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The effects of a quadrupole mass analyser on measurement of UHV residual gas composition
Vacuum/volume
47/number 2/pages 173 to 17611996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/96 $15.00+.00
Pergamon 0042-207x(95)00178-5
The effects of a quadrupole mass analyser on measurement of UHV residual gas composition Tom&S Jirshk and Vladimir Nikolajenko, J Heyrovskv lnstiture of Physical of the Czech Republic, DolejSkova 3, 182 23 Prague 8, Czech Republic received
27 May
Chemistry,
Academy
of Sciences
7995
The effect of both the quadrupole mass analyser tOMA) and the ion-getter-pump (IGP) on the residual gas composition and on the composition during gas dosing were investigated. At the background pressure the measured height of mass 16 was more than lo-14 rimes the sum of contributions of methane and oxygen ions from other molecules and a significant amount of inert gases were found at this pressure level. Pure oxygen, carbon monoxide, hydrogen and a mixture of carbon monoxide and hydrogen were introduced into the vacuum system while the IGP was running. The final concentration of the dosed gas depended on the ratio of its pumping speed to that of the inert gases. The pumping speeds followed the sequence: oxygen > carbon monoxide > hydrogen. Exceptional behaviour of mass 16 was observed during introduction of carbon monoxide and hydrogen.
1. Introduction For many vacuum studies knowledge of the total gas pressure in the system is insufficient and the chemical composition of residual atmosphere is also required. This information is usually obtained with quadrupole mass analysers (QMAs). As with other instruments measurement of partial pressures by a QMA can suffer from deficiencies which may place limitations on the data obtained. Experimental and theoretical papers’-* have been published showing that both a QMA and an ion gauge can influence the results of partial and total pressure measurements, respectively when detecting the residual gases in a carefully cleaned ultra high vacuum (UHV) system. It is known’ that vacuum gauges can both pump and evolve gases and, when oxygen is introduced into a system, the QMA can release methane, water, carbon monoxide and carbon dioxide.’ Hickmott,” Ellefson et al." and Dylla and Blanchard” have also shown that these gases appear in the QMA and ion gauges when operated in hydrogen. Additional evidence and phenomena, similar to that discussed above is described in the present work, which is a study of the QMA effects on a measured UHV gas composition. 2. Experimental All the experiments were performed in a UHV system with a base pressure p 5 x lo-‘” mbar; Figure 1 shows the vacuum system schematically. The vacuum vessel (volume 65 1) was directly connected to a diode ion-getter pump (IGP) (NMD - 0.41, produced together with the whole UHV system by mechanical workshops of the Academy of Sciences, USSR) with a maximum
Mechanical pump
Precision manometer
ILeak valve
Vacuum
Q”zr’e
Spectrometer chamber 65 M,echan!cal pump y;s;quld mtrogen trap
I Bayard-Alpert
o-Y BayardAlpert gauge Figure 1. Schematic diagram of the vacuum system (BAG 2 is a nude type gauge: quadrupole mass spectrometer has its ion source exposed into main chamber). IGP port is directly connected to the main chamber.
pumping speed of about 400 1s~’ at 1 x loo-’ mbar for air. Total pressure was measured by two Bayard-Alpert gauges (BAGS). One BAG was nude type (Delong Instruments, Czech Republic) 173
TJirshk
and V Nikolajenko:
Measurement
of UHV residual gas composition
and the second one was all glass type (Veeco Instruments) connected to the main vessel by a glass tube of a diameter d=4 cm and the length 1=20 cm. A quadrupole mass analyser (QMA) (Finnigan 400, USA) has been used for partial pressure measurements. Both the QMA and BAGS had tungsten cathodes. A side port was connected via an all-metal valve to a forevacuum system, consisting of a mechanical pump (pumping speed 2 m’/h) equipped with a LN2 trap, a sorption pump containing zeolites and a Pirani gauge. The second port was connected by a leak valve to a gas inlet system for controlled introduction of small quantities of gas from gas lecture bottles containing oxygen (99.995%), hydrogen (99.9993%) and carbon monoxide (99.97%) (all gases Linde Aktiengesellschaft, Germany), a mechanical pump equipped with a LN2 trap and a precision manometer (LEYBOLD 0 -2000 mbar). The all-metal UHV system was baked out at z 600 K while running the IGP for 5 h before each experiment. lonisation energy of electrons in the QMA was 70 eV. Both the fragmentation factor” and ionisation cross sectionI corresponding to the electron energy of 70 eV were taken into account during evaluation of measured data. 3. Results and discussion 3.1. Residual gases. Each experiment started with the registration of spectra of the residual gases in the vacuum chamber with both IGP and ion gauges in operation. The mean value of the measured total base pressures was 5 x 10~‘” mbar. It was found, that the major component of the spectra of the residual gases was hydrogen (mass 2), followed by helium (mass 4), neon (mass 20), carbon monoxide (mass 28) and methane (mass 15). Mass 15 was used to indicate methane since the measured peak of mass 16 besides CH,+ consisted of contributions of oxygen ions resulting from fragmentation of molecules of carbon dioxide (mass 44), oxygen (mass 32), carbon monoxide (mass 28) and water (mass 18) in the ion source of the mass spectrometer. In a simple residualgas spectrum, as in ours, mass 15 results solely from methane. Unusually high concentrations of the inert gases were found at the background pressure. Helium and neon formed 25-30% of the residual atmosphere. They could accumulate in the vacuum system because no molecular drag pump was applied. An extremely high peak of mass 16 was observed in the residual spectra. As was already mentioned, mass 16 consisted of methane-ion contributions and oxygen-ion contributions from carbon dioxide, oxygen, carbon monoxide and water. However, the concentrations of carbon monoxide and methane were low and furthermore we did not find any significant peak of masses 44,32 and 18 above the noise level. A sum of all mentioned contributions to mass 16 (so called calculated value Plhc) was IO-14 times lower (P,6m/P,c = 10-14) than the measured value of the mass 16 (Plhm). We have noticed that at the lower background pressure a higher discrepancy between calculated and measured values has been observed. It is known that atomic oxygen is the most abundant species which is produced on tungsten at temperatures above 2200 K.” This effect can influence the measured mass spectrum especially at the lowest pressures when the number of molecules entering the ionisation chamber of QMA is relatively small. For a more detail discussion of mass-16 behaviour see Section 3.3. 3.2. Effects of switching off the IGP. Measurements were performed for establishing the influence of switching off the IGP on 174
the spectra of residual gases. Both QMA and BAGS were running during the entire experiment. Figure 2 shows partial pressure changes with time for ten different gas masses. Time zero corresponds to the moment when the IGP was switched off and it can be seen that the main change in partial pressure occurs during the first 10 min. Thus the total pressure rose 522 times during the first 10 min whereas during further 31 h only 24 times. Final pressure after this time period was 5 x 10M6mbar and the dominant part of residual atmosphere formed was helium and neon. Thus, the increase of the partial pressures of these inert gases was mainly responsible for the increase of the total pressure in the vacuum system. This is not surprising because during a pumping process in a diode-type IGP the inert gases are buried under the sputtered titanium layer.16 If the pumping process is interrupted the inert-gas molecules are readily released. However, the highest rise during first 10 min after switching off IGP was observed for methane (mass 15). It can be explained by hydrogenation of carbon on the hot filament surface. During the first few minutes after switching off the IGP this process can prevail, because the release of inert gases from IGP is governed by a slow process-diffusion of molecules incorporated in the titanium layer to the surface and subsequent desorption. Since no molecular drag pump has been used, inert gases were accumulated in the vacuum system and consequently they could become a dominant part of the residual atmosphere after IGP switch off. Furthermore, it can be seen in Figure 2 that between 8 and 9 h after slow fall of pressures of masses 14, 15 and 16 (corresponding mainly to methane) one can observe a fast rise in these three partial pressures, on average three times the original value, and afterwards these pressures decrease slowly again. This effect was able to be reproduced and depended on the history of the chamber since the last baking. When this experiment was performed without introduction of any gas after baking out the apparatus, the pressure rise of methane has been observed after longer period than in the case when CO, 02, H, or their mixture were introduced repeatedly into the chamber. A detailed explanation of this effect is beyond the scope of this paper. Behaviour of the mass 16 was also unexpected. Within the time period from 10 min to 6 h the partial pressure of mass 16 corresponded nearly fully to methane as judged from the ratio of the peak heights of masses 15 to 16, and in a further period the
1
Figure 2. Total pressure rise and partial pressure changes vs time after IGP was switched off (numbers at the symbols of the curves represent mass-to-charge ratio M/Q, index Tot denotes the total pressure).
T Jirstik and V Nikolajenko: Measurement
of UHV residual gas composition
contribution of oxygen (O+) increased. It is interesting that the calculated peak height of mass 16 (when the contribution of Of fragments from other molecules and CH4+ fragments are considered-see Section 3.1.) was larger than the experimentally observed peak height of mass 16 and this difference grew to the value of P,,m/P ,; =0.5 at the end of the experiment. This difference of calculated and measured peak heights of the mass 16 appeared to arise from oxygen ions released by fragmentation of carbon monoxide. The sum of partial pressures of the other contributors to mass 16 (e.g. carbon dioxide, oxygen, water and methane) was lower than 2% of the total pressure. The only significant contributor to mass 16 was carbon monoxide. Moreover, its partial pressure grew to the end of experiment simultaneously with the difference of P,6m/P,6C. Thus, we suppose carbon monoxide was responsible for this discrepancy between measured and calculated value. Quite the opposite effect was observed at the background pressures of residual gases when the measured value exceeded the calculated one lo-14 times (see Section 3.1.).
Table 1. Concentration of gases at the highest total pressure during gas introduction (IGP on). Concentration (%)
-He
--cCH4
t-H20
-Ne
Inert gases
0,
15.7
0.1
co HZ CO + H,
14.4 2.9 5.4
37.4 0.0 0.0
O2
-N2
-02
-CO
lo-'0 10'9 nl
10-7
IO"
,#,d
-Ar
HZ
26.1 65.2 2.2 25.8
20.2 12.1 94.7 65.6
104
-002 lo-5
u
IO-'
,,,,,,A
b) C
co
x 40%. In the case of carbon monoxide (Figure 3(b)) and hydrogen (Figure 3(c)) maximum concentrations were x 65% and more than 95%, respectively. This can be explained in terms of different pumping speeds (sum of pumping speeds of IGP, of the ion gauges and of the chamber walls) for individual gases and of surface reactions. As shown in Section 3.2. the inert gases were mainly responsible for pressure rise after switching off the IGP. When the pressure level inside the vacuum chamber is increased due to gas introduction, a memory effect of the IGP is general affected. In our case the inert gases are released from the wall of the IGP. If the gas exhibiting high pumping speed is introduced, its molecules are pumped down very quickly and the released inert gases contribute significantly to the residual atmosphere. It can be seen in Figure 3 and Table 1 that the concentrations of inert gases really correspond to the sequence of pumping speeds for individual active gases (0, > CO > H2). After introduction of a mixture of carbon monoxide and hydrogen in the ratio 1: 1 (Figure 3(d)) this ratio changed to 1: 2.25 for the highest total pressure. Figure 3(a) shows a large increase of carbon monoxide when oxygen was added. It is known9 that oxygen reacts with carbon on the surface of hot filaments producing carbon monoxide and
3.3. Gas dosing. Oxygen, carbon monoxide, hydrogen and mixture of carbon monoxide and hydrogen in the ratio 1:l were introduced into the system in 8 steps (2.8 x 10d9, 1.0 x lo-*, 2.4x lo-*, 9.0x lo-“, 2.2x lo-‘, 7.8 x lo-‘, 2.0x 10-6, 7.6x 10mh mbar) to observe the production of other gases by the system. IGP, QMS and BAGS were running throughout the measurement. Figure 3 shows stationary concentrations of main components as a function of total pressure during the continuous introduction of pure gases: (a) oxygen, (b) carbon monoxide, (c) hydrogen and (d) mixture of hydrogen and carbon monoxide in the ratio l:l, respectively. It can be seen that, in this dynamic system, completely different concentrations of gases under investigation resulted. If pure oxygen was introduced into the vacuum system (Figure 3(a)) QMA indicated maximum of oxygen concentration
-r-H2
Gas introduced
100
-
80
0
60
‘5 8.
40
-
40
20
-
20
- 60
‘3
E
s
0
-0
100
-
26
EO-
c .o
60
-
.% B
40
-
a
E S
2::
i&wM++u 1
lo"O
IO”
w
“““1
IO”
10-7
8 q-
““‘7
104
104
104
Total pressure (mbar) Total pressure (mbar) Figure 3. Stationary concentration of gases in the vacuum system as a function of the total pressure for continuous introduction of: (a) oxygen, (b) carbon monoxide, (c) hydrogen and (d) mixture of carbon monoxide + hydrogen CO/H, = 1 (IGP on - dynamic equilibrium). Data for the lowest pressure correspond to the background level. 175
TJirsBk and V Nikolajenko:
Measurement
of UHV residual gas composition
Table 2. Values of the ratio PK/PF6 for different gases and total pressures. (Pys-partial pressure of mass 16 measured; Pf,-sum of calculated contributions to mass 16 due to oxygen ions O+ from carbon dioxide (mass 44), oxygen (mass 32), carbon monoxide (mass 28) and water (mass 18); methane molecules (mass 16 judged from mass 15 according to the fragmentation factor [14]). Values of the ratio Pt/PCh Total pressure Gas introduced
O2
co
H*
CO + Hz
14.3 5.6 1.8 1.1 0.48 0.34 0.26 0.22 0.22
11.7 12.7 9.6 5.1 1.8 1.1 0.71 0.63 0.61
12.2 4.1 1.5 0.74 0.41 0.29 0.25 0.23 0.24
_.. background 2.8 x 1O-9 1.0 x lomx 2.4 x lo-’ 9.0 x lomx 2.2 x lo-’ 7.8 x 10m7 2.0 x 10m6 7.6x 10.’
10.1 3.6 2.5 1.6 1.3 1.2 1.2 1.1 1
carbon dioxide. Atomic oxygen released from a hot cathode can also produce the mentioned gases by interaction with carbon on the walls.” Table 2 shows the behaviour of mass 16 during introduction of gases (oxygen, hydrogen, carbon monoxide and mixture of carbon monoxide (1:l)) into the vacuum system. As mentioned above, the background pressure of measured mass 16 exceeded by l&14 times the sum of calculated possible contributions to the peak of mass 16, i.e. methane ions CH,+ and oxygen ions O+ resulting from dissociation of molecules of carbon dioxide, oxygen, carbon monoxide and water. The methane contribution to mass 16 was calculated from the mass 15 which, nearly of the same height, is characteristic for methane and the ratio of other methane fragments (i.e. 12, 13 and 14) corresponded to the published fragmentation pattern of methane. We have mentioned above that the production of atomic oxygen can prevail at the lowest pressures when the number of molecules entering the ionisation chamber of the QMA is small. Introduction of a gas increases the number of molecules which reach the QMA ion source and the production of oxygen on the hot tungsten cathode becomes negligible. This is true for all investigated gases in our experiments (Table 2). It can be seen in Table 2 that the addition of carbon monoxide leads to rapid decreasing of mass-16 excess with increasing amount of CO dosed. At the highest pressures a deficit of mass 16 was observed. Carbon monoxide formed the main part of mass 16-peak height. The other contributions to mass 16 were negligible. It seemed likely that carbon monoxide caused a deficit of mass 16. Similarly, when hydrogen was dosed the ratio Plhm/P,; decreased and the deficit at highest pressures was observed again. Carbon monoxide exhibited the second highest rise of the partial pressure during hydrogen dosing and it represented the main part of the noise level and main part of mass 16. We interpret this as a reason for the deficit of mass 16. Similar effects have been
176
observed during switching off the IGP or during carbon monoxide dosing. In all these cases carbon monoxide formed the main part of peak height of mass 16 and the measured value was lower than the calculated one. However, a detailed explanation of this behaviour is not possible using our simple method of measurement. When large amounts of oxygen were introduced the ratio P,,“/P,,C approached the value p,,m/p,,c = 1. Oxygen was a dominant part of the residual atmosphere and the deficit of mass 16, which seemed to be caused by carbon monoxide, did not influence the mass spectrum. Introduction of a mixture of carbon monoxide and hydrogen resulted in an intermediate composition between the cases when pure carbon monoxide or hydrogen were introduced separately. 4. Conclusions Chemical processes in the ionisation chamber of a QMA influence substantially the measured residual gas composition. Oxygen ions produced from various gas molecules (0,, H,O, CO, CO,) incident on a hot tungsten cathode of a QMA increase the mass 16 signal at ultimate pressures. During introduction of carbon monoxide and hydrogen the experimentally observed height peak of mass 16 was lower than that which could expected from the calculated sum of its possible constituents. Acknowledgements The support of this work by the Grant Agency of Czech Republic (No. 203/93/0245) is gratefully acknowledged. The authors also express their thanks to Professor L Holland and Dr Z Knor for useful comments to the manuscript and Dr Z Dolejsek for helpful discussion. References ’ W E Austin, F M Mao, J M Yang and J H Leek, J Vat Sci Technol, A5, 2631 (1987). ‘F M Mao, J M Yang, W E Austin and J H Leek, Vacuum, 37, 335 (1987). ’ W E Austin and J H Leek, Vacuum, 41,200l (1990). ‘W E Austin, J H Leek and J H Batey, J Vat Sci Technol, AlO, 3563 (1992). ‘J H Batey, Vacuum, 43, 15 (1992). ’ M C Cowen, W Allison and J H Batey, Meas Sci Technol, 4,72 (1993). ‘J R J Bennet and R J Elsey, Vacuum, 44,647 (1993). ’ L Lieszkovszky, A R Filippelli and C R Tilford, J Vat Sci Technol, AS, 3838 (1990). “P A Redhead, J P Hobson and E V Kornelson, The Physical Basis oJ Ultrahigh Vacuum. Chapman and Hall, London (1968). ‘“T W Hickmott, J Chem Phys, 32,810 (1960). ” R E Ellefson, W E Moddeman and H F Dylla, J Vat Sci Technol, 18, 1062 (1981). ” H F Dylla and W R Blanchard, J Vat Sci Technol, Al, 1297 (1983). ” INFICON Leybold-Heraeus, Spectra Library Card. id R W Kiser, Introduction to Mass Spectrometry and Its Applications, p. 300. Prentice-Hall Inc, New Jersey (1965). “P 0 Schissel and 0 C Trulson, J Chem Phys, 43,737 (1965). “A Roth, Vacuum Technology, p 230. North-Holland Publishing Company, Amsterdam (1976). “J H Singleton, J Chem Phys, 45,2819 (1966).
47/number 2/pages 173 to 17611996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/96 $15.00+.00
Pergamon 0042-207x(95)00178-5
The effects of a quadrupole mass analyser on measurement of UHV residual gas composition Tom&S Jirshk and Vladimir Nikolajenko, J Heyrovskv lnstiture of Physical of the Czech Republic, DolejSkova 3, 182 23 Prague 8, Czech Republic received
27 May
Chemistry,
Academy
of Sciences
7995
The effect of both the quadrupole mass analyser tOMA) and the ion-getter-pump (IGP) on the residual gas composition and on the composition during gas dosing were investigated. At the background pressure the measured height of mass 16 was more than lo-14 rimes the sum of contributions of methane and oxygen ions from other molecules and a significant amount of inert gases were found at this pressure level. Pure oxygen, carbon monoxide, hydrogen and a mixture of carbon monoxide and hydrogen were introduced into the vacuum system while the IGP was running. The final concentration of the dosed gas depended on the ratio of its pumping speed to that of the inert gases. The pumping speeds followed the sequence: oxygen > carbon monoxide > hydrogen. Exceptional behaviour of mass 16 was observed during introduction of carbon monoxide and hydrogen.
1. Introduction For many vacuum studies knowledge of the total gas pressure in the system is insufficient and the chemical composition of residual atmosphere is also required. This information is usually obtained with quadrupole mass analysers (QMAs). As with other instruments measurement of partial pressures by a QMA can suffer from deficiencies which may place limitations on the data obtained. Experimental and theoretical papers’-* have been published showing that both a QMA and an ion gauge can influence the results of partial and total pressure measurements, respectively when detecting the residual gases in a carefully cleaned ultra high vacuum (UHV) system. It is known’ that vacuum gauges can both pump and evolve gases and, when oxygen is introduced into a system, the QMA can release methane, water, carbon monoxide and carbon dioxide.’ Hickmott,” Ellefson et al." and Dylla and Blanchard” have also shown that these gases appear in the QMA and ion gauges when operated in hydrogen. Additional evidence and phenomena, similar to that discussed above is described in the present work, which is a study of the QMA effects on a measured UHV gas composition. 2. Experimental All the experiments were performed in a UHV system with a base pressure p 5 x lo-‘” mbar; Figure 1 shows the vacuum system schematically. The vacuum vessel (volume 65 1) was directly connected to a diode ion-getter pump (IGP) (NMD - 0.41, produced together with the whole UHV system by mechanical workshops of the Academy of Sciences, USSR) with a maximum
Mechanical pump
Precision manometer
ILeak valve
Vacuum
Q”zr’e
Spectrometer chamber 65 M,echan!cal pump y;s;quld mtrogen trap
I Bayard-Alpert
o-Y BayardAlpert gauge Figure 1. Schematic diagram of the vacuum system (BAG 2 is a nude type gauge: quadrupole mass spectrometer has its ion source exposed into main chamber). IGP port is directly connected to the main chamber.
pumping speed of about 400 1s~’ at 1 x loo-’ mbar for air. Total pressure was measured by two Bayard-Alpert gauges (BAGS). One BAG was nude type (Delong Instruments, Czech Republic) 173
TJirshk
and V Nikolajenko:
Measurement
of UHV residual gas composition
and the second one was all glass type (Veeco Instruments) connected to the main vessel by a glass tube of a diameter d=4 cm and the length 1=20 cm. A quadrupole mass analyser (QMA) (Finnigan 400, USA) has been used for partial pressure measurements. Both the QMA and BAGS had tungsten cathodes. A side port was connected via an all-metal valve to a forevacuum system, consisting of a mechanical pump (pumping speed 2 m’/h) equipped with a LN2 trap, a sorption pump containing zeolites and a Pirani gauge. The second port was connected by a leak valve to a gas inlet system for controlled introduction of small quantities of gas from gas lecture bottles containing oxygen (99.995%), hydrogen (99.9993%) and carbon monoxide (99.97%) (all gases Linde Aktiengesellschaft, Germany), a mechanical pump equipped with a LN2 trap and a precision manometer (LEYBOLD 0 -2000 mbar). The all-metal UHV system was baked out at z 600 K while running the IGP for 5 h before each experiment. lonisation energy of electrons in the QMA was 70 eV. Both the fragmentation factor” and ionisation cross sectionI corresponding to the electron energy of 70 eV were taken into account during evaluation of measured data. 3. Results and discussion 3.1. Residual gases. Each experiment started with the registration of spectra of the residual gases in the vacuum chamber with both IGP and ion gauges in operation. The mean value of the measured total base pressures was 5 x 10~‘” mbar. It was found, that the major component of the spectra of the residual gases was hydrogen (mass 2), followed by helium (mass 4), neon (mass 20), carbon monoxide (mass 28) and methane (mass 15). Mass 15 was used to indicate methane since the measured peak of mass 16 besides CH,+ consisted of contributions of oxygen ions resulting from fragmentation of molecules of carbon dioxide (mass 44), oxygen (mass 32), carbon monoxide (mass 28) and water (mass 18) in the ion source of the mass spectrometer. In a simple residualgas spectrum, as in ours, mass 15 results solely from methane. Unusually high concentrations of the inert gases were found at the background pressure. Helium and neon formed 25-30% of the residual atmosphere. They could accumulate in the vacuum system because no molecular drag pump was applied. An extremely high peak of mass 16 was observed in the residual spectra. As was already mentioned, mass 16 consisted of methane-ion contributions and oxygen-ion contributions from carbon dioxide, oxygen, carbon monoxide and water. However, the concentrations of carbon monoxide and methane were low and furthermore we did not find any significant peak of masses 44,32 and 18 above the noise level. A sum of all mentioned contributions to mass 16 (so called calculated value Plhc) was IO-14 times lower (P,6m/P,c = 10-14) than the measured value of the mass 16 (Plhm). We have noticed that at the lower background pressure a higher discrepancy between calculated and measured values has been observed. It is known that atomic oxygen is the most abundant species which is produced on tungsten at temperatures above 2200 K.” This effect can influence the measured mass spectrum especially at the lowest pressures when the number of molecules entering the ionisation chamber of QMA is relatively small. For a more detail discussion of mass-16 behaviour see Section 3.3. 3.2. Effects of switching off the IGP. Measurements were performed for establishing the influence of switching off the IGP on 174
the spectra of residual gases. Both QMA and BAGS were running during the entire experiment. Figure 2 shows partial pressure changes with time for ten different gas masses. Time zero corresponds to the moment when the IGP was switched off and it can be seen that the main change in partial pressure occurs during the first 10 min. Thus the total pressure rose 522 times during the first 10 min whereas during further 31 h only 24 times. Final pressure after this time period was 5 x 10M6mbar and the dominant part of residual atmosphere formed was helium and neon. Thus, the increase of the partial pressures of these inert gases was mainly responsible for the increase of the total pressure in the vacuum system. This is not surprising because during a pumping process in a diode-type IGP the inert gases are buried under the sputtered titanium layer.16 If the pumping process is interrupted the inert-gas molecules are readily released. However, the highest rise during first 10 min after switching off IGP was observed for methane (mass 15). It can be explained by hydrogenation of carbon on the hot filament surface. During the first few minutes after switching off the IGP this process can prevail, because the release of inert gases from IGP is governed by a slow process-diffusion of molecules incorporated in the titanium layer to the surface and subsequent desorption. Since no molecular drag pump has been used, inert gases were accumulated in the vacuum system and consequently they could become a dominant part of the residual atmosphere after IGP switch off. Furthermore, it can be seen in Figure 2 that between 8 and 9 h after slow fall of pressures of masses 14, 15 and 16 (corresponding mainly to methane) one can observe a fast rise in these three partial pressures, on average three times the original value, and afterwards these pressures decrease slowly again. This effect was able to be reproduced and depended on the history of the chamber since the last baking. When this experiment was performed without introduction of any gas after baking out the apparatus, the pressure rise of methane has been observed after longer period than in the case when CO, 02, H, or their mixture were introduced repeatedly into the chamber. A detailed explanation of this effect is beyond the scope of this paper. Behaviour of the mass 16 was also unexpected. Within the time period from 10 min to 6 h the partial pressure of mass 16 corresponded nearly fully to methane as judged from the ratio of the peak heights of masses 15 to 16, and in a further period the
1
Figure 2. Total pressure rise and partial pressure changes vs time after IGP was switched off (numbers at the symbols of the curves represent mass-to-charge ratio M/Q, index Tot denotes the total pressure).
T Jirstik and V Nikolajenko: Measurement
of UHV residual gas composition
contribution of oxygen (O+) increased. It is interesting that the calculated peak height of mass 16 (when the contribution of Of fragments from other molecules and CH4+ fragments are considered-see Section 3.1.) was larger than the experimentally observed peak height of mass 16 and this difference grew to the value of P,,m/P ,; =0.5 at the end of the experiment. This difference of calculated and measured peak heights of the mass 16 appeared to arise from oxygen ions released by fragmentation of carbon monoxide. The sum of partial pressures of the other contributors to mass 16 (e.g. carbon dioxide, oxygen, water and methane) was lower than 2% of the total pressure. The only significant contributor to mass 16 was carbon monoxide. Moreover, its partial pressure grew to the end of experiment simultaneously with the difference of P,6m/P,6C. Thus, we suppose carbon monoxide was responsible for this discrepancy between measured and calculated value. Quite the opposite effect was observed at the background pressures of residual gases when the measured value exceeded the calculated one lo-14 times (see Section 3.1.).
Table 1. Concentration of gases at the highest total pressure during gas introduction (IGP on). Concentration (%)
-He
--cCH4
t-H20
-Ne
Inert gases
0,
15.7
0.1
co HZ CO + H,
14.4 2.9 5.4
37.4 0.0 0.0
O2
-N2
-02
-CO
lo-'0 10'9 nl
10-7
IO"
,#,d
-Ar
HZ
26.1 65.2 2.2 25.8
20.2 12.1 94.7 65.6
104
-002 lo-5
u
IO-'
,,,,,,A
b) C
co
x 40%. In the case of carbon monoxide (Figure 3(b)) and hydrogen (Figure 3(c)) maximum concentrations were x 65% and more than 95%, respectively. This can be explained in terms of different pumping speeds (sum of pumping speeds of IGP, of the ion gauges and of the chamber walls) for individual gases and of surface reactions. As shown in Section 3.2. the inert gases were mainly responsible for pressure rise after switching off the IGP. When the pressure level inside the vacuum chamber is increased due to gas introduction, a memory effect of the IGP is general affected. In our case the inert gases are released from the wall of the IGP. If the gas exhibiting high pumping speed is introduced, its molecules are pumped down very quickly and the released inert gases contribute significantly to the residual atmosphere. It can be seen in Figure 3 and Table 1 that the concentrations of inert gases really correspond to the sequence of pumping speeds for individual active gases (0, > CO > H2). After introduction of a mixture of carbon monoxide and hydrogen in the ratio 1: 1 (Figure 3(d)) this ratio changed to 1: 2.25 for the highest total pressure. Figure 3(a) shows a large increase of carbon monoxide when oxygen was added. It is known9 that oxygen reacts with carbon on the surface of hot filaments producing carbon monoxide and
3.3. Gas dosing. Oxygen, carbon monoxide, hydrogen and mixture of carbon monoxide and hydrogen in the ratio 1:l were introduced into the system in 8 steps (2.8 x 10d9, 1.0 x lo-*, 2.4x lo-*, 9.0x lo-“, 2.2x lo-‘, 7.8 x lo-‘, 2.0x 10-6, 7.6x 10mh mbar) to observe the production of other gases by the system. IGP, QMS and BAGS were running throughout the measurement. Figure 3 shows stationary concentrations of main components as a function of total pressure during the continuous introduction of pure gases: (a) oxygen, (b) carbon monoxide, (c) hydrogen and (d) mixture of hydrogen and carbon monoxide in the ratio l:l, respectively. It can be seen that, in this dynamic system, completely different concentrations of gases under investigation resulted. If pure oxygen was introduced into the vacuum system (Figure 3(a)) QMA indicated maximum of oxygen concentration
-r-H2
Gas introduced
100
-
80
0
60
‘5 8.
40
-
40
20
-
20
- 60
‘3
E
s
0
-0
100
-
26
EO-
c .o
60
-
.% B
40
-
a
E S
2::
i&wM++u 1
lo"O
IO”
w
“““1
IO”
10-7
8 q-
““‘7
104
104
104
Total pressure (mbar) Total pressure (mbar) Figure 3. Stationary concentration of gases in the vacuum system as a function of the total pressure for continuous introduction of: (a) oxygen, (b) carbon monoxide, (c) hydrogen and (d) mixture of carbon monoxide + hydrogen CO/H, = 1 (IGP on - dynamic equilibrium). Data for the lowest pressure correspond to the background level. 175
TJirsBk and V Nikolajenko:
Measurement
of UHV residual gas composition
Table 2. Values of the ratio PK/PF6 for different gases and total pressures. (Pys-partial pressure of mass 16 measured; Pf,-sum of calculated contributions to mass 16 due to oxygen ions O+ from carbon dioxide (mass 44), oxygen (mass 32), carbon monoxide (mass 28) and water (mass 18); methane molecules (mass 16 judged from mass 15 according to the fragmentation factor [14]). Values of the ratio Pt/PCh Total pressure Gas introduced
O2
co
H*
CO + Hz
14.3 5.6 1.8 1.1 0.48 0.34 0.26 0.22 0.22
11.7 12.7 9.6 5.1 1.8 1.1 0.71 0.63 0.61
12.2 4.1 1.5 0.74 0.41 0.29 0.25 0.23 0.24
_.. background 2.8 x 1O-9 1.0 x lomx 2.4 x lo-’ 9.0 x lomx 2.2 x lo-’ 7.8 x 10m7 2.0 x 10m6 7.6x 10.’
10.1 3.6 2.5 1.6 1.3 1.2 1.2 1.1 1
carbon dioxide. Atomic oxygen released from a hot cathode can also produce the mentioned gases by interaction with carbon on the walls.” Table 2 shows the behaviour of mass 16 during introduction of gases (oxygen, hydrogen, carbon monoxide and mixture of carbon monoxide (1:l)) into the vacuum system. As mentioned above, the background pressure of measured mass 16 exceeded by l&14 times the sum of calculated possible contributions to the peak of mass 16, i.e. methane ions CH,+ and oxygen ions O+ resulting from dissociation of molecules of carbon dioxide, oxygen, carbon monoxide and water. The methane contribution to mass 16 was calculated from the mass 15 which, nearly of the same height, is characteristic for methane and the ratio of other methane fragments (i.e. 12, 13 and 14) corresponded to the published fragmentation pattern of methane. We have mentioned above that the production of atomic oxygen can prevail at the lowest pressures when the number of molecules entering the ionisation chamber of the QMA is small. Introduction of a gas increases the number of molecules which reach the QMA ion source and the production of oxygen on the hot tungsten cathode becomes negligible. This is true for all investigated gases in our experiments (Table 2). It can be seen in Table 2 that the addition of carbon monoxide leads to rapid decreasing of mass-16 excess with increasing amount of CO dosed. At the highest pressures a deficit of mass 16 was observed. Carbon monoxide formed the main part of mass 16-peak height. The other contributions to mass 16 were negligible. It seemed likely that carbon monoxide caused a deficit of mass 16. Similarly, when hydrogen was dosed the ratio Plhm/P,; decreased and the deficit at highest pressures was observed again. Carbon monoxide exhibited the second highest rise of the partial pressure during hydrogen dosing and it represented the main part of the noise level and main part of mass 16. We interpret this as a reason for the deficit of mass 16. Similar effects have been
176
observed during switching off the IGP or during carbon monoxide dosing. In all these cases carbon monoxide formed the main part of peak height of mass 16 and the measured value was lower than the calculated one. However, a detailed explanation of this behaviour is not possible using our simple method of measurement. When large amounts of oxygen were introduced the ratio P,,“/P,,C approached the value p,,m/p,,c = 1. Oxygen was a dominant part of the residual atmosphere and the deficit of mass 16, which seemed to be caused by carbon monoxide, did not influence the mass spectrum. Introduction of a mixture of carbon monoxide and hydrogen resulted in an intermediate composition between the cases when pure carbon monoxide or hydrogen were introduced separately. 4. Conclusions Chemical processes in the ionisation chamber of a QMA influence substantially the measured residual gas composition. Oxygen ions produced from various gas molecules (0,, H,O, CO, CO,) incident on a hot tungsten cathode of a QMA increase the mass 16 signal at ultimate pressures. During introduction of carbon monoxide and hydrogen the experimentally observed height peak of mass 16 was lower than that which could expected from the calculated sum of its possible constituents. Acknowledgements The support of this work by the Grant Agency of Czech Republic (No. 203/93/0245) is gratefully acknowledged. The authors also express their thanks to Professor L Holland and Dr Z Knor for useful comments to the manuscript and Dr Z Dolejsek for helpful discussion. References ’ W E Austin, F M Mao, J M Yang and J H Leek, J Vat Sci Technol, A5, 2631 (1987). ‘F M Mao, J M Yang, W E Austin and J H Leek, Vacuum, 37, 335 (1987). ’ W E Austin and J H Leek, Vacuum, 41,200l (1990). ‘W E Austin, J H Leek and J H Batey, J Vat Sci Technol, AlO, 3563 (1992). ‘J H Batey, Vacuum, 43, 15 (1992). ’ M C Cowen, W Allison and J H Batey, Meas Sci Technol, 4,72 (1993). ‘J R J Bennet and R J Elsey, Vacuum, 44,647 (1993). ’ L Lieszkovszky, A R Filippelli and C R Tilford, J Vat Sci Technol, AS, 3838 (1990). “P A Redhead, J P Hobson and E V Kornelson, The Physical Basis oJ Ultrahigh Vacuum. Chapman and Hall, London (1968). ‘“T W Hickmott, J Chem Phys, 32,810 (1960). ” R E Ellefson, W E Moddeman and H F Dylla, J Vat Sci Technol, 18, 1062 (1981). ” H F Dylla and W R Blanchard, J Vat Sci Technol, Al, 1297 (1983). ” INFICON Leybold-Heraeus, Spectra Library Card. id R W Kiser, Introduction to Mass Spectrometry and Its Applications, p. 300. Prentice-Hall Inc, New Jersey (1965). “P 0 Schissel and 0 C Trulson, J Chem Phys, 43,737 (1965). “A Roth, Vacuum Technology, p 230. North-Holland Publishing Company, Amsterdam (1976). “J H Singleton, J Chem Phys, 45,2819 (1966).