MKS Stabil-Ion 370 Series, Instruction Manual, Page: 102 / 122

6 Theory of Operation and Specifications
6-2
Series 370 Stabil-Ion Instruction Manual - 370119
proportional to the density of molecules (pressure) in the gauge for pressures below approximately
1 x 10
-3
Torr. The strength of the ion current is then indicated on an electrometer that is calibrated in
units of pressure.
Because the pressure indication is linear, the hot cathode B-A gauge is generally considered to be
the most accurate continuous indicator for pressures below 1 x 10
-3
Torr.
The development of the Stabil-Ion Gauge brought the accuracy of B-A technology to the 4% to 6%
range. Although prior B-A style vacuum gauges were usually inaccurate as much as 20% to 50%,
they were still the best, commonly available vacuum gauge in the 1 x 10
-3
to 2 x 10
-11
Torr range.
6.1.2 X Ray Limit of Bayard-Alpert Gauges
The low end of the operating range of a B-A gauge is determined by the x ray limit of this type of
gauge. The x ray limit varies with different gauge designs. X rays are produced when the electrons
emitted by the cathode impact the grid (anode). Because of the geometry of the B-A gauge, only a
small fraction of the x rays emitted from the grid are intercepted by the ion collector. When the x
rays strike the collector they cause electrons to be photoelectrically ejected from the collector. This
photoelectron current from the ion collector is detected the same as positive ions arriving at the ion
collector and consequently adds to the ion current. This x ray current limits the pressures that can
be measured, and is equivalent to a pressure reading in the 10
-10
to 10
-11
Torr ranges. The x ray
limit refers to the lowest pressure indication that may be obtained in a gauge when all the output
current is due to x ray induced photoemission and there is an absence of gas.
The x ray limit of an Extended Range Stabil-Ion Gauge is approximately 2x10
-10
Torr. To measure
below this limit, an ultrahigh vacuum (UHV) Stabil-Ion gauge can be used. The UHV Stabil-Ion
gauge has an x ray limit of approximately 2x10
-11
Torr. This lower x ray limit is achieved by
modifying two elements of the standard B-A gauge design. First, the diameter of the collector is
reduced. The smaller cross-sectional area reduces the probability that the x rays created at the grid
will strike the collector. Second, the helical grid structure is replaced with a fine-wire mesh grid
structure, and there is also a fine-wire structure across both ends of the grid. The fine grid wires
provide a more transparent grid for longer electron path lengths, and the grid ends confine the
positive ions for better ion collection. Together, these two modifications cause a higher gauge
sensitivity for ions from the gas phase which causes the x ray current to be converted into a smaller
pressure indication (i.e., a lower x ray limit).
Some processes can result in deposits on the electrodes. These deposits can lead to an increase in x
ray limit since more electrons are released under x ray bombardment. Process deposits can
generally be removed by degassing the electrodes. See Degassing on the next page.
6.1.3 Filaments (Cathodes) used in Bayard-Alpert Gauges
There are two types of materials commonly used for filaments: tungsten and iridium. And, there are
two types of coatings used on the filaments: thoria and yttria.
Generally, filaments are yttria-coated iridium, thoria-coated iridium, or uncoated tungsten. The
most common style is coated iridium because they operate at a lower temperature than tungsten,
therefore less reactive. Coated iridium filaments are also more burnout resistant when exposed to
atmospheric pressure while power is on. Tungsten filaments will burn out immediately if exposed to
pressures of 1 x 10
-2
Torr or higher while they're ON. However, tungsten filaments are the best type
to use when the chemistry (such as halogen compounds) of the vacuum process causes premature
failure of coated iridium filaments.