.
.
IP
LUL
.
.
D
11. •
UNCLASSIFIED
ORNL
A
a
In
818
.
1
.
ILI
12
.
W
01
CW
.
!
"
W
ORNA-p-818
WWW
Paper to the presented at the Eleventh National Vacuum Symposium
in Chicago, Illinois, on October 2, 1964.
JAN 2, 1965
ATIES
CONF-640913.2
RELEASE OF GASES FROM SURFACES BY ENERGETIC ELECTRONS*
NASIL
Robert E. Clausing
Oak Ridge National Laboratory
Oak Ridge, Tennessee
-
--
-
-LEGAL NOTICE -
Tuo report w momand w
al Onvenuto narud work. Merther the Unid
mem, w we Owino, www porno iting
a ll w Corninatoa:
A. Makes my writy armowatation, entend a lowed. nu repect to the accu.
i dey, completeness, er woefulness of the information contained in this report, or that the ne
of way w eather, hent n o w pocoa deland to wompor may not latrice
miniaty wnd has a
1. Aronene meny Hamilto oth roupact to the won or lor damage rond thing from the
são el may information, apparatus, method, or proceso dlaclosed in this report.
Ao wand on the whomo, por este moment of the Chinet" mecludere may no.
magns or cinct of Counterton, a naplegue a much connector, to the open wat
much employee or tractor of the Commission, pleyes of work contractor womparsa,
onman. w mom mo m moration
Moto
counter
wa Contou, or MI plagal mo mantektur.

*Research sponsored by the U. S. Atomic Energy Commission under
contract with the Union Carbide Corporation.
En
el
. 2 -
INTRODUCTION
I am going to describe a very interesting and potentially useful, but
not generally well-known process. I hope I can point out some of its
possibilities and stimulate more interest in both application and research.
To introduce the subject I will state two experimental observations and
indicate their possible importance.
First: Bombardment of surfaces with electrons having
energies from 10 to 70,000 ev causes gases to
..
.
V
be released independent of heating effects.
-
2
st
Second:
The amount and kind of gases de sorbed depends
-
-
12
-
upon the amount and kind of material sorbed on
.
.
.
..-
the surface.
-
T.
These two observations provide the basis for several useful vacuum
ti
I.
techniques.
They may also provide useful techniques for research in
.
W
surface chemistry and physics. Let me elaborate briefly:
.
.
kul
. First the vacuum techniques: Electron bombardment may be
useå (either with thermal baking or by itself)
.
Als
in
V
2.
SU
1
T
to deplete the primary source of gas in ultra-
high vacuum systems, namely the gases sorbed on
surfaces. (In "dirty" systems as many as ten
gas molecules may be desorbed for each incident
I
!
W
AT.
.
electron.) Electron bombardment may also be used
-
-
;
to indicate the progress of cleaning operations
SIN
.
or surface contamination by periodically bombard-
.
:
:
.
ing the surfaces of interest and observing the
2.-
gas desorption.
3
.
.
WA
.
.
.
.
..!
yer
.
.
.
I
l
"W
"
.
.''
::
.
.
!
.
.
S
- 3 -
As for research tools: Electron bombardment provides
an easy technique for monitoring the progress
of some surface processes involving surface
.
diffusion, sticking coefficient measurements,
.
surface reaction rates, etc.
The removal of gases by electron bombardment
opens the possibility of cleaning adsorbed
-
gases from surfaces without heating or radi-
ation damage to the substrate.
The knowledge that electron bombardment can cause the release of
gases from surfaces is not new, but little data has appeared in the
literature. Most practical applications have involved heating, and the
research has been centered mostly around studies of plasma and ion gauge
phenomena. Some quantitative data have begun to appear in the litera-
ture recently for electrons with up to 500 v energy. Peterman,
Redhead, 24 Moore,' and Minzel and Gomer, have all published interest.
ing papers on specific gas-metal combinations recently. Others have
reported on the decomposition and polymerization of hydrocarbons by
electrons.
This information is provocat:lve but not sufficient to allow one to
evaluate the usefulness of electron bombardment as a vacuum or research
technique.
John Mark of Radio Corporation of America indicated to me

at last year's AVS meeting that he had successfully used electron
bombardmeni to reduce the base pressure in a vacuum system. Clearly
with this information in hand a further investigation was indicated.
. 4-
Town
OBJECTIVES OF THE INVESTIGATION
The objectives of the initial experiments were :
First: To determine in a semiquantitative way the number
of gas molecules de sorbed by each electron as a
function of electron energy.
Second: Whether or not thermal effects were directly
involved.
Third: Whether or not an electronic cleanup could be
achieved.
If these objectives were satisfactorily answered and the process still
appeared useful, it would then be appropriate to launch a more complete
investigation.
EXPERIMENTAL TECHNIQUES
The initial experiments were based on a very simple experimental
concept shown schematically in the first slide. A vacuum vessel is
bombarded on its inside surfaces with electrons from a simple unfocused
filament.
The gases liberated from the surfaces are monitored with an
ion gage or mass spectrometer. The quantity of gas liberated is calcu-
lated from the pressure rise and the volumetric pumping speed from the
chamber. These calculations assume that the de sorbed gases are not
55
appreciably readsorbed inside the vacuum vessel. A yield of gas per
.
electron is obtained by dividing the quantity of de sorbed gases by the
electron current. This "yield" or efficiency is then determined as a
*
*
function of current, voltage, and surface conditions.
.
PE
.
.
3
- 5 -
The first experiment used a 304 stainless steel vacuum chamber
with a volume of 2.5 liters and a surface area of approximately 1000 cm.
Its temperature was controlled with water and air cooling. The joints
:-
were sealed with copper gaskets and the whole system was bakeable. A
tungsten filament was used to provide electrons. The filament tempera.
--
ture was maintained constant, and the accelerating voltage was pulsed
.
ten
to turn the electron current on and off. Electron energies from 10 to
*
***...
30,000 ey were used. Electron currents from less than l ua to 30 ma
were used. A 0.1 cm
orifice was used to provide a known pumping speed
from the chamber. A 15 1/8 Vacion pump provided a "clean" pumping system
of adequate capacity. The system was not baked but it cleaned up during
electron bombardment until the base pressure was 5 x 10°, which was
the base pressure of the unbaked sputter ion pump.
RESULTS
The data in the second figure were taken when the base pressure
had dropped to ~ 5 x 1029.
The figure shows the pressure increase in
.
.
.
2.... sent...
.
this system as the result of electron bombardment as a function of
electron energy. The scale on the right shows an estimated yield of
gas molecules per electron based on the assumptions that: (1) the gas
1s "nitrogen-like", 1.e., its mass and ion gage sensitivity are similar
to those for nitrogen; and (2) the gas is not appreciably readsorbed in
the test chamber. The errors introduced as the result of these assump-
tions will lead to values of yield lower than the real values by as much
as a factor of ten if the gas evolved 18 hydrogen. Note that the experi-
mental results give yields of about 10 at low energies and about 10-2
at energies above a few thousand volts,
eori

S
"
..
.
.
.
1
L
-6-
2
The data shown were obtained after bombardment with 1026 or more
electrons per cm?. Initial yields, particularly those at voltages above
20,000 volts, were higher than these values by as much as two orders of
magnitude. The yield of gas per electron 1s nearly independent of
current from 1 to 30 ma at 3 kev.
**
*
:
-
1
=
Gas yields derived from data by Redhead, Todd, and Peterman are
.
included for comparison.
a
DISCUSSION OF RESULTS
These results seemed to indicate: (1) Yields of between 10
and
10º- gas molecules per electron could be obtained even in relatively
clean systems if electron energies of 2000 to 3000 ev were used.
(2) Thermal heating by the electrons was a separate effect since the
yield per electron seemed to be independent of the total electron
. .
.
current when the temperature of the surfaces was not allowed to rise.
(3) Electronic cleanup might be achieved since the yield of gas
decreased with total bombardment and so did the base pressure of the
•
•
•
•
system.
Additional Experiments in Large Systems
As the result of this experiment and two related ones it was
possible to persuade two co-workers at ORNL to try electron cleaning
experiments.
These experiments by E. R. Wells end R. A. Strehlow are
SES
reported with the experiment I just described in ORNL-3652. Figure 3
in
shows the results of some of these experiments.
It is shown to illus-
SE
trate the wide range of yields obtained and the effects of electronic
1
.
1.
and thermal cleaning. These data are based upon gage readings and
WO
.
.
.
.
.
.
AR
:
.
2
.
'
4
.
-
-
-
- 7 -
estimated pumping speeds, assuming that the gases are nitrogen-like.
The data from the UHV test facility show yield curves for a large copper
-
surface contaminated with diffusion pump oil and for the same surface
after a mild therinal bake (20 hr at temperatures from 100 to 145°c).
The yield dropped a factor of 20 after the bake. Additional baking
.
I
-
ii
decreased the yields still further until they were at least 2 orders of
magnitude lower than the initial value.
The data from the DCX-] end
.
-
-
region illustrate the yields obtained after extensive bombardment in a
*
*
*
large, complex system of stainless steel and copper. The yields at
3 kev were initially about 10 molecules per electror: but decayed to the
curve shown after electron bombardment of the exposed surfaces. The
data for the DCX-1 liner were obtained after a 12-hr pake at 400°c.
Although the surface had been previously coated with titanium, the
getter film was no longer active and no fresh titanium was evaporated
after baking.
The most important things to remember from these data and the
preceding data are:
(1) the yield values are relatively large,
(2) the yields decreased with bombardment time indicating a depletion
of gas- producing material as the result of bombardment, (3) cleaning
by thermal baking also reduced the yields, and (4) the yie 18 per
electron were independent of current at a given voltage so long as
heating was avoided.
I must add that in the last two systems the indicated base pressures

did not decrease appreciably as the result of bombardment. The explana-
tion seems to be that the gases desorbed were fragments from hydrocarbons.
=
E
These hydrocarbons contributed an undesirable but small part of the ion
gege pressure readings. At this point I should add that some things
'
.
'
.-
.-
!
.:
"
.
..
"
.
TY
.
!
V1
.8.
will be very effectively desorbed by electron bombardment while others
may have quite small de sorption rates.
Identification of Gases Evolved by Electron Bombardment
When it became apparent that electronic desorption processes might
have many practical consequences, a General Electric Company Model 22
P T 120 mass spectrometer was added to the vacuum system described
earlier. Other modifications were also made so that the system appeared
as in the fourth figure. A helium pump was added and the ion gage was
moved down stream from ihe bombardment chamber. Studies in both this
system and the UHV test facility have identified the gases present as
the result of electron bombardment of various surfaces.
The gases observed during the bombardment of surfaces contaminated
with hydrocarbons are predominantly hydrogen, methane, and light hydro-
carbons.
The composition varies with the type of contaminant and the
length of bombardment time. A relatively nonvolatile carbonaceous
residue is formed on the surfaces. A reduction of hydrocarbon vapors
reaching the mass spectrometer was observed as the result of electron
bombardment in at least two experiments. Table 1 shows the decrease in
the hydrocarbon peaks during electron bombardment with 1 kv electrons.
These data were taken when a new 304 stainless steel system was first
1
bombarded.
The gases desorbed from stainless steel surfaces were mostly hydrogen.
OF
*
.
*
*1
No major change in the composition of the gases desorbed occurred as a
function of electron energy between 100 and 40,000 v. Table 2 shows
some typical spectra obtained after prolonged bombardment with electrons
-
3
.
.
"
having energies up to 15,000 v.
3
.
1
.
C
M
.
PP
*
.
ki
7.
,
.
A
va
"
- 9-
The gases observed as the result of bombarding a tungsten target in
an unbaked giass chamber were mostly carbon monoxide. Again no major
change in the composition of the desorbed gases occurred for electrons
having energies between 100 and 40,000 v.
The major peaks and their
dependence on electron energy are shown in Table 3.
Only a small amount of hydrogen was observed when a very clean
platinum surface was bombarded.
The yield in this case was about 10%
hydrogen molecules per electron, but increase'l rapidly as the amount of
hydrogen allowed to accumulate on the surface increased.
In all of the above experiments it seems that the nature of the
gases adsorbed on the surface was more important than the substrate in
determining the amount and kind of gas evolved. In each instance the
composition of the gases evolved were similar to the residual gases in
the system, except when oil was present.
Bombardment of Titanium Getter Films
Several other exploratory experiments have been attempted with
varying degrees of success. I would like to briefly mention some
interesting results of one of them.
Measurements of the yield of gas from titanium getter films
partially loaded with different gases were attempted.
The interpreta-
tion of the results in terms of yield values depends upon the estimated
sticking factors for readsorption of gases on the getter and thus are
*
.
.
:
only approximate. The results are, however, especially interesting
because of the wide-spread use of titanium in sputter ion pumps and
th
plasma experiments.
w
*
radi
- 10 -
The procedure in each of these experiments was to coat the inside
· walls of a bombardment chamber, similar to the one used in the first
experiment I described, with an evaporated titanium film; admit a known
quantity of the desired gas and then bombard the film with 500 ev
electrons.
The film contaminated with approximately 10+ hydrogen molecules/cm
had an estimated yield of between one and two hydrogen molecules per
electron.
A film exposed to 10-4 torr of methane at room temperature adsorbed
very little if any CH. The same fllm exposed to Ch, at -195°c adscrbed
about 1075 CH, molecules/cm². Electron bombardment of this film while
it was still at -195°C caused the evolution of only a small amount of
hydrogen, about 5 x 10-2 molecules per electron.
The small methane
peak decreased rapidly during bombardment. The getter surface of about
600 cm was bombarded with 30 ma of 500 v electrons for a total of 10%
electrons. Only 10 to 20% of the methane was recovered when the film
warmed up. The hydrogen yield after the filin warmed up was about
2 x 10°C (about " at would have been expected from a new deposit), thus
the electrons seemed to "digest" the methane and allow the titanium to
adsorb the fragments.
In summary I want to stress four points:
(1) The electron bombardment desorption reported in this
paper is not a bulk-heating effect.
TV
(2) Electron bombardment desorption may be a valuable
aid in producing clean surfaces without heating or
9
.
causing radiation damage to the substrate.
.
.
-
N
I
!
.
.
""
.
'
'
YI
- ll -
(3) The partial pressure of hydrocarbon vapors in vacuum
volumes may be reduced by electron bombardment.
.
(4) The kind and quantity of gases desorbed by electron
bombardment are dependent upon the type and amount
of surface contamination and thus provide a means of
characterizing or monitoring the condition of the
surface.
This data is not sufficient to permit me to say how generally useful
the techniques may be, but I hope it illustrates some of the possi-
bilities and will stimulate you to consider unique capabilities or
electron bombardment whether you are doing research or are designing
vacuum systems.
7
.
:-
.
new-.
to a
-
12
-
d
a
REFERENCES
way h
1. L. A. Peterman, "Residual Gases in Electron Tubes," Proc. 2nd Intl.
Sympi, Milan, Italy, 1963.
in
ithered
2. P. A. Redhead, Vacuum 13, 253 (1963).
meni
3. P. A. Redhead, Can. J. Phys. (in press).
..
4. P. A. Redhead, Appl. Phys. Letters 4, 166 (1964).
5. G. E. Moore, J. Appl. Phys. 32, 1241 (1961).
6. D. Minzel and R. Gomer, J. Chem. Phys. 40, 1164 (1964).
-
-
-
.
.
.
-
-
7. B. J. Todd, J. L. Lineweaver, and J. T. Kerr, J. Appl. Phys. 31,
51 (1960).
*
**
*
WU!
'
I
WYM
UNDRUM
IN
4
.
1.
1
.
W
in L
.
D
YM ROU
WON
.
.
n.
""
.
V
Table 1. Decrcase in llydrocarbon Penka During Boinbar linent :ith One Kilovo.lt Electrons
us a function of the Tutal Bombardinert Current,

•
.
.
Muss
Charge
Total Current (1 Minutes)
105
425
2
15
1500
470000
37000
2.3000
3000
11100
340
2600
900
550
180
2000
1100
2:0
10000
5500
1200
600
120
1000
500
11100
750
2:00
220
30
1800
1100
780
A P(torr)
x 10-6
19.1 x 10-6
1.11 X 10
6.6 x 10-7
x 10-7
1.5 x 10-71
Me
Note:
(1) Peuk ile ights are in arbitrary units, but in all cases the total pressure can be
equate to the hycirocion pressure with only a small error.
(2)
The AP/mil is an uncorrected ion (ope renuiinc nornalized to in bombardinent current.
. 14 .
Table 2. Typical Spectra from 304 Stainless Steel after prolonged
Electron Bombardment but no Thermal Baking. (Spectra prior to
bombardment are provided for comparison.)

Ma88
Charge
0.1 ma
Initial
Base
Spectron
0.1 ma
109 v
104 v
Original Spectra
First Electron Bombardment
0.1 ma
Base
1.5 x 103 v
6100
20000
HO00000
17000
330
50
120
Sū ū 6 wra
500
1000
300000
120
810
5000
160000
V
V
V
V
.
r'.
205
220
3700
6000
1200000
'r''
V
V
V
10
43
44
si
8
190
190
2050
300000
(torr) 6.7 x 10-10 7.2 x 10-9 2.2 x 10-8
2 x 10-8
3 x 10-6
AC
"
'
.
".
"
..
1" :
.
.
..
"
IR
.
.
...
i
H
W
F
.
Pop
0
Art
BE
E-
.
.
Table 3.
Spectra Due to Blectron ''oinbarı!ent of a lungsten Tiurget in rj1235 Tube.

Mass
Charưe
I mal • 5 kV
Before Thuring,
lon. No kv
lerore During
I im
clore
. I kv
Durinc
lim
!'ctore
. Svo v
rrin,
In.
''efilc
V
During
Oil In • 100 V
Core During
1.00
3.0)
1.00
3.00)
1.000
1.60
:),15
0.13
1.')
0.381
0.15
1.50
0.12
1.10
0.13
0.00
0.06
0.16
0.ol
Golg
0.05
0.06
1.05
.
0,74
.
2.50 13.00
0.25 5.60
0.06 0.880
0.16 3.00
0.1:0 5.20
2.00 6.20
2.0) 10.00
0.ch 2,80
11.00 22.00
0.110 8.00
0.12 1.50
0.32 760
0.30 12.00
11.60 6.60
3.60 59.00
0.16 7.50
1.10
Joilo
oliv)
(1.1%)
0,1:
..
• 15 -
1.10
1.09
1.00
0.13
0.80
1.0
void
1.10
vegu
19.00
0.60
0.08.
0.16
1.00
2.30
0.16
:).13
0.36
1.00
11.10
0.733
Tel.0
9.00
1.40
09.16
15.00
0.00
0.11
0.18
Ion
Gage
~1x10-? max 10-6 | 7x10-7~1.5x10-
6:10-18 s. 5:10M? | 1x1v-** .,!x1327
5x10=1};.::dei
“x 1112007
Pressure
1..
LY!
vi
1
2
.01.
14
- 16 -
2
.
.
-
..
..
-
LIST OF FIGURES
-
Y
.
Fig. 1. Block Diagram of the First Electron Bombardment Experiment.
Fig. 2. Pressure Increase Per Milliampere of Electron Current as a
WED
Function of the Electron Energy.
22
-
-
Fig. 3. Comparison of Data from Various Electron Bombardment
E
Experiments.
.
Fig. 4. Block Diagram of the Mark IV Electron Bombardment Experiment.
.
.
.
.
-
*
*
*
*
17.1.
Saga
UNCLASSIFIED
ORNL-DWG 64-5223

FILAMENT
SUPPLY
METERED
HIGH VOLTAGE
SUPPLY
ROUGHING
VALVE
.
TON
GAUGE
o turi
000000000
VACION
PUMP
WATER
COOLING
v
+- 2
AIR
COOLING
CORIFICE WITH A
3 mm SQUARE
OPENING
2+
A
Fig. 1
l eve *
Roaxia
X
> Sen
UNCLASSIFIED
ORML -OWG 64.5224

FIRST EXPERIMENT
- SECONO EXPERWENT
THIRD EXPERIMENT
O SERIES OF 15-20 min STEPS 10 mo DC
0 5 sec PULSES 10 mo OC
• 6 sec PULSES AC (PEAK VOLTAGE GIVEN)
Imo AVERAGC OC CURRENT
A 5 sec PULSES AC (PEAK VOLTAGE GIVEN)
0.6 TO 1.0 mo AVERAGE OC CURRENT
A NO sec PULSES AC (PEAK VOLTAGE GIVENI
15 MO AVERAGE OC CURRENT EXCEPT AS
NOTED ONTO A 0.4 cm2 TUNGSTEN TARGET
TODOS or ol. FOR GLASS
I REDHEAD O2 ON MO AND W
V REDHEADY CO . PHASE ON MO
PETERNANO CO FROM NI
LITHIA - ALUMINA -
SILICATE
-
-
&
-
in........
SOD
--
i
i 10 ouo
..
---.
POTASH-SODA !
LEAD
logono
..--
APlma (tore)
-----
GAS VIELO PER ELECTRON (molecules/
....
.
..
.
-
..
.
MO
.
.
..
O
FUSED SILICA
21104
311006
3810'
9
102
5
105
103 2
ELECTRON ENERGY ( volts)
Fig. 2
e
į
UNCLASSIFIED
ORNL-DWG 64-5225

-
-
-
i nas Hii harnis
UHV TEST FACILITY (BEFORE CLEANING)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
UHV TEST FACILITY (AFTER A MILD BAKE)
-
-
-
-
10:1
+ TAI
GAS YIELD/electron
OCX-1 END REGION
AFTER ~1017 electrons/cm2
-- EB II AFTER
~1016 electrons/cm2
EB III AFTER
~1020 electrons/cm2
4
.
:,
U
>
RO
DCX-1 LINER
- AFTER 12 hr AT 400
TI
....
L
EB TEST I AFTER
~1017 electrons /cm2
-
10-3
EB TEST I AFTER
- 1018 electrons/cm2
NA
.
>
1004
0.04
0.1
0.2
0.5
10
20
50
100
1 2 5
ELECTRON VOLTAGE (KV)
ES
Fig. 3
|
Tr'
'
|
''
UNCLASSIFIED
ORNL-DWG 64-5226

FILAMENT
SUPPLY
METERED
HIGH VOLTAGE
SUPPLY
VACION
PUMP
1
LIQUID HELIUM
CRYOPUMP
MASS
SPECTROMETER
Vuorovo
ION
GAUGE
SORPTION
ROUGHING
PUMP
Fig. 4
Tek
?
.
.
.
>
i
ha
IR
..

mi
.
35
GALE
NA
W1
?
"
S
.
*
SET
"
E. SE
DATE FILMED
13 / 18 /65
.
V
:
.
..
.
-:
.
.
i
.
"
R
RE
.
.
-
.
Wit!
- LEGAL NOTICE -
This report was prepared as an account of Government sponsored work. Neither the United
States, nor the Commission, nor any person acting on behall of the Commission:
A. Makes any warranty or representation, expressod or implied, with respect to the accu-
racy, complotenors, or usefulness of the information contained in this roport, or that the use
of uny Information, apparatus, mothod, or proces disclosed in this report may not infringe
privately ownod rights; or
B. Assumos any liabilities with respect to the use of, or for damages resulting from the
use of any information, apparatus, mothod, or process disclosed in this report.
As usod in the above, “person acting on behalf of the Commission" Includes any om-
ployoo or contractor of the Commission, or omployce of such contractor, to the oxtent that
such employee or contractor of the Commission, or employee of such contractor prepares,
disseminates, or provides access to, any Information pursuant to his employment or contract
with the Commission, or his omploymont with such contractor.
ta
25
.
LR
.
.
www.
WIN
.
..
***
"
.
d.
*
HAW
be
+
TIM
MIL
.
tu
du
:
2141
PE
und
END
.
.
.
i'1
11
.
..
'
VI
12
!
12
W
m
.
.