- Email: [email protected]
Application of an impregnated cathode coated with W-Sc2O3 to a high current density electron gun
1200
Applied Surface Science 33/34 (1988) 1200-1207 North-Holland, Amsterdam
APPLICATION OF AN IMPREGNATED CATHODE COATED WITH W-Sc203 TO A HIGH CURRENT DENSITY ELECTRON GUN S. Y A M A M O T O , S. S A S A K I , S. T A G U C H I , I. W A T A N A B E Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tol~vo 185, Japan
and N. K O G A N E Z A W A Mobara Works, Hitachi, Ltd., Mobara, Chiba 297, Japan
Received 23 August 1987; accepted for publication 1 October 1987
The electron emission properties of a previously proposed [1,2] W Sc203-coated impregnated cathode are evaluated in a high current density electron gun. It is found that the cathode can be operated at a temperature 100-150 °C lower than that of Os-coated impregnated cathodes usually operated at 1000°C for a beam average current density of - 5 A/cm 2. The superiority of this cathode increases when used in high resolution, high brightness electron guns where the electron extracting field is high at the cathode surface.
1. Introduction High resolution display tubes a n d pick-up tubes require a high c u r r e n t d e n s i t y ( > 1 0 A / c m 2) electron source. This source should be able to w i t h s t a n d ion b o m b a r d m e n t , while at the same time o p e r a t i n g at as low a t e m p e r a t u r e as possible. It has already been reported [1,2] that a W - S c 2 0 3 - c o a t e d impregn a t e d cathode exhibits a low work function, due to the presence of a m o n o a t o m i c order surface layer c o m p o s e d of Ba, Sc a n d O. This cathode can be operated at much lower temperatures t h a n c o n v e n t i o n a l i m p r e g n a t e d cathodes a n d is thought to be resistive to gas c o n t a m i n a t i o n [3]. The advantages of applying the W - S c 2 0 3 cathode to a high c u r r e n t d e n s i t y electron g u n are investigated a n d the p a r a m e t e r s for o p t i m u m o p e r a t i o n are determined.
2. Fundamental properties of the W-Sc203-coated impregnated cathode The structure a n d f u n d a m e n t a l properties of the W - S c z O 3 coated impregn a t e d cathode have been reported elsewhere [1,2], a n d therefore only a brief 0 1 6 9 - 4 3 3 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
S. Yamamoto et al. / Impregnated cathode coated with W - Sc2O3
1 OOO [
900
800
I
1201
°C
I
, o
- -
3 --coated
5
_
o/-
o..
O.
oa.eo
:"%.
S:iJib!. :
1 8
9
]O*/T
lO
( K -1 )
Fig. 1. Saturation emission current density Js of various impregnated cathodes.
description is given here. The cathode is composed of two parts, a basic impregnated cathode and a thin tungsten film 50-400 nm thick containing 2.5-6.5 wt% Sc203. The saturation electron emission current density Js of this cathode is plotted in fig. 1 as a function of inverse cathode temperature together with that of other impregnated cathodes such as a basic impregnated cathode and an Os-coated impregnated cathode. From this figure the work function can be estimated to be about 1.2 eV. This agrees with the value measured recently by Hasker [4]. At high current density operation, such as Js > 10 A / c m 2, the cathode temperature for W-Sc203-coated cathodes can be reduced by as much as 1 0 0 - 1 5 0 ° C compared to that for the Os-coated impregnated cathodes. The emission current characteristics at relatively low electric field are shown in fig. 2 compared with Os-coated impregnated cathode. Measurements are made in a diode configuration for cathodes 1.3 mm in diameter separated 3 mm from the anode. The solid line indicates the theoretical calculation when the Child-Langmuir's equation is applied to this particular cathode-anode configuration. In the region of low applied potential, the emission current of
S. Y a m a m o t o et al. / Impregnated cathode coated with W - S c 2 0 ;
1202
500
J
I
i
i
Temperature
C a t h o d e
850
°C b
/ W
100
S c .,O:~ c o a t e d
,
[3" C ALC ULAT
50
\
I ON ~/,n(
.,P
m t> T.
/
/
~o~O~ o ~
OJ
I0
/ /o
5
0 o3 .-I
E
S/
Os
coated
L~
1
O. 5
O.
1
0
L
I
L
50
I00
500
I I000
Potential Difference b e t w e e n C a t h o d e and Anode
55000
(V)
Fig. 2. E ~ s s i o n current dependency on c a t h o d e - a n o d e p o t e n t i ~ difference.
both cathodes is space-charge limited and is in good agreement with the 3 / 2 power law of the equation. As the potential is raised, the emission current level begins to fall below the calculated level and tends to shift towards the temperature-limited region, particularly for the Os-coated impregnated cathode. In general, deviation from the calculation starts at lower potentials, for lower cathode temperatures. In the case of the W-SczO3-coated impregnated cathode, the deviation is much less and the emission current is not really temperature limited, even at higher potentials. This latter point is thought to be due to the "patch effect", which is usually observed in cathodes containing Sc203 [1-3,5].
S. Yamamotoet a L / Impregnated cathode coated with W-Sc203
1203
3. Experimental procedures 3.1. Cathode fabrication
The details of the cathode fabrication have been published elsewhere [1,2] and only a brief description is given here. The cathode is composed of two parts. The basic impregnated cathode is made of a porous tungsten body impregnated with electron emissive materials. The basic cathode is coated with W-Sc203 by R F sputtering of a tungsten target on which Sc203 pellets are placed. The Sc203 content in the W-coated film can be controlled to within 2.5-6.5 wt% by varying the number of Sc203 pellets placed on the target. The Os coated impregnated cathodes are made by depositing Os evaporated films approximately 400 nm on basic impregnated cathodes. Both types of cathodes are 1.3 m m in diameter and are installed in an electron gun; an Os-coated cathode is used in the red gun and two W-Sc203coated cathodes are used in the Green and Blue guns. 3.2. Electron emission measurement
Measurements are made under triode operation in the electron gun configuration shown in fig. 3. High voltage pulses are applied on grid G 2. Grid G 1 is G6
04 N
H
"':"odost Fig. 3. Electron gun configuration for beam current measurement.
1204
S. Yamamoto et aL / Impregnated cathode coated with W - Sc:O¢
set at ground potential. The emission current measurement is made by varying either the applied potential or the cathode temperature while keeping the other factors constant. In actual operation, as the cathode is usually positively biased, the emission current (beam current) is actually much less than is measured by this method. 3.3. Space-charge current calculation
The beam current has been calculated for the geometry shown in fig. 3, where the aperture size of the first grid is 0.4 m m and the separation between the cathode and the first grid is 60/~m. The Poisson equation has been solved with the aid of a program devised by Weber [6,7]; the axial initial velocities are taken equal to zero and the transverse-velocity distribution as a function of cathode temperature is assumed to be Maxwellian.
4. Results 4.1. Beam current dependency on cathode temperature
The beam current through the first 0.4 m m diameter grid is measured at various cathode temperatures for three different potentials of the second grid. The results are shown in fig. 4, where the Os-coated cathode and the 100 il2nd. Grid PoLential 1400V
+Z
/"
10
roov
-,
/,. ..,
/d
/
..
. _2oo v
" I ~ . , , ~; i ,
0.1 800
900
Cathode
l OOO
Te.lperature
l l O0
1200
(~C. ~ )
Fig. 4. Maximum beam current dependency on cathode temperature.
S. Yamamoto et al. / Impregnated cathode coated with W - Sc20~
1205
W-Sc203-coated are shown by o and D, respectively. (The basic cathode is also shown by A as a reference.) The beam current is space-charge limited at high cathode temperatures and shifts towards the temperature-limited condition as the cathode temperature is lowered. The transition starts at a higher temperature when a higher potential is applied to the second grid. The difference in the beam current characteristics is obvious between the Os-coated and W-Sc203-coated cathodes; the transition temperature is much lower for the W-Sc203-coated cathode. Furthermore, the reduction in emission rate below the transition temperature is much less for the W-Sc203-coated cathode. 4.2. Beam current dependency on applied potential The beam current dependency on applied potential under triode operation is shown in fig. 5, where the beam current density is plotted as a function of the potential of the second grid for two cathode temperatures, 850 and 1 0 0 0 ° C b . The sofid line is the calculation under the space-charge limited condition. At low applied potentials, the beam current follows the calculation. As the potential is raised, the beam current begins to diverge from the calculation and falls behind, which starts at lower potentials for lower cathode temperatures especially for the Os-coated cathode. 100
lltl]
t
t
t
i
t Jtt]
I
I
~
I
I Itt
)1 ( 3 0 o " ¢
•
N .J co)
E
," *;;::;7 ,',7' .
0.1
,,
,,,,E 1 O0
~
,
L , ,,x,[ 1000
2nd. Grid P o t e n t i a l
I
,
1,.i
L
t ,,,I
I
d I IlL,
10000
(V)
Fig. 5. Beam current dependency on applied second grid potential.
1206
S. Yamarnoto et a L / Impregnated cathode coated with W - Sc.O¢
5. Discussion
It has already been pointed out that the W-Sc203-coated cathode is superior to the Os-coated cathode under a high electron extraction field. However, due to the patch field, this difference may be markedly diminished under practical operating conditions. A direct way of evaluating the cathodes is to apply the cathode to a real electron gun and to operate it in a triode configuration. The results in fig. 4 yield the minimum cathode temperature under which the emission current is space-charge limited. This temperature is usually called the minimum operation temperature of the cathode. The potential of the second grid is determined by the required beam current and the aperture size of the first grid, which in turn determines the b e a m spot size. In the configuration used in this work for the normal applied potential of about 700 V (average beam current density J = 5 A / c m 2 ) , the minimum operating temperature is determined to be 1000°Cb for the Os-coated cathodes. The peak beam current density in this case can be estimated from the calculated beam profile [6] and is calculated to be 12.5 A / c m 2. In the case of the W-Sc203-coated cathode, the minimum operating temperature can be reduced to as low as 850-900°Cb . To obtain electron beams of much higher resolution and intensity, a higher potential has to be applied. In this case, it can be seen from fig. 5 that the W-Sc203-coated cathode offers more advantages than the Os-coated cathode. The W-Sc203-coated cathode shows a lower rate of beam current reduction below the transition temperature than the Os coated cathode (fig. 4). One possible reason is due to the low work function of the cathode. The change in emission current with change in cathode temperature is much less for low work function cathodes. Another possible reason is that the W-Sc203-cathode is resistive to gas contamination which becomes important at low cathode temperatures. 6. Conclusion
It has been demonstrated in an electron gun configuration that W Sc203coated cathodes can be operated at 100-150 ° C lower than Os-coated cathodes for an average beam current of - 5 A / c m 2 (peak beam current of 12.5 A/cm2). It should be noted that the superiority of the W-Sc203-coated cathode increases as the electron extracting field increases and hence, this cathode is suitable for high resolution, high intensity electron gun applications. The role and origin of the patch field are not yet clear but it is certain that the electron emission characteristics can be further improved by reducing the patch field.
S. Yamamoto et al. / Impregnated cathode coated with W - Sc:O 3
1207
Acknowledgements T h e a u t h o r s w o u l d like to t h a n k D r s . K a t s u k i M i y a u c h i a n d T s u n e o Suganuma for stimulating discussions throughout the course of this work.
References [1] S. Yamamoto, S. Taguchi, I. Watanabe and S. Kawase, Japan J. Appl. Phys. 25 (1986) 971. [2] S. Yamamoto, S. Taguchi, I. Watanabe and S. Kawase, J. Vacuum Sci. Technol. A 5 (1987) 1299. [3] J. Hasker, J. van Esdonk and J.E. Crombeen, Appl. Surface Sci. 26 (1986) 173. [4] J. Hasker, private communication. [5] A. van Oostrom and L. Augustus, Appl. Surface Sci. 2 (1979) 173. [6] C. Weber, Philips Res. Rept. Suppl. No. 6 (1967). [7] J. Hasker and N.C.J. van Hijningen, Appl. Surface Sci. 24 (1985) 318.
Applied Surface Science 33/34 (1988) 1200-1207 North-Holland, Amsterdam
APPLICATION OF AN IMPREGNATED CATHODE COATED WITH W-Sc203 TO A HIGH CURRENT DENSITY ELECTRON GUN S. Y A M A M O T O , S. S A S A K I , S. T A G U C H I , I. W A T A N A B E Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tol~vo 185, Japan
and N. K O G A N E Z A W A Mobara Works, Hitachi, Ltd., Mobara, Chiba 297, Japan
Received 23 August 1987; accepted for publication 1 October 1987
The electron emission properties of a previously proposed [1,2] W Sc203-coated impregnated cathode are evaluated in a high current density electron gun. It is found that the cathode can be operated at a temperature 100-150 °C lower than that of Os-coated impregnated cathodes usually operated at 1000°C for a beam average current density of - 5 A/cm 2. The superiority of this cathode increases when used in high resolution, high brightness electron guns where the electron extracting field is high at the cathode surface.
1. Introduction High resolution display tubes a n d pick-up tubes require a high c u r r e n t d e n s i t y ( > 1 0 A / c m 2) electron source. This source should be able to w i t h s t a n d ion b o m b a r d m e n t , while at the same time o p e r a t i n g at as low a t e m p e r a t u r e as possible. It has already been reported [1,2] that a W - S c 2 0 3 - c o a t e d impregn a t e d cathode exhibits a low work function, due to the presence of a m o n o a t o m i c order surface layer c o m p o s e d of Ba, Sc a n d O. This cathode can be operated at much lower temperatures t h a n c o n v e n t i o n a l i m p r e g n a t e d cathodes a n d is thought to be resistive to gas c o n t a m i n a t i o n [3]. The advantages of applying the W - S c 2 0 3 cathode to a high c u r r e n t d e n s i t y electron g u n are investigated a n d the p a r a m e t e r s for o p t i m u m o p e r a t i o n are determined.
2. Fundamental properties of the W-Sc203-coated impregnated cathode The structure a n d f u n d a m e n t a l properties of the W - S c z O 3 coated impregn a t e d cathode have been reported elsewhere [1,2], a n d therefore only a brief 0 1 6 9 - 4 3 3 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
S. Yamamoto et al. / Impregnated cathode coated with W - Sc2O3
1 OOO [
900
800
I
1201
°C
I
, o
- -
3 --coated
5
_
o/-
o..
O.
oa.eo
:"%.
S:iJib!. :
1 8
9
]O*/T
lO
( K -1 )
Fig. 1. Saturation emission current density Js of various impregnated cathodes.
description is given here. The cathode is composed of two parts, a basic impregnated cathode and a thin tungsten film 50-400 nm thick containing 2.5-6.5 wt% Sc203. The saturation electron emission current density Js of this cathode is plotted in fig. 1 as a function of inverse cathode temperature together with that of other impregnated cathodes such as a basic impregnated cathode and an Os-coated impregnated cathode. From this figure the work function can be estimated to be about 1.2 eV. This agrees with the value measured recently by Hasker [4]. At high current density operation, such as Js > 10 A / c m 2, the cathode temperature for W-Sc203-coated cathodes can be reduced by as much as 1 0 0 - 1 5 0 ° C compared to that for the Os-coated impregnated cathodes. The emission current characteristics at relatively low electric field are shown in fig. 2 compared with Os-coated impregnated cathode. Measurements are made in a diode configuration for cathodes 1.3 mm in diameter separated 3 mm from the anode. The solid line indicates the theoretical calculation when the Child-Langmuir's equation is applied to this particular cathode-anode configuration. In the region of low applied potential, the emission current of
S. Y a m a m o t o et al. / Impregnated cathode coated with W - S c 2 0 ;
1202
500
J
I
i
i
Temperature
C a t h o d e
850
°C b
/ W
100
S c .,O:~ c o a t e d
,
[3" C ALC ULAT
50
\
I ON ~/,n(
.,P
m t> T.
/
/
~o~O~ o ~
OJ
I0
/ /o
5
0 o3 .-I
E
S/
Os
coated
L~
1
O. 5
O.
1
0
L
I
L
50
I00
500
I I000
Potential Difference b e t w e e n C a t h o d e and Anode
55000
(V)
Fig. 2. E ~ s s i o n current dependency on c a t h o d e - a n o d e p o t e n t i ~ difference.
both cathodes is space-charge limited and is in good agreement with the 3 / 2 power law of the equation. As the potential is raised, the emission current level begins to fall below the calculated level and tends to shift towards the temperature-limited region, particularly for the Os-coated impregnated cathode. In general, deviation from the calculation starts at lower potentials, for lower cathode temperatures. In the case of the W-SczO3-coated impregnated cathode, the deviation is much less and the emission current is not really temperature limited, even at higher potentials. This latter point is thought to be due to the "patch effect", which is usually observed in cathodes containing Sc203 [1-3,5].
S. Yamamotoet a L / Impregnated cathode coated with W-Sc203
1203
3. Experimental procedures 3.1. Cathode fabrication
The details of the cathode fabrication have been published elsewhere [1,2] and only a brief description is given here. The cathode is composed of two parts. The basic impregnated cathode is made of a porous tungsten body impregnated with electron emissive materials. The basic cathode is coated with W-Sc203 by R F sputtering of a tungsten target on which Sc203 pellets are placed. The Sc203 content in the W-coated film can be controlled to within 2.5-6.5 wt% by varying the number of Sc203 pellets placed on the target. The Os coated impregnated cathodes are made by depositing Os evaporated films approximately 400 nm on basic impregnated cathodes. Both types of cathodes are 1.3 m m in diameter and are installed in an electron gun; an Os-coated cathode is used in the red gun and two W-Sc203coated cathodes are used in the Green and Blue guns. 3.2. Electron emission measurement
Measurements are made under triode operation in the electron gun configuration shown in fig. 3. High voltage pulses are applied on grid G 2. Grid G 1 is G6
04 N
H
"':"odost Fig. 3. Electron gun configuration for beam current measurement.
1204
S. Yamamoto et aL / Impregnated cathode coated with W - Sc:O¢
set at ground potential. The emission current measurement is made by varying either the applied potential or the cathode temperature while keeping the other factors constant. In actual operation, as the cathode is usually positively biased, the emission current (beam current) is actually much less than is measured by this method. 3.3. Space-charge current calculation
The beam current has been calculated for the geometry shown in fig. 3, where the aperture size of the first grid is 0.4 m m and the separation between the cathode and the first grid is 60/~m. The Poisson equation has been solved with the aid of a program devised by Weber [6,7]; the axial initial velocities are taken equal to zero and the transverse-velocity distribution as a function of cathode temperature is assumed to be Maxwellian.
4. Results 4.1. Beam current dependency on cathode temperature
The beam current through the first 0.4 m m diameter grid is measured at various cathode temperatures for three different potentials of the second grid. The results are shown in fig. 4, where the Os-coated cathode and the 100 il2nd. Grid PoLential 1400V
+Z
/"
10
roov
-,
/,. ..,
/d
/
..
. _2oo v
" I ~ . , , ~; i ,
0.1 800
900
Cathode
l OOO
Te.lperature
l l O0
1200
(~C. ~ )
Fig. 4. Maximum beam current dependency on cathode temperature.
S. Yamamoto et al. / Impregnated cathode coated with W - Sc20~
1205
W-Sc203-coated are shown by o and D, respectively. (The basic cathode is also shown by A as a reference.) The beam current is space-charge limited at high cathode temperatures and shifts towards the temperature-limited condition as the cathode temperature is lowered. The transition starts at a higher temperature when a higher potential is applied to the second grid. The difference in the beam current characteristics is obvious between the Os-coated and W-Sc203-coated cathodes; the transition temperature is much lower for the W-Sc203-coated cathode. Furthermore, the reduction in emission rate below the transition temperature is much less for the W-Sc203-coated cathode. 4.2. Beam current dependency on applied potential The beam current dependency on applied potential under triode operation is shown in fig. 5, where the beam current density is plotted as a function of the potential of the second grid for two cathode temperatures, 850 and 1 0 0 0 ° C b . The sofid line is the calculation under the space-charge limited condition. At low applied potentials, the beam current follows the calculation. As the potential is raised, the beam current begins to diverge from the calculation and falls behind, which starts at lower potentials for lower cathode temperatures especially for the Os-coated cathode. 100
lltl]
t
t
t
i
t Jtt]
I
I
~
I
I Itt
)1 ( 3 0 o " ¢
•
N .J co)
E
," *;;::;7 ,',7' .
0.1
,,
,,,,E 1 O0
~
,
L , ,,x,[ 1000
2nd. Grid P o t e n t i a l
I
,
1,.i
L
t ,,,I
I
d I IlL,
10000
(V)
Fig. 5. Beam current dependency on applied second grid potential.
1206
S. Yamarnoto et a L / Impregnated cathode coated with W - Sc.O¢
5. Discussion
It has already been pointed out that the W-Sc203-coated cathode is superior to the Os-coated cathode under a high electron extraction field. However, due to the patch field, this difference may be markedly diminished under practical operating conditions. A direct way of evaluating the cathodes is to apply the cathode to a real electron gun and to operate it in a triode configuration. The results in fig. 4 yield the minimum cathode temperature under which the emission current is space-charge limited. This temperature is usually called the minimum operation temperature of the cathode. The potential of the second grid is determined by the required beam current and the aperture size of the first grid, which in turn determines the b e a m spot size. In the configuration used in this work for the normal applied potential of about 700 V (average beam current density J = 5 A / c m 2 ) , the minimum operating temperature is determined to be 1000°Cb for the Os-coated cathodes. The peak beam current density in this case can be estimated from the calculated beam profile [6] and is calculated to be 12.5 A / c m 2. In the case of the W-Sc203-coated cathode, the minimum operating temperature can be reduced to as low as 850-900°Cb . To obtain electron beams of much higher resolution and intensity, a higher potential has to be applied. In this case, it can be seen from fig. 5 that the W-Sc203-coated cathode offers more advantages than the Os-coated cathode. The W-Sc203-coated cathode shows a lower rate of beam current reduction below the transition temperature than the Os coated cathode (fig. 4). One possible reason is due to the low work function of the cathode. The change in emission current with change in cathode temperature is much less for low work function cathodes. Another possible reason is that the W-Sc203-cathode is resistive to gas contamination which becomes important at low cathode temperatures. 6. Conclusion
It has been demonstrated in an electron gun configuration that W Sc203coated cathodes can be operated at 100-150 ° C lower than Os-coated cathodes for an average beam current of - 5 A / c m 2 (peak beam current of 12.5 A/cm2). It should be noted that the superiority of the W-Sc203-coated cathode increases as the electron extracting field increases and hence, this cathode is suitable for high resolution, high intensity electron gun applications. The role and origin of the patch field are not yet clear but it is certain that the electron emission characteristics can be further improved by reducing the patch field.
S. Yamamoto et al. / Impregnated cathode coated with W - Sc:O 3
1207
Acknowledgements T h e a u t h o r s w o u l d like to t h a n k D r s . K a t s u k i M i y a u c h i a n d T s u n e o Suganuma for stimulating discussions throughout the course of this work.
References [1] S. Yamamoto, S. Taguchi, I. Watanabe and S. Kawase, Japan J. Appl. Phys. 25 (1986) 971. [2] S. Yamamoto, S. Taguchi, I. Watanabe and S. Kawase, J. Vacuum Sci. Technol. A 5 (1987) 1299. [3] J. Hasker, J. van Esdonk and J.E. Crombeen, Appl. Surface Sci. 26 (1986) 173. [4] J. Hasker, private communication. [5] A. van Oostrom and L. Augustus, Appl. Surface Sci. 2 (1979) 173. [6] C. Weber, Philips Res. Rept. Suppl. No. 6 (1967). [7] J. Hasker and N.C.J. van Hijningen, Appl. Surface Sci. 24 (1985) 318.