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Measurement of uhv and xhv by hot cathode ionization gauge with higher sensitivity
Vacuum~volume 44/numbers 5-7/pages 555 to 557/1993
0042-207X/9356 00+ 00 © 1993 PergamonPress Ltd
Printed )n Great Britain
Measurement of uhv and xhv by hot cathode ionization gauge with higher sensitivity ¥ H K u o , Department of Physics, Peking University, 100871 Betting, People's Republic of China and T K a n a j i , Faculty of Engineering, Kobe Umverslty, Rokko, Nada, Kobe 657, Japan
For pressure measurement m the regmns of ultra-high vacuum or extreme-high vacuum ( <~ 10-~oPa), several Studms on new mmzatmn gauges have been accomphshed m recent years m Japan and m China. The common feature of the new gauges/s their high sens/tlwty
1. Introduction In recent years, the measurement of extreme-high vacuum (xhv < 10 -j° Pa) has attracted considerable attention in the field of surface physics We already know that we can conventionally obtain gauges that can be used for the measurement of pressures in the 10 ~ Pa region However, in order to measure more accurately and easily, to measure lower pressures, or to manufacture xhv gauges more cheaply, some new, interesting studies on the hot-cathode ionization gauge are under way in Japan and China In the new studies, computers are used to determine the electrode structure, and the c o m m o n feature of the new gauges is their high sensitivity
2. Principle The 'true ion current (I,)' generated in an ionizauon gauge at a pressure (P) is given by
t, = SP/e
O)
Here, S is the sensitlwty and le is the emlssmn current Because the mean free path of the electrons in the residual gas is very long in the xhv region, it ~s not easy to obtam sufficient signal intensity in the xhv regmn The pressure at which the signal current is equal to the noise current is usually called the 'lowest measurable pressure (Pu)' o f the gauge It is well known that, m many cases, the most important source of noise are the soft X-rays generated at the anode surface by electron bombardment, and the pnnclpal purpose of the past several studies was decreasing or separation of the soft X-ray photocurrent (Ix) But in the xhv region, the other noises are also important F o r example, Pu is hmlted often by the apparent signal Current (Id), which corresponds to the gases desorbed from the working electrodes by heating or electron bombardment This phenomenon is sometimes observed m high-pressure regions also, but in such a case this effect can be decreased suffimently by the well-known process of 'degassmg' However, in xhv measurement, the gas desorptlon must be completely decreased, and sometimes too much time is required for degasslng Therefore, in order to measure very low pressures quickly or easily, this problem must be solved
Another important no~se source m the xhv region is the electron-stimulated ion desorptlon from the anode surface If the coverage (0) of the gas molecules stroking to the anode surface is high, the magnitude of the noise current due to these desorbed Ions (Is) is comparable with that of the 'true ion' current Several other noises are also known, but the above three (Ix, ld, and I~) are important and it must be pointed out that Ix and la are closely related to the emission current This means that Ix can be reduced when the magnitude of Ic is decreased, and the effect of Id can be reduced also through a decrease in the heating power of the cathode and a decrease in the number of electrons that bombard the anode The effects of vapour pressure of the cathode material and some other noise sources are also reduced by a decrease of Ic The relation between I, and I~ is not simple because the value of 0 is not independent of Ic, but there are several methods that are effective for decreasing I, In order to suppress the effects of noises while keeping signal intensity at a sufficient level, it is a good method to develop a new gauge that has high sensitivity (S) and operates with httle Ic There have already been several well-known studies on highsensitivity gauges, such as the Lafferty gauge t, the orbitron gauge 2, and the oscillation gauge 3, but the authors wish to introduce the latest interesting studies in the following
3. Several new studies in Japan and China 3.1. Improvement of the Heimer gauge (IHG). In the past studies on the Helmer gauge, the deflection angle of the ions was usually 90 °4, but in a study, recently completed by Oshlma et al of Japan, the aberration of the mn deflector was calculated and an angle of 254 6 ° was chosen as the best angle (Figure 1) With this improved deflector, not only a higher transmission probability for the 'true ions' is obtained, but also the desorbed tons (I~) are separated, and the ion collector is located farther away from the X-ray source In order to avoid the effect of Id, this gauge works with a rather small emission current (0 1 m A for example), and the number of ions arriving at the detector (channeltron) is counted The lineanty of this gauge has been confirmed down to 1 x 10- ~t Pa Because the resolution of the deflector is good, this gauge can also be used to study various phenomena that occur m the xhv gauge 555
YHKuo and TKanalt Hot cathode ,onlzatmn gauge Channeltron
18+ 15,) 0~
Suppressor
0v
Filament
"G;id ]
1'/ /~
o0: . . . .
200v 400v
500v 0v Figure 1. Improved Helmer gauge lons are deflected by an ~mproved deflector
3.2. Axial e m i s s i o n magnetron gauge ~AEMG). This was studmd a b o u t ten years ago 5 In this gauge, the filament, the grid, a n d the m n collector are installed on a n axis, a n d the emitted electrons are rejected into the grid parallel to the axis The magnetic field is a p p h e d parallel to the axis (Figure 2), a n d the lengths of the electron trajectories before they are a b s o r b e d on the a n o d e are elongated by the effect of this field In this gauge, the value o f I~ c a n n o t be controlled freely, but tt ts d e t e r m m e d by the mtenstty o f the field a n d the geometry of the electrodes Then, instead o f S, the value of K (= I•/P) is used for e v a l u a t m n just as m the cold c a t h o d e m a g n e t r o n gauge or the Lafferty gauge The value of K of this gauge was a b o u t 10 3 A Pa J when the value of 1~ was several # A The h n e a n t y was confirmed d o w n to the regmn ofl0 )°Pa 3.3. C a s c a d e static lens g a u g e ( C S L G ) . The study o f this gauge is only partmlly completed Th~s gauge also has all its electrodes on a n axis, I e several a p e r t u r e d discs are installed between the c a t h o d e a n d the m n collector, a n d the d~scs form cascade stauc lenses (Figure 3) The trajectories of the electrons are hm~ted near the ax~s by the effect of these lenses, a n d the electrons repeat o s c d l a t m n between the c a t h o d e a n d the m n collector The electron trajectories are elongated by th~s o s c d l a u o n Because there are m a n y electrodes, it ~s necessary to determine the
Figure 3. Cascade static lens gauge Electrons oscillate many rimes along
the axls
geometry a n d potent]Ms ol the electrodes using a cornputer ~' In a p r e h m l n a r y experiment, the value of S was a b o u t 30 Pa ~w~th I~ = 3 #A The effects of noise are beheved to be shght because the value of L is small, but the hnearlty has been confirmed on b down to 5 x 10 '~ Pa, a n d the operating charactensUcb m the xhv region have not yet been c o n h r m e d 3.4. Improved axial emission g a u g e (IAEG). In thl~ gauge, the electrons t h a t have passed t h r o u g h the effeeuve ~omzauon zone are then collected by a special electrode of the 'electron collector (Figure 4) Because a large part o f the electrons b o m b a r d only this electrode, the ion collector is effecuvely shmlded from X - r a ) s and the desorbed ions from the electron collector, using a simple metal c y h n d e r In a c o m p u t e r s l m u l a u o n study, almost all electrons arrwed at the electron collector, but m a p r e h m m a r y experiment in the 10 ~ Pa reglom a b o u t 70% of the electrons arrived and the value o f S ~ a s a b o u t 1 Pa ~with L = 80/~A This stud) will be c o n t i n u e d m J a p a n a n d in C h i n a sunultaneou~ly 4
Conclusion
It is expected that, m the near luture, considerable d e v e l o p m e n t wdl be seen in the field o f x h v m e a s u r e m e n t The values obtained in the m e a s u r e m e n t of pressure m the xhv regmn will become more rehable, a n d the m e a s u r e m e n t of xhv itself w.ll become
Shzeld L I,I , ~
~
0V 500V 350V 250V 60V 20V 0V -100V 200V-250V-90V
Ion Collector"
//
------Magnet
@~ Hehcal6rld •
H
•
~ --------GlassEnvelope
12V F
z Z //
F-] Hehcal
Fd~ment Figure 2 Axial emlsston magnetron gauge 556
H
L1
L2
L3
L8
L9
L18 L4
L5
L6
L7
Figure 4. Improved axial emission gauge Electrons are collected by an electron collector
Y H Kuo and T Kanal/ Hot cathode ionization gauge
much easier The authors hope that the above studies will contribute to such development
References ~J M Lafferty, 7th Vac Syrup Trans (Edited by C R Meisser), p 97 Pergamon Press, Oxford (1962)
2E A Meyer and R G Herb, J Vac Scl Technol, 4, 63 (1967) 3S M Atklnson, R K Fitch and G J Rushton, Vacuum, 21,453 (1971) 4C BenvenuU and M Hauer, Proc 8th Int Vac Congr, Cannes, Vol II, p 199 (1980) 5j Z Chen, C D Suen and Y H Kuo, J Vac Sci Technol, A3, 14 (1985) °T Kanajl, T Urano and S Hongo, Vacuum, 41, 2144 (1990)
557
0042-207X/9356 00+ 00 © 1993 PergamonPress Ltd
Printed )n Great Britain
Measurement of uhv and xhv by hot cathode ionization gauge with higher sensitivity ¥ H K u o , Department of Physics, Peking University, 100871 Betting, People's Republic of China and T K a n a j i , Faculty of Engineering, Kobe Umverslty, Rokko, Nada, Kobe 657, Japan
For pressure measurement m the regmns of ultra-high vacuum or extreme-high vacuum ( <~ 10-~oPa), several Studms on new mmzatmn gauges have been accomphshed m recent years m Japan and m China. The common feature of the new gauges/s their high sens/tlwty
1. Introduction In recent years, the measurement of extreme-high vacuum (xhv < 10 -j° Pa) has attracted considerable attention in the field of surface physics We already know that we can conventionally obtain gauges that can be used for the measurement of pressures in the 10 ~ Pa region However, in order to measure more accurately and easily, to measure lower pressures, or to manufacture xhv gauges more cheaply, some new, interesting studies on the hot-cathode ionization gauge are under way in Japan and China In the new studies, computers are used to determine the electrode structure, and the c o m m o n feature of the new gauges is their high sensitivity
2. Principle The 'true ion current (I,)' generated in an ionizauon gauge at a pressure (P) is given by
t, = SP/e
O)
Here, S is the sensitlwty and le is the emlssmn current Because the mean free path of the electrons in the residual gas is very long in the xhv region, it ~s not easy to obtam sufficient signal intensity in the xhv regmn The pressure at which the signal current is equal to the noise current is usually called the 'lowest measurable pressure (Pu)' o f the gauge It is well known that, m many cases, the most important source of noise are the soft X-rays generated at the anode surface by electron bombardment, and the pnnclpal purpose of the past several studies was decreasing or separation of the soft X-ray photocurrent (Ix) But in the xhv region, the other noises are also important F o r example, Pu is hmlted often by the apparent signal Current (Id), which corresponds to the gases desorbed from the working electrodes by heating or electron bombardment This phenomenon is sometimes observed m high-pressure regions also, but in such a case this effect can be decreased suffimently by the well-known process of 'degassmg' However, in xhv measurement, the gas desorptlon must be completely decreased, and sometimes too much time is required for degasslng Therefore, in order to measure very low pressures quickly or easily, this problem must be solved
Another important no~se source m the xhv region is the electron-stimulated ion desorptlon from the anode surface If the coverage (0) of the gas molecules stroking to the anode surface is high, the magnitude of the noise current due to these desorbed Ions (Is) is comparable with that of the 'true ion' current Several other noises are also known, but the above three (Ix, ld, and I~) are important and it must be pointed out that Ix and la are closely related to the emission current This means that Ix can be reduced when the magnitude of Ic is decreased, and the effect of Id can be reduced also through a decrease in the heating power of the cathode and a decrease in the number of electrons that bombard the anode The effects of vapour pressure of the cathode material and some other noise sources are also reduced by a decrease of Ic The relation between I, and I~ is not simple because the value of 0 is not independent of Ic, but there are several methods that are effective for decreasing I, In order to suppress the effects of noises while keeping signal intensity at a sufficient level, it is a good method to develop a new gauge that has high sensitivity (S) and operates with httle Ic There have already been several well-known studies on highsensitivity gauges, such as the Lafferty gauge t, the orbitron gauge 2, and the oscillation gauge 3, but the authors wish to introduce the latest interesting studies in the following
3. Several new studies in Japan and China 3.1. Improvement of the Heimer gauge (IHG). In the past studies on the Helmer gauge, the deflection angle of the ions was usually 90 °4, but in a study, recently completed by Oshlma et al of Japan, the aberration of the mn deflector was calculated and an angle of 254 6 ° was chosen as the best angle (Figure 1) With this improved deflector, not only a higher transmission probability for the 'true ions' is obtained, but also the desorbed tons (I~) are separated, and the ion collector is located farther away from the X-ray source In order to avoid the effect of Id, this gauge works with a rather small emission current (0 1 m A for example), and the number of ions arriving at the detector (channeltron) is counted The lineanty of this gauge has been confirmed down to 1 x 10- ~t Pa Because the resolution of the deflector is good, this gauge can also be used to study various phenomena that occur m the xhv gauge 555
YHKuo and TKanalt Hot cathode ,onlzatmn gauge Channeltron
18+ 15,) 0~
Suppressor
0v
Filament
"G;id ]
1'/ /~
o0: . . . .
200v 400v
500v 0v Figure 1. Improved Helmer gauge lons are deflected by an ~mproved deflector
3.2. Axial e m i s s i o n magnetron gauge ~AEMG). This was studmd a b o u t ten years ago 5 In this gauge, the filament, the grid, a n d the m n collector are installed on a n axis, a n d the emitted electrons are rejected into the grid parallel to the axis The magnetic field is a p p h e d parallel to the axis (Figure 2), a n d the lengths of the electron trajectories before they are a b s o r b e d on the a n o d e are elongated by the effect of this field In this gauge, the value o f I~ c a n n o t be controlled freely, but tt ts d e t e r m m e d by the mtenstty o f the field a n d the geometry of the electrodes Then, instead o f S, the value of K (= I•/P) is used for e v a l u a t m n just as m the cold c a t h o d e m a g n e t r o n gauge or the Lafferty gauge The value of K of this gauge was a b o u t 10 3 A Pa J when the value of 1~ was several # A The h n e a n t y was confirmed d o w n to the regmn ofl0 )°Pa 3.3. C a s c a d e static lens g a u g e ( C S L G ) . The study o f this gauge is only partmlly completed Th~s gauge also has all its electrodes on a n axis, I e several a p e r t u r e d discs are installed between the c a t h o d e a n d the m n collector, a n d the d~scs form cascade stauc lenses (Figure 3) The trajectories of the electrons are hm~ted near the ax~s by the effect of these lenses, a n d the electrons repeat o s c d l a t m n between the c a t h o d e a n d the m n collector The electron trajectories are elongated by th~s o s c d l a u o n Because there are m a n y electrodes, it ~s necessary to determine the
Figure 3. Cascade static lens gauge Electrons oscillate many rimes along
the axls
geometry a n d potent]Ms ol the electrodes using a cornputer ~' In a p r e h m l n a r y experiment, the value of S was a b o u t 30 Pa ~w~th I~ = 3 #A The effects of noise are beheved to be shght because the value of L is small, but the hnearlty has been confirmed on b down to 5 x 10 '~ Pa, a n d the operating charactensUcb m the xhv region have not yet been c o n h r m e d 3.4. Improved axial emission g a u g e (IAEG). In thl~ gauge, the electrons t h a t have passed t h r o u g h the effeeuve ~omzauon zone are then collected by a special electrode of the 'electron collector (Figure 4) Because a large part o f the electrons b o m b a r d only this electrode, the ion collector is effecuvely shmlded from X - r a ) s and the desorbed ions from the electron collector, using a simple metal c y h n d e r In a c o m p u t e r s l m u l a u o n study, almost all electrons arrwed at the electron collector, but m a p r e h m m a r y experiment in the 10 ~ Pa reglom a b o u t 70% of the electrons arrived and the value o f S ~ a s a b o u t 1 Pa ~with L = 80/~A This stud) will be c o n t i n u e d m J a p a n a n d in C h i n a sunultaneou~ly 4
Conclusion
It is expected that, m the near luture, considerable d e v e l o p m e n t wdl be seen in the field o f x h v m e a s u r e m e n t The values obtained in the m e a s u r e m e n t of pressure m the xhv regmn will become more rehable, a n d the m e a s u r e m e n t of xhv itself w.ll become
Shzeld L I,I , ~
~
0V 500V 350V 250V 60V 20V 0V -100V 200V-250V-90V
Ion Collector"
//
------Magnet
@~ Hehcal6rld •
H
•
~ --------GlassEnvelope
12V F
z Z //
F-] Hehcal
Fd~ment Figure 2 Axial emlsston magnetron gauge 556
H
L1
L2
L3
L8
L9
L18 L4
L5
L6
L7
Figure 4. Improved axial emission gauge Electrons are collected by an electron collector
Y H Kuo and T Kanal/ Hot cathode ionization gauge
much easier The authors hope that the above studies will contribute to such development
References ~J M Lafferty, 7th Vac Syrup Trans (Edited by C R Meisser), p 97 Pergamon Press, Oxford (1962)
2E A Meyer and R G Herb, J Vac Scl Technol, 4, 63 (1967) 3S M Atklnson, R K Fitch and G J Rushton, Vacuum, 21,453 (1971) 4C BenvenuU and M Hauer, Proc 8th Int Vac Congr, Cannes, Vol II, p 199 (1980) 5j Z Chen, C D Suen and Y H Kuo, J Vac Sci Technol, A3, 14 (1985) °T Kanajl, T Urano and S Hongo, Vacuum, 41, 2144 (1990)
557