New Hot-Cathode Ion Tubes
GFV Tube Types
Our tubes are manufactured in a variety of configurations. GFV provides several tube types which are widely used as original equipment in ion implantation equipment, physical vapor deposition equipment and other equipment used in the manufacture of semiconductor devices and integrated circuits. All GFV tubes use resistance outgassing.
- The BA-1 uses tungsten filaments operating at high temperature. It is the most stable tube. For more information click BA-1.
- The BA-2 uses a thoria-coated iridium filament operating at low temperature. For more information click BA-2.
- The BA-4 is a nude tube, usually used where tube breakage is a problem, For more information click BA-4.
- The BA-5 is a broad-range tube with somewhat better high pressure performance than the BA-2. For more information click BA-5.
- The BA-6 is a BA-2 with an internal shield, useful where electrical noise pickup is expected. For more information click BA-6.
- The CT tube has excellent high pressure performance. For more information click CT.
- GFV sells a generic thermocouple tube which can be used with our IGC-10 Ion Gauge Control. For more information click TC.
How Hot-Cathode Ion Gauge Tubes Work
Ionization gauge tubes have been used to measure pressure in vacuum systems since 1916 when Buckley first described the device. These conventional triode or CT tubes used prior to 1950 were limited to measurement of pressure in the high vacuum range until Robert Bayard and Daniel Alpert introduced a revolutionary change in tube design allowing the measurement of pressure in the ultrahigh vacuum range. This modern tube is called the Bayard-Alpert or BA tube.
All tube designs share a basic concept, shown in the above figure. A hot filament boils out an emission current of electrons which are accelerated to a positively charged grid, usually in the form of a wire spiral. Some of the electrons pass through the spaces between the spiral and find themselves in a retarding field which reverses their direction before they strike the collector electrode. However, while in the space between the grid and the collector, a few of the electrons strike gas molecules, ionizing them by knocking off one or more of the molecules' peripheral electrons. The positive ions thus created are attracted to the collector; they form a current (ion current) which is proportional to the pressure of gas molecules and the emission current of electrons:
Ion Current = G � Emission Current � Pressure.
The constant G is called the gauge factor; when ion and emission currents are expressed in Amps and pressure in Torr, G has the dimension Torr-1. The value of G depends on the type of gas, the dimensions of the tube, and the voltages applied to the electrodes.
The most sophisticated work on gauge factors has been carried out by Charles Tilford and his group at the National Institute of Standards and Technology (NIST). They define a tube's gauge factor with filament bias 30 V, grid bias 180 V, collector bias 0 V, and pure nitrogen as gas. They further recommend an emission current of 1 mA. The gauge factors quoted below for GFV tubes have all been measured using these NIST values.
All tube designs share a basic concept, shown in the above figure. A hot filament boils out an emission current of electrons which are accelerated to a positively charged grid, usually in the form of a wire spiral. Some of the electrons pass through the spaces between the spiral and find themselves in a retarding field which reverses their direction before they strike the collector electrode. However, while in the space between the grid and the collector, a few of the electrons strike gas molecules, ionizing them by knocking off one or more of the molecules' peripheral electrons. The positive ions thus created are attracted to the collector; they form a current (ion current) which is proportional to the pressure of gas molecules and the emission current of electrons:
Ion Current = G � Emission Current � Pressure.
The constant G is called the gauge factor; when ion and emission currents are expressed in Amps and pressure in Torr, G has the dimension Torr-1. The value of G depends on the type of gas, the dimensions of the tube, and the voltages applied to the electrodes.
The most sophisticated work on gauge factors has been carried out by Charles Tilford and his group at the National Institute of Standards and Technology (NIST). They define a tube's gauge factor with filament bias 30 V, grid bias 180 V, collector bias 0 V, and pure nitrogen as gas. They further recommend an emission current of 1 mA. The gauge factors quoted below for GFV tubes have all been measured using these NIST values.
Tube Installation and Precautions
The glass envelopes used with GFV tubes are robust and rarely cause a problem. However, as with any device with a glass envelope, exercise reasonable care.
GFV tubes with glass or Kovar sleeve are designed for high vacuum service. They may be installed using an elastomer-sealed compression fitting. When installing a tube, be sure the compression fitting used to seal the tube to the vacuum system has the correct diameter for the tube used. Do not force the tube. Tubes should not be attached using a compression fitting if the system is pressurized during the process cycle; the tube may pop out.
GFV tubes with Conflat� flanges may be used for ultrahigh vacuum service. Follow the directions of the flange manufacturer to make up this connection. Dress the electrical cable to the tube to prevent an inadvertent yank to the tube.
Electronic equipment to supply the bias voltages, to supply the filament heating voltage, and to read the ion current from the tube are termed �controls.� Note that the accelerating voltage applied to the grid by most controls is 180 V; take care not to touch the connection to this electrode while it is connected to the control. If the tubes are used with a control which uses electron bombardment outgassing (see below), be especially careful since even higher voltages, up to 1,000 V, are applied to the tube.
Note that under some circumstances an electrical breakdown of the gas in the vacuum system may occur; under these circumstances the gas in the vacuum system becomes an electrical conductor, allowing the voltage on any exposed electrode in the vacuum system to connect to the metal wall of the vacuum system. To prevent electrical shocks under this condition, be sure the gauge control and the vacuum system are separately grounded to a third-wire or water-pipe ground with cables which can carry at least 10 Amp.
GFV tubes with glass or Kovar sleeve are designed for high vacuum service. They may be installed using an elastomer-sealed compression fitting. When installing a tube, be sure the compression fitting used to seal the tube to the vacuum system has the correct diameter for the tube used. Do not force the tube. Tubes should not be attached using a compression fitting if the system is pressurized during the process cycle; the tube may pop out.
GFV tubes with Conflat� flanges may be used for ultrahigh vacuum service. Follow the directions of the flange manufacturer to make up this connection. Dress the electrical cable to the tube to prevent an inadvertent yank to the tube.
Electronic equipment to supply the bias voltages, to supply the filament heating voltage, and to read the ion current from the tube are termed �controls.� Note that the accelerating voltage applied to the grid by most controls is 180 V; take care not to touch the connection to this electrode while it is connected to the control. If the tubes are used with a control which uses electron bombardment outgassing (see below), be especially careful since even higher voltages, up to 1,000 V, are applied to the tube.
Note that under some circumstances an electrical breakdown of the gas in the vacuum system may occur; under these circumstances the gas in the vacuum system becomes an electrical conductor, allowing the voltage on any exposed electrode in the vacuum system to connect to the metal wall of the vacuum system. To prevent electrical shocks under this condition, be sure the gauge control and the vacuum system are separately grounded to a third-wire or water-pipe ground with cables which can carry at least 10 Amp.
Outgassing
All GFV tubes provide connection to both ends of the spiral grid structure; they therefore work with controls which pass a current through the spiral when operated in the "outgas" mode. This resistance heating of the grid creates an incandescent glow, removing layers of adsorbed gas. This procedure may shorten the time required for stable pressure readings.
If the control uses electron bombardment outgassing rather than the safer, more convenient resistance outgassing method, note the warning above.
If the control uses electron bombardment outgassing rather than the safer, more convenient resistance outgassing method, note the warning above.
Range of Operation
Ionization tubes generate noise, the �X-ray limit.� For the CT tubes the limit corresponds to a pressure of a few times 10-8 Torr, for BA tubes to a pressure of a few times 10-10 Torr. This noise establishes the lower limit to the pressure range over which these tubes are useful. Space charge and other phenomena limit the upper end of their range to 1x10-2 Torr for CT tubes and a few times 10-4 Torr for BA tubes. The broad-range BA tube (see below) provides useful output to somewhat higher pressure than the standard BA types.
NIST research has established tube output as accurately linear with pressure between these limits.
NIST research has established tube output as accurately linear with pressure between these limits.
Correction for Gases Other than Nitrogen
The true pressure for gases other than nitrogen may be estimated by multiplying the control reading by the factors measured by NIST for tubes equivalent to GFV's twin tungsten BA tubes:
Xenon: 0.35
Krypton: 0.52
Argon: 0.72
Water Vapor: 1.4
Hydrogen: 2.4
Neon: 3.36
Helium: 6.06
Xenon: 0.35
Krypton: 0.52
Argon: 0.72
Water Vapor: 1.4
Hydrogen: 2.4
Neon: 3.36
Helium: 6.06
Accuracy of Measurement
The most important source of error is tube-to-tube variation in gauge factor. For new tungsten filament tubes, this variation is �10% of nominal gauge factor; for new thoria-coated iridium filament tubes, this variation is �20% of nominal gauge factor. Tubes which have been in service for some time show even larger variations in gauge factor.
Control/tube system calibration to a primary standard is available from NIST, A55 Metrology, Quince Orchard Blvd., Gaithers�burg, MD 20899. Uncertainty in NIST's primary standard is less than �2% over the high vacuum range. Calibration to a secondary standard, traceable to NIST, is available with uncertainty of less than �5% over the high vacuum range from GFV Associates. For more information click Calibration.
Control/tube system calibration to a primary standard is available from NIST, A55 Metrology, Quince Orchard Blvd., Gaithers�burg, MD 20899. Uncertainty in NIST's primary standard is less than �2% over the high vacuum range. Calibration to a secondary standard, traceable to NIST, is available with uncertainty of less than �5% over the high vacuum range from GFV Associates. For more information click Calibration.