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Discharge cleaning and gas trapping by use of a glow-mode plasma source
Vacuum/volume 41/numbers 7-9/pages 1977 to 1979/1990
0042-207X/90S3.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
Discharge cleaning and gas trapping by use of a glow-mode plasma source T Kawabe, M Kaminaga*, M Fukazawa, K Nakazato, T Hayashi and S Sato, Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki, 305 Japan and A I t o h , Mechanical Engineering Laboratory, Hitachi Ltd, Tsuchiura, Ibaraki, 300 Japan
A glow-mode plasma source made the hydrogen gas pressure in glow discharge cleaning as low as 0.27 Pa, and it made discharge voltage as low as 20 V. By using this plasma source, the dependencies of the hydrogen gas trapping at the surface of stainless steel (SUS304) on the energy of the bombarding ions (up to 500 V), on the gas pressure in a range from 0.27 to 21 Pa, and on the temperature of the surface (in a range from 50 to 400°C) in glow discharge cleaning have been also studied experimentally. It was found that the gas trapping rate, which was evaluated by Temperature Programmed Desorption method, was reduced by lowering the discharge voltage below a critical value ( 5 0 - 1 0 0 V) at low pressure operation, as expected, and by raising the gas pressure to more than 1 Pa, unexpectedly. By introducing a theoretical model of the ion energy distribution in the sheath on the surface of the vacuum chamber, a qualitative interpretation is given for the latter phenomenon. Substantial reduction of the trapping rate was also observed when the temperature of the metal surface is higher than 200°C, which indicates the effectiveness of the glow discharge cleaning under the baking condition to avoid trapping of hydrogen gas on the vacuum chamber.
1. Introduction Glow discharge cleaning of vacuum chambers L2 has been one of the powerful methods for the conditioning of the walls of chambers for many applications which require good vacuum. This method does not require a magnetic field as in the ECR discharge cleaning or Taylor discharge cleaning for the Tokamak device in the nuclear fusion research. On the other hand, the ECR discharge cleaning has advantages, such as less damage on the surface of the wall due to the bombardment of the charged particles in the case of the glow discharge cleaning, and operation with a lower pressure of the driving gas. In the case of ECR discharge cleaning (ECRDC), the pressure can be lower than 10 -2 Pa, while in the case of GDC, the pressure must be higher than 10° Pa to sustain the discharge. We have been investigating the glow discharge cleaning of the vacuum chamber by hydrogen gas to obtain good vacuum conditions, and we have invented an effective method to improve the operation of the glow discharge cleaning method to reach the one for ECRDC without using a magnetic field. This new method is to run the glow discharge cleaning with assistance of an additional plasma source 3. This additional plasma is made by another glow discharge with hot cathode. Hydrogen gas is an attractive feature as the discharge gas for cleaning, since we can expect chemical processes besides the physical one. In this paper, we present the experimental results *On leave of absence from Atsugi Unisia Co. Ltd., Atsugi, Kanagawa, Japan.
on the glow discharge cleaning with glow-mode plasma source. We will show the effectiveness of the plasma source for the starting and sustaining of the glow discharge, and control of the plasma potential, the effects on the cleaning, and trapping in the walls. An attempt to interpret the experimental results on the driving gas pressure dependence of the trapping rate has been carried out by introducing the ion energy distribution in the sheath.
2. Experimental arrangement and procedure The experimental apparatus is shown in Figure 1. It consists of the vacuum chamber, pumping system (turbomolecular pump with a pumping speed of 310 l s-~), glow discharge system with glow-mode plasma source, specimen (type SUS 304 with a size of 120ram × 10mm x 0.05mm) for thermal desorption method, as well as diagnostics including a Langmuir probe. The vacuum chamber is made of SUS 304, with an inner surface and volume of 0.73 m 2 and 0.024 m 3, respectively. The detailed description of the procedure of the discharge can be found in the previous paper 3.
3. Condition of glow discharge and plasma parameters The glow discharge with assistance of a glow-mode plasma source (GMPS) has a wider range of the discharge condition in its operation. The gas pressure can be less than 0.3 Pa for sustaining the discharge and there is no limit of the pressure for the breakdown of the gas, since GMPS provides the plasma for 1977
T Kawabe et al." Glow-mode plasma s o u r c e
~
-
recorder
~10-1 L
(31 ¢-
6_ ~1 0 -2
•
L.
~
A:probe
L
~
B:CA ihermocouple
c
1 pressure
[ c p o e r supply for bias
C: specimen
10 ( Pa )
100
Figure 2. The dependence of the trapping rate on the discharge gas pressure. The solid circles are experimental results and the curve is a theoretical one.
D: glow mode plasma source
Figure 1. The experimental arrangement. r
start up. Another characteristic of the glow discharge with GMPS is the range of the variable plasma potential. It is found that by the use of GMPS the space potential can be as low as 80 V. In the case when the discharge gas is argon, the lowest voltage was 20 V.
~n
i
i
J
J
6
| "5-sd
•I o
4. Evaluation of cleaning effect
J
I
I
200
o
400
potential d i f f e r e n c e
The cleaning effect of the GDC with GMPS is evaluated by the temporal variation of the total and partial pressure of the gas in the vacuum chamber during the pumping-out phase. The experimental results indicate that the pressure in the case of GDC is lower than that without GDC by several I0%, at about 5 h after the start of the pumping. In other words, the pumping time to the objective pressure, such a s l 0 - 4 Pa, is reduced by 8 to 9 h by use of GDC in the present experimental conditions. The results also show that the water is removed by this GDC, as is observed in GDC without GMPS. 5. Trapping of hydrogen gas
The amount of the trapped hydrogen gas in the specimen has been evaluated by the thermal desorption method. The current to the specimen has been measured during the GDC. This current is considered as the ion current, since the specimen is biased strongly negative to the plasma potential. By dividing the amount of the trapped gas by the ion current and the time of the discharge cleaning, one can obtain the trapping rate (the trapping probability for the ions hitting the specimen). The solid circles in Figure 2 show the experimental results on the pressure dependence of the trapping rate of hydrogen for the case of the potential difference of 540 V. This indicates that the trapping rate increases as the pressure decreases from 0.27 to 21 Pa. Figure 3 shows the dependence of the trapping rate on the potential difference between the specimen and the plasma under the discharge gas pressure of 0.27 Pa. From this figure, one can find that the trapping rate increases with the potential difference, and that the trapping rate saturates when the potential difference is larger than about 200 V. Another experimental result shows that the trapping rate is reduced substantially when the temperature of the specimen is raised to higher than 250°C during the GDC as shown in Figure 4. 1978
600
( V )
Figure 3. The dependence of the trapping rate on the potential difference between the plasma and the wall.
|0 "I
• ~0
•
0.27 Pa
0
2L3 Pa
,.~ 10~
C o Q_
r~ L.. I0-3
10-4
t
I
100
i
200
temperature
I
i
300
(°C)
Figure 4. The dependence of the trapping rate on the temperature of the specimen during the glow discharge.
6. Discussion
To interpret the increase of the trapping rate with decreasing gas pressure, we have investigated a model of the trapping mechanism, which is as follows. The total trapping rate is assumed to depend on the ion energy distribution function and on the dependence of the trapping probability vs energy.
T Kawabe et al: Glow-mode plasma source
The energy distribution function of the ions, passing through the sheath on the surface of the wall, is a function of the gas pressure. This is due to the collision of the ions with the gas molecules. The essential contribution is due to charge exchange under the present experimental conditions. The energy distribution function of this case had been investigated in the past by Davis and Vanderslice 4. The theoretical trapping rate is obtained by integrating the trapping probability of the individual ion, and multiplying the ion energy distribution function over the energy of the ions. The results of this calculation for a linear trapping efficiency dependence on the ion energy is shown in Figure 2, and is compared with the experimental results. Many calculations have been made assuming different dependences of the trapping efficiency on ion energy, and the results show the same characteristics as the one in Figure 2. From this it can be said that there is good agreement, qualitatively, between the experimental results and theoretical (semi-empirical) model.
7. Conclusion
The characteristics of the glow discharge cleaning with assistance of the glow-mode plasma source have been investigated experimentally. It was found that, (l) the discharge gas pressure can be reduced to values as low as 0.1 Pa. (2) the voltage difference between the metal wall and the discharge plasma is reduced to as few as l0 V. It is observed that the trapping rate of the hydrogen gas
increases as the gas pressure decreases. However, the trapping rate is reduced even in the low pressure discharge by (1) reducing the potential difference between the wall and the plasma by controlling the power supply of GMPS, or (2) raising the temperature of the wall higher than 200°C. The latter corresponds to the case of GDC with simultaneous baking of the vacuum chamber wall. A theoretical interpretation of the dependence of the total trapping rate on gas pressure has been carried out by introducing the ion energy distribution in the sheath at the surface of the wall. A qualitative agreement is obtained between the theoretical and experimental results. Acknowledgements
The authors would like to acknowledge the members of the Plasma Research Center at the University of Tsukuba and Prof Y Sakamoto of Toyo University for their kind discussions and encouragement. They would also like to thank Dr K Kitajima of the National Laboratory of Metal for his fruitful discussions. M Kaminaga is grateful to the University of Tsukuba for the research contractor system of the Ministry of Education, Culture and Science. References
H F Dylla, J Vac Sci Technol, A6, 1276 (1988). 2 T Kawabe, H Hirata, M Ichimura, H Kanda, N Yamaguchi, K Yatsu and Y Sakamoto, J Nucl Materials, 128/129, 893 (1984). 3A Itoh, K Ishikawa and T Kawabe, J Vac Sci Technol, A6, 2421 (1988). 4W D Davis and T A Vanderslice, Phys Rev, 131, 219 (1963).
1979
0042-207X/90S3.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
Discharge cleaning and gas trapping by use of a glow-mode plasma source T Kawabe, M Kaminaga*, M Fukazawa, K Nakazato, T Hayashi and S Sato, Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki, 305 Japan and A I t o h , Mechanical Engineering Laboratory, Hitachi Ltd, Tsuchiura, Ibaraki, 300 Japan
A glow-mode plasma source made the hydrogen gas pressure in glow discharge cleaning as low as 0.27 Pa, and it made discharge voltage as low as 20 V. By using this plasma source, the dependencies of the hydrogen gas trapping at the surface of stainless steel (SUS304) on the energy of the bombarding ions (up to 500 V), on the gas pressure in a range from 0.27 to 21 Pa, and on the temperature of the surface (in a range from 50 to 400°C) in glow discharge cleaning have been also studied experimentally. It was found that the gas trapping rate, which was evaluated by Temperature Programmed Desorption method, was reduced by lowering the discharge voltage below a critical value ( 5 0 - 1 0 0 V) at low pressure operation, as expected, and by raising the gas pressure to more than 1 Pa, unexpectedly. By introducing a theoretical model of the ion energy distribution in the sheath on the surface of the vacuum chamber, a qualitative interpretation is given for the latter phenomenon. Substantial reduction of the trapping rate was also observed when the temperature of the metal surface is higher than 200°C, which indicates the effectiveness of the glow discharge cleaning under the baking condition to avoid trapping of hydrogen gas on the vacuum chamber.
1. Introduction Glow discharge cleaning of vacuum chambers L2 has been one of the powerful methods for the conditioning of the walls of chambers for many applications which require good vacuum. This method does not require a magnetic field as in the ECR discharge cleaning or Taylor discharge cleaning for the Tokamak device in the nuclear fusion research. On the other hand, the ECR discharge cleaning has advantages, such as less damage on the surface of the wall due to the bombardment of the charged particles in the case of the glow discharge cleaning, and operation with a lower pressure of the driving gas. In the case of ECR discharge cleaning (ECRDC), the pressure can be lower than 10 -2 Pa, while in the case of GDC, the pressure must be higher than 10° Pa to sustain the discharge. We have been investigating the glow discharge cleaning of the vacuum chamber by hydrogen gas to obtain good vacuum conditions, and we have invented an effective method to improve the operation of the glow discharge cleaning method to reach the one for ECRDC without using a magnetic field. This new method is to run the glow discharge cleaning with assistance of an additional plasma source 3. This additional plasma is made by another glow discharge with hot cathode. Hydrogen gas is an attractive feature as the discharge gas for cleaning, since we can expect chemical processes besides the physical one. In this paper, we present the experimental results *On leave of absence from Atsugi Unisia Co. Ltd., Atsugi, Kanagawa, Japan.
on the glow discharge cleaning with glow-mode plasma source. We will show the effectiveness of the plasma source for the starting and sustaining of the glow discharge, and control of the plasma potential, the effects on the cleaning, and trapping in the walls. An attempt to interpret the experimental results on the driving gas pressure dependence of the trapping rate has been carried out by introducing the ion energy distribution in the sheath.
2. Experimental arrangement and procedure The experimental apparatus is shown in Figure 1. It consists of the vacuum chamber, pumping system (turbomolecular pump with a pumping speed of 310 l s-~), glow discharge system with glow-mode plasma source, specimen (type SUS 304 with a size of 120ram × 10mm x 0.05mm) for thermal desorption method, as well as diagnostics including a Langmuir probe. The vacuum chamber is made of SUS 304, with an inner surface and volume of 0.73 m 2 and 0.024 m 3, respectively. The detailed description of the procedure of the discharge can be found in the previous paper 3.
3. Condition of glow discharge and plasma parameters The glow discharge with assistance of a glow-mode plasma source (GMPS) has a wider range of the discharge condition in its operation. The gas pressure can be less than 0.3 Pa for sustaining the discharge and there is no limit of the pressure for the breakdown of the gas, since GMPS provides the plasma for 1977
T Kawabe et al." Glow-mode plasma s o u r c e
~
-
recorder
~10-1 L
(31 ¢-
6_ ~1 0 -2
•
L.
~
A:probe
L
~
B:CA ihermocouple
c
1 pressure
[ c p o e r supply for bias
C: specimen
10 ( Pa )
100
Figure 2. The dependence of the trapping rate on the discharge gas pressure. The solid circles are experimental results and the curve is a theoretical one.
D: glow mode plasma source
Figure 1. The experimental arrangement. r
start up. Another characteristic of the glow discharge with GMPS is the range of the variable plasma potential. It is found that by the use of GMPS the space potential can be as low as 80 V. In the case when the discharge gas is argon, the lowest voltage was 20 V.
~n
i
i
J
J
6
| "5-sd
•I o
4. Evaluation of cleaning effect
J
I
I
200
o
400
potential d i f f e r e n c e
The cleaning effect of the GDC with GMPS is evaluated by the temporal variation of the total and partial pressure of the gas in the vacuum chamber during the pumping-out phase. The experimental results indicate that the pressure in the case of GDC is lower than that without GDC by several I0%, at about 5 h after the start of the pumping. In other words, the pumping time to the objective pressure, such a s l 0 - 4 Pa, is reduced by 8 to 9 h by use of GDC in the present experimental conditions. The results also show that the water is removed by this GDC, as is observed in GDC without GMPS. 5. Trapping of hydrogen gas
The amount of the trapped hydrogen gas in the specimen has been evaluated by the thermal desorption method. The current to the specimen has been measured during the GDC. This current is considered as the ion current, since the specimen is biased strongly negative to the plasma potential. By dividing the amount of the trapped gas by the ion current and the time of the discharge cleaning, one can obtain the trapping rate (the trapping probability for the ions hitting the specimen). The solid circles in Figure 2 show the experimental results on the pressure dependence of the trapping rate of hydrogen for the case of the potential difference of 540 V. This indicates that the trapping rate increases as the pressure decreases from 0.27 to 21 Pa. Figure 3 shows the dependence of the trapping rate on the potential difference between the specimen and the plasma under the discharge gas pressure of 0.27 Pa. From this figure, one can find that the trapping rate increases with the potential difference, and that the trapping rate saturates when the potential difference is larger than about 200 V. Another experimental result shows that the trapping rate is reduced substantially when the temperature of the specimen is raised to higher than 250°C during the GDC as shown in Figure 4. 1978
600
( V )
Figure 3. The dependence of the trapping rate on the potential difference between the plasma and the wall.
|0 "I
• ~0
•
0.27 Pa
0
2L3 Pa
,.~ 10~
C o Q_
r~ L.. I0-3
10-4
t
I
100
i
200
temperature
I
i
300
(°C)
Figure 4. The dependence of the trapping rate on the temperature of the specimen during the glow discharge.
6. Discussion
To interpret the increase of the trapping rate with decreasing gas pressure, we have investigated a model of the trapping mechanism, which is as follows. The total trapping rate is assumed to depend on the ion energy distribution function and on the dependence of the trapping probability vs energy.
T Kawabe et al: Glow-mode plasma source
The energy distribution function of the ions, passing through the sheath on the surface of the wall, is a function of the gas pressure. This is due to the collision of the ions with the gas molecules. The essential contribution is due to charge exchange under the present experimental conditions. The energy distribution function of this case had been investigated in the past by Davis and Vanderslice 4. The theoretical trapping rate is obtained by integrating the trapping probability of the individual ion, and multiplying the ion energy distribution function over the energy of the ions. The results of this calculation for a linear trapping efficiency dependence on the ion energy is shown in Figure 2, and is compared with the experimental results. Many calculations have been made assuming different dependences of the trapping efficiency on ion energy, and the results show the same characteristics as the one in Figure 2. From this it can be said that there is good agreement, qualitatively, between the experimental results and theoretical (semi-empirical) model.
7. Conclusion
The characteristics of the glow discharge cleaning with assistance of the glow-mode plasma source have been investigated experimentally. It was found that, (l) the discharge gas pressure can be reduced to values as low as 0.1 Pa. (2) the voltage difference between the metal wall and the discharge plasma is reduced to as few as l0 V. It is observed that the trapping rate of the hydrogen gas
increases as the gas pressure decreases. However, the trapping rate is reduced even in the low pressure discharge by (1) reducing the potential difference between the wall and the plasma by controlling the power supply of GMPS, or (2) raising the temperature of the wall higher than 200°C. The latter corresponds to the case of GDC with simultaneous baking of the vacuum chamber wall. A theoretical interpretation of the dependence of the total trapping rate on gas pressure has been carried out by introducing the ion energy distribution in the sheath at the surface of the wall. A qualitative agreement is obtained between the theoretical and experimental results. Acknowledgements
The authors would like to acknowledge the members of the Plasma Research Center at the University of Tsukuba and Prof Y Sakamoto of Toyo University for their kind discussions and encouragement. They would also like to thank Dr K Kitajima of the National Laboratory of Metal for his fruitful discussions. M Kaminaga is grateful to the University of Tsukuba for the research contractor system of the Ministry of Education, Culture and Science. References
H F Dylla, J Vac Sci Technol, A6, 1276 (1988). 2 T Kawabe, H Hirata, M Ichimura, H Kanda, N Yamaguchi, K Yatsu and Y Sakamoto, J Nucl Materials, 128/129, 893 (1984). 3A Itoh, K Ishikawa and T Kawabe, J Vac Sci Technol, A6, 2421 (1988). 4W D Davis and T A Vanderslice, Phys Rev, 131, 219 (1963).
1979