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Behavior of catalytic probes at low pressure
~cuum/volume 44/numbers 5-7/pages 459 to 460/1993 'flnted in Great Br=tam
0042-207X/93$6 00+ 00 © 1993 Pergamon Press Ltd
3ehavior of catalytic probes at low pressure B r e c e l j , M M o z e t i ~ and K Zupan, Ingtltut za elektromko m vakuumsko tehniko, Teslova 30, S L 0 - 6 1 0 0 0 iubljana, Slovenia
|nd D ro b niE, Inst/tut Jo2ef Stefan, Jamo va 32, SL O- 61000 Ljubljana, SIo vema
~m/tattons of the use of catalyttc probes for determination of atomic hydrogen dens/ty and/ts part/a~ pressure are ~scussed. Measurements have been performed at total pressures below 10 -1 Pa We found out that the atomic ~ydrogen dens/ty can be determ/ned down to the value of I x 10 m m -3 prowded that the total pressure is less than ~tO-2 Pa. The probe activat/on process at low pressures has been measured and discussed as well [
i lIntroduction l ~ d w temperature reduction of metal oxide thin layers has become important technology of final cleamng of vacuum devices 1-3 lde layers are effectively reduced through the interaction of rogen ions and neutral atoms with surfaces The application lons is often limited because of the damage of the surfaces sed by sputtering It is more convenient to use thermal neutral rmc hydrogen It is produced in atomic hydrogen sources, h as low pressure weakly ionized plasmas Atomic hydrogen let through vacuum components to the device to be cleaned the way, some of the atoms recombine on components rfaces The density of atomic hydrogen in the device is therefore nerally lower then In the source The density of atomic hydrogen can be measured by the use ff catalytic probes. A catalytic probe IS a small disc made of a letal with a high recomblnalaon coefficient, connected to very aln thermocouple wires 4 Atormc hydrogen reaclung the disc .'combines on the surface In this process, energy equal to the issoclatlon energy of a hydrogen molecule is dissipated, so that ae temperature of the disc rases substantially over the temrature of the surrounding gas It has been pointed out that the perature of the disc is a measure of the density of atonuc drogen in the vicinity of the probe 4 In the present paper, we cuss the apphcablhty of probes used to determine the atormc drogen density at pressures lower than 10- ~ Pa
]2. Experimental I. lExpenments on the behavior of catalyUc probes at low pressure Were carried out in a vacuum system, which is schematically ~ o w n in Figure 1 It is pumped by an oll diffusion pump with IChe pumping speed of 150 1 s- ~ backed by a mechanical rotary ~ump with the pumping speed of I 1 1 s- I The base pressure In ttae system is 1 x 10- 3 Pa The effectwe pumping speed throughout the measunng vessel, a glass cylindrical tube w~th the diamnter of 30 mm and the length of 300 ram, is 60 1 s - ~ A catalytic probe is mounted into the center of the measunng vessel The probe is made of a mckel disc with the radms of 1 turn and the thickness of 0 1 ram, connected to thermocouple wires chromelalumel with the diameter of 25/~m. Atonuc hydrogen is produced in the discharge vessel, wtuch is a glass cyhndncal tube with the diameter of 20 mm and the length of 200 mm Low pressure
!
~
• ~
measuring vessel
discharge
• J vessel
Figure 1. Schematic of the vacuum system
weakly ionized hydrogen plasma in the discharge vessel is produced by an inductively coupled RF generator First, the equlllbnum temperature of the probe was determined at different total pressure in the measunng vessel. This is plotted m Figure 2 Second, the decrease of the temperature of the disc after turmng off the atomic hydrogen source was measured, and the atomic hydrogen density m the vicinity of the probe was calculated using the following equation s
4Mcv
dT
n =
(1)
x/(SkTm)TWonr 2 Here, M is the mass of the disc, cv the specific thermal capacity of nickel ; dT/dt is the first derivation of the measured T = T(t) curve just after the atozmc hydrogen source was turned off, x/(8kT/zm) is the average random velocity of hydrogen atoms, y the recombination coefficient of hydrogen atoms on nickel surface 6, r is the radius of the disc and Wo is the dissociation
459
F BreceO et al Behawor of catalytm probes
T
[K]
Tt 300
30
Y
Ix lOiSm 3]
200
100
8 ' oo
/
'
0 ~4
'
0 b8
'
0 ]2
.
. 0 1 .6 .
.
020
8'00
024
'
r 001
'
0 0' g
'
0
'12 '
0
'16 '
0 20
'
021
ptot [P a] Figure 2 Steady state temperature of the catalytic probe vs total pressure
Figure 3 Atomic hydrogen den,,~ly ,,s total pzessul~_
energy o f a h y d r o g e n molecule, i e Wo = 4 5 eV The calculated values o f the atomic h y d r o g e n density as a function of total pressure are plotted in Figure 3
This is valid for o u r case, where the degree o f dissociation is high a n d the total pressure ts a b o u t twice the partial pressure o f atomic hydrogen In the case of weakly dissociated hydrogen, the low pressure hmlt o f the use of the catalytic probes is correspondlngl~ higher Catalytic probes, which are m o u n t e d into a vacuum systeln should first be activated The surface o f nickel is Initially covered with a native layer of oxide, which has a small r e c o m b i n a t i o n coefficient a n d must be removed This is carried out by exposing the disc surface to a flux o f atomic hydrogen A t o m i c hydrogen reduces the oxide layer The efficiency o f reduction at room t e m p e r a t u r e was f o u n d to be o f the order of 10 4 (ref 5) At low total pressures the density of atomic hydrogen IS low and thus the activation process is very slow At the total pressure of 1 x 10 ~ Pa the activation process took 90 rain so we r e c o m m e n d p e r f o r m i n g the activation process at a higher pressure The atomic hydrogen c o n c e n t r a t i o n is determined as soon as the p r o b e reaches the e q u l h b r l u m t e m p e r a t u r e This can h a p p e n a few seconds after t u r n i n g on the atomic hydrogen source, providing that the total pressure is high However, m the lou pressure regime, this may take m u c h more time In the case of,t degree of dissociation of 60%, the disc reaches the equIhbrlum t e m p e r a t u r e in a b o u t 1 mln at a total pressure o f l x 10 ~ P,t a n d m 1 0 m m at a pressure of 1 x l0 2 Pa
3. Discussion M e a s u r e m e n t s of atomic h y d r o g e n density with catalytic probes at low pressures were p e r f o r m e d The equilibrium t e m p e r a t u r e of the disc as a function o f total pressure between 1 x 10 2 Pa and 2 x 10 t Pa is plotted m Figure 2 It is noticeable that the t e m p e r a t u r e o f the disc exceeds the t e m p e r a t u r e o f the surr o u n d i n g gas for a few 100°C T h e e q u l h b r l u m t e m p e r a t u r e o f the disc increases with increasing total pressure, which displays the fact that the density o f atomic h y d r o g e n increases a n d thus provides more power dissipated o n the disc surface due to the H A - H ~ H2 reaction A t a very low pressure the slope of the T = T ( p ) curve is steeper t h a n at m o d e r a t e pressures, since a higher pressure also m e a n s higher t h e r m a l conductivity o f the s u r r o u n d i n g gas a n d thus more intensive cooling of the disc (At even higher pressures the T = T ( p ) curve finally reaches the m a x i m u m a n d in the high pressure regime the slope of the curve is negative ) A t o m i c h y d r o g e n density is calculated according to (1) from the d a t a o b t a i n e d by m e a s u r i n g the decrease o f the disc temperature after t u r n i n g off the atomic h y d r o g e n source T h e density increases fairly hnearly with increasing pressure, which means that the ratio between the atomic a n d molecular h y d r o g e n density remains c o n s t a n t In o u r case, a b o u t 6 0 % o f h y d r o g e n is dissociated This is very different f r o m the high pressure regime, where a strong decrease o f the d~ssoclatmn ratio as a function of pressure has been reported 4 A c o m p a r i s o n of Figures 2 a n d 3 shows t h a t even at the density of a t o m m hydrogen, 1 x 10 TM m ~, the t e m p e r a t u r e o f the p r o b e remains 100°C over the t e m p e r a t u r e of the s u r r o u n d i n g gas Therefore, the p r o b e gives r a t h e r p u n c t u a l data on the atomic h y d r o g e n density even m this low pressure regime T a k i n g into account the relation p = n k T , we find t h a t the density o f atomic h y d r o g e n is determined to a r a t h e r high degree o f accuracy then d o w n to the atomic h y d r o g e n partial pressure o f 5 x 10 ~ Pa
460
References ] Y Sakamoto, Y Oshlbe K Yano and H Oyama I ,~uH Mater 93 and 94, 333 (1980) -'Y Matsuzakl, N Suzuki and T H~rayama Jap J 4pp[ Phls 25, 253 (1986) ~F Brecelj and M MozeU6, Va(uum 40, 177 (t990) 4M Mozetl6, M Drobme, F Brecelj and M Kveder, Pro( lOth IS'P( Bochum, 2 1 20 (1991) ~M Drobnl6, M MozetlC, F Brecelj and M K'ceder Pto( ~ ¥ ICPIG Plsa, p 313 (1991) 6M Mozetl6, M Drobm6 A Paulln, M Kveder and 1 Brecelj, Vuoto (to be pubhshed) 7H Wise and B J Wood, In Adtame~ m Atomt( and Moh,~ular Ph}~t(s (Edited by D R Bates and l Estermann), Vol 3, p 291 Academic Press New York (1967)
0042-207X/93$6 00+ 00 © 1993 Pergamon Press Ltd
3ehavior of catalytic probes at low pressure B r e c e l j , M M o z e t i ~ and K Zupan, Ingtltut za elektromko m vakuumsko tehniko, Teslova 30, S L 0 - 6 1 0 0 0 iubljana, Slovenia
|nd D ro b niE, Inst/tut Jo2ef Stefan, Jamo va 32, SL O- 61000 Ljubljana, SIo vema
~m/tattons of the use of catalyttc probes for determination of atomic hydrogen dens/ty and/ts part/a~ pressure are ~scussed. Measurements have been performed at total pressures below 10 -1 Pa We found out that the atomic ~ydrogen dens/ty can be determ/ned down to the value of I x 10 m m -3 prowded that the total pressure is less than ~tO-2 Pa. The probe activat/on process at low pressures has been measured and discussed as well [
i lIntroduction l ~ d w temperature reduction of metal oxide thin layers has become important technology of final cleamng of vacuum devices 1-3 lde layers are effectively reduced through the interaction of rogen ions and neutral atoms with surfaces The application lons is often limited because of the damage of the surfaces sed by sputtering It is more convenient to use thermal neutral rmc hydrogen It is produced in atomic hydrogen sources, h as low pressure weakly ionized plasmas Atomic hydrogen let through vacuum components to the device to be cleaned the way, some of the atoms recombine on components rfaces The density of atomic hydrogen in the device is therefore nerally lower then In the source The density of atomic hydrogen can be measured by the use ff catalytic probes. A catalytic probe IS a small disc made of a letal with a high recomblnalaon coefficient, connected to very aln thermocouple wires 4 Atormc hydrogen reaclung the disc .'combines on the surface In this process, energy equal to the issoclatlon energy of a hydrogen molecule is dissipated, so that ae temperature of the disc rases substantially over the temrature of the surrounding gas It has been pointed out that the perature of the disc is a measure of the density of atonuc drogen in the vicinity of the probe 4 In the present paper, we cuss the apphcablhty of probes used to determine the atormc drogen density at pressures lower than 10- ~ Pa
]2. Experimental I. lExpenments on the behavior of catalyUc probes at low pressure Were carried out in a vacuum system, which is schematically ~ o w n in Figure 1 It is pumped by an oll diffusion pump with IChe pumping speed of 150 1 s- ~ backed by a mechanical rotary ~ump with the pumping speed of I 1 1 s- I The base pressure In ttae system is 1 x 10- 3 Pa The effectwe pumping speed throughout the measunng vessel, a glass cylindrical tube w~th the diamnter of 30 mm and the length of 300 ram, is 60 1 s - ~ A catalytic probe is mounted into the center of the measunng vessel The probe is made of a mckel disc with the radms of 1 turn and the thickness of 0 1 ram, connected to thermocouple wires chromelalumel with the diameter of 25/~m. Atonuc hydrogen is produced in the discharge vessel, wtuch is a glass cyhndncal tube with the diameter of 20 mm and the length of 200 mm Low pressure
!
~
• ~
measuring vessel
discharge
• J vessel
Figure 1. Schematic of the vacuum system
weakly ionized hydrogen plasma in the discharge vessel is produced by an inductively coupled RF generator First, the equlllbnum temperature of the probe was determined at different total pressure in the measunng vessel. This is plotted m Figure 2 Second, the decrease of the temperature of the disc after turmng off the atomic hydrogen source was measured, and the atomic hydrogen density m the vicinity of the probe was calculated using the following equation s
4Mcv
dT
n =
(1)
x/(SkTm)TWonr 2 Here, M is the mass of the disc, cv the specific thermal capacity of nickel ; dT/dt is the first derivation of the measured T = T(t) curve just after the atozmc hydrogen source was turned off, x/(8kT/zm) is the average random velocity of hydrogen atoms, y the recombination coefficient of hydrogen atoms on nickel surface 6, r is the radius of the disc and Wo is the dissociation
459
F BreceO et al Behawor of catalytm probes
T
[K]
Tt 300
30
Y
Ix lOiSm 3]
200
100
8 ' oo
/
'
0 ~4
'
0 b8
'
0 ]2
.
. 0 1 .6 .
.
020
8'00
024
'
r 001
'
0 0' g
'
0
'12 '
0
'16 '
0 20
'
021
ptot [P a] Figure 2 Steady state temperature of the catalytic probe vs total pressure
Figure 3 Atomic hydrogen den,,~ly ,,s total pzessul~_
energy o f a h y d r o g e n molecule, i e Wo = 4 5 eV The calculated values o f the atomic h y d r o g e n density as a function of total pressure are plotted in Figure 3
This is valid for o u r case, where the degree o f dissociation is high a n d the total pressure ts a b o u t twice the partial pressure o f atomic hydrogen In the case of weakly dissociated hydrogen, the low pressure hmlt o f the use of the catalytic probes is correspondlngl~ higher Catalytic probes, which are m o u n t e d into a vacuum systeln should first be activated The surface o f nickel is Initially covered with a native layer of oxide, which has a small r e c o m b i n a t i o n coefficient a n d must be removed This is carried out by exposing the disc surface to a flux o f atomic hydrogen A t o m i c hydrogen reduces the oxide layer The efficiency o f reduction at room t e m p e r a t u r e was f o u n d to be o f the order of 10 4 (ref 5) At low total pressures the density of atomic hydrogen IS low and thus the activation process is very slow At the total pressure of 1 x 10 ~ Pa the activation process took 90 rain so we r e c o m m e n d p e r f o r m i n g the activation process at a higher pressure The atomic hydrogen c o n c e n t r a t i o n is determined as soon as the p r o b e reaches the e q u l h b r l u m t e m p e r a t u r e This can h a p p e n a few seconds after t u r n i n g on the atomic hydrogen source, providing that the total pressure is high However, m the lou pressure regime, this may take m u c h more time In the case of,t degree of dissociation of 60%, the disc reaches the equIhbrlum t e m p e r a t u r e in a b o u t 1 mln at a total pressure o f l x 10 ~ P,t a n d m 1 0 m m at a pressure of 1 x l0 2 Pa
3. Discussion M e a s u r e m e n t s of atomic h y d r o g e n density with catalytic probes at low pressures were p e r f o r m e d The equilibrium t e m p e r a t u r e of the disc as a function o f total pressure between 1 x 10 2 Pa and 2 x 10 t Pa is plotted m Figure 2 It is noticeable that the t e m p e r a t u r e o f the disc exceeds the t e m p e r a t u r e o f the surr o u n d i n g gas for a few 100°C T h e e q u l h b r l u m t e m p e r a t u r e o f the disc increases with increasing total pressure, which displays the fact that the density o f atomic h y d r o g e n increases a n d thus provides more power dissipated o n the disc surface due to the H A - H ~ H2 reaction A t a very low pressure the slope of the T = T ( p ) curve is steeper t h a n at m o d e r a t e pressures, since a higher pressure also m e a n s higher t h e r m a l conductivity o f the s u r r o u n d i n g gas a n d thus more intensive cooling of the disc (At even higher pressures the T = T ( p ) curve finally reaches the m a x i m u m a n d in the high pressure regime the slope of the curve is negative ) A t o m i c h y d r o g e n density is calculated according to (1) from the d a t a o b t a i n e d by m e a s u r i n g the decrease o f the disc temperature after t u r n i n g off the atomic h y d r o g e n source T h e density increases fairly hnearly with increasing pressure, which means that the ratio between the atomic a n d molecular h y d r o g e n density remains c o n s t a n t In o u r case, a b o u t 6 0 % o f h y d r o g e n is dissociated This is very different f r o m the high pressure regime, where a strong decrease o f the d~ssoclatmn ratio as a function of pressure has been reported 4 A c o m p a r i s o n of Figures 2 a n d 3 shows t h a t even at the density of a t o m m hydrogen, 1 x 10 TM m ~, the t e m p e r a t u r e o f the p r o b e remains 100°C over the t e m p e r a t u r e of the s u r r o u n d i n g gas Therefore, the p r o b e gives r a t h e r p u n c t u a l data on the atomic h y d r o g e n density even m this low pressure regime T a k i n g into account the relation p = n k T , we find t h a t the density o f atomic h y d r o g e n is determined to a r a t h e r high degree o f accuracy then d o w n to the atomic h y d r o g e n partial pressure o f 5 x 10 ~ Pa
460
References ] Y Sakamoto, Y Oshlbe K Yano and H Oyama I ,~uH Mater 93 and 94, 333 (1980) -'Y Matsuzakl, N Suzuki and T H~rayama Jap J 4pp[ Phls 25, 253 (1986) ~F Brecelj and M MozeU6, Va(uum 40, 177 (t990) 4M Mozetl6, M Drobme, F Brecelj and M Kveder, Pro( lOth IS'P( Bochum, 2 1 20 (1991) ~M Drobnl6, M MozetlC, F Brecelj and M K'ceder Pto( ~ ¥ ICPIG Plsa, p 313 (1991) 6M Mozetl6, M Drobm6 A Paulln, M Kveder and 1 Brecelj, Vuoto (to be pubhshed) 7H Wise and B J Wood, In Adtame~ m Atomt( and Moh,~ular Ph}~t(s (Edited by D R Bates and l Estermann), Vol 3, p 291 Academic Press New York (1967)