Hot Cathode Ionization Vacuum Gauges

The absolute pressure, defined as force per unit area, is difficult to determine in high vacuum, where the force acting on an object becomes too small. A highly practicable, but indirect method to measure pressure in this low range is therefore to ‘count’ the gas particles. This is achieved by ionizing the residual gas in a defined volume and measuring the generated total ion current. Ions are produced by a bombardment of gas particles with thermally induced electrons from a filament (hot cathode). In this way, the particle number density can be determined. The pressure is directly proportional to this value (p=nkT), and can be measured with a good accuracy over a wide range.

  • The proportionality factor kT is constant as long as the gas temperature is constant. Changing the temperature induces also a pressure change, but not the particle-number density (if the volume and the particle number remain the same).

The idea is to ionize the gas, because ions can be easily detected with a collector electrode. Therefore, electrons are accelerated to cause collision ionizations. This is possible in a wide pressure range from medium up to extreme high vacuum, where the particles have a sufficiently high mean free path.

  • Hot cathode: Thermal emission from a hot cathode generates electrons. That leave the filament because their thermal energy is higher than the characteristic work function of the filament material (or coating).
  • Alternative method: Gas discharge in a cold cathode, see section Cold cathode - ionization vacuum gauges.

The electron emission from the cathode is restricted to a constant current Ie, e.g. 1 mA. The cathode is brought to a temperature of 1500 °C releasing several watt of electrical power in the form of heat. The electrons accelerate to an anode grid, where most electrons pass through and ionize the gas inside the grid. The ions are directed towards the collector and neutralize there. The neutralization generates a current Ic, which can be measured. This collector current is proportional to the particle number density, hence to the pressure. The fundamental equation for hot cathode gauges is p = Ic/(Ie*S), with S describing the sensitivity of the gauge. It has to be taken into account that the sensitivity depends on the gas composition as well, due to the different ionization probabilities for different chemical species.

Physical effects, limitation factors
The pressure range of a hot cathode, where the pressure is directly proportional to the collector current, is limited. Depending on the design of the gauge the upper limit varies between 0.1 mbar and 10-4 mbar. This constraint is imposed by e.g. space-charge effects. The lower limit ranges between 10 -8 mbar and 10-13 mbar, since the collector current contains a residual component, which cannot be totally eliminated. The residual current sums up the contribution from various error effects). The so called x-ray limit is generally used as a synonym for the residual current in hot-cathode pressure gauges.

Collector current:
Measurement
1.)    Ion current (electron released from the collector to neutralize impacting ions. The current is proportional to the particle-number density, hence to the pressure)
Error-effects (not linear with pressure)
2.)    X-ray effect (Bremsstrahlung from an electron impact on the anode with following photoemission at the collector)
3.)    Electron stimulated desorption (ESD) from particles at the anode (a real local pressure increase)
4.)    Outgassing of heated sensor parts (local pressure increase)
5.)    Inverse x-ray effect (Bremsstrahlung generated by electron bombardment of the anode; following photoemission of electrons from the outer wall; electron absorption at the collector, i.e. inverse to the
x-ray effect)

Application
Hot cathode gauges are offered in various designs. They exhibit high stability, accuracy and reproducibility. However, the sensitivity is influenced by several factors:

  • Changes, which affect the electron trajectories (e.g. external magnetic fields, geometrical changes of the electrodes or the outer wall – especially for nude gauges);
  • Contamination of electrodes, e.g. by material from the vacuum system (thus a  frequent use of the DEGAS-function is important);
  • Gas type (specific ionization probabilities can be considered with a correction factor)

The lower range of hot cathode gauges is limited by the residual current. Therefore, the measurement has an increasingly high uncertainty, if the pressure is smaller than 3 to 4 times the x-ray limit.

The life time of the hot cathode gauge is typically limited by the contamination by the vacuum system or by the depletion of the filament coating layer. A frequent degassing of the gauge increases its life time. For coating applications it is recommended to use a baffle in order to slow down the contamination process. Under harsh industrial conditions (e.g. sputtering processes at relatively high pressure) problematic glow discharges or electric flashovers between the electrodes can occur. In those cases grids or baffles help to protect the sensor from charge carriers and electromagnetic fields.

The established iridium-cathodes with yttrium oxide coating are stable in oxygen environments (air intrusion). In general the filaments have a life time of over 10000 h, which can be lowered by reductive gas species (e.g. hydrogen; halogens). In these cases tungsten filaments have to be used, which on the other side are unstable in an oxygen atmosphere. Most hot cathode gauges have an extra cathode, in case the first one is broken.

Further Information

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