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Degassing a vacuum system with in-situ UV radiation

November 2012
Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 30(6)

DOI:10.1116/1.4754292

Authors:

Sean Koebley

Thermo Fisher Scientific

Ronald Outlaw

William & Mary

Randy Dellwo

RBD Instruments

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 Multiplication-Sign 10{sup -10} Torr L s{sup -1} cm{sup -2} was reached 3 h sooner in pumpdowns with UV assistance compared to those without UV, while a rate of 1.2 Multiplication-Sign 10{sup -10} Torr L s{sup -1} cm{sup -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.

Schematic of the all-metal seal, stainless steel vacuum system.

…

(Color online) Outgassing rate after 0 (control), 30, 60, and 120 min of UV radiation.

…

Figures - uploaded by Sean Koebley

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Content uploaded by Sean Koebley

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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

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LETTERS

Degassing a vacuum system with in-situ UV radiation

Sean R. Koebley

and Ronald A. Outlaw

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

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

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

or deposition of an adlayer

coating

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

but may also produce diffusion and subse-

quent desorption of species from the bulk.

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,

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

110

10

Torr, a surface area of 4300 cm

, and a volume of

8100 cm

. 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

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.

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.

The lamp was centered in the system to provide line of sight

Electronic mail: srkoeb@email.wm.edu

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

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

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

2

. For example, a

rate of 3 10

10

Torr L s

1

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

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

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function of time. As indicated by the similarity between the

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

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

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

molecules per monolayer, we determined that the

UV desorbed 22 H

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,

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

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Article

Jul 1977

An upper bound to the outgassing rate of low-surface-area metal surfaces after an atmospheric exposure is deduced from a recently derived model. The result is Q/sub max/=1.72 x 10â»âµ/t (Torr l)/(s cmÂ²) where t is in seconds. Comparisons of outgassing data with this estimate are made for (1) various low-surface-area materials, (2) a given surface at elevated temperatures, and (3) a variation in the real pumping speed for a particular surface. Q/sub max/ has been found to be an upper bound to the outgassing data under a variation of the above parameters provided that the material is operated at approximately-less-than100degreeC for times between 15 min and 7 days.

Photon stimulated desorption of an unbaked stainless steel chamber by 3.75 keV critical energy photons

Article

Mar 1999

A stainless steel vacuum chamber, which had been going through a usual laboratory ultrahigh vacuum process, was exposed to a white synchrotron radiation photon beam, 3.75 keV critical energy, 70 W power at DCI. The vessel was however not baked “in situ” in order to test the necessity of this intermediate step. This is an issue with far reaching consequences for new synchrotron radiation sources. Beam scrubbing from a photon beam dose of 2.3×1023 photons m-1 was found to decrease the molecular yields from an initial value of 10-2 molecules per photon to below 10-5 for the main species CO and CO2. The cleaning effect of the secondaries was clearly demonstrated, thus justifying the definition of the photon beam dose. It also explains the strong reduction of the static pressure. Wall pumping developed by the irradiation of the 3.6 m long tube was found to develop a pumping speed of, respectively, 750 and 1900 l s-1 for CO and CO2 and nil for H2,CH4, and N2. The resulting pumping capacity amounts to an equivalent of 10-4 monolayer for both CO and CO2 to be compared to a total of 2 monolayers removed by beam scrubbing. These results are finally applied to predict the gas beam lifetime for a new synchrotron radiation source SOLEIL. It appears that a reasonable lifetime can be reached in a short time in the absence of in situ bakeout. © 1999 American Vacuum Society.

An overview of methods to suppress hydrogen outgassing rate from austenitic stainless steel with reference to UHV and EXV

Article

Jan 2003
VACUUM

The methods to suppress hydrogen outgassing rate from austenitic stainless steel with reference to ultrahigh vacuum (UHV) and extreme high vacuum (EXV) were discussed. The work included thermal treatment and modelling, permeation rate and role of surfaces, diffusion barriers and coatings. The results on measurements of the hydrogen concentration in stainless steel taken by the nuclear methods showed that the amount of hydrogen could be much higher than expected from Sieverts data taken by the conventional methods.

Comparison of photodesorption yields from aluminum, stainless and Cu-plated beam tubes

Article

Dec 1991
VACUUM

Photon Stimulated Desorption (PSD) from the walls of beam ducts constitutes the most serious gas load in storage rings and therefore exerts a great influence on both beam quality and lifetime. With the exception of the SSC which plans to use a Cu-plated beam tube, all existing and proposed machines use either aluminum or stainless steel beam tubes. In order to develop techniques for producing surfaces with the lowest PSD, we have studied the above materials using dedicated beam line at the National Synchroton Light Source (NSLS). Three meter long tubes were exposed to white light beams having critical energy of 500 eV and the desorption yields were measured as a function of beam dose. Several treatments commonly used for cleaning surfaces (prebaked, baked, fired, chemically cleaned and dc glow discharged) were investigated, and their relative merits are discussed. The results from sample tube measurements are compared with those from the X-ray ring Al beam tube during operation. The agreement is found to be quite good. Glow discharge cleaned stainless steel exhibited the lowest desorption yield.

Mechanisms of desorption due to electrons or photons

Article

Dec 1979
SURF SCI

David Lichtman

It is well established that both electrons and photons can, by quantum processes, cause the desorption of atoms or molecules from suitable solid surfaces. Many research papers and detailed reviews exist in the literature. The basic processes involved are described and some examples given. The review literature is cited.

Outgassing measurement of an LHC collimator and estimation for the NEG performances

Article

Jun 2011
VACUUM

The outgassing rate of the collimators in the Large Hadron Collider (LHC) at CERN has an important role for the life-time of the Non-Evaporable Getter (NEG), and an accurate analysis allows the definition of future activities, like NEG vacuum activation. For these reasons, both, total outgassing rate and gas composition of a secondary collimator have been measured in the laboratory. The outgassing rate decreases by about two orders of magnitude by after bake-out and moreover, repeated bake-out further reduced the outgassing rate. The gas transmission through the NEG coated beam pipes and the resulting pressure distributions near the collimator were also measured in a dedicated setup. It is found that the main gas component after just 2 m of NEG coated beam pipe is CH4 due to the extreme pumping speed of NEG for the other gases. Large amount of outgassing for H2 and carbon related molecules are released when moving the collimator jaws. It is found that the NEG is very effective even in such case with large gas load.