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Hialvac and Teflon outgassing under ultra-high vacuum conditions
Hialvac vacuum received
and Teflon conditions
30 June
K F Poole,
outgassing
under
ultra-high
1980
Department
of Electronic
Engineering,
University
of Natal,
South
Africa
and M M Michaelis,
Department
of Physics,
University
of Natal.
South
Africa
A quadrupole mass spectrometer was used to determine the outgassing characteristics of Hialvac glaze and Teflon by analysing the residual gases in an ultra-high vacuum (1 O-9 Pa range) system. The main gases evolved by a sample of extruded Teflon were H,O, CO and COZ. The outgassing from Hialvac glaze and machined Teflon was below the detectable limit.
The glazeis fired in air at 1000°Cuntil it runs and is allowedto cool slowly. The disc shapedsamplesof Hialvac glaze and the Polytetrafluoroethylene(Teflon) rod sampleswereapproximately 11cm’ in area and were degreasedin methanol before placing in the uhv system.
Introduction
The maincriterion which determinesthe suitability of a material for usein an ultra-high vacuumsystemis the degreeand type of contamination introduced by the material into the residual gasesof the system. A previous note’ reported some other properties of Hialvac and the purposeof these tests was to determinethe ultra-high vacuum performanceof a sampleof Hialvac glaze.
Material
Vacuum system
A fully bakeable(300°C)stainlesssteelvacuumsystemapproximately 60 1in volume and fitted with sorption roughingpumps, a 150I/s ion pumpand a titanium sublimationpump (approximately 3000cm2)wasusedin the experiments.A sketch of the systemis shownin Figure 1. The systemisfitted with a Bayard-
samples
As describedpreviously’ the powder is mixed with water and the liquid can be appliedto a surfaceby brushingwith a brush.
Chamber /
Specimen
Mass port
port
spectrometer
Metol
-
Ion gauge port
_
vake
1
I,
,-
i
Sorption w-w l+l
-w”mp
Sublimation
pump
’
well
Figure 1. Ultra-highvacuumsystem. Vacuum/volume
30/number
10. Pergamon
Press Ltd/Printed
in Great
Britain
415
K F Poole
and M M Michaelis:
Hialvac and Teflon outgassing
under ultra-high
Alpert nude ionization gauge with an X-ray limit of (2 x IO- ’ ’ tot-r) 2.7 x 10e9 Pa and a quadrupole mass spectrometer with a partial pressure limit of (IO-l3 torr) 1.3 x 10-r’ Pa and a mass range from 2-l 20. The pump down procedure uses two sorption pumps in series, the first pump is used to flush the system of inert gases and is valved off 5 min after opening to the system. The second sorption pump is used to reduce the pressure to 0.13 Pa (10m3 torr) and the ion pump is then switched on. As no isolation valve is fitted between the chamber and the ion pump, the ion pump must be restarted after letting the system down to air. In a leaktight system, a pressure of 2.7 x 10e6 Pa (2 x IO-’ torr) is reached after 2 h of pumping and after an 8 h bakeout at 200°C a pressure of 6.7 x 1Om8 Pa (5 x lo-” torr) is achieved. The sublimation pump reduces the pressure to 2.7 x 10y9 Pa (2 x lo-” tot-r).
vacuum conditions
spectrometer filament and a general trend can be seen for the clean system. The results for a clean system are compared with those for the system plus Teflon sample (Table 2) and the system plus Hialvac sample (Table 3).
Table 1. Results for a clean system: Total pump down time before switching on the mass spectrometer (including bake out) was 20 h Column 1 shows the spectrum taken 0.75 h after the mass spectrometer was switched on Column 2 shows the spectrum taken 3 h after the mass spectrometer was switched on Column 3 shows a spectrum taken 8 h after the mass spectrometer was switched on and represents the reference spectrum
Mass number
Procedure The system was always let down to dry nitrogen and the same pump-down procedure followed every time. Initially a number of experiments were performed with the clean system to determine the residual gases in the system and the effects of the mass spectrometer and the ion gauge on the residual gases. This was followed by tests with the Teflon sample in the system and the results compared with other measurements. A check was again made on the clean system and finally residual gas analysis measurements were made with a Hialvac glaze sample in the system. All residual gas analysis measurements were taken under similar vacuum conditions after thoroughly outgassing the ion gauge and mass spectrometer filaments. A Y-T plotter was used to record the mass spectra.
2 4 12 14 16 17 18 19 20 27 28 36 44 92 93 Total
Peak heights Column 1
Column 2
Recorder units
Recorder units
2
%
2 1 19
1.2 0.6 1.2 0.6 11.5
22 14 2 5 38 2 29 14 10
13.3 8.4 1.2 3.0 22.9 1.2 17.5 8.4 6.0
166
Column 3
%
3
2.5
1
0.8
18
Recorder units
%
1
1.9
15.1
12
22.2
14 7 2 4 24
11.8 5.9 1.7 3.4 20.2
5 2
9.3 3.7
5
9.3
18 16 12
15.1 13.5 10.1
2 15 12
3.7 27.8 22.2
119
54
Technique The approach used was to characterize the residual gases in the system without any sample materials present, that is, a reference residual gas spectrum was obtained for the clean system. Spectra of the residual gases obtained when small samples of the materials under investigation were placed inside the system were then compared with the spectrum for a clean system. The technique was similar to that described by Barton and Govie? and this facilitated the comparison with their results for Teflon. As the pumps could not be isolated from the chamber or their conductance controlled, other standard techniques for measuring the outgassing rates could not be followed. However, for the purpose of determining the residual gases evolved by the specimens, the technique of using small samples at low pressures is satisfactory. AS the sensitivity of the mass spectrometer is a function of the ionization probability of the gas, the determination of the true partial pressures of the gases present requires the calibration factors to be taken into account. However, the quadrupole mass spectrometer is not suited to quantitative analysis, but rather yields a qualitative result, so to simplify the results and to determine the gases evolved by the samples, tables showing the actual mass numbers recorded and the contribution ofeach peak to the total are given. Results The first results (Table I) show the effect of outgassing 416
the mass
The total pressures as measured on the ion gauge are not identical, varying from 1.4 x 10e9 Pa (2 x lo-” torr) to <1.3 x 10m9 Pa (IO-” torr). As the gauge cannot be relied upon at these pressures only qualitative results can be made. Discussion The results are reliable to within one recorder unit hence constituents of up to two recorder units serve only to indicate the presence of these in the system. Whilst the error on the small peaks can be large, the comparison of the ‘fingerprinV3 spectra yields useful data. The results for the clean system show clearly the gradually reducing 18 peak and the increase in the 16 peak. The first column indicates the 17 peak in the correct ratio with the 18 peak as expected for water; however in the subsequent columns the 17 peak was not easily resolved and hence it was ignored. The final spectrum is reproducible and the ‘fingerprint’ pattern of the residual gases in the clean system is compared with the patterns obtained with samples in the system. Teflon. The results for the Teflon sample in the as-received condition agree well with those obtained by Barton and Govie? who found the major outgassing peaks to be 18, 28 and 44 without bake-out. Table 2 shows that after bakeout the most significant peaks are 18, 28 and 44. The presence of water, 8 h after switching on the mass spectrometer indicates a large quantity of adsorbed water. This test was repeated with samples from
K F Poole
Hialvac and Teflon outgassing under ultra-high
and M M Michaelis:
Table 2. Results from Teflon samples. Total pump down time before switching on the mass spectrometer (including bakeout) was 20 h Column 1. System with Teflon sample specimen as received from supplier 8 h after the mass spectrometer was switched on Column 2. System with Teflon sample specimen with machined surface 8 h after the mass spectrometer was switched on Column 3. Clean system (as for Table 1)
Mass number
2 4 12 14 16 17 18 19 20 27 28 36 44 92 93
Peak heights Column 1
Column 2
Recorder units
Recorder units
%
%
5 2
2.3 0.9
2
2.1
1.9
50
23.4
23
24.5
22.2
20
7
7.5
9.3
5
9.4 4.7 2.3
1
1.1
3.7
32
15.0
9
9.6
9.3
21 38 31
9.8 17.8 14.5
1 28 23
1.1
3.7 27.8 22.2
10
Total
%
Column 3
214
different suppliers
29.8 24.5
94
and the results found to agreewith those in
Table 2. After 48 h of pumping, the residualgaseswereas for a clean system,that is the outgassing had dropped below the detectable limit. One samplewas removed from the uhv systemafter a 48-h pumping period and allowed to remain in the atmosphere for 72 h. It was then re-introduced into the systemand similar resultswere obtained. This indicated that moisturewas readsorbedon the surfaceof the Teflon. Another Teflon samplewaspreparedfrom the samestock by machining1.Omm off the diameter.This sampleyieldedsurprisingly different resultsascan beseenin column2. Eight hoursafter switching on the massspectrometer,the outgassingfrom this samplewas below the detectablelimit and comparisonof the two residualgasspectrumsof column2 and column 3 in Table 2 showsno difference. Hialvac. The fired sampleof Hialvac glazeshowedno detect-
able outgassingwhen comparedwith the cleansystemascan be seenby comparing column 1 and column 3 in Table 3. Comparisonwith K-ramic, a materialofsimilarcompositionreported by Moore4, indicated that Hialvac, which requires a higher firing temperature,performsbetter underuhv conditions. Moore reports that after bakeout, Hz, CO and CH4 are presentwith K-ramic in the system.However, from the resultsit is not possible to ascertain the contribution to these peaks from other sourcessuchasthe viton or to determinethe effect of comparing the RCA of a cleanedsystembaked at 200°C and the system with a sampleof K-ramic baked at 140°C. These and other factors make the comparisonunsatisfactory. A noticeabledifferencebetweenthe experimentsof the previous workers is that measurements were done at higher press-
vacuum conditions
uresand a residualgasanalysisof the clean systemswas not included in their results. Conclusion
The glaze known as Hialvac is suitablefor ultra-high vacuum work. The resultsof the testsperformedshowthat the outgassing is low and could not be detectedagainstthe backgroundof residualgasesin a cleansystem. Teflon in the as-receivedextruded form showedsignificant outgassingand required an extended pumping period of 48 h before the level of outgassingwasreducedto below the detectable limit. The Teflon samplewith a machinedsurfaceshowed no detectableoutgassing:this result may be explained by considering the nature of the surface of the two Teflon samples. Under microscopicexamination the extruded surface showed evidenceof cracksand tearswhilst the machiningremovedthese surfacedefects.The conclusionis that moistureis trapped in the cracksand it takestime to remove it in the uhv systemasthese act as virtual leaks.
Table 3. Results for Hialvac specimen. Total pump down time before switching on the mass spectrometer (including bakeout) was 20 h Column 1. System with Hialvac glaze sample after the mass spectro-
meterwasswitchedon for 8 h Column 3. Clean system (as for Table 1)
Mass number
Peakheights Column 1 Recorder units
2 4 12 14 16 17 18 19 20 27 28 36 44 92 93
Total
Column3
:/.
o/ IO
I
0.9
1.9
27
23.5
22.2
4
8.7 3.5
9.3 3.7
10
8.7
9.3
4 33 26
3.5 28.7 22.6
3.7 27.8 22.2
10
115
Acknowledgements
The authors gratefully acknowledge the financial assistance given by the Council for Scientific and Industrial Research, the University of Natal ResearchFund and the Atomic Energy Board. References ’ M M Michaelis, Vuc~unt, 28, 1978, 369. * R S Barton and R P Govier, J Vat Sci Technol, 2, 1965, 113. ’ R D Craig and E H Harden, Vuc~tum, 16, 1966, 67. ’ R Moore, J Vat Sci Techtrol, 16, 1979, 748. 417
and Teflon conditions
30 June
K F Poole,
outgassing
under
ultra-high
1980
Department
of Electronic
Engineering,
University
of Natal,
South
Africa
and M M Michaelis,
Department
of Physics,
University
of Natal.
South
Africa
A quadrupole mass spectrometer was used to determine the outgassing characteristics of Hialvac glaze and Teflon by analysing the residual gases in an ultra-high vacuum (1 O-9 Pa range) system. The main gases evolved by a sample of extruded Teflon were H,O, CO and COZ. The outgassing from Hialvac glaze and machined Teflon was below the detectable limit.
The glazeis fired in air at 1000°Cuntil it runs and is allowedto cool slowly. The disc shapedsamplesof Hialvac glaze and the Polytetrafluoroethylene(Teflon) rod sampleswereapproximately 11cm’ in area and were degreasedin methanol before placing in the uhv system.
Introduction
The maincriterion which determinesthe suitability of a material for usein an ultra-high vacuumsystemis the degreeand type of contamination introduced by the material into the residual gasesof the system. A previous note’ reported some other properties of Hialvac and the purposeof these tests was to determinethe ultra-high vacuum performanceof a sampleof Hialvac glaze.
Material
Vacuum system
A fully bakeable(300°C)stainlesssteelvacuumsystemapproximately 60 1in volume and fitted with sorption roughingpumps, a 150I/s ion pumpand a titanium sublimationpump (approximately 3000cm2)wasusedin the experiments.A sketch of the systemis shownin Figure 1. The systemisfitted with a Bayard-
samples
As describedpreviously’ the powder is mixed with water and the liquid can be appliedto a surfaceby brushingwith a brush.
Chamber /
Specimen
Mass port
port
spectrometer
Metol
-
Ion gauge port
_
vake
1
I,
,-
i
Sorption w-w l+l
-w”mp
Sublimation
pump
’
well
Figure 1. Ultra-highvacuumsystem. Vacuum/volume
30/number
10. Pergamon
Press Ltd/Printed
in Great
Britain
415
K F Poole
and M M Michaelis:
Hialvac and Teflon outgassing
under ultra-high
Alpert nude ionization gauge with an X-ray limit of (2 x IO- ’ ’ tot-r) 2.7 x 10e9 Pa and a quadrupole mass spectrometer with a partial pressure limit of (IO-l3 torr) 1.3 x 10-r’ Pa and a mass range from 2-l 20. The pump down procedure uses two sorption pumps in series, the first pump is used to flush the system of inert gases and is valved off 5 min after opening to the system. The second sorption pump is used to reduce the pressure to 0.13 Pa (10m3 torr) and the ion pump is then switched on. As no isolation valve is fitted between the chamber and the ion pump, the ion pump must be restarted after letting the system down to air. In a leaktight system, a pressure of 2.7 x 10e6 Pa (2 x IO-’ torr) is reached after 2 h of pumping and after an 8 h bakeout at 200°C a pressure of 6.7 x 1Om8 Pa (5 x lo-” torr) is achieved. The sublimation pump reduces the pressure to 2.7 x 10y9 Pa (2 x lo-” tot-r).
vacuum conditions
spectrometer filament and a general trend can be seen for the clean system. The results for a clean system are compared with those for the system plus Teflon sample (Table 2) and the system plus Hialvac sample (Table 3).
Table 1. Results for a clean system: Total pump down time before switching on the mass spectrometer (including bake out) was 20 h Column 1 shows the spectrum taken 0.75 h after the mass spectrometer was switched on Column 2 shows the spectrum taken 3 h after the mass spectrometer was switched on Column 3 shows a spectrum taken 8 h after the mass spectrometer was switched on and represents the reference spectrum
Mass number
Procedure The system was always let down to dry nitrogen and the same pump-down procedure followed every time. Initially a number of experiments were performed with the clean system to determine the residual gases in the system and the effects of the mass spectrometer and the ion gauge on the residual gases. This was followed by tests with the Teflon sample in the system and the results compared with other measurements. A check was again made on the clean system and finally residual gas analysis measurements were made with a Hialvac glaze sample in the system. All residual gas analysis measurements were taken under similar vacuum conditions after thoroughly outgassing the ion gauge and mass spectrometer filaments. A Y-T plotter was used to record the mass spectra.
2 4 12 14 16 17 18 19 20 27 28 36 44 92 93 Total
Peak heights Column 1
Column 2
Recorder units
Recorder units
2
%
2 1 19
1.2 0.6 1.2 0.6 11.5
22 14 2 5 38 2 29 14 10
13.3 8.4 1.2 3.0 22.9 1.2 17.5 8.4 6.0
166
Column 3
%
3
2.5
1
0.8
18
Recorder units
%
1
1.9
15.1
12
22.2
14 7 2 4 24
11.8 5.9 1.7 3.4 20.2
5 2
9.3 3.7
5
9.3
18 16 12
15.1 13.5 10.1
2 15 12
3.7 27.8 22.2
119
54
Technique The approach used was to characterize the residual gases in the system without any sample materials present, that is, a reference residual gas spectrum was obtained for the clean system. Spectra of the residual gases obtained when small samples of the materials under investigation were placed inside the system were then compared with the spectrum for a clean system. The technique was similar to that described by Barton and Govie? and this facilitated the comparison with their results for Teflon. As the pumps could not be isolated from the chamber or their conductance controlled, other standard techniques for measuring the outgassing rates could not be followed. However, for the purpose of determining the residual gases evolved by the specimens, the technique of using small samples at low pressures is satisfactory. AS the sensitivity of the mass spectrometer is a function of the ionization probability of the gas, the determination of the true partial pressures of the gases present requires the calibration factors to be taken into account. However, the quadrupole mass spectrometer is not suited to quantitative analysis, but rather yields a qualitative result, so to simplify the results and to determine the gases evolved by the samples, tables showing the actual mass numbers recorded and the contribution ofeach peak to the total are given. Results The first results (Table I) show the effect of outgassing 416
the mass
The total pressures as measured on the ion gauge are not identical, varying from 1.4 x 10e9 Pa (2 x lo-” torr) to <1.3 x 10m9 Pa (IO-” torr). As the gauge cannot be relied upon at these pressures only qualitative results can be made. Discussion The results are reliable to within one recorder unit hence constituents of up to two recorder units serve only to indicate the presence of these in the system. Whilst the error on the small peaks can be large, the comparison of the ‘fingerprinV3 spectra yields useful data. The results for the clean system show clearly the gradually reducing 18 peak and the increase in the 16 peak. The first column indicates the 17 peak in the correct ratio with the 18 peak as expected for water; however in the subsequent columns the 17 peak was not easily resolved and hence it was ignored. The final spectrum is reproducible and the ‘fingerprint’ pattern of the residual gases in the clean system is compared with the patterns obtained with samples in the system. Teflon. The results for the Teflon sample in the as-received condition agree well with those obtained by Barton and Govie? who found the major outgassing peaks to be 18, 28 and 44 without bake-out. Table 2 shows that after bakeout the most significant peaks are 18, 28 and 44. The presence of water, 8 h after switching on the mass spectrometer indicates a large quantity of adsorbed water. This test was repeated with samples from
K F Poole
Hialvac and Teflon outgassing under ultra-high
and M M Michaelis:
Table 2. Results from Teflon samples. Total pump down time before switching on the mass spectrometer (including bakeout) was 20 h Column 1. System with Teflon sample specimen as received from supplier 8 h after the mass spectrometer was switched on Column 2. System with Teflon sample specimen with machined surface 8 h after the mass spectrometer was switched on Column 3. Clean system (as for Table 1)
Mass number
2 4 12 14 16 17 18 19 20 27 28 36 44 92 93
Peak heights Column 1
Column 2
Recorder units
Recorder units
%
%
5 2
2.3 0.9
2
2.1
1.9
50
23.4
23
24.5
22.2
20
7
7.5
9.3
5
9.4 4.7 2.3
1
1.1
3.7
32
15.0
9
9.6
9.3
21 38 31
9.8 17.8 14.5
1 28 23
1.1
3.7 27.8 22.2
10
Total
%
Column 3
214
different suppliers
29.8 24.5
94
and the results found to agreewith those in
Table 2. After 48 h of pumping, the residualgaseswereas for a clean system,that is the outgassing had dropped below the detectable limit. One samplewas removed from the uhv systemafter a 48-h pumping period and allowed to remain in the atmosphere for 72 h. It was then re-introduced into the systemand similar resultswere obtained. This indicated that moisturewas readsorbedon the surfaceof the Teflon. Another Teflon samplewaspreparedfrom the samestock by machining1.Omm off the diameter.This sampleyieldedsurprisingly different resultsascan beseenin column2. Eight hoursafter switching on the massspectrometer,the outgassingfrom this samplewas below the detectablelimit and comparisonof the two residualgasspectrumsof column2 and column 3 in Table 2 showsno difference. Hialvac. The fired sampleof Hialvac glazeshowedno detect-
able outgassingwhen comparedwith the cleansystemascan be seenby comparing column 1 and column 3 in Table 3. Comparisonwith K-ramic, a materialofsimilarcompositionreported by Moore4, indicated that Hialvac, which requires a higher firing temperature,performsbetter underuhv conditions. Moore reports that after bakeout, Hz, CO and CH4 are presentwith K-ramic in the system.However, from the resultsit is not possible to ascertain the contribution to these peaks from other sourcessuchasthe viton or to determinethe effect of comparing the RCA of a cleanedsystembaked at 200°C and the system with a sampleof K-ramic baked at 140°C. These and other factors make the comparisonunsatisfactory. A noticeabledifferencebetweenthe experimentsof the previous workers is that measurements were done at higher press-
vacuum conditions
uresand a residualgasanalysisof the clean systemswas not included in their results. Conclusion
The glaze known as Hialvac is suitablefor ultra-high vacuum work. The resultsof the testsperformedshowthat the outgassing is low and could not be detectedagainstthe backgroundof residualgasesin a cleansystem. Teflon in the as-receivedextruded form showedsignificant outgassingand required an extended pumping period of 48 h before the level of outgassingwasreducedto below the detectable limit. The Teflon samplewith a machinedsurfaceshowed no detectableoutgassing:this result may be explained by considering the nature of the surface of the two Teflon samples. Under microscopicexamination the extruded surface showed evidenceof cracksand tearswhilst the machiningremovedthese surfacedefects.The conclusionis that moistureis trapped in the cracksand it takestime to remove it in the uhv systemasthese act as virtual leaks.
Table 3. Results for Hialvac specimen. Total pump down time before switching on the mass spectrometer (including bakeout) was 20 h Column 1. System with Hialvac glaze sample after the mass spectro-
meterwasswitchedon for 8 h Column 3. Clean system (as for Table 1)
Mass number
Peakheights Column 1 Recorder units
2 4 12 14 16 17 18 19 20 27 28 36 44 92 93
Total
Column3
:/.
o/ IO
I
0.9
1.9
27
23.5
22.2
4
8.7 3.5
9.3 3.7
10
8.7
9.3
4 33 26
3.5 28.7 22.6
3.7 27.8 22.2
10
115
Acknowledgements
The authors gratefully acknowledge the financial assistance given by the Council for Scientific and Industrial Research, the University of Natal ResearchFund and the Atomic Energy Board. References ’ M M Michaelis, Vuc~unt, 28, 1978, 369. * R S Barton and R P Govier, J Vat Sci Technol, 2, 1965, 113. ’ R D Craig and E H Harden, Vuc~tum, 16, 1966, 67. ’ R Moore, J Vat Sci Techtrol, 16, 1979, 748. 417