. There are three general purpose gauges: the Ion Gauge, the Pirani Gauge and the Capacitance Gauge. The ion gauge works by ionization of the gas molecules, and the fine wire collector reduces the low pressure limit due to X-ray emission of electrons, which mimics an ion current. This gauge is sometimes referred to as the Bayard-Alpert gauge, after the inventors. It works well below 10-3 mbar, and has a lower limit typically below 10-11 mbar, depending on the design.

    The Pirani gauge utilizes the thermal conductivity of the gas molecules, and works over a relatively narrow range above 10-3 mbar; it typically is used for qualitative monitoring of the fore-vacuum. A capacitance gauge is extremely precise above 10-4 mbar, but requires different heads for different pressure ranges. This is sometimes referred to as a Baratron, but this is in fact the trade name of the dominant company for such equipment; these gauges are used very widely in all aspects of pressure measurement, process and flow control, for example in Chemical Vapor Deposition (CVD) reactors. Luth gives a description of such process equipment on pages 50-55.

    A list of such gauges, taken from Roth (chap 6, pages 281) is given in diagram 49. This list is not exhaustive; there are some newer ones, including the Spinning Rotor Gauge, based on gas viscosity, which have been developed and marketed since Roth was published. The basic gauge is still the ionization gauge. It is typically calibrated for N2. Other gases have different sensitivities, as set out in diagram 50.

    The determination of gas composition is also very important, and is typically done with a compact mass spectrometer known as a residual gas analyzer, or RGA. This produces a characteristic mass spectrum, for example in diagram 51. It is helpful to record this spectrum, and store examples of when your system is working well, as the spectrum when you have a real leak is typically quite different from if you have performed an inadequate bakeout, or have let 'nasties' into your system. As we have implied in section 2.1, the vacuum composition for a well outgassed system is typically dominated by H2, CO and H2O, very different from the atmosphere (see diagram 52, from Roth, page 6). With a real leak, the O2 peak at mass 32 is much higher than in these examples, where it is very (or immeasurably) small.

    Most of the less expensive RGA's are based on a Quadrupole Mass Spectrometer, or QMS, whose principle is explained by Luth on page 64-65. Higher mass resolution is obtained in more specialized Magnetic Sector or Poschenrieder instruments, which are typically attached to specialist facilities for e.g. cluster research or atom probe microanalysis. Both of these techniques would be suitable topics for an assignment related to section 3 of the course.


    Continue to section 2.4

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