Mass Spectroscopy Study of Hydrocarbon Removal by an
Argon-Oxygen DC-Glow Discharge
Jiang-Tao Li, Zhen-Bin Wang, Da-Wei Hu, and Yi-Kang Pu
Department of Engineering Physics, Tsinghua University, Beijing 100084, China
Abstract: Hydrocarbon contamination has been one of the most concerned issues by
researchers who work on UHV systems. In this work, we report the results of
removing some unknown hydrocarbons from a previously contaminated device
mounted on the UHV chamber by alternate baking and argon-oxygen dc-glow
discharges. By analyzing the temporal behaviors of various peaks in the residual gas
mass spectra, some of the strong peaks can be identified. It is found that, after an
argon-oxygen dc-glow discharge, mass peak 19 can be the strongest peak due to the
electron stimulated desorption of H3O+, and that, different hydrocarbon peaks show
different temporal behaviors during a baking period, due to gas phase diffusion and
surface diffusion.
Keywords: mass spectrum, hydrocarbon, oxygen, diffusion, H3O+
1. Introduction
Hydrocarbon contamination has been one
of the most concerned issues by researchers
who work on UHV systems [1-4]. For ones
who work on accelerators, synchrotrons, or
other beam storage rings, hydrocarbons can
cause undesirable beam loss due to electron
stimulated desorption (ESD) [5] and
gas-phase scattering [6-7]. For ones who
work on scanning electron microscope
(SEM), Auger electron spectroscopy (AES)
or other surface sensitive analytical
apparatuses, hydrocarbons can cause carbon
contamination of the sample surface or even
reduced sensitivity [8-9]. Various methods
have been tried to remove hydrocarbons,
including baking and discharges. In
particular,
oxygen/argon-oxygen
glow
discharge is found to be an effective way to
do the work [3, 6-7, 10].
In this work, we report the results of
removing some unknown hydrocarbons
from a previously contaminated electron
energy analyzer (Omicron EA125), mounted
on our UHV chamber. We did alternate
baking and argon-oxygen dc-glow discharge
cleaning cycles, and used an ionization
gauge and a residual gas analyzer (RGA), to
monitor the total pressure and the mass
spectrum of the residual gas in the chamber.
By analyzing the temporal evolution of the
mass spectrum peaks and the correlations
between them, we tried to identify what they
represent or what kind of physical or
chemical processes they participate in. The
effectiveness of hydrocarbon removal by the
argon-oxygen discharge is proved again in
this case. It is found that, mass peak 19 is
one of the strongest after discharges due to
the ESD of H3O+, and that different
hydrocarbons show different diffusion
behaviors during a baking period relating to
gas phase diffusion and surface diffusion.
2. Experimental details
A schematic diagram of the apparatus is
shown in Figure 1. The EA125 (Omicron)
was mounted on the UHV chamber. The
total pressure of the chamber and the mass
spectrum of the residual gas in it were
monitored by an ionization gauge (Inficon
BPG400) and a RGA (Stanford Research
System RGA300) respectively. The system
was pumped by a turbo-molecular pump
(Pfeiffer TMU-261, with a pump speed of
210 l/s for Nitrogen) and a scroll pump
(Varian TriScroll300).
When we do the baking procedure, the
EA125 and the chamber are wrapped with
six heating strips, with aluminum foil
covering the whole system from outside.
The baking temperature is 210 oC.
We use a mixture of flowing argon and
oxygen (ratio 2:1) to do the discharge,
which is under the pressure of 1.8 Pa (so
that the plasma can spread over the whole
chamber). A home-made high voltage
dc-module provides a discharge current of
40 mA.
We do baking and argon-oxygen dc-glow
discharge alternatively so that the amount of
Figure 1. A schematic diagram of the apparatus. For the
sake of illustration, the ports of the UHV chamber for EA125,
RGA300, and gauge are rotated 45o, 44.5o, and 9o
respectively, around the axis (shown in dash line).
hydrocarbons left on the EA125 continues
to decrease with each cleaning cycle.
3. Results and Discussion
3.1 Hydrocarbon removal by argon-oxygen
discharge
Figure 2 (a) shows a RGA spectrum of the
chamber after baking. The spectrum has lots
of peaks very high but scarcely seen in
Figure 2 (b). These peaks are mostly related
to hydrocarbons from the EA125.
When the chamber is clean, as is shown
Figure 2. (a) The residual gas mass spectrum of the UHV
chamber after a 67-hour baking, which was measured under
the pressure of 8.8e-6 Pa. (b) The residual gas mass
spectrum of the UHV chamber after a succeeding 3–hour, 1.8
Pa, 40mA dc-glow discharge, which was measured under the
pressure of 2.8e-6 Pa.
by Figure 2 (b), we bake the whole system,
so that, the hydrocarbons which stay mostly
on the surface of the EA125 tend to desorb
and diffuse into the chamber and further into
the vacuum pump. The RGA spectrum will
evolve towards the pattern shown in Figure
2 (a). If we do a discharge to clean the
chamber then, the RGA spectrum of the
chamber will go back to the pattern shown
in Figure 2 (b).
We apply a short baking to monitor the
temporal evolution of the dominating peaks
shown in Figure 2 (b). Figure 3 shows the
result.
a) Mass peaks: 12, 28, 44.
They are mainly from CO and CO2, since
mass peak 12 can only be C+, and that the
close relationships between mass peaks 12
and 28, 12 and 44 exclude other candidates
which have significant contributions to mass
peaks 28 and 44.
b) Mass peaks: 14, 30.
They are mainly from NO. Although CH2+
could also have a peak at mass 14, its
accompanying peaks CH+ (mass 13) and
CH3+ (mass 15) are not clearly seen.
Even though NO2 (mass 46) and N2O
(mass 44) could also contribute to mass
peaks 14 and 30, in this case, there is not a
significant peak at mass 46 and the behavior
of mass peak 44 is very different from those
of mass peaks 14 and 30.
c) Mass peaks: 16, 17, 18.
Mass peak 18 and its by-peaks 17 and 16
are from H2O. But the temporal behavior of
mass peak 16 is a little different from those
of mass peaks 17 and 18 due to its other
contributions from CO, CO2, NO, and O2.
d) Mass peaks: 19, 20.
Mass peak 19 is one of the controversial
Figure 3. The temporal evolution of some dominating peaks
in RGA after the argon-oxygen dc glow discharge when a
short baking is applied (marked on the time axis). (a), (b), (c),
and (d) show four groups of mass peaks. The arbitrary time
unit is 1 minute and 23 seconds here.
peaks in the RGA spectrum of the vacuum
chamber [11]. The main candidates are F+
and H3O+. Here, we tend to believe that
mass peak 19 are mainly from H3O+, which
is generated by the electrons from the RGA
filament through ESD from the local surface
near the RGA ionizer. There are several
reasons for us to believe that. First, mass
peak 19 is the highest peak (see Figure 2
(b)), which is hard to believe if 19 is F+,
since F+ is so electronegative. Second, by
slowly shutting down the valve between the
chamber and pump, we find that, all peaks
whose contributions are from the residual
gases increased in the RGA spectrum due to
the decrease of their loss rate, but mass peak
19 kept constant. Another important proof is
the temporal correlation between mass peak
19 and hydrocarbon peaks 98 and 99, which
we will show below.
Mass peak 20 could be from HF+,
H2O(18)+, and Ar2+. However, for argon, the
double ionization mass peak 20 should be
an order of magnitude smaller than mass
peak 40. Besides, mass peak 20 is not as
sensitive to temperature as mass peaks 18
and 40. So there should be a contribution
from HF+, which is possibly from the
plasma interaction with O-rings.
e) Mass peaks: 2, 32, 40.
Mass peaks 2, 32 and 40 are mainly from
H2, O2 and Ar, respectively. H2 is one of the
dominating residual gases in the UHV
chamber, since it is easy for H2 to penetrate
the chamber wall from outside. Besides, O2
and Ar are the discharge gases we used.
3.2 Temporal behaviors of various peaks in
RGA spectrum during baking
As the baking starts, the peaks of the RGA
generally have two kinds of response which
are different in timescale (seen in Figure 4).
The first is a quick rise and decay, which
lasts for about 10 hours or less, reflecting
desorption rate change of various species
adsorbed on the surface of the chamber.
This kind of response can be seen on the
mass peaks 14，17, 18 and so on. The
second
is
a
slow
increase
of
hydrocarbon-related peaks, reflecting the
out-coming of the hydrocarbons from the
EA125. This kind of response can be seen
on mass peaks 13, 14, as well as other high
mass peaks such as 98 and 99.
Even with the same mass peak, there may
be different gas species which mainly
contribute to it at different time.
a) Mass peaks: 12, 28, 44.
As can be seen in Figure 4 (a), mass peaks
Figure 4. The temporal evolution of some dominating peaks
in RGA during a 67-hour baking after the argon-oxygen dc
glow discharge. (a), (b), and (c) compare the evolution
behavior of three groups of species, while (d) shows the
temporal correlation between mass peak 19 and 98, 99. The
baking starts at time = 0.
12 and 44 keep close relationship, but mass
peak 28 deviates a little towards the
behavior pattern of mass peak 14. This
suggests that, mass peaks 12, and 44 are
mainly from residual gases of CO and CO2,
but mass peak 28 has another contribution
from N2.
b) Mass peaks: 13, 14, 15.
As reasoned above, mass peaks 14 and 28
have contribution from N2, but according to
Figure 4 (b), mass peak 14 should have a
contribution from CH2+, to behave like mass
peaks 13 and 15 which are mainly from CH+
and CH3+.
The first kind of response mentioned
above can be clearly seen in Figure 4 (b). In
this time region, mass peak 14 mainly
represents the behavior of NO.
c) Mass peaks: 16, 17, 18.
Mass peaks 17 and 18 show the temporal
behavior of H2O in the chamber, which only
have the first kind of response. But mass
peak 16 doesn’t, due to its other
contributions from, such as, CH4, CO, CO2
and so on.
d) Mass peaks: 19, 98, 99.
As we see in Figure 4 (d), there is an
obvious temporal correlation between mass
peaks 19 and 98, 99. Mass peaks 98 and 99
are typical hydrocarbon mass peaks in our
case. When the mass peaks 98 and 99 are
high, mass peak 19 decreases severely. This
is a repeatable phenomenon in our
experiment, illustrating the occurrence of
proton transfer reaction [12], supporting our
identification of mass peak 19 as H3O+.
3.3 Two kinds of temporal behaviors shown
by hydrocarbon mass peaks during
baking
Table 1. A summary of the main contributors to some
dominating mass peaks shown in RGA in different time
periods of the cleaning cycle.
Mass Peak
(amu)
Figure 5. Two typical temporal behaviors of hydrocarbon
mass peaks in four succeeding baking periods with
argon-oxygen discharges “cleaning” the chamber before
each baking period. Baking periods start at time = 0. (a) and
(b) show the temporal behaviors of mass peaks 105 and 99,
respectively.
As mentioned above, we did alternate
baking and argon-oxygen discharges. Figure
5 shows two typical temporal behaviors of
hydrocarbon mass peaks.
The temporal behaviors of mass peaks
105 and 99 are quite different. Mass peak
105 shows almost linear increase during
each baking period, while mass peak 99
seems to have a time “threshold” before an
abrupt increase.
The “linear increase” behavior can be
understood if the chamber accumulates
hydrocarbons linearly through gas phase
diffusion, and these hydrocarbons in the
chamber contribute to the gas phase partial
pressure of mass peak 105. However, the
time “threshold” behavior seems to show
another kind of diffusion mechanism, which
is like a slow but great tidal wave. This kind
of diffusion is hard to understand if we
consider only gas phase diffusion. However,
with surface diffusion considered in the
diffusion process, the “tidal wave” picture is
easier to understand since the RGA records
the concentration of hydrocarbons locally
near the surface of the RGA ionizer due to
the thermal desorption processes and
electron stimulated desorption processes
caused by the RGA filament.
4. Conclusions
In our work, we prove again the
effectiveness of argon-oxygen plasma on
hydrocarbon removal, and analyze the main
contributors to different mass peaks in
different time periods of a cleaning cycle
(see Table 1). It is found that, after an
argon-oxygen dc glow discharge, mass peak
19 is so dominating due to the ESD of H3O+
near the RGA ionizer. During a baking
period, hydrocarbon peaks 105 and 99 show
different temporal behaviors, probably
relating to different diffusion mechanisms.
2
12
13
14
Main Contributors to the Mass Peaks
After
Plasma
Baking
Initial Period
Baking
Final Period
H2
CO, CO2
H2
CO, CO2
hydrocarbons
NO, N2,
hydrocarbons
hydrocarbons
H2O, NO，
CO, CO2 …，
hydrocarbons
H2O
H2O
H3O+
HF, Ar
CO, CO2, N2
NO, CH2O
O2
Ar
CO2
H2
CO, CO2
hydrocarbons
N2,
hydrocarbons
hydrocarbons
H2O, CO,
CO2 …，
hydrocarbons
H2O
H2O
H3O+
HF
CO, CO2, N2
CH2O
NO
15
16
17
18
19
20
28
30
32
40
44
H2O, NO,
CO,
CO2 …
H2O
H2O
H3O+
HF, Ar
CO, CO2
NO
O2
Ar
CO2
CO2
Acknowledgements
This work is supported by the NSFC grant (No
10935006).
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