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Degassing a vacuum system with in-situ UV radiation
Sean R. Koebley, Ronald A. Outlaw, and Randy R. Dellwo
Citation: J. Vac. Sci. Technol. A 30, 060601 (2012); doi: 10.1116/1.4754292
View online: http://dx.doi.org/10.1116/1.4754292
View Table of Contents: http://avspublications.org/resource/1/JVTAD6/v30/i6
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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LETTERS
Degassing a vacuum system with in-situ UV radiation
Sean R. Koebley
a)
and Ronald A. Outlaw
b)
College of William and Mary, Department of Applied Science, 325 McGlothlin Street Hall, Williamsburg,
Virginia 23187
Randy R. Dellwo
RBD Instruments, 2437 Northeast Twin Knolls Drive, Bend, Oregon 97701
(Received 6 June 2012; accepted 7 September 2012; published 21 September 2012)
Photon-stimulated desorption (PSD) from a high-powered ultraviolet source was investigated as a
technique to degas a vacuum system. A stainless steel vacuum system was pumped down from
atmosphere with different time doses of 185 nm light, and the resulting outgassing rates were compared
to that of a control pumpdown without UV assistance. PSD was found to provide a factor of 2
advantage in pumpdown pressure after only 30 min of UV exposure, with no additional advantage
observed for longer irradiation times. Specifically, an outgassing rate of 3 10
10
Torr L s
1
cm
2
was reached 3 h sooner in pumpdowns with UV assistance compared to those without UV, while a rate
of 1.2 10
10
Torr L s
1
cm
2
was reached 16 h sooner in UV runs. The authors calculated that
about 22 monolayers of water were desorbed after 30 min of UV exposure. The results indicate that
PSD by a 40 W 185 nm UV source can serve as a nonthermal technique to significantly speed the
pumpdown of a vacuum system from atmosphere after only 30 min. V
C2012 American Vacuum
Society. [http://dx.doi.org/10.1116/1.4754292]
I. INTRODUCTION
Thermal desorption (bakeout) of vacuum systems at a tem-
perature up to 450 C is a well-known technique to promote
the diffusion and desorption of molecules from the bulk and
surface of vacuum walls and is the method of choice for attain-
ing ultra-high vacuum (UHV).
1–3
Such bakeouts, however, are
time consuming and high-temperature limited because of com-
ponents that can be damaged by heat. Furthermore, in very
large systems, e.g., accelerators, bakeouts are impractical. For
more manageable laboratory systems, a short term (<20 h)
modest bakeout of less than 150 Cisusedtodesorbsurface
water, which allows the system to recover a modest opera-
tional base pressure. A thin wall
4
or deposition of an adlayer
coating
5
have been suggested as methods to reduce the perma-
nent outgassing rate of a system, and several degassing treat-
ments besides bakeout have been investigated,
6–8
but after
exposure to atmosphere, physisorbed water is replenished and
pumping times in the hours remain necessary.
Photon-stimulated desorption (PSD) is another vacuum
degassing option that can serve as an attractive alternative to
bakeout. PSD has been studied extensively in the context of
synchrotron beam chambers, where pressure due to PSD lim-
its the beam lifetime.
9–12
Light-induced atomic desorption
from various substrates has also attracted attention as a non-
thermal control of the partial pressure of Na, Rb, K, or other
atomic species,
13,14
a capability with wide applications.
15–17
PSD is a nonthermal
18,19
process that occurs primarily at
surface defects
20
but may also produce diffusion and subse-
quent desorption of species from the bulk.
21
Because of its
nonthermal nature and ease of application, a PSD source is
less damaging to vacuum components, simpler to implement,
and confers its benefit over a course of minutes instead of
hours or days. The comparison has previously been made
between thermal desorption and photon-stimulated desorption
for short-term degassing in a small experimental system,
22
but
details like the optimal time of irradiation were neglected.
This suggests the need for further study in order to fully real-
ize PSD for its degassing benefit. In this paper, we show that
there is a significant benefit of PSD, that the phenomenon is
indeed nonthermal, and that the advantage is realized after
only 30 min of photon irradiation.
II. METHODS
The stainless steel vacuum system schematically shown
in Fig. 1is an all-metal seal chamber with pumps and
instrumentation shown in the figure, a base pressure of
110
10
Torr, a surface area of 4300 cm
2
, and a volume of
8100 cm
3
. The conductance of the system limited the effec-
tive pumping speed of the turbomolecular pump to 36 L/s at
the SRS IGC ion gauge (IG). Since water is the predominant
source of vapor pressure after volume gas is removed,
1,8
a
photon source of wavelength 185 nm (6.7 eV) ultraviolet
light was used to excite water vapor at an efficient wave-
length in its absorption curve.
23
A high-intensity 40 W RBD
Phototron lamp served as the UV source, as PSD has been
found to linearly depend on the incident photon flux.
24
The lamp was centered in the system to provide line of sight
a)
Electronic mail: srkoeb@email.wm.edu
b)
Electronic mail: raoutl@wm.edu
060601-1 J. Vac. Sci. Technol. A 30(6), Nov/Dec 2012 0734-2101/2012/30(6)/060601/3/$30.00 V
C2012 American Vacuum Society 060601-1
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radiation to roughly 80% of the system’s total surface area.
Before each pumpdown, the system was first leaked to
atmosphere, with laboratory temperature and humidity
noted. After 30 min of atmospheric exposure, the system was
sealed. The time origin of each run was set at the initiation
of the roughing pump. The UV lamp was also activated at
the time origin and run for 30, 60, and 120 min. In the
control test, the UV lamp was not activated. Ten minutes
into each run, the turbomolecular pump valve was opened,
followed by the activation of the SRS 100 residual gas ana-
lyzer (RGA) and IG. The IG was previously compared with
a spinning rotor gauge, but the RGA was not calibrated and
therefore only used to observe qualitative trends.
Following the pressure tests, another control test was
run using thermocouples to measure the temperature of the
apparatus during degassing. By replacing the UV lamp
with a resistance heater located in the same position, a cur-
rent of 13 A was found to produce a temperature that
matched the maximum system wall temperature observed
during the two hour UV simulated run (35 C). Pump-
downs with the resistance heater in place of the UV lamp
yielded pressure curves equivalent to those of the control
run, confirming the nonthermal nature of degassing with
this UV apparatus.
III. RESULTS AND DISCUSSION
Results of all tests are summarized in Fig. 2. The control
outgassing, for which no UV light was administered, exhib-
ited a linear decrease on a log–log scale in agreement with
the literature.
8,25
Each UV test yielded a similarly shaped
curve during irradiation, but higher outgassing rates were
observed due to PSD generated desorption of surface H
2
O.
After the UV lamp was extinguished in each test, outgassing in
the UV-assisted tests dropped to half the outgassing of the
control test, reflecting the lower pressures attained in the UV
runs for equivalent pumpdown times. The factor of 2 advantage
conferred by the UV persisted for at least 8 h after initial pump-
down, until the advantage lessened to a factor of 1.8 at 24 h.
Administering UV light for 120 min versus 30 min granted no
additional outgassing benefit, suggesting that most of the multi-
layer H
2
O was desorbed rapidly by the UV down to the near
surface interface, where the water is more strongly bound by
the dipole moment of the oxide and long range forces. The sav-
ings in pumpdown time accomplished with the use of UV light
is potentially substantial, especially for the attainment of out-
gassing rates below 1 10
9
Torr L s
1
cm
2
. For example, a
rate of 3 10
10
Torr L s
1
cm
2
was reached 3 h sooner in
pumpdowns with UV assistance compared to those without
UV, while a rate of 1.2 10
10
Torr L s
1
cm
2
was reached
16 h sooner in UV runs.
Figure 3shows the total pressure according to the IG
and the partial pressure of each major gas species as meas-
ured by the RGA. The sum of the partial pressures does not
accurately reflect the IG total pressure due to the noncali-
brated RGA. However, the RGA does show the trends of
the individual gas species relative to one another as a
FIG. 1. Schematic of the all-metal seal, stainless steel vacuum system.
FIG. 2. (Color online) Outgassing rate after 0 (control), 30, 60, and 120 min
of UV radiation.
060601-2 Koebley, Outlaw, and Dellwo: Degassing a vacuum system with in-situ UV radiation 060601-2
J. Vac. Sci. Technol. A, Vol. 30, No. 6, Nov/Dec 2012
Downloaded 21 Sep 2012 to 128.239.166.177. Redistribution subject to AVS license or copyright; see http://avspublications.org/jvsta/about/rights_and_permissions
function of time. As indicated by the similarity between the
H
2
O partial pressure and total pressure, outgassing of water
is the most significant contributor to the total pressure
decrease on this time scale. A UV source that targets water
vapor is therefore the ideal candidate to speed the pump-
down process.
To quantify the outgassing contribution of UV, we esti-
mated the desorption yield and the number of monolayers of
water desorbed during irradiation. The total number of H
2
O
molecules desorbed by the UV was found by integrating the
change in water vapor pressure (as indicated by the cali-
brated IG) between the control and UV-assisted pumpdowns
during the period of UV activation. A rough estimate of the
desorption yield after a 10
20
photon dose, g¼5.8 10
3
molecules/photon, is in good agreement with past stainless
steel PSD measurements.
9,12
Employing the approximation
of 10
15
molecules per monolayer, we determined that the
UV desorbed 22 H
2
O monolayers over the course of the
30 min run, 28 monolayers during the 60 min run, and
56 monolayers during the 120 min run. As the total number
of adsorbed water monolayers at atmosphere has been previ-
ously found to be as high as 600 monolayers,
26
it is plausible
that our calculations reflect an impressive desorption effect
of high-power UV radiation.
IV. SUMMARY AND CONCLUSIONS
We found that degassing a vacuum system with this par-
ticular UV source conferred a factor of 2 advantage in the
outgassing rate, which indicates a significantly expedited
pumpdown. If a vacuum system requires frequent exposure
to atmosphere, UV treatment may be attractive as a nonther-
mal, easily installed alternative or supplement to bakeout
that achieves a desired pressure with hours saved in pump-
down time. The UV benefit was observed following 30 min
of UV irradiation, and no additional advantage was observed
for treatment times in excess of 30 min. The primary gas
species desorbed in this short time period was found to be
water (as expected). UV sources with higher flux density
will likely increase desorption rates
12,13,21,27
and further
decrease pumpdown time.
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FIG. 3. (Color online) Total pressure and noncalibrated partial pressure of
each gas species for the pumpdown with 30 min of UV treatment.
060601-3 Koebley, Outlaw, and Dellwo: Degassing a vacuum system with in-situ UV radiation 060601-3
JVST A - Vacuum, Surfaces, and Films
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