VISUAL AIDS for instruction in VACUUM TECHNOLOGY AND APPLICATIONS

VISUAL AIDS for instruction in VACUUM TECHNOLOGY AND APPLICATIONS
Module 2: Total Pressure Measurement in Vacuum Second Edition Section 4 Indirect Measurement of Pressure II Ionisation Gauges

Indirect Measurement of Pressure Ionisation
The most convenient method of measuring pressures below about 0.1 Pa is to ionise the remaining gas molecules, collect the ions and measure the ion current Ionisation can be effected by various means but the two most common are to use either a beam of low energy electrons, often between 50eV and 250eV a plasma (gas) discharge of some sort In this chapter examples of gauges using both these techniques will be given

Indirect Measurement of Pressure
Ionisation There are two important points to note when using gauges based on gas ionisation Such gauges measure number density of gas molecules, not pressure Therefore they must be calibrated Ionisation cross sections are species dependent, so such gauges will give readings which are dependent on the gases present The next few slides describe the basic ionisation process

Ionisation Ionisation occurs when outer electrons are removed from an atom by an energetic particle or photon This gives positive ions Polyatomic molecules may break up if they receive enough energy in such a process

Ionisation It is also possible to remove more than one electron giving a multiply charged positive ion If an electron is excited to a higher energy level but does not have enough energy to escape, this excited atom can decay with emission of an electron or photon

Ionisation Here, we can see how the energy required to create a singly charged positive ion varies for some selected atomic species (not all are gases)

Ionisation In this slide, we see a plot of the ionisation energy for a wide range of elements – some are identified The local maxima correspond to atoms where all electron energy shells are full

Ionisation The ionisation probability for a gas atom by an electron depends not only on the species, but also on the energy of the incident electron Here we see this probability plotted for a number of common gases

Glow Discharge If a voltage is applied to a pair of parallel plates in vacuum, breakdown can occur. Electrons are stripped from the gas atoms by the field and a glow discharge may be ignited Here we see such a discharge tube and a plot of the breakdown voltage as a function of pressure This is known as Paschen’s Law

Glow discharge If the voltage between two plates is fixed at a few thousand volts, then the discharge current is a function of pressure and may be used as a measure of the pressure Such a tube is useful over the pressure range 100 Pa to 0.5 Pa

Glow discharge When a discharge is struck, the colour of the glow is characteristic of the gas – it is pink for air The discharge varies along the tube and appears to form bands which blend into each other This slide shows a discharge at a few tens of Pascals Below about 0.1 Pa, the discharge goes “black”

Cold Cathode Ionisation Gauge An important class of gauge in practical vacuum work in the medium to high vacuum ranges is based on a cold gas discharge in combined electric and magnetic fields In such discharges, free electrons are accelerated by the electric field and are trapped by the magnetic field so that they have very long path lengths – much longer than the gauge dimensions This means that even at low pressures, these electrons have a good chance of ionising a gas molecule Therefore a discharge can be maintained to much lower pressures than in a simple discharge tube

Cold Cathode Ionisation Gauge There have been many configurations of such gauges and a few are illustrated in the next slide These are often referred to as Penning Gauges, since the most popular configurations are based on the Penning discharge The useful range of standard Penning gauges is between 0.1 Pa and 10-6 Pa Discharge gauges are susceptible to contamination The accuracy of such gauges is not very good, especially at low pressures. Large changes in sensitivity are not uncommon

Cold Cathode Ionisation Gauge

Cold Cathode Ionisation Gauge This is the classic Penning discharge configuration. It operates at fixed voltage and fixed magnetic field Ions are collected on the ring anode Also shown is the gauge characteristic as a function of pressure for a few gas species It should be noted that at low pressures the discharge is unstable and the calibration can change abruptly

Cold Cathode Ionisation Gauge This a commercial realisation of the Penning gauge Special versions of such gauges can be obtained which will operate down to below 10-7 Pa

Cold Cathode Ionisation Gauge A development of the cold cathode gauge based on a different configuration known as the Inverted Magnetron Gauge has become quite popular This gauge can operate down to 10-9 Pa or lower The accuracy and repeatability are similar to the Penning gauge However, like all discharge gauges, the discharge can be reluctant to strike at very low pressures This means that starting times (i.e. before the gauge actually measures pressure) can be quite long, often several hours

Cold Cathode Ionisation Gauge This is the construction of the Inverted Magnetron Gauge as constructed by Redhead

Hot Cathode Ionisation Gauge The hot cathode ionisation gauge was developed to provide a convenient method of measuring pressures in the high vacuum and later the ultra high vacuum regimes In such a gauge, a heated filament generates a beam of electrons which ionise the gas molecules The ions are collected on a negatively biased collector and the resultant current is a measure of the pressure These gauges were based on electron tube designs and the first such gauge to be widely used was the triode ionisation gauge

Hot Cathode Ionisation Gauge The upper figure is the conventional electron tube triode amplifier tube configuration. Here, the grid potential is varied to regulate the current flowing to the cylindrical anode If the polarity of grid and outer cylinder are reversed, then the electrons are collected on the grid and the ions created are collected on the outer cylinder

Hot Cathode Ionisation Gauge The ion current i+ is proportional to the emission current i- and the pressure p, so that where  is a gauge constant with units of Pa-1 and K is the gauge sensitivity with units of Amp Pa-1 Values of  are in the range of 0.1 to 0.2 Pa-1 The useful pressure range is 0.1 Pa to 10-5 Pa

Hot Cathode Ionisation Gauge In this slide, we see how the sensitivity of a hot cathode ionisation gauge depends strongly on the gas species being measured The exact values will depend on the particular gauge involved

Hot Cathode Ionisation Gauge This slide illustrates the variation of ion current with pressure which is found in a typical triode gauge for three different gases There are three clear regions and it will be noted that only region 2 is really useful for pressure measurement In this region, the characteristic is linear

Hot Cathode Ionisation Gauge In region 3 a glow discharge may strike and the gauge may be damaged In region 1, the output is independent of pressure The reasons for this will be discussed later

Electron Stimulated Desorption
Indirect Measurement of Pressure Hot Cathode Ionisation Gauge First, we look at some of the physical processes which occur in a hot cathode ionisation gauge. These are Soft X-ray emission Photoemission Electron Stimulated Desorption

Hot Cathode Ionisation Gauge Therefore it is best to use a high atomic number metal for the anode of a gauge and a relatively low electron energy for the ionising electrons

Hot Cathode Ionisation Gauge When an electron impinges on a solid surface, it can excite the surface atoms. These may then decay by emitting a soft X-ray The photon energy increases with the atomic number of the excited atom and the photon flux increases as a power, n, of the incident energy

Hot Cathode Ionisation Gauge If a beam of photons of energy E=h. impinges on a surface, the electrons in the surface can absorb the full energy of a photon If the energy transferred is sufficiently high, the electron can leave the surface This is the process of photoemission

Hot Cathode Ionisation Gauge There is a threshold energy for this process – called the work function, . This is the energy needed for the electron to escape The energy of the emitted electron is therefore given by E=h. – 

Hot Cathode Ionisation Gauge This slide illustrates the process of electron stimulated desorption in which electrons of sufficient energy can cause gases adsorbed on a surface to be emitted The gas can be desorbed as an ion or as a neutral. These can be complete molecules or fragments of the adsorbed molecule

Hot Cathode Ionisation Gauge If the number of adsorbed molecules per unit area is n and the number of incident electrons per unit time is ne, we can write the desorbed current as i varies from to cm-2

Hot Cathode Ionisation Gauge As seen earlier, the desorbed current is Ii where i will be + for positive ions, 0 for neutrals and – for negative ions Since negative ion production is small compared to the other two processes we shall ignore it here

Hot Cathode Ionisation Gauge We now see how these processes affect the operation of a triode hot cathode ionisation gauge. The three main contributions to the collector current are shown In the first, top left, the ion current is the desired analogue of pressure and is proportional to the number of gas atoms present (or to the pressure) This could be described as the ‘normal’ operation of the gauge

Hot Cathode Ionisation Gauge Again the three main contributions to the collector current in a triode ionisation gauge are shown The second contribution comes from the so-called X-ray effect Here, the electrons arriving at the grid generate soft X-rays When these X-rays strike the cylindrical collector, they cause photoemisssion which gives a collector current that is independent of pressure

Hot Cathode Ionisation Gauge Electrons emitted by the filament are collected by the wire grid (1) These electrons generate soft X-rays (2) which strike the collector The X-rays then cause photemission of electrons (3) which return to the grid This photoelectron current, IR, leaving the collector gives an apparent increase in the current generated by the positive ions (4) arriving at the collector, I+

Hot Cathode Ionisation Gauge Thus the indicated pressure is given by Where K is the gauge sensitivity defined earlier These processes limit the useful low pressure of typical triode gauges to about 10-5 Pa

Hot Cathode Ionisation Gauge This slide again illustrates the three main contributions to the measured collector current in a triode gauge The third mechanism is somewhat more indirect Electrons or ions may desorb gas from the structure of the gauge, giving rise to a local increase in number density of gas molecules which in turn may be ionised, increasing the ion current to the collector

Hot Cathode Ionisation Gauge The ESD process is analogous to that described previously for the X-ray mechanism At (2) the electrons emit ions rather than X-rays and these are collected directly at the collector The result is an offset to the ion current so that

Hot Cathode Ionisation Gauge In this slide are plotted a set of characteristics for a number of different hot cathode triode gauges showing the indicated pressure PI against the true pressure PTR It will be seen that although the detail varies, the shape of the curve is basically the same

Hot Cathode Ionisation Gauge At low pressures, as we have seen, the X ray effect dominates, although there may be some desorption also contributing to the reading by increasing the local number density in the gauge head

Hot Cathode Ionisation Gauge At high pressures the curve is no longer single valued, and the pressure reading is ambiguous This is due to lower net ionisation at high pressures caused by recombination and also by scattering of the ions from residual gas in the gauge head

Hot Cathode Ionisation Gauge A major improvement to low pressure measurement capability came with the development of the Bayard-Alpert Gauge In this gauge, the triode structure is inverted and a very thin central wire is used as the ion collector The provision of two filaments increases reliability

Hot Cathode Ionisation Gauge The improvement in performance is because the solid angle subtended by the collector is very small, so that the number of X-rays which strike it is very much reduced The grid can be made finer so that ESD is somewhat reduced It is also easier to outgas the gauge head to remove desorbed gas The X-ray limit of such a gauge may be better than 10-9 Pa with a sensitivity of 0.1 to 0.2 Pa-1

Hot Cathode Ionisation Gauge Further improvements to the low pressure limit can be made by moving the collector outside the ionisation region in the extractor gauge Ions are extracted by an electric field and reflected onto a very small collector Both X-ray and ESD currents are much reduced The low pressure limit is reduced to about Pa

Hot Cathode Ionisation Gauge These are examples of commercial Bayard-Alpert gauge heads (left) and an extractor gauge head (above)

VISUAL AIDS for instruction in VACUUM TECHNOLOGY AND APPLICATIONS
Module 2: Total Pressure Measurement in Vacuum Second Edition End of Section 4 Indirect Measurement of Pressure II Ionisation Gauges

VISUAL AIDS for instruction in VACUUM TECHNOLOGY AND APPLICATIONS

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