Operation
Ionization Gauge Instruction Manual - 274026 - Rev. B
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may be obtained in a gauge when all the output current is due to X-ray
induced photo emission and there is an absence of gas.
The X-ray limit of standard glass or nude B-A gauges is approximately
3x10
-10
Torr. To measure below this limit, an ultrahigh vacuum (UHV)
nude B-A gauge can be used. The UHV nude 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.
3.5
Degas Procedure
The deposition of elements or compounds on exposed gauge surfaces can
result from some processes, such as sputtering or coating operations. Water
vapor is another compound that can collect inside the gauge when a
vacuum chamber is exposed to atmosphere. During normal operation these
materials will slowly come off the gauge surfaces, increasing the local
pressure in the gauge. To more rapidly get the pressure in the gauge in
equilibrium with the pressure in the chamber, "degassing" can be used to
drive the molecules on the inner walls and surfaces from the gauge back
into the chamber where they can be pumped out of the system. Degassing
can be done as required or as part of a regular pumpdown sequence.
Regular degassing helps prevent process deposits from collecting and
allows the gauge to provide lower and more repeatable pressure indications
by bringing the pressure in the gauge closer to equilibrium with the
chamber.
There are two types of degassing techniques: Electron Bombardment (EB)
and Resistive (I
2
R). EB degas must be used for UHV nude gauges with fine
wire mesh grids and can also be used for glass or nude gauges with helical
coil grids. I
2
R degas can only be used for gauges with helical grids.
Although similar in result, each degassing technique employs a different
mechanism to perform this beneficial function.
• EB degassing is accomplished by increasing the anode voltage and the
emission current to bombard the gauge with electrons of sufficient
quantity and energy to displace deposited molecules. EB degassing is
