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Experimental investigation of current fluctuation at a short emitter to anode distance
Ultramicroscopy 73 (1998) 119—124
Experimental investigation of current fluctuation at a short emitter to anode distance Kazuto Ashihara*, Shinya Tsuchida, Hideaki Nakane, Hiroshi Adachi Department of Electrical and Electronic Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050, Japan Received 7 July 1997; received in revised form 10 November 1997
Abstract It was experimentally observed that the number of spike-like noise was increased as the anode approached the emitter, under bad vacuum conditions of 10~6 Torr. The duration of the stable emission also decreased upon approaching the anode to the emitter. It is numerically solved that the number of generated gas molecules and ions by electron-stimulated desorption on the anode surface come to the emitter surface. Under UHV (ultra-high vacuum), the number was not much affected by the distance. On the other hand, under bad vacuum conditions, the number was increased largely as the anode approached the emitter. It suggests that the gas molecules and/or ions generated at the anode surface hitting the emitting area on the emitter is affected by the emitter to anode distance. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Field emitter arrays (FEAs), which has been very actively investigated as a new device recently, has extremely short emitter to gate distances of order of lm. However, there are a few reports of the fluctation of field emission in such a short spacing of electrode. We have investigated the fluctuation using a model electrode structure of which emitter to anode distance is variable. It is believed that the fluctuation of field emission current is caused by migration of absorbed gas molecules over the emitter surface, and ion bombardment to the emitter surface [1—4]. So the fluc-
* Corresponding author. Tel.: #81 143 44 4181 ext. 2540; fax: #81 143 47 3300; e-mail: [email protected].
tuation can be increased as the impinging rates of the gas molecules and ions are increased. Even if the anode approaches the emitter, it is expected that the number of the residual gas molecules and ions in the space surrounding the emitter and that hit the emitting area is not affected by the emitter to anode distance, since the density of the residual gas molecules in the vacuum does not change. However, it is possible that the number of gas molecules and ions generated at the anode and hitting the emitting area is affected by the change in the emitter to anode distance. We assumed that the generated gas molecules and ions by electron-stimulated desorption on the anode surface come to the emitter surface. And a balance equation between the incident gas molecules and the desorbed gas molecules by electron stimulated desorption on the anode surface is
0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 4 4 - 7
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K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
numerically solved. The calculated results shows that the incident gas molecules are desorbed as soon as they hit the surface in ultra-high vacuum condition. That is, the anode is just a reflector with reflectance 1. This implies that no effect due to the emitter to anode distance is expected in this condition. We had also experimentally investigated the current fluctuation as a function of the emitter to anode distance in ultra-high vacuum condition of 10~10 Torr, the amplitude of the fluctuation was hardly dependent on the distance [5]. The calculated results also suggest that the anode may be a gas supplier in bad vacuum condition. When the anode approaches the emitter, both the absorbed gas molecules on the emitter surface and the bombarding ion to the emitter surface may be increased. That means that the emission current may be unstable as the anode approaches the emitter. To confirm this situation in the bad vacuum condition, experiment was performed in 10~6 Torr. The results showed that the duration of the stable region decreased with the anode approaching the emitter. In addition, the number of spike-like noise increased as the anode approaches the emitter in stable emission region. We suggest that the number of the gas molecules and/or ions hitting the emitting area and the emitter surface can increase as the distance is decreased.
2. Estimation The absorbed molecules on the anode surface will be desorbed by electron stimulation, when the field-emitted electrons hit the anode surface. Some of the desorbed molecules hit the emission area and the emitter surface. It is sure that some of the desorbed molecules are ionized. When the emitter to anode distance is very short, the desorbed molecules from anode can hit easily the emitting area. On the other hand, when the distance is long, it is difficult for the desorbed molecules to hit the emitting area. Therefore, hitting probability is affected by the distance. The hitting probability can be approximately expressed as the solid angle X of the desorbed gas molecules which see the electron emitting area on the emitter surface
Fig. 1. Schematic image of solid angle X.
(Fig. 1):
P
nr2 X" dX" % , d2
(1)
where dX is the small angle on the anode where the molecules are desorbed, r is the effective radius of % the electron-emitting area and d is the emitter to anode distance and is a function of the emitter to anode distance. The number of the gas molecules which hit the emitting area N can be given as a product of the G solid angle X and the total number of desorbed gas molecules from the anode N . N has two cases. ESD ESD When the electron stimulation ability is much higher than the adsorption velocity on the anode surface, N is approximately expressed as the ESD number of absorbed gas molecules at anode N , A N "N "R A1nvN , (2) ESD A ! 4 where R is the adsorption rate; here it was as! sumed to be 1 for simplicity, A is the electron beam spot area estimated by a numerical calculation of electron trajectory, n is the density of residual gas molecules in the vacuum and v is the average speed of the molecules. In this case, the gas molecules can desorb as soon as they absorb on the anode surface. Therefore, it can be regarded that the number of desorbed gas molecules are equal to the adsorbed ones, i.e. the number of gas molecules hitting the emitting area is not affected by the emitter to anode distance. On the other hand, when the electron stimulation ability is much lower than the adsorption
K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
121
Hence, the total number of the desorbed gas molecules on the anode surface N is smaller than ESD N and N . For example, in the case of .!9 A 10~10 Torr, N is N . It means that the number ESD A of gas molecules hitting the emitter surface is not affected by the distance. Such a situation appears also under the region of the distance of 10~2 cm in the case of 10~6 Torr in Fig. 2a. In the other region of the distance, the number of gas molecules hitting the emitting area N decreases rapidly with the G distance as shown in Fig. 2b. It suggests that the field emission current might be unstable, when the distance is very short under a bad vacuum condition like 10~6 Torr.
3. Experimental procedure
Fig. 2. Calculated N
ESD
and N as a function of the distance. G
velocity on the anode surface, N is approximESD ately expressed as N , the number of adsorbed .!9 gas molecules should be restricted by the emission current: N "N "R n A, (3) ESD .!9 % % where R is the desorption rate of the electron % stimulation: here it was assumed to be 10~4 which is determined according to the experimental data given by Nishijima [6]. n is the number of hitting % electrons. Here the emitted current is assumed 64.6 lA in (Fig. 2a). N is determined by the .!9 number of electrons and is almost unaffected by the distance.
Our electrode structure, whose emitter to anode distance is variable, is schematically shown in Fig. 3. The distance is adjustable precisely by using three sets of micrometer screws. The emitter was made of tungsten, which was electrochemically etched to form a sharp needle. The apex has an equivalent radius of 0.3—0.4 lm. The anode was a 0.05 mm thick tantalum foil. Prior to the measurement of the current fluctuation, both the emitter and anode were cleaned by flash heating upto 2000°C. Then, the emitter to anode distance was set and emission was started by applying voltage. The emission current variation was recorded using a strip-chart recorded. The range of the emitter—anode distance was varied from about 50 lm to 1 mm. The vacuum condition in the measurement was 1.2]10~6 Torr.
4. Results and discussion Fig. 4 shows an example of the trace of emission current. The emitter to anode distance is 100 lm and the emission current is about 200 nA. The emission current was relatively stable first, the fluctuation of the stable region was mainly flicker noise and a few minutes periodic drift. Spike-like noise also occurred when the current was large. These trends were commonly observed at all the emitter to anode distances. After a while
122
K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
Fig. 3. Schematic diagram of the experimental equipment.
Fig. 4. Trace of emission current. Emitter—anode distance is 100 lm, and the vacuum is 1.2]10~6 Torr.
many strong spike-like noise appeared and the current increased gradually. Within one and a half hour, the current increase region did not appear in the case when the current was less than 100 nA. At 100 nA, the current increase region appeared only when the distance was 50 lm. At 200 nA, the increase region was observed for all distances, and stable emission decreased upon approaching the anode. Fig. 5 shows the duration of the stable emission as a function of the distance. The duration was determined at the time
when the strong spike-like noise appeared; here the strong spike-like noise was defined to mean that the amplitude of the spike was over 20% compared to the mean current. The duration decreased upon approaching the anode to the emitter. We suggest that the decrease of the duration of the stable region was caused by increase of the ion bombardment to the emitter surface. After the strong spike-like noise appeared, it seems that the shape of the emitter apex and/or emitting area is deformed according to Fowler—Nordheim plot.
K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
123
ment, so the spike-like noise can be created by the ion bombardment. The number of spike-like noise was plotted as a function of the distance (Fig. 6). It has a tendency to increase upon decreasing the distance. It means that the ion bombardment can increase. Many ion bombardment can deform the emitter shape in the early stage, it corresponds to the decrease of the duration of the stable region. That is, the ion bombardment to the emitter surface can increase when the emitter to anode distance is very short, under bad vacuum condition of 10~6 Torr.
5. Conclusions Fig. 5. Duration of the stable region as a function of the emitter to anode distance, current is 200 nA.
Fig. 6. The number of spike-like noise per 10 min during stable region.
When ion hits an emitter surface, adsorbed gas molecules or atoms of the emitter can be moved. Such phenomena can cause the change of the local work function on an emitter surface at that mo-
The number of gas molecules, which desorbed by electron stimulation at the anode surface hitting the emitting area on the emitter, was numerically calculated as a function of the emitter to anode distance. It suggested that the number of impinging molecules at the emitting area was hardly affected by the distance under ultra-high vacuum of 10~10 Torr. In addition, it was also suggested that it increases as the distance decreases under bad vacuum condition of 10~6 Torr. The stability of the field emission current was experimentally investigated as a parameter of the emitter to anode distance in 10~6 Torr. The duration of the stable emission was decreased upon the anode approaching the emitter. The phenomenon was discussed in connection with ion bombardment. The number of spike-like noise was increased as the distance was decreased, it can correspond to the increase of ion bombardment. The experimental result also suggests that the gas molecules and/or ions generated at the anode surface hitting the emitting area on the emitter is affected by the emitter to anode distance in bad vacuum condition of 10~6 Torr.
References [1] D.J. Swann, K.C.A. Smith, Proc. 6th Annual Scanning Electron Microscope Symp., Chicago, USA, April 1973, pp. 41—48.
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K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
[2] H. Todokoro, N. Saitou, S. Yamamoto, J. Appl. Phys. 10 (1982) 1513. [3] Ch. Kleint, Surf. Sci. 25 (1971) 394. [4] Robert Gomer, Surf. Sci. 38 (1973) 373.
[5] K. Ashihara, Y. Saito, S. Tsuchida, H. Nakane, H. Adachi, Trans. IEICE C-II J80-C-II (12) (1997) 434 (in Japanese). [6] M. Nishijima, Oyo Buturi 40 (2) (1971) 114 (in Japanese).
Experimental investigation of current fluctuation at a short emitter to anode distance Kazuto Ashihara*, Shinya Tsuchida, Hideaki Nakane, Hiroshi Adachi Department of Electrical and Electronic Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050, Japan Received 7 July 1997; received in revised form 10 November 1997
Abstract It was experimentally observed that the number of spike-like noise was increased as the anode approached the emitter, under bad vacuum conditions of 10~6 Torr. The duration of the stable emission also decreased upon approaching the anode to the emitter. It is numerically solved that the number of generated gas molecules and ions by electron-stimulated desorption on the anode surface come to the emitter surface. Under UHV (ultra-high vacuum), the number was not much affected by the distance. On the other hand, under bad vacuum conditions, the number was increased largely as the anode approached the emitter. It suggests that the gas molecules and/or ions generated at the anode surface hitting the emitting area on the emitter is affected by the emitter to anode distance. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Field emitter arrays (FEAs), which has been very actively investigated as a new device recently, has extremely short emitter to gate distances of order of lm. However, there are a few reports of the fluctation of field emission in such a short spacing of electrode. We have investigated the fluctuation using a model electrode structure of which emitter to anode distance is variable. It is believed that the fluctuation of field emission current is caused by migration of absorbed gas molecules over the emitter surface, and ion bombardment to the emitter surface [1—4]. So the fluc-
* Corresponding author. Tel.: #81 143 44 4181 ext. 2540; fax: #81 143 47 3300; e-mail: [email protected].
tuation can be increased as the impinging rates of the gas molecules and ions are increased. Even if the anode approaches the emitter, it is expected that the number of the residual gas molecules and ions in the space surrounding the emitter and that hit the emitting area is not affected by the emitter to anode distance, since the density of the residual gas molecules in the vacuum does not change. However, it is possible that the number of gas molecules and ions generated at the anode and hitting the emitting area is affected by the change in the emitter to anode distance. We assumed that the generated gas molecules and ions by electron-stimulated desorption on the anode surface come to the emitter surface. And a balance equation between the incident gas molecules and the desorbed gas molecules by electron stimulated desorption on the anode surface is
0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 4 4 - 7
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K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
numerically solved. The calculated results shows that the incident gas molecules are desorbed as soon as they hit the surface in ultra-high vacuum condition. That is, the anode is just a reflector with reflectance 1. This implies that no effect due to the emitter to anode distance is expected in this condition. We had also experimentally investigated the current fluctuation as a function of the emitter to anode distance in ultra-high vacuum condition of 10~10 Torr, the amplitude of the fluctuation was hardly dependent on the distance [5]. The calculated results also suggest that the anode may be a gas supplier in bad vacuum condition. When the anode approaches the emitter, both the absorbed gas molecules on the emitter surface and the bombarding ion to the emitter surface may be increased. That means that the emission current may be unstable as the anode approaches the emitter. To confirm this situation in the bad vacuum condition, experiment was performed in 10~6 Torr. The results showed that the duration of the stable region decreased with the anode approaching the emitter. In addition, the number of spike-like noise increased as the anode approaches the emitter in stable emission region. We suggest that the number of the gas molecules and/or ions hitting the emitting area and the emitter surface can increase as the distance is decreased.
2. Estimation The absorbed molecules on the anode surface will be desorbed by electron stimulation, when the field-emitted electrons hit the anode surface. Some of the desorbed molecules hit the emission area and the emitter surface. It is sure that some of the desorbed molecules are ionized. When the emitter to anode distance is very short, the desorbed molecules from anode can hit easily the emitting area. On the other hand, when the distance is long, it is difficult for the desorbed molecules to hit the emitting area. Therefore, hitting probability is affected by the distance. The hitting probability can be approximately expressed as the solid angle X of the desorbed gas molecules which see the electron emitting area on the emitter surface
Fig. 1. Schematic image of solid angle X.
(Fig. 1):
P
nr2 X" dX" % , d2
(1)
where dX is the small angle on the anode where the molecules are desorbed, r is the effective radius of % the electron-emitting area and d is the emitter to anode distance and is a function of the emitter to anode distance. The number of the gas molecules which hit the emitting area N can be given as a product of the G solid angle X and the total number of desorbed gas molecules from the anode N . N has two cases. ESD ESD When the electron stimulation ability is much higher than the adsorption velocity on the anode surface, N is approximately expressed as the ESD number of absorbed gas molecules at anode N , A N "N "R A1nvN , (2) ESD A ! 4 where R is the adsorption rate; here it was as! sumed to be 1 for simplicity, A is the electron beam spot area estimated by a numerical calculation of electron trajectory, n is the density of residual gas molecules in the vacuum and v is the average speed of the molecules. In this case, the gas molecules can desorb as soon as they absorb on the anode surface. Therefore, it can be regarded that the number of desorbed gas molecules are equal to the adsorbed ones, i.e. the number of gas molecules hitting the emitting area is not affected by the emitter to anode distance. On the other hand, when the electron stimulation ability is much lower than the adsorption
K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
121
Hence, the total number of the desorbed gas molecules on the anode surface N is smaller than ESD N and N . For example, in the case of .!9 A 10~10 Torr, N is N . It means that the number ESD A of gas molecules hitting the emitter surface is not affected by the distance. Such a situation appears also under the region of the distance of 10~2 cm in the case of 10~6 Torr in Fig. 2a. In the other region of the distance, the number of gas molecules hitting the emitting area N decreases rapidly with the G distance as shown in Fig. 2b. It suggests that the field emission current might be unstable, when the distance is very short under a bad vacuum condition like 10~6 Torr.
3. Experimental procedure
Fig. 2. Calculated N
ESD
and N as a function of the distance. G
velocity on the anode surface, N is approximESD ately expressed as N , the number of adsorbed .!9 gas molecules should be restricted by the emission current: N "N "R n A, (3) ESD .!9 % % where R is the desorption rate of the electron % stimulation: here it was assumed to be 10~4 which is determined according to the experimental data given by Nishijima [6]. n is the number of hitting % electrons. Here the emitted current is assumed 64.6 lA in (Fig. 2a). N is determined by the .!9 number of electrons and is almost unaffected by the distance.
Our electrode structure, whose emitter to anode distance is variable, is schematically shown in Fig. 3. The distance is adjustable precisely by using three sets of micrometer screws. The emitter was made of tungsten, which was electrochemically etched to form a sharp needle. The apex has an equivalent radius of 0.3—0.4 lm. The anode was a 0.05 mm thick tantalum foil. Prior to the measurement of the current fluctuation, both the emitter and anode were cleaned by flash heating upto 2000°C. Then, the emitter to anode distance was set and emission was started by applying voltage. The emission current variation was recorded using a strip-chart recorded. The range of the emitter—anode distance was varied from about 50 lm to 1 mm. The vacuum condition in the measurement was 1.2]10~6 Torr.
4. Results and discussion Fig. 4 shows an example of the trace of emission current. The emitter to anode distance is 100 lm and the emission current is about 200 nA. The emission current was relatively stable first, the fluctuation of the stable region was mainly flicker noise and a few minutes periodic drift. Spike-like noise also occurred when the current was large. These trends were commonly observed at all the emitter to anode distances. After a while
122
K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
Fig. 3. Schematic diagram of the experimental equipment.
Fig. 4. Trace of emission current. Emitter—anode distance is 100 lm, and the vacuum is 1.2]10~6 Torr.
many strong spike-like noise appeared and the current increased gradually. Within one and a half hour, the current increase region did not appear in the case when the current was less than 100 nA. At 100 nA, the current increase region appeared only when the distance was 50 lm. At 200 nA, the increase region was observed for all distances, and stable emission decreased upon approaching the anode. Fig. 5 shows the duration of the stable emission as a function of the distance. The duration was determined at the time
when the strong spike-like noise appeared; here the strong spike-like noise was defined to mean that the amplitude of the spike was over 20% compared to the mean current. The duration decreased upon approaching the anode to the emitter. We suggest that the decrease of the duration of the stable region was caused by increase of the ion bombardment to the emitter surface. After the strong spike-like noise appeared, it seems that the shape of the emitter apex and/or emitting area is deformed according to Fowler—Nordheim plot.
K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
123
ment, so the spike-like noise can be created by the ion bombardment. The number of spike-like noise was plotted as a function of the distance (Fig. 6). It has a tendency to increase upon decreasing the distance. It means that the ion bombardment can increase. Many ion bombardment can deform the emitter shape in the early stage, it corresponds to the decrease of the duration of the stable region. That is, the ion bombardment to the emitter surface can increase when the emitter to anode distance is very short, under bad vacuum condition of 10~6 Torr.
5. Conclusions Fig. 5. Duration of the stable region as a function of the emitter to anode distance, current is 200 nA.
Fig. 6. The number of spike-like noise per 10 min during stable region.
When ion hits an emitter surface, adsorbed gas molecules or atoms of the emitter can be moved. Such phenomena can cause the change of the local work function on an emitter surface at that mo-
The number of gas molecules, which desorbed by electron stimulation at the anode surface hitting the emitting area on the emitter, was numerically calculated as a function of the emitter to anode distance. It suggested that the number of impinging molecules at the emitting area was hardly affected by the distance under ultra-high vacuum of 10~10 Torr. In addition, it was also suggested that it increases as the distance decreases under bad vacuum condition of 10~6 Torr. The stability of the field emission current was experimentally investigated as a parameter of the emitter to anode distance in 10~6 Torr. The duration of the stable emission was decreased upon the anode approaching the emitter. The phenomenon was discussed in connection with ion bombardment. The number of spike-like noise was increased as the distance was decreased, it can correspond to the increase of ion bombardment. The experimental result also suggests that the gas molecules and/or ions generated at the anode surface hitting the emitting area on the emitter is affected by the emitter to anode distance in bad vacuum condition of 10~6 Torr.
References [1] D.J. Swann, K.C.A. Smith, Proc. 6th Annual Scanning Electron Microscope Symp., Chicago, USA, April 1973, pp. 41—48.
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K. Ashihara et al. / Ultramicroscopy 73 (1998) 119—124
[2] H. Todokoro, N. Saitou, S. Yamamoto, J. Appl. Phys. 10 (1982) 1513. [3] Ch. Kleint, Surf. Sci. 25 (1971) 394. [4] Robert Gomer, Surf. Sci. 38 (1973) 373.
[5] K. Ashihara, Y. Saito, S. Tsuchida, H. Nakane, H. Adachi, Trans. IEICE C-II J80-C-II (12) (1997) 434 (in Japanese). [6] M. Nishijima, Oyo Buturi 40 (2) (1971) 114 (in Japanese).