5–18
Transpector CPM Operating Manual
Sometimes there is a problem with mass spectral overlap—ions with differing
chemical composition form chemically distinct source molecules, but with the same
mass. For example, there is a problem with detecting small amounts of water vapor
in argon, as is often desired when monitoring a PVD process. Normally, water
vapor is monitored at 18 AMU (H
2
O
+
). Overlap from the doubly charged argon
36
isotope also shows up at mass-to-charge ratio 18 (most mass filters, including
quadrupoles, are really mass-to-charge ratio filters and not true mass filters).
Approximately 3,400 parts per million of all argon atoms are the mass 36 isotope.
Also, at 70 eV, the doubly-charged argon ion, Ar
2+
, has about 15% of the intensity
of the singly charged Ar
+
ion. Therefore, there will be approximately 510 ppm
(3,400 ppm times 15%) of
36
Ar
2+
at 18 AMU, making it impossible to detect several
ppm of water vapor at the same mass. Another ion from water vapor, the OH
+
at
17 AMU, could be used instead. Unfortunately, OH
+
at 17 AMU from water vapor
has an intensity of only 25% of that of the parent ion H
2
O
+
. To detect several ppm
of water vapor would require the detection of less than one ppm of current at
17 AMU. This is difficult because there will be some tailing of the 510 ppm
36
Ar
2+
peak at 18 AMU onto 17 AMU.
The best solution to this argon/water vapor problem is to make use of a property of
ions known as appearance potential. The appearance potential for an ion from a
given substance is the minimum electron energy required to produce that particular
type of ion from the specified substance. Table 5-3 lists appearance potentials for
various ions from common gases.
Table 5-3 Appearance potentials for various ions from common gases
Ion Gas Mass-to-Charge Appearance Potential (eV)
Ar
+
argon 40 15.7
Ar
2+
argon 20 43.5
Ar
3+
argon 13.3 >70
N
2
+
nitrogen 28 15.6
N
+
nitrogen 14 24.3
O
2
+
oxygen 32 12
O
+
oxygen 16 14.01
CO
2
+
carbon dioxide 44 13.8
CO
+
carbon dioxide 28 19.4
O
+
carbon dioxide 16 19.4
C
+
carbon dioxide 12 22.7
CO
+
carbon monoxide 28 14.1
O
+
carbon monoxide 16 20.9
C
+
carbon monoxide 12 20.9
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