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Production of ultrahigh vacuum by helium glow discharge cleaning in an unbaked vacuum chamber
Vacuum/volume 48fnumber 7-g/pages 767 to 770/1997 0 1997 Elsevier Science Ltd
Pergamon PII: SOO42-207X(97)00041-9
All rights reserved. Printed in Great Britain 0042-207x/97 $17.00+.00
Production of ultrahigh vacuum by helium glow discharge cleaning in an unbaked vacuum chamber KAkaishi,“K Ezaki,“O Motojima”and M Nakasuga,‘, aNational institute for Fusion Science, Chikusaku, Nagoya 464-01, Japan; *Plasma Physics Laboratory, Kyoto University, Kyoto 61 I, Japan accepted in revised form 18 December
7996
For the production of ultrahigh vacuum in an unbaked vacuum system the effect of helium glow discharge cleaning as a wall conditioning technique has been investigated experimentally. A test chamber made of 304 stainless steel was constructed. Before the pump-down of the chamber it was initialized by air exposure. Then the discharge cleaning was done at the constant helium ion fluence of 0.5 Coulomb/cm*. As an effect of the discharge cleaning it is shown that the pumping time to attain the pressure of order of magnitude of IO-’ Torr is shortened after the discharge cleaning by one fourth compared with the pumping time without discharge cleaning. But the ultimate pressure at the pumping time of 72 h after the discharge cleaning is rather the same as that without discharge cleaning. The role of helium glow discharge and technical aspects to be taken care for the production of ultrahigh vacuum in an unbaked vacuum system are discussed. 0 1997 Elsevier Science Ltd. All rights reserved
Introduction For the production of ultrahigh vacuum in a large vacuum system, we meet occasionally with the constrain that high temperature baking is not applicable for the system because of permissible limit to the thermal expansion, taking much time for the baking procedure, integrity of in vessel components and so on. In such situation our concern is how can we shorten the pumpdown time from atmosphere to a pressure of ultrahigh vacuum region in the pump-down of the large vacuum system without help of baking. How we can attain a pressure as low as possible in the pump-down is also an interesting matter. In this study we pay attention to the helium glow discharge cleaning as a wall conditioning technique and investigate experimentally the effect of this technique for the fast pump-down. Although we know that it is possible to use argon gas for the discharge cleaning of a vacuum system, we do not use this gas, for the reason that optical windows for plasma diagnostics in fusion device must be protected from the sputter coating of wall mateial due to high sputtering yield of argon ion. It has been considered so far that the helium glow discharge plays a role to clean the vacuum wall due to ion impact induced gas desorption. Therefore, we may expect that the amount of adsorbed molecules on the wall surface will be reduced by the helium glow discharge, namely the outgassing rate of the chamber will be reduced, and as a result the pump-down time to attain a very low pressure will be shortened. However, in the experiment using a test chamber, we have observed that the chamber pressure decreases rapidly to a minimum after the helium glow discharge cleaning but soon rises up again to a pressure higher than the
minimum. We discuss this pressure inversion. In addition, we describe the experiment on the improvement of ultimate pressure using a liquid nitrogen trap of 77 K.
Experimental Experimental apparatus. We constructed a test chamber for the experiment of helium glow discharge cleaning. Figure 1 shows the schematic picture of the apparatus. The test chamber consists of a cylindrical tube of 150 mm in diameter and 1 m long and a manifold chamber. This chamber was made of 304 stainless steel and the inner surface was electropolished. On the top of the manifold a liquid nitrogen trap was mounted through a gate valve. This trap is a simple cylindrical tube, 64mm in diameter and 250mm long and the effective surface area cooled down to 77K when the cylinder is filled with liquid nitrogen is approximately 400cm’. The surface area of the liquid nitrogen trap chamber including the gate valve is 5.1 x 103cm2. The conductance from the test chamber to the nitrogen trap through the opened gate valve is 5261/s. A pumping orifice of the pumping speed of lOliter/s was set in the bottom of the manifold. The downstream side of the orifice was pumped with a turbomolecular pump through a gate valve. A nude B-A gauge was set on the manifold. A Pirani gauge was used to monitor the helium gas pressure in the chamber during glow discharge. For the glow discharge the test chamber is grounded electrically and is used as the cathode, and a tungsten wire as an anode was set on the axis of the cylindrical tube. A dc power supply was connected electrically in series through a load resistor of lOOohm to the 767
KAkaishi
et a/: Production of ultrahigh vacuum
L-N trap
-
gas analyzer valve for air leak or roughing pure He gas through mass flow meter and L-N trap
nude B-A gauge I
SS 304
vacuum chamber -@
r1
tungsten wire anode
r-i
+
Pirani gauge
-I-
@
DC power supply I
1 To roughing pump
grounded
t 300 liter/s turbomolecular pump Figure 1. Schematic picture of experimental apparatus.
anode and the test chamber. The helium gas for glow discharge was supplied through a liquid nitrogen trap and a mass flow controller of the maximum flow rate of 100 cm3 STP per minute. The total surface area of the test chamber which is exposed to glow discharge plasma is about 7.3 x 103cm’. Experimental method. In order to investigate the effect of the
helium glow discharge cleaning, we initialized routinely the wall surface of the test chamber by air exposure for 1 h before the pump-down of the chamber. Then we isolated the nitrogen trap from the test chamber by closing the gate valve of the top port and started the pump-down of the test chamber. We compared the pressure versus time characteristics in the respective pumpdown when the helium glow discharges was carried out and not carried out in the chamber. The total measuring time in each test run was 72 h, i.e. 3 days. In the discharge cleaning of the chamber, we had to keep the glow discharge power as low as possible, since we can not ignore the heatup of the chamber due to the power deposition of discharge plasma on the wall. This care is very important as we need not consider the effect of thermal desorption and are able to evaluate only the effect of helium ion impact induced gas desorption. The parameters of the helium glow discharge were set as follows: the helium gas pressure was 2 x 10m2Torr, the anode voltage was around + 200V and the discharge current was 0.2 A. Since we set a standard helium ion fluence at 0.5 Coulomb/cm’ for the discharge cleaning of the wall by reffering the discharge cleaning experiment made by Li and Dylla,’ the discharge time was fixed usually to 5 h. Here the ion fluence of 0.5 C/cm2 is equivalent to about 3 x 10’8ions/cm2. We observed the final pressures of the chamber at pumping time 768
of 72 h for respective pump-downs with and without discharge cleaning. And then we opened the gate valve on the top port of the manifold and further pumped-down the chamber using the liquid nitrogen trap. For this experiment we mounted a residual gas analyzer on the side port of the liquid nitrogen trap. AS a result, we were able to know the behavior of the residual gases in the chamber. Experimental results
We have attempted many times discharge cleaning for the test chamber. The discharge cleaning of the test chamber was usually carried out for 5 h at the pumping time from 1 to 6 h after the start of the pump-down of the test chamber. The comparison of pressure versus time characteristics of the test chamber are shown in Figure 2, in which “without-DC” shows the pump-down curve without discharge cleaning, and # 1, 2 and 3 show the pumpdown curves after discharge cleaning were made for the test chamber. But the condition of discharge cleaning are not same between #2 and other two of # 1 and 3, namely the discharge current and time are 0.5 A and 2 h for # 2 but 0.2 A and 5 h for # 1 and 3. Figure 3 shows the change of total pressure and ion currents of residual gases (such as HZ, H20, CO+NI and COZ) in the test chamber after discharge cleaning before and after operation of the liquid nitrogen trap at 77 K at the pumping time of 72 h. In Figure 3, the total pressure was improved by one order of magnitude from 1.1 x lo-’ to 1.6 x 10-‘Torr. Since we did not bake the test chamber, it is natural that water vapor is the main residual gas in the chamber and hence the effective pumping of water molecules on the liquid nitrogen trap may result in the improvement of the ultimate pressure.
KAkaishietal:
Production of ultrahigh vacuum
t
:
::::I.
: : :
‘:‘,.;
I
lssI
d
, ; I
.:::;d III<
;I
100
0.1
pulnpiug
time lt$
Figure 2. Pump-down curves of the test chamber for without discharge cleaning and after discharge cleanings; during the helium glow discharge only the discharge voltage is kept constant at about 200 V but discharge current and discharge 0.5Ax2hfor #2.
Discussion-fast
time are set at 0.2A x 5 h for
# 1 and
3 and
pump-down and ultimate pressure
From the comparison of pump-down curves of without-DC and # 1 or # 3 in Figure 2, we can see that the chamber pressure after the discharge cleaning decreases more rapidly than that without discharge cleaning. For the curves of # 1 and 3, the attained pressures just after the discharge cleaning are 5 x 1O-’ Torr at the pumping time of 9 h and these pressures are almost the same as those at the time of 72 h. Therefore, we can say that the pumping time is shortened by one-eighth by the discharge cleaning. However, the ultimate pressures at the end of the pump-down after 72 h reaches a constant pressure of about 5x10-*Torr whether the discharge cleaning was carried out or not for the chamber. This result shows that it is not easy to produce ultrahigh vacuum of very low pressure in the unbaked vacuum chamber with only the help of the discharge cleaning. Namely, it seems that although the discharge cleaning is capable of removing adsorbed molecules from the chamber wall to a certain extent due to ion impact induced gas desorption, there exists a limitation for the removal action. This limitation may be asked for areas of the vacuum chamber which are not directly exposed to the glow discharge plasma, since such areas keep the state of very high
outgassing rate. However, since the test chamber scarcely has long sleeved-ports, we can consider only the elastomer O-ring (Viton) attached to the driving vane of the gate valve on the top of the manifold chamber as a gas source of high outgassing rate. Figure 2 also shows the influence of deposition of discharge power on the chamber wall to the pressure drop. The large pressure drop is attained after the discharge of low current and long time as made in # 1 and 3 and the pressure drop after the discharge of high current and short time as made in #2 is rather small. In. addition, there is observed pressure inversion that the chamber pressure once decreases to the minimum and then recovers to a higher pressure. We can relate the pressure inversion with the deposition of discharge power on the chamber, since we observed that the chamber was heated up to a temperature considerably higher than room temperature in the test of # 2. In this situation, the driving vane of the gate valve will be heated up to a temperature higher than that of the chamber wall, because of good thermal insulation in vacuum, and the cooling-down time of the vane to room temperature after the discharge will be longer compared with that of the chamber wall. Thus we can suppose that the pressure inversion occurs as a result of the significant outgassing from the Viton O-ring under a high temperature. Recently, we examined this point by baking only the gate valve at 110°C for 5 h. And we confirmed that by the residual gas analysis, main gases released from the gate valve during baking are water vapor and nitrogen and the ultimate pressure at the pumping time of 72 h after the baking is reduced from 5 x lo-’ to 1.1 x lo-* Torr. This result shows that when elastomer gaskets are used as invessel components in an unbaked vacuum system, it is important to degas elastomer gaskets for the achievment of ultimate pressure as low as possible. In order to improve further the ultimate pressure of the chamber after the discharge cleaning, we operated the liquid nitrogen trap at the pumping time of 72 h. As seen in Figure 3, the ultimate pressure is improved by one third from 5 x 10m8to 1.6 x lo-‘Torr using the trap of 77 K. The residual water is well pumped out by the trap, and at the pressure region less than 1.0 x lO_*Torr hydrogen instead ofwater vapor becomes the predominant desorbing gas in the chamber. Thus, for the production of ultrahigh vacuum in an unbaked vacuum system, it will be a good method to use a cryopump which has large pumping speeds for hydrogen and water vapor, in adding to carry out discharge cleaning for the chamber wall. Conclusion
z : b :a
350
We have investigated experimentally the effect of the helium glow discharge cleaning for the fast pump-down and the production of ultrahigh vacuum in an unbaked vacuum chamber. From the experiment using a test chamber the following results are obtained:
300 250
‘;: 200 f 150 2 B 100 L 3 3
5o 0
ABABABABAB
Figure 3. The comparison of total pressure and ion currents of residual gases such as H,, H,O, CO+N, and CO, before and after use of the liquid nitrogen trap at the pumping time of 72 h after discharge cleaning, where A shows the pump-down by the orifice of 101/s and B shows the pump-down by the orifice of 10 l/s + the L-N trap (77 K).
The discharge cleaning is effective to shorten the pump-down time. In particular when it was applied just after the start of the pump-down of the test chamber from atmosphere, the pumping time was shortened at least by one fourth to achieve the pressure of the order of magnitude of lo-‘Torr compared with the pump-down at room temperature when the discharge cleaning was not made. The ultimate pressure achieved after long time pump-down is not improved by the discharge cleaning. As a limitation for the improvement of the ultimate pressure, the gas release from the Viton O-ring which is used in the gate valve is considered. 769
KAkaishi eta/: Production of ultrahigh vacuum
3. For the improvement of the ultimate pressure after the discharge cleaning, the nitrogen trap of 77 K was operated at the pumping time of 72 h. The residual water is mainly pumped out by the trap and as a result, the ultimate pressure is improved by one-half. For the production of ultrahigh vacuum in an unbaked vacuum system, since hydrogen and water vapor are dominant desorbing gases in the system, it is rec-
ommended to use a cryopump which has large pumping speeds for these gases, in adding to carry out discharge cleaning for the system. Reference 1. Minxu, Li, Dylla, H. F., J Vat Sci Technol, 1995, A13, 571.
Pergamon PII: SOO42-207X(97)00041-9
All rights reserved. Printed in Great Britain 0042-207x/97 $17.00+.00
Production of ultrahigh vacuum by helium glow discharge cleaning in an unbaked vacuum chamber KAkaishi,“K Ezaki,“O Motojima”and M Nakasuga,‘, aNational institute for Fusion Science, Chikusaku, Nagoya 464-01, Japan; *Plasma Physics Laboratory, Kyoto University, Kyoto 61 I, Japan accepted in revised form 18 December
7996
For the production of ultrahigh vacuum in an unbaked vacuum system the effect of helium glow discharge cleaning as a wall conditioning technique has been investigated experimentally. A test chamber made of 304 stainless steel was constructed. Before the pump-down of the chamber it was initialized by air exposure. Then the discharge cleaning was done at the constant helium ion fluence of 0.5 Coulomb/cm*. As an effect of the discharge cleaning it is shown that the pumping time to attain the pressure of order of magnitude of IO-’ Torr is shortened after the discharge cleaning by one fourth compared with the pumping time without discharge cleaning. But the ultimate pressure at the pumping time of 72 h after the discharge cleaning is rather the same as that without discharge cleaning. The role of helium glow discharge and technical aspects to be taken care for the production of ultrahigh vacuum in an unbaked vacuum system are discussed. 0 1997 Elsevier Science Ltd. All rights reserved
Introduction For the production of ultrahigh vacuum in a large vacuum system, we meet occasionally with the constrain that high temperature baking is not applicable for the system because of permissible limit to the thermal expansion, taking much time for the baking procedure, integrity of in vessel components and so on. In such situation our concern is how can we shorten the pumpdown time from atmosphere to a pressure of ultrahigh vacuum region in the pump-down of the large vacuum system without help of baking. How we can attain a pressure as low as possible in the pump-down is also an interesting matter. In this study we pay attention to the helium glow discharge cleaning as a wall conditioning technique and investigate experimentally the effect of this technique for the fast pump-down. Although we know that it is possible to use argon gas for the discharge cleaning of a vacuum system, we do not use this gas, for the reason that optical windows for plasma diagnostics in fusion device must be protected from the sputter coating of wall mateial due to high sputtering yield of argon ion. It has been considered so far that the helium glow discharge plays a role to clean the vacuum wall due to ion impact induced gas desorption. Therefore, we may expect that the amount of adsorbed molecules on the wall surface will be reduced by the helium glow discharge, namely the outgassing rate of the chamber will be reduced, and as a result the pump-down time to attain a very low pressure will be shortened. However, in the experiment using a test chamber, we have observed that the chamber pressure decreases rapidly to a minimum after the helium glow discharge cleaning but soon rises up again to a pressure higher than the
minimum. We discuss this pressure inversion. In addition, we describe the experiment on the improvement of ultimate pressure using a liquid nitrogen trap of 77 K.
Experimental Experimental apparatus. We constructed a test chamber for the experiment of helium glow discharge cleaning. Figure 1 shows the schematic picture of the apparatus. The test chamber consists of a cylindrical tube of 150 mm in diameter and 1 m long and a manifold chamber. This chamber was made of 304 stainless steel and the inner surface was electropolished. On the top of the manifold a liquid nitrogen trap was mounted through a gate valve. This trap is a simple cylindrical tube, 64mm in diameter and 250mm long and the effective surface area cooled down to 77K when the cylinder is filled with liquid nitrogen is approximately 400cm’. The surface area of the liquid nitrogen trap chamber including the gate valve is 5.1 x 103cm2. The conductance from the test chamber to the nitrogen trap through the opened gate valve is 5261/s. A pumping orifice of the pumping speed of lOliter/s was set in the bottom of the manifold. The downstream side of the orifice was pumped with a turbomolecular pump through a gate valve. A nude B-A gauge was set on the manifold. A Pirani gauge was used to monitor the helium gas pressure in the chamber during glow discharge. For the glow discharge the test chamber is grounded electrically and is used as the cathode, and a tungsten wire as an anode was set on the axis of the cylindrical tube. A dc power supply was connected electrically in series through a load resistor of lOOohm to the 767
KAkaishi
et a/: Production of ultrahigh vacuum
L-N trap
-
gas analyzer valve for air leak or roughing pure He gas through mass flow meter and L-N trap
nude B-A gauge I
SS 304
vacuum chamber -@
r1
tungsten wire anode
r-i
+
Pirani gauge
-I-
@
DC power supply I
1 To roughing pump
grounded
t 300 liter/s turbomolecular pump Figure 1. Schematic picture of experimental apparatus.
anode and the test chamber. The helium gas for glow discharge was supplied through a liquid nitrogen trap and a mass flow controller of the maximum flow rate of 100 cm3 STP per minute. The total surface area of the test chamber which is exposed to glow discharge plasma is about 7.3 x 103cm’. Experimental method. In order to investigate the effect of the
helium glow discharge cleaning, we initialized routinely the wall surface of the test chamber by air exposure for 1 h before the pump-down of the chamber. Then we isolated the nitrogen trap from the test chamber by closing the gate valve of the top port and started the pump-down of the test chamber. We compared the pressure versus time characteristics in the respective pumpdown when the helium glow discharges was carried out and not carried out in the chamber. The total measuring time in each test run was 72 h, i.e. 3 days. In the discharge cleaning of the chamber, we had to keep the glow discharge power as low as possible, since we can not ignore the heatup of the chamber due to the power deposition of discharge plasma on the wall. This care is very important as we need not consider the effect of thermal desorption and are able to evaluate only the effect of helium ion impact induced gas desorption. The parameters of the helium glow discharge were set as follows: the helium gas pressure was 2 x 10m2Torr, the anode voltage was around + 200V and the discharge current was 0.2 A. Since we set a standard helium ion fluence at 0.5 Coulomb/cm’ for the discharge cleaning of the wall by reffering the discharge cleaning experiment made by Li and Dylla,’ the discharge time was fixed usually to 5 h. Here the ion fluence of 0.5 C/cm2 is equivalent to about 3 x 10’8ions/cm2. We observed the final pressures of the chamber at pumping time 768
of 72 h for respective pump-downs with and without discharge cleaning. And then we opened the gate valve on the top port of the manifold and further pumped-down the chamber using the liquid nitrogen trap. For this experiment we mounted a residual gas analyzer on the side port of the liquid nitrogen trap. AS a result, we were able to know the behavior of the residual gases in the chamber. Experimental results
We have attempted many times discharge cleaning for the test chamber. The discharge cleaning of the test chamber was usually carried out for 5 h at the pumping time from 1 to 6 h after the start of the pump-down of the test chamber. The comparison of pressure versus time characteristics of the test chamber are shown in Figure 2, in which “without-DC” shows the pump-down curve without discharge cleaning, and # 1, 2 and 3 show the pumpdown curves after discharge cleaning were made for the test chamber. But the condition of discharge cleaning are not same between #2 and other two of # 1 and 3, namely the discharge current and time are 0.5 A and 2 h for # 2 but 0.2 A and 5 h for # 1 and 3. Figure 3 shows the change of total pressure and ion currents of residual gases (such as HZ, H20, CO+NI and COZ) in the test chamber after discharge cleaning before and after operation of the liquid nitrogen trap at 77 K at the pumping time of 72 h. In Figure 3, the total pressure was improved by one order of magnitude from 1.1 x lo-’ to 1.6 x 10-‘Torr. Since we did not bake the test chamber, it is natural that water vapor is the main residual gas in the chamber and hence the effective pumping of water molecules on the liquid nitrogen trap may result in the improvement of the ultimate pressure.
KAkaishietal:
Production of ultrahigh vacuum
t
:
::::I.
: : :
‘:‘,.;
I
lssI
d
, ; I
.:::;d III<
;I
100
0.1
pulnpiug
time lt$
Figure 2. Pump-down curves of the test chamber for without discharge cleaning and after discharge cleanings; during the helium glow discharge only the discharge voltage is kept constant at about 200 V but discharge current and discharge 0.5Ax2hfor #2.
Discussion-fast
time are set at 0.2A x 5 h for
# 1 and
3 and
pump-down and ultimate pressure
From the comparison of pump-down curves of without-DC and # 1 or # 3 in Figure 2, we can see that the chamber pressure after the discharge cleaning decreases more rapidly than that without discharge cleaning. For the curves of # 1 and 3, the attained pressures just after the discharge cleaning are 5 x 1O-’ Torr at the pumping time of 9 h and these pressures are almost the same as those at the time of 72 h. Therefore, we can say that the pumping time is shortened by one-eighth by the discharge cleaning. However, the ultimate pressures at the end of the pump-down after 72 h reaches a constant pressure of about 5x10-*Torr whether the discharge cleaning was carried out or not for the chamber. This result shows that it is not easy to produce ultrahigh vacuum of very low pressure in the unbaked vacuum chamber with only the help of the discharge cleaning. Namely, it seems that although the discharge cleaning is capable of removing adsorbed molecules from the chamber wall to a certain extent due to ion impact induced gas desorption, there exists a limitation for the removal action. This limitation may be asked for areas of the vacuum chamber which are not directly exposed to the glow discharge plasma, since such areas keep the state of very high
outgassing rate. However, since the test chamber scarcely has long sleeved-ports, we can consider only the elastomer O-ring (Viton) attached to the driving vane of the gate valve on the top of the manifold chamber as a gas source of high outgassing rate. Figure 2 also shows the influence of deposition of discharge power on the chamber wall to the pressure drop. The large pressure drop is attained after the discharge of low current and long time as made in # 1 and 3 and the pressure drop after the discharge of high current and short time as made in #2 is rather small. In. addition, there is observed pressure inversion that the chamber pressure once decreases to the minimum and then recovers to a higher pressure. We can relate the pressure inversion with the deposition of discharge power on the chamber, since we observed that the chamber was heated up to a temperature considerably higher than room temperature in the test of # 2. In this situation, the driving vane of the gate valve will be heated up to a temperature higher than that of the chamber wall, because of good thermal insulation in vacuum, and the cooling-down time of the vane to room temperature after the discharge will be longer compared with that of the chamber wall. Thus we can suppose that the pressure inversion occurs as a result of the significant outgassing from the Viton O-ring under a high temperature. Recently, we examined this point by baking only the gate valve at 110°C for 5 h. And we confirmed that by the residual gas analysis, main gases released from the gate valve during baking are water vapor and nitrogen and the ultimate pressure at the pumping time of 72 h after the baking is reduced from 5 x lo-’ to 1.1 x lo-* Torr. This result shows that when elastomer gaskets are used as invessel components in an unbaked vacuum system, it is important to degas elastomer gaskets for the achievment of ultimate pressure as low as possible. In order to improve further the ultimate pressure of the chamber after the discharge cleaning, we operated the liquid nitrogen trap at the pumping time of 72 h. As seen in Figure 3, the ultimate pressure is improved by one third from 5 x 10m8to 1.6 x lo-‘Torr using the trap of 77 K. The residual water is well pumped out by the trap, and at the pressure region less than 1.0 x lO_*Torr hydrogen instead ofwater vapor becomes the predominant desorbing gas in the chamber. Thus, for the production of ultrahigh vacuum in an unbaked vacuum system, it will be a good method to use a cryopump which has large pumping speeds for hydrogen and water vapor, in adding to carry out discharge cleaning for the chamber wall. Conclusion
z : b :a
350
We have investigated experimentally the effect of the helium glow discharge cleaning for the fast pump-down and the production of ultrahigh vacuum in an unbaked vacuum chamber. From the experiment using a test chamber the following results are obtained:
300 250
‘;: 200 f 150 2 B 100 L 3 3
5o 0
ABABABABAB
Figure 3. The comparison of total pressure and ion currents of residual gases such as H,, H,O, CO+N, and CO, before and after use of the liquid nitrogen trap at the pumping time of 72 h after discharge cleaning, where A shows the pump-down by the orifice of 101/s and B shows the pump-down by the orifice of 10 l/s + the L-N trap (77 K).
The discharge cleaning is effective to shorten the pump-down time. In particular when it was applied just after the start of the pump-down of the test chamber from atmosphere, the pumping time was shortened at least by one fourth to achieve the pressure of the order of magnitude of lo-‘Torr compared with the pump-down at room temperature when the discharge cleaning was not made. The ultimate pressure achieved after long time pump-down is not improved by the discharge cleaning. As a limitation for the improvement of the ultimate pressure, the gas release from the Viton O-ring which is used in the gate valve is considered. 769
KAkaishi eta/: Production of ultrahigh vacuum
3. For the improvement of the ultimate pressure after the discharge cleaning, the nitrogen trap of 77 K was operated at the pumping time of 72 h. The residual water is mainly pumped out by the trap and as a result, the ultimate pressure is improved by one-half. For the production of ultrahigh vacuum in an unbaked vacuum system, since hydrogen and water vapor are dominant desorbing gases in the system, it is rec-
ommended to use a cryopump which has large pumping speeds for these gases, in adding to carry out discharge cleaning for the system. Reference 1. Minxu, Li, Dylla, H. F., J Vat Sci Technol, 1995, A13, 571.