6–8
Transpector CPM Operating Manual
When operating at high emission (2 mA at 70 eV), the pressure will read
approximately 20 times high. Therefore, the over-pressure will trip at a pressure
about 20 times lower. Typically, this is not an issue since high emission should only
be used when high sensitivity is needed, such as when leak checking a system.
When sampling at a process pressures, running at high emission will result in poor
linearity if the over-pressure does not trip. If 70 eV is desired for monitoring process
gas, run FabGuard Explorer, enter Sensor Configuration and change the electron
energy to 70 eV, leaving the emission current at 200 A.
Re-calibration requires calculation of the pressure in the CIS when Process Gas is
introduced at a known pressure. The relationship is:
P
CIS
= C
Orifice
P
Process
/ C
CIS
[1]
Table 6-5 lists the values for the exit conductance for CIS and for different orifices
used in CPM sampling systems. Equation [1] can be used to infer the target gas
pressure in the CIS that produces the total pressure measurement. Using the
process gas pressure (measured by the CDG or other gauge) and the conductance
of the orifice in use, the P
CIS
(Hi-P) and P
CIS
(Lo-P) values can be calculated to use
the automatic TW-32 Total Pressure Calibration software.
6.5.3 Filament Lifetime
Tungsten filament material is used in corrosive applications. The melting point of
tungsten is 3410°C; its boiling point is 5660°C. At normal operating temperatures
(approximately 2200°C), tungsten vapor pressure is approximately 1E-7 Torr,
which normally results in a reasonable lifetime. Various oxides of tungsten are
known.
The lowest oxide, WO
2
, is brown in color and has a sublimation temperature of
around 800°C. The intermediate oxides, W
2
O
5
and W
4
O
11
, which are blue-violet
in color, and sublimes between 800°C and 900°C.
Table 6-5 CIS and Orifice Conductances for use with Equation 1
Orifice Designation Orifice Diameter
(microns)
Conductance (L/s)
PCIS (Torr)
[@ Design
P
Process
]
CVD – CIS Exit ------- 1.0
10 mT 750 3.9E-2 3.9E-4
100 mT 200 2.8E-3 2.8E-4
1 Torr 62 2.7E-4 2.7E-4
3 Torr 35 8.6E-5 2.6E-4
6 Torr 25 4.4E-5 2.6E-4
10 Torr 20 2.8E-5 2.8E-4
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