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Hot vs. Cold Ionization Gauges
Every modern high vacuum and ultrahigh vacuum system relies on some form of ionization gauge for
pressure measurements under 10-3 Torr. There are currently two competing ionization gauge technologies
to choose from - the hot cathode gauge (HCG) and the cold cathode gauge (CCG). This application note
is designed to help vacuum users choose between the two competing ionization technologies. Each gauge
type has its own advantages and disadvantages. The best choice requires careful consideration of the
operating characteristics of both gauges and is dependent on the application.
In This Application Note
Introduction 3
Hot-Cathode Gauges (HCG) 4
Cold-Cathode Gauges (CCG) 6
Conclusions 8
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2 Hot vs. Cold Ionization Gauges
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Hot vs. Cold Ionization Gauges 3
Introduction
Every modern high vacuum and ultrahigh vacuum system relies on some form of
ionization gauge for pressure measurements under 10-3 Torr. There are currently two
competing ionization gauge technologies to choose from which are viable means for
pressure measurements between 10-2 and 10-10 Torr:
1. In the hot cathode gauge (HCG) ionizing electrons from a thermionic cathode are
accelerated by suitable electrodes into an ionizing space.
2. In the cold cathode gauge (CCG) ionization is caused by a circulating electron
plasma trapped in crossed electric and magnetic fields.
In both cases, the electrical current resulting from the collection of the positive ions
created inside the gauge is used as an indirect measure of gas density and pressure.
This application note is designed to help vacuum users choose between the two
competing ionization technologies. Each gauge type has its own advantages and
disadvantages. The best choice requires careful consideration of the operating
characteristics of both gauges and is dependent on the application.
For more detailed information on this subject consult the following publications:
1. J. M. Lafferty, Ed., "Foundations of Vacuum Science and Technology", p. 414,
section 6.9., John Wiley and Sons, NY, 1998. Note: This is an excellent book
recommended for any high vacuum question.
2. R. N. Peacock, N. T. Preacock, and D. S. Hauschulz, "Comparison of hot cathode
and cold cathode ionization gauges", J. Vac. Sci. Technol. A 9(3) (1991) 1977.
3. R. F. Kendall, "Cold cathode gauges for ultrahigh vacuum measurements", J. Vac.
Sci. Technol. A 15(3) (1997) 740.
4. R. F. Kendall, "Ionization Gauge Errors at Low Pressures", J. Vac. Sci. Technol. A
17(4) (1999) 2041. Note: Great paper that compares the performance of both gauges,
particularly at low pressures.
5. Vic. Comello, "Should your next ion gauge run hot or cold?", R&D Magazine, p. 65,
Nov. 1997.
6. Eric Bopp, "Pressure measurement in ion implanters", Solid State Technology,
February 2000, p. 51. Note: The special gauging requirements of ion implant
applications are nicely discussed in this article.
7. J. H. Singleton, "Practical Guide to the use of Bayard-Alpert Ionization Gauges",
J. Vac. Sci. Technol. A 19(4) (2001) 1712.
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4 Hot vs. Cold Ionization Gauges
Hot-Cathode Gauges (HCG)
The majority of commercially available HCGs are of the Bayard-Alpert design and are
compatible with the IGC100 controller.
A Bayard-Alpert gauge (BAG) boils electrons from a hot filament and accelerates them
toward a cylindrical grid cage. As the electrons traverse the space enclosed by the grid,
which is fully open to the vacuum chamber, they collide with gas molecules ionizing
some of them. A fine wire located at the center of the ionization volume collects the
resulting cations producing a current proportional to the gas density at the gauge. At
constant temperature, the collector current is linearly related to the gas pressure.
The useful operating range of a conventional BAG extends between 10-3 and 10-10 Torr,
corresponding to an impressive seven decades of dynamic range. Special gauge designs
are available to extend the lower limit to 10-11 Torr for UHV applications, or the upper
end to 10-1 Torr for process applications.
The strict linear dependence of collector current on pressure is one of the most important
advantages of HCGs over the competing ionization technology. It is generally possible to
approximate the 'collector current vs. pressure' response of a BAG to a straight line and
calculate pressures from a single linear proportionality factor (i.e. sensitivity factor)
stored in the gauge controller. A sensitivity factor calibrated at mid-range, can be used for
accurate and reproducible pressure measurements between 10-9 and 10-4 Torr. Deviations
from linearity typically amount to less than