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A cesium sputter negative ion source using a multilaminate wire mesh ionizer
Pergamon 0042-207x(95)00179-4
Vacuum/volume 47lnumber l/pages 23 to 2511996 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/96 $9.50+.00
A cesium sputter negative ion source using a multilaminate wire mesh ionizer Jiao Gui-Yue,” Wang Wen-Xun,” Ji Cheng-Zhou, b “Analytical and Testing Center, Beijing Normal University, Beijing 100875, P R China; blnstitute of Low Energy Nuclear Physics, Beijing Normal University, Beijing 100875, P R China received 24 March 7995
A new ventilated surface ionizer has been developed for a 860A cesium sputter negative ion source operated at BNU. The new ionizer consists of a multi-laminate nickel wire mesh and a spiral heater, and is spherical in shape. Low pressure cesium vapour diffuses through the wire mesh to the extraction side of the ionizer, meanwhile the neutral cesium can be ionized. The ionizer is capable of producing a positive ion current up to milliampere range. The negative ion current is 50 PA for P- and more than 150 PA for O- and Si-.
1. Introduction negative ion sources usually employ high work function metals to generate a positive cesium ion beam used to sputter a negatively biased target material. Cesium vapour comes from a cesium reservoir positioned outside of the source and can be efficiently ionized when contacting a hot, high work function surface. In the ionization process, an adsorbed atom leaves a valence electron attached to the metal surface upon evaporation from the surface. Since the work function 4 of most clean metal surfaces lie between 4.0 and 5.0 eV and the first ionization potential I’, of a cesium atom is 3.89 eV, the surface ionization is expected to be an efficient way to form a positive ion beam. Surface ionization is not identical to plane ionization. For example, if the temperature of a tip or a tiny wire is high enough, the energy supplied to the adsorbed atoms will enable them to be ionized and desorbed from the tip or wire surface. That is, if AH, - eV, + qS > AH, holds, the surface ionization process occurs.’ In this paper a multi-laminate wire mesh ionizer will be described and the performance of a 860A sputter source equipped with the new ionizer discussed. Sputter-type
2. A new ionizer Positive ion yield is governed by the threshold temperature and cesium flux on the ionizer surface.’ To increase the positive ion yield further, an advisable way is to raise the number of striking cesium atoms per unit area per unit time, in other words, to enhance the cesium flux striking the ionizer surface. The correlation between the cesium flux and cesium oven temperature is given by
d
(1) where T is the absolute temperature of ionization surface and PC represents the probability for capture at the ionizer surface, defined as the ratio of the number of particles captured by ionizer to the number of particles per unit volume around the ionization surface. Because both the sputter probe and ionizer are situated in the same ionization chamber, the cesium oven temperature not only controls the rate at which cesium atoms strike the ionizer surface, but determines a cesium flow rate which is sufficient for steady-state deposition of about one monolayer on the sputter probe surface. In order to optimize negative ion yield, the cesium oven is customarily operated at a fixed temperature which results in a saturation of positive ion yield for the previous source. With the above consideration, a gauze wire ionizer has been designed by Jiao. The ionizer consists of several layers of nickel or platinum gauze wires and a heater. The gauzes are spherically shaped with a diameter of 36 mm and are surrounded by the heater. The cesium vapour is fed from the cesium oven through a sealed tube. Cesium atoms are ionized when they diffuse through the gauze layers. In order to distinguish the new ionizer from the previous one, the surface ionization process in which both neutral cesium atoms to be ionized and cesium ions are located on the same side of the emission surface, is called rq7ected surface ionization;’ and the surface ionization process in which neutral cesium atoms pass through the emission surface and are ionized when leaving the surface is called transmitted surface ionization. As shown in Figure 1, the transmission construction 23
Jiao Gui-Yue ef al: A cesium sputter negative
ion source
Table 1. Performances of reflection and transmission surface ionizers in
SNICS Ionizers
Target ion current
Positive ion current/mA
Negative ion Erosion current/uA center/tjmm
Refl. Trans.
2.45 3.50
0.89 1.27
35.00 55.00
0.75 1.00
r o Transmission Figure 1. Schematic diagram of a transmission surface ionization source in SNICS.
,-
l
Reflection
I-
has not got a stagnant region for cesium vapour. The multilayered gauze becomes a passageway for cesium vapour, the majority of cesium atoms do not have to land on the ionization surface, they are fed directly to the ionization layers, At the same pressure, the cesium flux for the transmission ionization surface is greater than that for the reflection ionizer. As a result, positive ion yield for the transmission ionizer goes up. Assuming that the motion probability for cesium atoms is the same in all directions, the emission surface of the reflection ionizer will capture one sixth of the total particles per unit volume though its area is as large as the transmission ionizer, while the capture probability will be four sixths for the emission surface of the transmission ionizer.
3. Measurements and discussions During the sputtering process secondary electrons can also be ejected from the target material, and the coefficient for secondary electron emission lies between 0.3 and 3 electrons/ion.“ Thus, the measured target current IT comprises three parts: the positive ion current I+, the negative ion current I- and the secondary electron current Z,. IT = 1’ + f-
+ z, + . . = TZ1
(2)
O
0
I 2
4
6
I
I
I
8
10
12
I(A)
Figure 2. Relationship between heater current and negative-ion currents for reflection
and transmission
ionizers.
of time for a double layer nickel gauze to reach complete desorption of cesium atoms than a Nickel sheet, this leads to a difference in ionization probability. Above 9.OA, the cesium ions are desorbed rapidly and the work function is closer to the intrinsic value, the probability for ionization will be steadied, and the cesium atom density becomes imporatant for governing the positive ion yield. In addition to the large cesium flux, cesium atoms can get enough energy to be ionized because of the longer residence time resulting from multiple collisions in the metal gauze. Consequently, ionization probability for the transmission ionizer will increase when the temperature remains stable. Figure 3 shows the variation of negative ion current as a
Secondary electron current, in general, does not add to the negative current due to its deflection by the radial magnet placed outside the source body. If we take an average of the end point values for the secondary emission coefficients cited above, a value of 1.65 electrons/ion is obtained. Using this value for Z,, eqn 2 can be approximated as follows:5 I+ r0.37(1,
--I-)
(3)
Under the conditions that target voltages are 6.0-X.0 kV, ionizer current is 12.5A (z 1100°C) and cesium oven temperature is at x2OO”C, the performances of the sputter-ion source equipped respectively with reflection and transmission ionizer are compared in Table 1. The ionization material is nickel (sheet for reflection ionizer and gauze for transmission ionizer). The work function of nickel is 4.6leV. The target material to be sputtered is multicrtsicl-InP. Figure 2 shows the variation of negative ion beam intensity as the temperature at the ionizer surface rises. In the region where ionizer current is less than 9.OA, negative ion yield for the transmission ionizer is smaller than that for the reflection one, the reason may be that it will take a longer period 24
i0
Figure 3. Negative-ion current as a function of cesium oven temperature.
Jiao Gui-Yue et al: A cesium sputter negative ion source
function of cesium oven temperature. At x2OO”C, the negative ion yield has maximum values for both the reflection and transmission ionizer. This implies that at the same pressure of cesium vapour cesium flux for the transmission surface is larger than that of the reflection one when cesium vapour has the same pressure, thus negative ion beam intensity is higher. Because of space charge effects, the relationship between positive ion current I+ and ionizer voltage V, follows the ChildLangmuir law: I+ =
1200 -
l
250°C
A
230°C o 210°C
loo0 ,? 2 800 + 600 -
KV3;’
(4) I where K the perveance, is a function of the geometry of the electrode system and the mass of the cesium ion. In our system K= I .9 x IO-’ A/V3.‘2.6The positive and negative ion current versus ionizer voltage at a cesium oven temperature of 200°C is shown in Figure 4. As noted in the space-charge-limited region, positive current increases with an increasing ionizer voltage. When the voltage goes beyond 8 kV, there exists a critical saturation value. Positive ion current begins at a differing value to the Child-Langmuir value, probably due to the acceleration of positive ions, i.e. the reduction of space
Saha surface ionization theory, ionization probability is 100% in the cesium oven temperature-limited regime. For as positive ion current reaches a saturated constant value their ions are extracted as quickly as they are formed. In the unsaturated region, ionization probability is the ratio of ion current at a given ionizer voltage to the saturated
100-
current,
0 Positive
“0
I
2
3
4
5
6
7
8
9
10
Vi (kV)
Figure 5. Positive-ion current vs ionizer voltage for cesium oven temperature at 250,230.210 and 19O’C.
P,(T/,,T)=
(5)
z+(vi~~)/r+(r)
On the basis of the data shown in Figure 5, there exists a simple relationship between ion current I+(T) and cesium oven temperature for the transmission ionizer. I+(T)
= 1.3 x lo-‘T(K)
-
5.24
(6)
Thus the probability of cesium surface ionization can be determined by equating eqns (5) and (6), indicating clearly the effect of space charge on ionization probability in a sputter ion source
equipped with a transmission
surface ionizer.
4. Conclusions The ventilated wire mesh electrode has been developed for a surface ionizer to generate a high intensity Cs beam which requires a lower oven temperature and the usual charge for cesium vapour. Because of this advantage the sputter-ionization chamber has reduced the need for frequent cleaning of neutralized cesium and sputtered, sample material. The disadvantage is that the wire mesh becomes misshapen and dissolved partly from heating after the operating period, but this never interferes with the cesium current intensity. This type of wire mesh electrode can probably be developed for a large-diameter beam formation electrode, such as a MEVVA ion source, and accelerating-decelerating grids for ion space propulsion. References
0
I
2345678910’
Vi (kV Figure 4. Positive and negative-ion current vs ionizer voltage for a 860A SNICS equipped with an ionizer of the transmission-type.
’ G D Alton, Nucl Ins@ Meth, B73,221 (1993). ‘G D Alton, Rev Sci Ins@, 59, 1039 (1988). ‘G Y Jiao, Vacuum, 45,951 (1994). 4 P E Burd and M D Fridman, Handbook of ENiptic Integralsfor Engineers and Physicists. Springer, Berlin, 1954. ‘G Y Jiao, Nucl Techniques, 16,143 (1993). ’ G Y Jiao, C Z Ji and G F Wang, Journal of Beijing Normal University (Natural Science), 31,66 (1995).
25
Vacuum/volume 47lnumber l/pages 23 to 2511996 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/96 $9.50+.00
A cesium sputter negative ion source using a multilaminate wire mesh ionizer Jiao Gui-Yue,” Wang Wen-Xun,” Ji Cheng-Zhou, b “Analytical and Testing Center, Beijing Normal University, Beijing 100875, P R China; blnstitute of Low Energy Nuclear Physics, Beijing Normal University, Beijing 100875, P R China received 24 March 7995
A new ventilated surface ionizer has been developed for a 860A cesium sputter negative ion source operated at BNU. The new ionizer consists of a multi-laminate nickel wire mesh and a spiral heater, and is spherical in shape. Low pressure cesium vapour diffuses through the wire mesh to the extraction side of the ionizer, meanwhile the neutral cesium can be ionized. The ionizer is capable of producing a positive ion current up to milliampere range. The negative ion current is 50 PA for P- and more than 150 PA for O- and Si-.
1. Introduction negative ion sources usually employ high work function metals to generate a positive cesium ion beam used to sputter a negatively biased target material. Cesium vapour comes from a cesium reservoir positioned outside of the source and can be efficiently ionized when contacting a hot, high work function surface. In the ionization process, an adsorbed atom leaves a valence electron attached to the metal surface upon evaporation from the surface. Since the work function 4 of most clean metal surfaces lie between 4.0 and 5.0 eV and the first ionization potential I’, of a cesium atom is 3.89 eV, the surface ionization is expected to be an efficient way to form a positive ion beam. Surface ionization is not identical to plane ionization. For example, if the temperature of a tip or a tiny wire is high enough, the energy supplied to the adsorbed atoms will enable them to be ionized and desorbed from the tip or wire surface. That is, if AH, - eV, + qS > AH, holds, the surface ionization process occurs.’ In this paper a multi-laminate wire mesh ionizer will be described and the performance of a 860A sputter source equipped with the new ionizer discussed. Sputter-type
2. A new ionizer Positive ion yield is governed by the threshold temperature and cesium flux on the ionizer surface.’ To increase the positive ion yield further, an advisable way is to raise the number of striking cesium atoms per unit area per unit time, in other words, to enhance the cesium flux striking the ionizer surface. The correlation between the cesium flux and cesium oven temperature is given by
d
(1) where T is the absolute temperature of ionization surface and PC represents the probability for capture at the ionizer surface, defined as the ratio of the number of particles captured by ionizer to the number of particles per unit volume around the ionization surface. Because both the sputter probe and ionizer are situated in the same ionization chamber, the cesium oven temperature not only controls the rate at which cesium atoms strike the ionizer surface, but determines a cesium flow rate which is sufficient for steady-state deposition of about one monolayer on the sputter probe surface. In order to optimize negative ion yield, the cesium oven is customarily operated at a fixed temperature which results in a saturation of positive ion yield for the previous source. With the above consideration, a gauze wire ionizer has been designed by Jiao. The ionizer consists of several layers of nickel or platinum gauze wires and a heater. The gauzes are spherically shaped with a diameter of 36 mm and are surrounded by the heater. The cesium vapour is fed from the cesium oven through a sealed tube. Cesium atoms are ionized when they diffuse through the gauze layers. In order to distinguish the new ionizer from the previous one, the surface ionization process in which both neutral cesium atoms to be ionized and cesium ions are located on the same side of the emission surface, is called rq7ected surface ionization;’ and the surface ionization process in which neutral cesium atoms pass through the emission surface and are ionized when leaving the surface is called transmitted surface ionization. As shown in Figure 1, the transmission construction 23
Jiao Gui-Yue ef al: A cesium sputter negative
ion source
Table 1. Performances of reflection and transmission surface ionizers in
SNICS Ionizers
Target ion current
Positive ion current/mA
Negative ion Erosion current/uA center/tjmm
Refl. Trans.
2.45 3.50
0.89 1.27
35.00 55.00
0.75 1.00
r o Transmission Figure 1. Schematic diagram of a transmission surface ionization source in SNICS.
,-
l
Reflection
I-
has not got a stagnant region for cesium vapour. The multilayered gauze becomes a passageway for cesium vapour, the majority of cesium atoms do not have to land on the ionization surface, they are fed directly to the ionization layers, At the same pressure, the cesium flux for the transmission ionization surface is greater than that for the reflection ionizer. As a result, positive ion yield for the transmission ionizer goes up. Assuming that the motion probability for cesium atoms is the same in all directions, the emission surface of the reflection ionizer will capture one sixth of the total particles per unit volume though its area is as large as the transmission ionizer, while the capture probability will be four sixths for the emission surface of the transmission ionizer.
3. Measurements and discussions During the sputtering process secondary electrons can also be ejected from the target material, and the coefficient for secondary electron emission lies between 0.3 and 3 electrons/ion.“ Thus, the measured target current IT comprises three parts: the positive ion current I+, the negative ion current I- and the secondary electron current Z,. IT = 1’ + f-
+ z, + . . = TZ1
(2)
O
0
I 2
4
6
I
I
I
8
10
12
I(A)
Figure 2. Relationship between heater current and negative-ion currents for reflection
and transmission
ionizers.
of time for a double layer nickel gauze to reach complete desorption of cesium atoms than a Nickel sheet, this leads to a difference in ionization probability. Above 9.OA, the cesium ions are desorbed rapidly and the work function is closer to the intrinsic value, the probability for ionization will be steadied, and the cesium atom density becomes imporatant for governing the positive ion yield. In addition to the large cesium flux, cesium atoms can get enough energy to be ionized because of the longer residence time resulting from multiple collisions in the metal gauze. Consequently, ionization probability for the transmission ionizer will increase when the temperature remains stable. Figure 3 shows the variation of negative ion current as a
Secondary electron current, in general, does not add to the negative current due to its deflection by the radial magnet placed outside the source body. If we take an average of the end point values for the secondary emission coefficients cited above, a value of 1.65 electrons/ion is obtained. Using this value for Z,, eqn 2 can be approximated as follows:5 I+ r0.37(1,
--I-)
(3)
Under the conditions that target voltages are 6.0-X.0 kV, ionizer current is 12.5A (z 1100°C) and cesium oven temperature is at x2OO”C, the performances of the sputter-ion source equipped respectively with reflection and transmission ionizer are compared in Table 1. The ionization material is nickel (sheet for reflection ionizer and gauze for transmission ionizer). The work function of nickel is 4.6leV. The target material to be sputtered is multicrtsicl-InP. Figure 2 shows the variation of negative ion beam intensity as the temperature at the ionizer surface rises. In the region where ionizer current is less than 9.OA, negative ion yield for the transmission ionizer is smaller than that for the reflection one, the reason may be that it will take a longer period 24
i0
Figure 3. Negative-ion current as a function of cesium oven temperature.
Jiao Gui-Yue et al: A cesium sputter negative ion source
function of cesium oven temperature. At x2OO”C, the negative ion yield has maximum values for both the reflection and transmission ionizer. This implies that at the same pressure of cesium vapour cesium flux for the transmission surface is larger than that of the reflection one when cesium vapour has the same pressure, thus negative ion beam intensity is higher. Because of space charge effects, the relationship between positive ion current I+ and ionizer voltage V, follows the ChildLangmuir law: I+ =
1200 -
l
250°C
A
230°C o 210°C
loo0 ,? 2 800 + 600 -
KV3;’
(4) I where K the perveance, is a function of the geometry of the electrode system and the mass of the cesium ion. In our system K= I .9 x IO-’ A/V3.‘2.6The positive and negative ion current versus ionizer voltage at a cesium oven temperature of 200°C is shown in Figure 4. As noted in the space-charge-limited region, positive current increases with an increasing ionizer voltage. When the voltage goes beyond 8 kV, there exists a critical saturation value. Positive ion current begins at a differing value to the Child-Langmuir value, probably due to the acceleration of positive ions, i.e. the reduction of space
Saha surface ionization theory, ionization probability is 100% in the cesium oven temperature-limited regime. For as positive ion current reaches a saturated constant value their ions are extracted as quickly as they are formed. In the unsaturated region, ionization probability is the ratio of ion current at a given ionizer voltage to the saturated
100-
current,
0 Positive
“0
I
2
3
4
5
6
7
8
9
10
Vi (kV)
Figure 5. Positive-ion current vs ionizer voltage for cesium oven temperature at 250,230.210 and 19O’C.
P,(T/,,T)=
(5)
z+(vi~~)/r+(r)
On the basis of the data shown in Figure 5, there exists a simple relationship between ion current I+(T) and cesium oven temperature for the transmission ionizer. I+(T)
= 1.3 x lo-‘T(K)
-
5.24
(6)
Thus the probability of cesium surface ionization can be determined by equating eqns (5) and (6), indicating clearly the effect of space charge on ionization probability in a sputter ion source
equipped with a transmission
surface ionizer.
4. Conclusions The ventilated wire mesh electrode has been developed for a surface ionizer to generate a high intensity Cs beam which requires a lower oven temperature and the usual charge for cesium vapour. Because of this advantage the sputter-ionization chamber has reduced the need for frequent cleaning of neutralized cesium and sputtered, sample material. The disadvantage is that the wire mesh becomes misshapen and dissolved partly from heating after the operating period, but this never interferes with the cesium current intensity. This type of wire mesh electrode can probably be developed for a large-diameter beam formation electrode, such as a MEVVA ion source, and accelerating-decelerating grids for ion space propulsion. References
0
I
2345678910’
Vi (kV Figure 4. Positive and negative-ion current vs ionizer voltage for a 860A SNICS equipped with an ionizer of the transmission-type.
’ G D Alton, Nucl Ins@ Meth, B73,221 (1993). ‘G D Alton, Rev Sci Ins@, 59, 1039 (1988). ‘G Y Jiao, Vacuum, 45,951 (1994). 4 P E Burd and M D Fridman, Handbook of ENiptic Integralsfor Engineers and Physicists. Springer, Berlin, 1954. ‘G Y Jiao, Nucl Techniques, 16,143 (1993). ’ G Y Jiao, C Z Ji and G F Wang, Journal of Beijing Normal University (Natural Science), 31,66 (1995).
25