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An ionization gauge with a cold electron source for an extremely high vacuum
Vacuum/volume44/numbers 5-7/pages 595 to 597/1993
0042-207X/9356 00+ 00 @ 1993 Pergamon Press Ltd
Pnnted )n Great Bntam
An ionization g a u g e w i t h a cold e l e c t r o n source f o r an e x t r e m e l y high v a c u u m C O s h i m a , T S a t o h and A O t u k a , Department of Apphed Physics, Waseda University, Okubo 3-4-1, ShmlYuku,
Tokyo 169, Japan
An ionization gauge with a cold electron source has been constructed, and its performance has been evaluated The Spmdt emitter used as the cold source has shown the stable emmslon current of 1 mA, and the low outgas condmon has been obtained after the long operation of I mA emission for approximately 1 month. Between the cold and hot electron sources, no large difference concerning the stability of the emission and the outgas has been detected m extremely high vacuum of I x 10 -1o Pa, and the senstttwty of the gauge with the cold electron source (1 9x 101° cps Pa -I at O. 1 mA reduced for mtrogen gas) was shghtly higher than that of the gauge with the hot electron source 1, Introduction An extremely high vacuum (xhv) of 10-Jt Pa has a very low molecule density of ~ 103 molecules cm 3 in space Therefore, sUch an xhv c o n d m o n is established only after enormous efforts Of decreasing the outgas of the chamber walls, valves, pumps and the other components in vacuum F o r measurement of such a 10w pressure, in general, we unavoidably waste power, which p ~ s u m a b l y produces thermal and electronic desorptlon of chemisorbed gases resulting in the disturbance to xhv In the hot sOurce operation of our gauge, for instance, the main power of 21)--30 W is consumed for heating the filament, which is three orders of magnitude larger than the power consumed in the molecule lomzation process If we use a cold electron source instead of a hot source, therefore, the consumed power is reduced by a factor of 1000, which seems to be a promising technique tO measure the extremely low pressure by the ionization gauge without disturbing the pressure in xhv Recently, in fact, we detected the outgas from the hot filament made of Th-tungsten above the emission of 0 5 m A at 1 x 10 - t l Pa I In th~s work, therefore, we have constructed the lomzatlon gauge with two kinds of electron source, a conventional hot filament and a cold source We have compared the fundamental performance of two kinds of electron source under the xhv condillon of I x 10- 10 Pa, namely, the stabdlty of the cold emission alad the outgas from the cold source Lastly, the sensmvlty of the gauge with two sources have been compared experimentally under the xhv condition of 1 x 10- ~o Pa
electron sources, an lomzatlon cage (grid), an ion deflector, and a channeltron as an ion detector Since the ion deflector of the deflection angle, 256 4 °, possesses the function of the energy analysis by the cylindrical electrostatic fields, the signal ions are efficiently discnnunated from both the X-ray noise and the electron stimulated desorption (ESD) ions 2'3, which are the main noises in the conventional iomzatlon gauge for xhv 4 These noise levels were estimated to be an order of 10-13 Pa in the normal operation of our gauge t The use of the channeltron makes it possible to increase the sensltwlty of the ion detection, and hence, the emission current of 0 1 m A is large enough to measure the total pressure of 1 x 10 t i Pa precisely, which we have reported in a previous paper ~ A m o n g the known cold electron sources, the Splndt emitter is used in this experiment, because the large emission current is
Channeltron
FieldEmitter -Gnd Hot -F~lament
2. New ionization gauge and a cold emitter The pressure in this experiment has been measured by the extract0r gauge (Leybold IE514), the sensitivity of which was calibrated by the spmnmg rotor gauge The vacuum chamber used in this e~penment is made of special stainless steel ( N K K clean Z) with 10w impurity of hydrogen, and all the inner walls of chamber Were polished by electro-mechanical buffering (Ultra Clean Tech ) The vacuum system is evacuated routinely to xhv by a d~ffusion pump w~th a liquid mtrogen trap and a tltamum sUbhmation p u m p with a liquid nitrogen cooled shroud Figure 1 shows the schematic diagram o f the gauge constructed for pressure measurement in xhv, which consists of two kinds of
i
i
liu
u
u
i
ii
Figure 1. Schematic dmgram of the lomzatlon gauge with two kmds of electron sources
595
C Oshtrna et al Iomzatlon gauge with cold electron source
~
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v
~-12
Z W
-9 10
or ~_)
g
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iCollector
tO tO
O
103 b3 t'~
--{-
hi I-< rr
to U3 kO
UJ
(D Z
161°
~04
102U 7
-Gate
I.-O I---
69V
O r,,.)
[~10 4 Ttps
p= 5X1() 8 Pa I
I
5
10
I
15
I
20 TIME(h)
Figure 2. Typical stablhty of total emission current from the Spmdt emitter The gate is 69 V A schematic diagram of the emission measurement ~s inserted
routinely obtained against the low extractmg voltage ~ The Splndt emitters used were supplied from Stanford Research InternaUonal A huge n u m b e r o f micro-field-emission guns, ~ l04 fabricated on a slhcon wafer o f I mmqb with the silicon technology, provide the stable large emission owing to the statlshcal effect o f the s u m m a t i o n as shown in Figure 2 In Figure 2 the total emission current from the Spindt emitter has been shown against the operating time, the time dependence o f the emission was measured for 24 h, 7 days after starting the emission o f 1 mA Since the gap space between the gate and the ttps is several /~m, m addition, the voltage o f 69 V applied between those electrodes Is large enough to get the large current o f ~ 1 2 mA The hfetlme o f this emitter was larger than 10 ~ h In contrast to the first expectation, we have detected a huge a m o u n t o f outgas from the Splndt emttter This seems to come l¥om the ESD gas o f the gate s , when the Splndt emitter is exposed to air, the gases chemlsorb on the electrode surfaces In the contrast to a one-tip field emitter, however, S p m d t emitter has no specml way to remove the chemlsorbed gases on the gate except by the long operation Even 2 days after starting the operation o f I m A emission, we have detected the pressure Increase from 1 x 10 .0 Pa to 1 x 10 '~ Pa with Increasing the emission up to 1 m A ~ After the extremely long operation o f I m A for ~ 1 m o n t h , the outgas was decreased Figure 3 shows the result obtained after the long o p e r a U o n , the pressure indicated by extractor gauge (the open circle) and counting rate o f the stgnal ions measured In our gauge (the filled circles), are plotted against the emission The measured pressure in F~gure 3 is almost constant independent o f the emission, and the counting rate o f the signal ions is correctly proportional to the emission Both data exhibit that the outgas from the electron gun can be neglected under the expertmental c o n d i t i o n , i e the m a x i m u m emission is 1 mA, and the lowest pressure lb 1 x 10 TM Pa The s~mflar data concerning outgas from the hot source were obtained after the long degas process In other words, no clear difference regarding xts outgas was found between two electron sources until the pressure was below 1 x 10 ~0 Pa In the prewous paper, however, we have reported that the hot source clearly disturbs the vacuum c o n d m o n o f I × 10 ' ' Pa above the emission
596
1(~11
........
10-2
I
,
10-I EMiSSiON
I
i ~,1
,
101
10 0 CURRENT (mA)
hgure 3 The indicated pressure by the extractor gauge and the counting rate of our gauge as a funchon ol the emJsslon current from the Spmdt emitter
of 0 5 m A The outgas character ot the cold source m 10 '' Pa ~s o f importance in the pressure m e a s u r e m e n t o f xhv
3. Performance of the ionization gauge with a cold electron source The typical ion spectra obtained In our gauge are shown in Figure 4 The spectra (a) and (b) are measured by using a cold electron source and a hot electron source, respectively, Here, the emission current is 01 m A m both cases, and the pressure IS 1 × 10 t 0 Pa It should be noted that the counting rate o f the spectrum (a) is slightly higher than that o f the spectrum (b) The m a x i m u m s e n s l t l v l t y ( 1 9 x l 0 ~ ° c p s P a ~ a t 0 1 m A , o r l 8 x 1 0 ~6cpsPa ' A ' reduced for nitrogen gas) m the case o f the cold source was reahzed by adjusting the potential o f the hot filament placed between the cold source and grids as s h o w n in Ftgure 1 On the other hand, no large change was found by changing the potential of the cold electron source and grids in the operation o f the hot filament Hence, the special fields formed by the potential o f a
P=I X 1(510 Po (a) Cold Source
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( b ) Hot Source
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'
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1
100 ION ENERGY (eV)
200
Figure 4 Typical spectra obtained by using a cold electron source (a) and a hot electron source (b)
C Osh/ma et al Iomzat)on gauge wrth cold electron source
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. . . .
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. . . .
10-9 168 PRESSURE ( P o )
Fipre 5. Countmg rate of the signal runs wnth two electron sources at various pressures (O) a hot electron source, and ((3) a cold electron source
hot filament (0 V) m a y provide the long m e a n free p a t h of cold electrons, e n h a n c i n g the sensltlwty E S D peaks were so small t h a t we did not detect t h e m in those spoctra If there are E S D ions, we detect them clearly in those spectra within a n accuracy of the o n e - h u n d r e d t h of the m a i n peak ~ The flat b a c k g r o u n d In Figure 4 originates from the soft X . r a y radiation ~ F r o m those spectra, we have o b t a i n e d only the n ~ m b e r o f the signal ions correctly by s u b t r a c t i o n o f the X-ray b a c k g r o u n d a n d the E S D ions Figure 5 shows the c o u n t i n g rate o f the pure signal ions measured using our gauge with two electron sources at various pressures The pressures were m e a s u r e d by a n extractor gauge placed near the new gauge The open circles have been o b t a i n e d using the cold source a n d the filled circles have been measured using the h o t source A b o v e the l0 9 Pa, two curves increase along the 45 ° straight llaes, which m e a n s t h a t they are linear functions of the pressure
Consequently, It 1s concluded t h a t the two kinds o f gauges operate correctly in the pressure region a b o v e l0 -9 Pa Below 8 x l 0 - 10 Pa, o n the o t h e r h a n d , two curves bent d o w n w a r d from the 45 ° straight lines, which is the same tendency as observed previously I It is well k n o w n t h a t the extractor gauge has X-ray noises o f 1-2 × l0 ~0 p a 6 O n the contrary, the c o u n t i n g n u m b e r of the signals in o u r gauge does not include X-ray a n d ESD noises Hence, o u r gauge correctly indicates m u c h lower pressures t h a n ~ 10- ~0 Pa, while the indication o f the extractor gauge c a n n o t follow the pressure below 8 × 10- ~0 Pa This is the reason for the d o w n w a r d bending of the two observed curves in Figure 4 The lowest estimated pressure by our gauge is ~ 1 × l0 t0 Pa reduced for nitrogen gas s h o w n in Figure 5, which IS one order of m a g n i t u d e higher t h a n t h a t of the last time t It m a y be due to the r o o m t e m p e r a t u r e difference o f 20°C between winter a n d summer
4. Summary A n ionization gauge with two kinds of electron sources has been constructed The emissxon stability, the outgas a n d the sensitivity of the gauges with cold a n d hot electron sources were examined N o large difference in the outgas a n d the stability of emission were detected at the pressures above 1 × l0 ~0 Pa In a previous paper, we observed clearly the outgas from the hot electron source at 1 × 10- i i Pa Since cold emitters are promising electron sources for the ionization gauge for xhv, the outgas experiment at 1 × i0 ~ P a is now in progress In our l a b o r a t o r y
References A Otuka and C Oshlma, (m preparatnon) 2A Otuka and C Oshlma, J Vacuum Set Technol m press (1993) C A Spmdt, I Bro&e, L Humphrey and E R Westerberg, J Appl Phys, 47, 5248 (1976) 4p A Redhead, J Vacuum Sct Technol, 7, 182 (1969) 5C Oshlma and A Otuka, Techmcal Dtgest of the Fourth Internauonal Vacuum Mtcroelectromc Conference (E&ted by S Nanba, Y Nannlchl and T Utsum0, p 102 Nagahama (1991) 66 Grosse and G Messer, Proc 8th Int Vacuum Congr Cannes, Vol 2, p 255 (1980)
597
0042-207X/9356 00+ 00 @ 1993 Pergamon Press Ltd
Pnnted )n Great Bntam
An ionization g a u g e w i t h a cold e l e c t r o n source f o r an e x t r e m e l y high v a c u u m C O s h i m a , T S a t o h and A O t u k a , Department of Apphed Physics, Waseda University, Okubo 3-4-1, ShmlYuku,
Tokyo 169, Japan
An ionization gauge with a cold electron source has been constructed, and its performance has been evaluated The Spmdt emitter used as the cold source has shown the stable emmslon current of 1 mA, and the low outgas condmon has been obtained after the long operation of I mA emission for approximately 1 month. Between the cold and hot electron sources, no large difference concerning the stability of the emission and the outgas has been detected m extremely high vacuum of I x 10 -1o Pa, and the senstttwty of the gauge with the cold electron source (1 9x 101° cps Pa -I at O. 1 mA reduced for mtrogen gas) was shghtly higher than that of the gauge with the hot electron source 1, Introduction An extremely high vacuum (xhv) of 10-Jt Pa has a very low molecule density of ~ 103 molecules cm 3 in space Therefore, sUch an xhv c o n d m o n is established only after enormous efforts Of decreasing the outgas of the chamber walls, valves, pumps and the other components in vacuum F o r measurement of such a 10w pressure, in general, we unavoidably waste power, which p ~ s u m a b l y produces thermal and electronic desorptlon of chemisorbed gases resulting in the disturbance to xhv In the hot sOurce operation of our gauge, for instance, the main power of 21)--30 W is consumed for heating the filament, which is three orders of magnitude larger than the power consumed in the molecule lomzation process If we use a cold electron source instead of a hot source, therefore, the consumed power is reduced by a factor of 1000, which seems to be a promising technique tO measure the extremely low pressure by the ionization gauge without disturbing the pressure in xhv Recently, in fact, we detected the outgas from the hot filament made of Th-tungsten above the emission of 0 5 m A at 1 x 10 - t l Pa I In th~s work, therefore, we have constructed the lomzatlon gauge with two kinds of electron source, a conventional hot filament and a cold source We have compared the fundamental performance of two kinds of electron source under the xhv condillon of I x 10- 10 Pa, namely, the stabdlty of the cold emission alad the outgas from the cold source Lastly, the sensmvlty of the gauge with two sources have been compared experimentally under the xhv condition of 1 x 10- ~o Pa
electron sources, an lomzatlon cage (grid), an ion deflector, and a channeltron as an ion detector Since the ion deflector of the deflection angle, 256 4 °, possesses the function of the energy analysis by the cylindrical electrostatic fields, the signal ions are efficiently discnnunated from both the X-ray noise and the electron stimulated desorption (ESD) ions 2'3, which are the main noises in the conventional iomzatlon gauge for xhv 4 These noise levels were estimated to be an order of 10-13 Pa in the normal operation of our gauge t The use of the channeltron makes it possible to increase the sensltwlty of the ion detection, and hence, the emission current of 0 1 m A is large enough to measure the total pressure of 1 x 10 t i Pa precisely, which we have reported in a previous paper ~ A m o n g the known cold electron sources, the Splndt emitter is used in this experiment, because the large emission current is
Channeltron
FieldEmitter -Gnd Hot -F~lament
2. New ionization gauge and a cold emitter The pressure in this experiment has been measured by the extract0r gauge (Leybold IE514), the sensitivity of which was calibrated by the spmnmg rotor gauge The vacuum chamber used in this e~penment is made of special stainless steel ( N K K clean Z) with 10w impurity of hydrogen, and all the inner walls of chamber Were polished by electro-mechanical buffering (Ultra Clean Tech ) The vacuum system is evacuated routinely to xhv by a d~ffusion pump w~th a liquid mtrogen trap and a tltamum sUbhmation p u m p with a liquid nitrogen cooled shroud Figure 1 shows the schematic diagram o f the gauge constructed for pressure measurement in xhv, which consists of two kinds of
i
i
liu
u
u
i
ii
Figure 1. Schematic dmgram of the lomzatlon gauge with two kmds of electron sources
595
C Oshtrna et al Iomzatlon gauge with cold electron source
~
E
v
~-12
Z W
-9 10
or ~_)
g
13-
c~a
<__1__
W cr
iCollector
tO tO
O
103 b3 t'~
--{-
hi I-< rr
to U3 kO
UJ
(D Z
161°
~04
102U 7
-Gate
I.-O I---
69V
O r,,.)
[~10 4 Ttps
p= 5X1() 8 Pa I
I
5
10
I
15
I
20 TIME(h)
Figure 2. Typical stablhty of total emission current from the Spmdt emitter The gate is 69 V A schematic diagram of the emission measurement ~s inserted
routinely obtained against the low extractmg voltage ~ The Splndt emitters used were supplied from Stanford Research InternaUonal A huge n u m b e r o f micro-field-emission guns, ~ l04 fabricated on a slhcon wafer o f I mmqb with the silicon technology, provide the stable large emission owing to the statlshcal effect o f the s u m m a t i o n as shown in Figure 2 In Figure 2 the total emission current from the Spindt emitter has been shown against the operating time, the time dependence o f the emission was measured for 24 h, 7 days after starting the emission o f 1 mA Since the gap space between the gate and the ttps is several /~m, m addition, the voltage o f 69 V applied between those electrodes Is large enough to get the large current o f ~ 1 2 mA The hfetlme o f this emitter was larger than 10 ~ h In contrast to the first expectation, we have detected a huge a m o u n t o f outgas from the Splndt emttter This seems to come l¥om the ESD gas o f the gate s , when the Splndt emitter is exposed to air, the gases chemlsorb on the electrode surfaces In the contrast to a one-tip field emitter, however, S p m d t emitter has no specml way to remove the chemlsorbed gases on the gate except by the long operation Even 2 days after starting the operation o f I m A emission, we have detected the pressure Increase from 1 x 10 .0 Pa to 1 x 10 '~ Pa with Increasing the emission up to 1 m A ~ After the extremely long operation o f I m A for ~ 1 m o n t h , the outgas was decreased Figure 3 shows the result obtained after the long o p e r a U o n , the pressure indicated by extractor gauge (the open circle) and counting rate o f the stgnal ions measured In our gauge (the filled circles), are plotted against the emission The measured pressure in F~gure 3 is almost constant independent o f the emission, and the counting rate o f the signal ions is correctly proportional to the emission Both data exhibit that the outgas from the electron gun can be neglected under the expertmental c o n d i t i o n , i e the m a x i m u m emission is 1 mA, and the lowest pressure lb 1 x 10 TM Pa The s~mflar data concerning outgas from the hot source were obtained after the long degas process In other words, no clear difference regarding xts outgas was found between two electron sources until the pressure was below 1 x 10 ~0 Pa In the prewous paper, however, we have reported that the hot source clearly disturbs the vacuum c o n d m o n o f I × 10 ' ' Pa above the emission
596
1(~11
........
10-2
I
,
10-I EMiSSiON
I
i ~,1
,
101
10 0 CURRENT (mA)
hgure 3 The indicated pressure by the extractor gauge and the counting rate of our gauge as a funchon ol the emJsslon current from the Spmdt emitter
of 0 5 m A The outgas character ot the cold source m 10 '' Pa ~s o f importance in the pressure m e a s u r e m e n t o f xhv
3. Performance of the ionization gauge with a cold electron source The typical ion spectra obtained In our gauge are shown in Figure 4 The spectra (a) and (b) are measured by using a cold electron source and a hot electron source, respectively, Here, the emission current is 01 m A m both cases, and the pressure IS 1 × 10 t 0 Pa It should be noted that the counting rate o f the spectrum (a) is slightly higher than that o f the spectrum (b) The m a x i m u m s e n s l t l v l t y ( 1 9 x l 0 ~ ° c p s P a ~ a t 0 1 m A , o r l 8 x 1 0 ~6cpsPa ' A ' reduced for nitrogen gas) m the case o f the cold source was reahzed by adjusting the potential o f the hot filament placed between the cold source and grids as s h o w n in Ftgure 1 On the other hand, no large change was found by changing the potential of the cold electron source and grids in the operation o f the hot filament Hence, the special fields formed by the potential o f a
P=I X 1(510 Po (a) Cold Source
~
( b ) Hot Source
200
u v W ,<
t'
c.O Z
0
F--IO0 Z 0 U
¢,
{,f 0
L ...'r
100
•
x.
'
200
1
100 ION ENERGY (eV)
200
Figure 4 Typical spectra obtained by using a cold electron source (a) and a hot electron source (b)
C Osh/ma et al Iomzat)on gauge wrth cold electron source
Q. u
3
I.<
O z
102 0 L.)
101
I
1610
. . . .
I
. . . .
10-9 168 PRESSURE ( P o )
Fipre 5. Countmg rate of the signal runs wnth two electron sources at various pressures (O) a hot electron source, and ((3) a cold electron source
hot filament (0 V) m a y provide the long m e a n free p a t h of cold electrons, e n h a n c i n g the sensltlwty E S D peaks were so small t h a t we did not detect t h e m in those spoctra If there are E S D ions, we detect them clearly in those spectra within a n accuracy of the o n e - h u n d r e d t h of the m a i n peak ~ The flat b a c k g r o u n d In Figure 4 originates from the soft X . r a y radiation ~ F r o m those spectra, we have o b t a i n e d only the n ~ m b e r o f the signal ions correctly by s u b t r a c t i o n o f the X-ray b a c k g r o u n d a n d the E S D ions Figure 5 shows the c o u n t i n g rate o f the pure signal ions measured using our gauge with two electron sources at various pressures The pressures were m e a s u r e d by a n extractor gauge placed near the new gauge The open circles have been o b t a i n e d using the cold source a n d the filled circles have been measured using the h o t source A b o v e the l0 9 Pa, two curves increase along the 45 ° straight llaes, which m e a n s t h a t they are linear functions of the pressure
Consequently, It 1s concluded t h a t the two kinds o f gauges operate correctly in the pressure region a b o v e l0 -9 Pa Below 8 x l 0 - 10 Pa, o n the o t h e r h a n d , two curves bent d o w n w a r d from the 45 ° straight lines, which is the same tendency as observed previously I It is well k n o w n t h a t the extractor gauge has X-ray noises o f 1-2 × l0 ~0 p a 6 O n the contrary, the c o u n t i n g n u m b e r of the signals in o u r gauge does not include X-ray a n d ESD noises Hence, o u r gauge correctly indicates m u c h lower pressures t h a n ~ 10- ~0 Pa, while the indication o f the extractor gauge c a n n o t follow the pressure below 8 × 10- ~0 Pa This is the reason for the d o w n w a r d bending of the two observed curves in Figure 4 The lowest estimated pressure by our gauge is ~ 1 × l0 t0 Pa reduced for nitrogen gas s h o w n in Figure 5, which IS one order of m a g n i t u d e higher t h a n t h a t of the last time t It m a y be due to the r o o m t e m p e r a t u r e difference o f 20°C between winter a n d summer
4. Summary A n ionization gauge with two kinds of electron sources has been constructed The emissxon stability, the outgas a n d the sensitivity of the gauges with cold a n d hot electron sources were examined N o large difference in the outgas a n d the stability of emission were detected at the pressures above 1 × l0 ~0 Pa In a previous paper, we observed clearly the outgas from the hot electron source at 1 × 10- i i Pa Since cold emitters are promising electron sources for the ionization gauge for xhv, the outgas experiment at 1 × i0 ~ P a is now in progress In our l a b o r a t o r y
References A Otuka and C Oshlma, (m preparatnon) 2A Otuka and C Oshlma, J Vacuum Set Technol m press (1993) C A Spmdt, I Bro&e, L Humphrey and E R Westerberg, J Appl Phys, 47, 5248 (1976) 4p A Redhead, J Vacuum Sct Technol, 7, 182 (1969) 5C Oshlma and A Otuka, Techmcal Dtgest of the Fourth Internauonal Vacuum Mtcroelectromc Conference (E&ted by S Nanba, Y Nannlchl and T Utsum0, p 102 Nagahama (1991) 66 Grosse and G Messer, Proc 8th Int Vacuum Congr Cannes, Vol 2, p 255 (1980)
597