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Low energy ion beam transport through apertures
NUCLEAR
INSTRUMENTS
AND
METHODS
143 ( 1 9 7 7 )
1-6 ;
(~)
NORTH-HOLLAND
PUBLISHING
CO.
LOW ENERGY ION BEAM TRANSPORT THROUGH APERTURES K. W1TTMAACK
Gesellschafi fiir Strahlen- und UmweltJorschung mbH, Physikalisch-Technische Abteilung, D-8042 Neuherberg, W. Germany Received 3 January 1977 Argon ion transport through a beam defining aperture has been investigated at energies between 0.1 and 15 keV. It was found that the current which can be fed through the aperture depends strongly upon the operation parameters of the ion source. For optimum source performance the maximum current transmitted is limited by space charge expansion of the beam. This interpretation of the experimental results is supported by simple calculations. The measurements additionally provide information about the plasma potential.
1. Introduction In a previous paper ~) beam formation in a triode ion gun gas been described in detail. The properties of the respective ion source have also been studied thoroughly2,3). To ease comparison all investigations ~-3) were carried out at an (argon) ion energy of 40 keV. The results were applied in designing the ion gun for a UHV secondary ion mass spectrometer4). Such a gun has to be operated at ion energies much less than 40 keV because of the fact that the depth resolution in secondary ion mass spectrometry (SIMS) decreases with increasing primary ion energyS,6). To conserve adequate depth resolution the sputter ion energy should not exceed 15 keV. In many cases SIMS investigations at variable primary ion energy provide useful information7,8). It is of interest, therefore, to have available an ion gun which can be operated at optimum output between some 100 eV and about 15 keV. Studies on the properties of ion guns as a function of beam energy are desirable, however, not only for application purposes. Such investigations are also of basic interest from an ion optical point of view since they allow a better understanding of the influence of the various operation parameters on the beam characteristics. It has been pointed out recently ~) that it is not very instructive to measure merely the energy dependence of the maximum current that can be extracted from a certain ion source. Instead it is necessary to determine the beam quality simultaneously. For convenience profile measurements have been used previously to characterize the beam~-3). In this study we try to optimize the beam quality by operating the ion gun such that the beam current which can be fed through a cer-
tain aperture attains the maximum value. If apertures of different size are placed at a fixed position in the drift region of the beam far outside the acceleration field the measurements can be expected to yield quantitative data about the properties of the source plasma, the ion energy distribution and the amount of space charge expansion in the drift region.
2. Experimental The measurements were carried out using the ion gun section of the DIDA secondary ion mass spectrometer4,9). The experimental set-up is shown schematically in fig. 1. The ion source is of the magnetically confined, oscillating electron bombardment type with hot cathode and extraction of ions parallel to the magnetic fleld3,1°-~2). Ion acceleration and beam formation takes place in the field of the immersion lens made up by a cup extractor and an acceleration tube. The triode ion gun used in this study constitutes a modified "version of the more flexible set-up investigated recentlyL2). The tube diameter D has been enlarged (now D = 100 mm) and the extractor length L and the extraction gap distance d are fixed (L/D= 1.2 and d = 2 mm). The diameter of the ion source opening is 1.5 mm as before1-3). .ran source
~ /
Extractton electrode
Acceleration electrode
Beam centering plates
J-
Beam defining aperture
Faraday cup
I
Fig. 1. Schematic drawing of the experimental set-up.
2
K, WITTMAACK
Different from the previous studies ~-3) the extraction and acceleration voltage, V 1 and I/2, are now defined with respect to the anode potential (see fig. 1). The reason for this change is that preliminary investigations indicated the plasma potential, Vp, to be close to the anode potential. This result has also been reported by other authors~l). In low energy studies an accurate knowledge of the plasma potential is desirable since Vp determines the zero of the ion energy. To determine the beam quality the ions were fed through an aperture (diameter a) placed at a distance of 85 cm from the ion source exit. In the DIDA system this aperture serves as a pressurestep between ion gun vessel and UHV target chamber. Ion transport through the aperture is optimized by adequate setting of the extraction voltage. Accurate centering of the beam with respect to the aperture is provided by two pairs of deflection plates. To get information as complete as possible apertures of different size were used. The beam current fed through the apertures was measured by means of a Faraday cage. The source pressure could not be measured directly. In the normal SIMS operation mode this is not a drawback because the discharge characteristics 3A°-12) supply sufficient information about the source pressure. In the present study the source pressure Ps was calculated from the steady state increase in pressure in the target chamber, Ap,
dpg
=
p~ C1 C2/S1 $2,
(2)
where n is the number of argon atoms sputtered per second. In steady state, h = 1Je,
3. Results Prior to an investigation of the energy dependence of the target current I~, i.e. the current transmitted through the beam defining aperture, optimum operation conditions of the ion source had to be evaluated. Obviously it was of major importance in this study to run the ion source such that the transverse energies and the energy spread of the ions were as small as possible. It was found that this can be achieved by use of relatively large source pressures and low magnetic fields. To maximize the target current the argon pressure had to be increased up to about 2 × 10-3 mbar. This is about an order of magnitude larger than the minimum pressure3,1°-12). At either smaller or larger source pressures the maximum current through the aperture decreases, the effect being most pronounced at low ion energies. An example for the influence of the discharge (emission) current and the magnetic field in the ion source on the target current is shown in fig.
(l)
where C~ and C2 are the gas flow conductances of the ion source exit and the beam defining aperture, respectively, and S~ and $2 are the pumping speeds of the pumps evacuating the ion gun vessel and the target chamber, respectively. For argon, S~ = 200 1/s (turbomolecular pump), $ 2 = 6 0 1 / s (ion pump, 2001/s for nitrogen), Cl =0.15 1/s and C2 =0.3 1/s (2 mm diam. aperture) 13) so that A p g = 4 x 10-6ps. A typical source pressure of 7 x 10-4 mbar (= 5 x 10-4 torr) 3J°-~2) will thus enhance the chamber pressure by only 3 x 10 -9 mbar. This may be compared with the increase in chamber pressure, Ape, produced during continuous bombardment of a target by sputtering of previously implanted argon, Ape = hiS2,
where It is the Ar + current entering the target chamber and e the elementary charge. For the above pumping speed, Apc[mbar] = 4 x 10-9 I~[#A], a result which has been confirmed experimentally. From these estimates it is obvious that SIMS investigations can be carried out at total background pressures in the 10 -9 mbar range even in case that beam currents of up to about 2 #A are applied.
(3)
2.5
I
I 20p/
-[
31
t.5
lO~
~ 30
0.5 0 '
0
~
~
~
m
m
20 40 Emissioncurrent [mA]
60
Fig. 2. Target current as a function of the emission (discharge) current. Parameter is the axial magnetic field in the ion source.
LOW
ENERGY
ION
2 for an ion energy of 4 keV and a 2 mm diam. beam defining aperture. One can see that It increases the more rapidly with increasing discharge current the strongerthe magnetic field B. This is reasonable because the plasma density on the source axis increases with increasing B3). However, I t increases only up to a certain optimum discharge current ioo,(B) and then decreases. Obviously the plasma density at the source exit should not exceed a certain upper limit. Moreover the source magnetic field should be low for maximum I~. This effect is known from beam profile measurements which showed that the beam spot size at a certain monitor position increases with increasing B (at constant beam current)2). The variation of the optimum target current, loot(B) , with increasing magnetic field is shown in fig. 3. The results are deduced from fig. 2 and similar measurements at 5 keV. One can see that loo t exhibits a maximum, Ira, at a weak magnetic field, B~. For B>Bm, /opt decreases nonmonotonically. The reason for the 'step-wise' variation of [opt is not known in detail. The main effect is likely to result from a nonmonotonic variation of the source plasma parameters, i.e. plasma density and in particular electron and ion temperature. This idea is supported by the fact that details of curves such as in fig. 3 depend upon the discharge voltage V~. (Note that the ion source was operated in the constant voltage mode. Variations of the dis-
BEAM T R A N S P O R T
3
charge current were achieved by varying the filament heating power.) The results shown in figs. 2 and 3 were obtained by optimizing the extraction voltage II1 at a given acceleration voltage V2. As shown in a previous studyl), recording of V1 provides useful information about the beam transport. The extraction voltages required to achieve the respective target currents in fig. 2 are plotted in fig. 4. The data in figs. 2 and 4 represent a set of the main operation parameters of the ion gun at a given acceleration voltage and a given position and size of the beam defining aperture. The results of fig. 4 look rather complicated at first sight. Comparison with fig. 2, however, indicates a systematic behaviour. This becomes more obvious if we plot V1 as a function of the emission current (fig. 5). Except for very low and very high plasma densities the extraction voltage has to be increased with increasing emission current, i.e. with increasing plasma density. If the plasma density at the ion source exit exceeds a certain upper limit the target current decreases although the total beam current increases furtherb. Deviations from this behaviour occur at very low plasma density at which the curvature of the plasma boundary may change from convex to concaveb. To account for this change the extraction voltage has to be increased (dashed line in figs 4 and 5). 200
I
I
j, 30 3
,
a = 2ram
~
",~
/-
.~
,12
180
v~ = 5 ~
160 ~3
3
140
v~ : ~ k v
~.
z
120
v. :3ov
V2 = 4 k V £I
0 L 0
I 20 Magnettc field
t 40 strength
60 [mT]
Fig. 3. M a x i m u m target current as a function of the axial magnetic field in the ion source. Parameters are the acceleration voltage V2 and the anode (discharge) voltage Va.
100 0
f 1 Target
= 2mtT~
I 2 current
3 [HA]
Fig. 4. O p t i m u m extraction voltage vs target current. Parameter is the axial magnetic field in the ion source.
4
K. W I T T M A A C K
2O0
I
I /2
180
/--
........ 30 4
160
3
c 140
2
t~ 120 12 = 2mro 100 0
1
I
20 40 Emission current [mA]
60
Fig. 5. O p t i m u m extraction voltage v e r s u s e m i s s i o n (discharge) current. P a r a m e t e r is the axial magnetic field in the ion
source. At very high plasma densities the extraction voltage has to lowered to achieve optimum ion transport through the beam defining aperture (dashed-dotted line in figs. 4 and 5). Using the data of ref. 1 the current density at the ion source exit can be estimated to be about 5 0 / i A / m m 2 for an emission current of 3 0 m A and B = 3 0 mT ( = 3 0 0 G ) . This is about ten times the space charge limited current density between plane electrodes in case that the. electrode separation is equal to the extraction gap distance and the voltage between the electrodes is equal to the extraction voltage ( ~ 180 V, see fig. 5). Accordingly one must expect that the plasma markedly diffuses out of the source under the above conditions. The resuiting plasma boundary will be characterized by a strongly curved meniscus which in turn will lead to a large angle of divergence of the extracted ion beaml0. Pronounced profile distortions have been observed under such conditions2). It is not astonishing, therefore, that at high plasma densities the extraction voltage has to be set in an unexpected way. It is interesting to realize (fig. 4) that for not too large source magnetic fields (B<20 roT) the extraction voltage required to transport a certain ion current through the beam defining aperture depends only little upon B [for i
ly to indicate that under the above conditions the beam profiles are similar. Measurements as those presented in figs. 2-5 have been carried out for acceleration voltages between 0.1 and 15 keV and diameters of the beam defining aperture between 2 and 6 mm. The maximum currents observed under the respective conditions are plotted in fig. 6 as a function of the acceleration voltage with the aperture size as a parameter. The data represent optimum values at low magnetic field. As can be seen from fig. 6, Im increases rapidly with increasing ion energy e V 2. Within certain energy intervals the energy dependence can be approximated by power laws, I m o~ V~, where ~ _
a[mm]. •
4
At÷ / / /
J
I°-6~
•~ tO-7
E
Ol Acceleration
1 I0 voltage ~ (RV]
Fig. 6. M a x i m u m target current as a function of the acceleration voltage. Parameter is the diameter of the beam defining aperture.
LOW E N E R G Y
500
I
I
a[mm]: 400 -
ION BEAM T R A N S P O R T
•
2
•
4
that for not too small apertures the upper limit of current is governed by space charge expansion of the beam in the drift region outside the acceleration field. This idea is supported by a quantitative estimate. Suppose the ion beam converges after having passed through the immersion lens made up by cup extractor and acceleration tube, the waist being formed at the position of the beam defining aperture. To determine the contour of the beam in the drift region we reverse the point of view and calculate the expansion of a beam emerging from the aperture and travelling towards the ion source, a procedure which is justified because of symmetry~S). The relative beam expansion due to space charge effects, r/ro, is given by~5):
/
"~ 300
°2oo
0 0
[
I
5
10
Acceleration
voltage
5
15 V2 [kV]
Fig. 7. Extraction voltage required for transport of the maximum target current vs acceleration voltage. Parameter is the diameter of the beam defining aperture.
ture had been covered with the same uniform current density. The results of fig. 6 may thus indicate that profile distortions are produced under the condition of maximum current transport through (small) apertures. In such a case the distorted beam profiles are characterized by radial current density distributions with a strong central peak and much less intense shoulders~). Information about the ion source plasma can be obtained from fig. 7 in which the extraction voltages V1 required to feed the maximum beam current through the respective aperture are plotted as a function of the acceleration voltage I/2. One can see that V2/V ~ is nearly constant through two orders of magnitude in ion energy. At a given ion energy eV 2 the extraction voltage has to be increased with increasing aperture size. This is due to the fact that for maximum target current the plasma density had to be increased with increasing aperture size. 4. Discussion The experimental set-up used in this study represents a very simple configuration not only for investigating formation and transport of ion beams but also for determining characteristics of the source plasma and the ion beam. The main results are contained in figs. 6 and 7. From the V~ dependence in fig. 6 we conclude
f
r/,o dp = 6.4x .)1 (lnp)~
10 - 3
z
1/}
(4)
r o'
where r0 is the minimum beam radius (in this case r0 = ½a), r is the beam radius at a distance z from the position of the minimum radius, l[/tA] is the beam current, eV~[keV] is the ion energy, M[u] is the ion mass and n ~s the charge state of the ion. With the help of the tabulated integral in eq. (4) one can plot a universal curve for the space-charge spreading of an ion beam in field-free space~5). As an example we apply the results of fig. 6 in the region where I m oc 1/~. For a = 6 mm we have lm/l~ = 0.76 ~ A / k e V i. Moreover M = 40, n = 1 and z-~ 650 mm (i.e. z/ro ~-220). Using these data the right hand side (rh) in eq. (4) yields rh = 3.1. From the universal curve ~5) the respective space charge expansion is found to be r/ro ~-2.9 so that r = 8.7 ram. Similarly the results for a = 2 m m are Ira/V~ = 0.38/zA/keVL rh = 6.5, r/ro = 7.1, r = 7 . 1 mm. According to the above estimates the converging beam starts with an initial diameter of at least 14-18 mm. This corresponds to a filling factor of the immersion lens of up to 20%. Under such conditions pronounced aberrations must be expected. The calculations thus demonstrate that for the geometry used in this study the maximum target currents represent an upper limit set by space charge effects and lens aberrations. Gaussian optics or waist-to-waist transfer 16) are less important under the condition of maximum beam current. Although, for example, the magnification could be calculated from the aperture position and by use of the focal properties of twotube electrostatic lenses ~7) this is of little use be3
6
K. W I T T M A A C K
cause the object size is not known. Image size or waist diameter must be taken into account merely to realize that the above estimates for the space charge effects represent a lower limit. Attributing a certain part of the beam diameter to particle optics would lead to a stronger space charge broadening and, therefore, to a larger initial beam diameter. On the other hand, the energy spread of the ions is likely to play a role, in particular at low energies and. with small apertures. This idea is supported by the results of fig. 6. Quantitative estimates cannot be given on the basis of the present data. One can expect, however, that more pronounced effects due to the energy spread would have been observed if the energy dependence of the maximum current had not been measured under optimum conditions (as deduced from figs. 2 and 3). Finally we discuss the results of fig. 7 which showed that V 2 / V ~ is constant under the respective conditions. This proportionality indicates that the plasma potential is equal to or very close to the anode potential. Moreover it supports the idea that the beam contours were very similar irrespective of the ion energy. 5. Conclusions The present study has shown that a lot of information about ion transport, beam quality and plasma properties can be obtained by means of a very simple experimental set-up. Maximum current transport through a given aperture can be achieved only after optimizing the operation parameters of the ion source. The upper limit of the current through the aperture is then set by space charge effects.
Further optimization of ion gun performance will require a quantitative investigation of source properties such as energy spread and emittance as a function of source pressure, magnetic field, discharge current and discharge voltage.
References I) K. Wittmaack, Nucl. Instr. and Meth. 118 (1974) 99. 2) K. Wittmaack and F. Schulz, J. Vac. Sci. Technol. 10 (1973) 918. 3) K. Wittmaack and F. Schulz, in Proc. 2nd Int. Conf. on Ion sources (eds. F. Viehb6ck, H. Winter and M. Bruck; SGAE, Vienna, 1972) p. 863. 4) K. Wittmaack, J. Maul and F. Schulz, in Proc. 6th lnt. Conf. on Electron and ion beam science and technology (ed. R. Bakish; The Electrochemical Society, Princeton, 1974) p. 164. 5) F. Schulz, K. Wittmaack and J. Maul, Rad. Effects 18 (1973) 211. 6) j. A. McHugh, Rad. Effects 21 (1974) 209. 7) K. Wittmaack and G. Staudewha, ier,. Appl. Phys. Lett.. 27 (1975) 318. 8) K. Wittmaack, Surface Sci. 53 (1975) 626. 9) K. Wittmaack, in Ion beam su(/ace layer analysis (eds. O. Meyer, G. Linker and F. KO_ppeler; Plenum Press, New York, 1976) p. 649. 10) O. AIm6n and K. O. Nielsen, Nucl. Instr. and Meth. ! (1957) 302. ll) C. E. Carlston and G. D. Magnuson, Rev. Sci. Instr. 33 (1962) 905. 12) K. Wittmaack and W. Wach, Nucl. Instr. and Meth., this issue, p. 7. 13) M. V. Ardenne, Tabellen zur angewandten Physik, vol. 2 (VEB Deutscher Verlag,der Wissenschaften, Berlin, 1964). 14) I. Chavet and R. Bernas~ Nucl. Instr. and Meth. 47 (1967) 77. 15) R. Hutter, in Focusing of charged particles, vol. 2 (ed. A. Septier; Academic Press, New York, 1967) p. 3. 16) H. F. Glavish, Nucl. Instr. and Meth. 99 (1972) 109. 27) D. DiChio, S. V. Natali and C. E. Kuyatt, Rev. Sci. lnstr. 45 (1974) 559.
INSTRUMENTS
AND
METHODS
143 ( 1 9 7 7 )
1-6 ;
(~)
NORTH-HOLLAND
PUBLISHING
CO.
LOW ENERGY ION BEAM TRANSPORT THROUGH APERTURES K. W1TTMAACK
Gesellschafi fiir Strahlen- und UmweltJorschung mbH, Physikalisch-Technische Abteilung, D-8042 Neuherberg, W. Germany Received 3 January 1977 Argon ion transport through a beam defining aperture has been investigated at energies between 0.1 and 15 keV. It was found that the current which can be fed through the aperture depends strongly upon the operation parameters of the ion source. For optimum source performance the maximum current transmitted is limited by space charge expansion of the beam. This interpretation of the experimental results is supported by simple calculations. The measurements additionally provide information about the plasma potential.
1. Introduction In a previous paper ~) beam formation in a triode ion gun gas been described in detail. The properties of the respective ion source have also been studied thoroughly2,3). To ease comparison all investigations ~-3) were carried out at an (argon) ion energy of 40 keV. The results were applied in designing the ion gun for a UHV secondary ion mass spectrometer4). Such a gun has to be operated at ion energies much less than 40 keV because of the fact that the depth resolution in secondary ion mass spectrometry (SIMS) decreases with increasing primary ion energyS,6). To conserve adequate depth resolution the sputter ion energy should not exceed 15 keV. In many cases SIMS investigations at variable primary ion energy provide useful information7,8). It is of interest, therefore, to have available an ion gun which can be operated at optimum output between some 100 eV and about 15 keV. Studies on the properties of ion guns as a function of beam energy are desirable, however, not only for application purposes. Such investigations are also of basic interest from an ion optical point of view since they allow a better understanding of the influence of the various operation parameters on the beam characteristics. It has been pointed out recently ~) that it is not very instructive to measure merely the energy dependence of the maximum current that can be extracted from a certain ion source. Instead it is necessary to determine the beam quality simultaneously. For convenience profile measurements have been used previously to characterize the beam~-3). In this study we try to optimize the beam quality by operating the ion gun such that the beam current which can be fed through a cer-
tain aperture attains the maximum value. If apertures of different size are placed at a fixed position in the drift region of the beam far outside the acceleration field the measurements can be expected to yield quantitative data about the properties of the source plasma, the ion energy distribution and the amount of space charge expansion in the drift region.
2. Experimental The measurements were carried out using the ion gun section of the DIDA secondary ion mass spectrometer4,9). The experimental set-up is shown schematically in fig. 1. The ion source is of the magnetically confined, oscillating electron bombardment type with hot cathode and extraction of ions parallel to the magnetic fleld3,1°-~2). Ion acceleration and beam formation takes place in the field of the immersion lens made up by a cup extractor and an acceleration tube. The triode ion gun used in this study constitutes a modified "version of the more flexible set-up investigated recentlyL2). The tube diameter D has been enlarged (now D = 100 mm) and the extractor length L and the extraction gap distance d are fixed (L/D= 1.2 and d = 2 mm). The diameter of the ion source opening is 1.5 mm as before1-3). .ran source
~ /
Extractton electrode
Acceleration electrode
Beam centering plates
J-
Beam defining aperture
Faraday cup
I
Fig. 1. Schematic drawing of the experimental set-up.
2
K, WITTMAACK
Different from the previous studies ~-3) the extraction and acceleration voltage, V 1 and I/2, are now defined with respect to the anode potential (see fig. 1). The reason for this change is that preliminary investigations indicated the plasma potential, Vp, to be close to the anode potential. This result has also been reported by other authors~l). In low energy studies an accurate knowledge of the plasma potential is desirable since Vp determines the zero of the ion energy. To determine the beam quality the ions were fed through an aperture (diameter a) placed at a distance of 85 cm from the ion source exit. In the DIDA system this aperture serves as a pressurestep between ion gun vessel and UHV target chamber. Ion transport through the aperture is optimized by adequate setting of the extraction voltage. Accurate centering of the beam with respect to the aperture is provided by two pairs of deflection plates. To get information as complete as possible apertures of different size were used. The beam current fed through the apertures was measured by means of a Faraday cage. The source pressure could not be measured directly. In the normal SIMS operation mode this is not a drawback because the discharge characteristics 3A°-12) supply sufficient information about the source pressure. In the present study the source pressure Ps was calculated from the steady state increase in pressure in the target chamber, Ap,
dpg
=
p~ C1 C2/S1 $2,
(2)
where n is the number of argon atoms sputtered per second. In steady state, h = 1Je,
3. Results Prior to an investigation of the energy dependence of the target current I~, i.e. the current transmitted through the beam defining aperture, optimum operation conditions of the ion source had to be evaluated. Obviously it was of major importance in this study to run the ion source such that the transverse energies and the energy spread of the ions were as small as possible. It was found that this can be achieved by use of relatively large source pressures and low magnetic fields. To maximize the target current the argon pressure had to be increased up to about 2 × 10-3 mbar. This is about an order of magnitude larger than the minimum pressure3,1°-12). At either smaller or larger source pressures the maximum current through the aperture decreases, the effect being most pronounced at low ion energies. An example for the influence of the discharge (emission) current and the magnetic field in the ion source on the target current is shown in fig.
(l)
where C~ and C2 are the gas flow conductances of the ion source exit and the beam defining aperture, respectively, and S~ and $2 are the pumping speeds of the pumps evacuating the ion gun vessel and the target chamber, respectively. For argon, S~ = 200 1/s (turbomolecular pump), $ 2 = 6 0 1 / s (ion pump, 2001/s for nitrogen), Cl =0.15 1/s and C2 =0.3 1/s (2 mm diam. aperture) 13) so that A p g = 4 x 10-6ps. A typical source pressure of 7 x 10-4 mbar (= 5 x 10-4 torr) 3J°-~2) will thus enhance the chamber pressure by only 3 x 10 -9 mbar. This may be compared with the increase in chamber pressure, Ape, produced during continuous bombardment of a target by sputtering of previously implanted argon, Ape = hiS2,
where It is the Ar + current entering the target chamber and e the elementary charge. For the above pumping speed, Apc[mbar] = 4 x 10-9 I~[#A], a result which has been confirmed experimentally. From these estimates it is obvious that SIMS investigations can be carried out at total background pressures in the 10 -9 mbar range even in case that beam currents of up to about 2 #A are applied.
(3)
2.5
I
I 20p/
-[
31
t.5
lO~
~ 30
0.5 0 '
0
~
~
~
m
m
20 40 Emissioncurrent [mA]
60
Fig. 2. Target current as a function of the emission (discharge) current. Parameter is the axial magnetic field in the ion source.
LOW
ENERGY
ION
2 for an ion energy of 4 keV and a 2 mm diam. beam defining aperture. One can see that It increases the more rapidly with increasing discharge current the strongerthe magnetic field B. This is reasonable because the plasma density on the source axis increases with increasing B3). However, I t increases only up to a certain optimum discharge current ioo,(B) and then decreases. Obviously the plasma density at the source exit should not exceed a certain upper limit. Moreover the source magnetic field should be low for maximum I~. This effect is known from beam profile measurements which showed that the beam spot size at a certain monitor position increases with increasing B (at constant beam current)2). The variation of the optimum target current, loot(B) , with increasing magnetic field is shown in fig. 3. The results are deduced from fig. 2 and similar measurements at 5 keV. One can see that loo t exhibits a maximum, Ira, at a weak magnetic field, B~. For B>Bm, /opt decreases nonmonotonically. The reason for the 'step-wise' variation of [opt is not known in detail. The main effect is likely to result from a nonmonotonic variation of the source plasma parameters, i.e. plasma density and in particular electron and ion temperature. This idea is supported by the fact that details of curves such as in fig. 3 depend upon the discharge voltage V~. (Note that the ion source was operated in the constant voltage mode. Variations of the dis-
BEAM T R A N S P O R T
3
charge current were achieved by varying the filament heating power.) The results shown in figs. 2 and 3 were obtained by optimizing the extraction voltage II1 at a given acceleration voltage V2. As shown in a previous studyl), recording of V1 provides useful information about the beam transport. The extraction voltages required to achieve the respective target currents in fig. 2 are plotted in fig. 4. The data in figs. 2 and 4 represent a set of the main operation parameters of the ion gun at a given acceleration voltage and a given position and size of the beam defining aperture. The results of fig. 4 look rather complicated at first sight. Comparison with fig. 2, however, indicates a systematic behaviour. This becomes more obvious if we plot V1 as a function of the emission current (fig. 5). Except for very low and very high plasma densities the extraction voltage has to be increased with increasing emission current, i.e. with increasing plasma density. If the plasma density at the ion source exit exceeds a certain upper limit the target current decreases although the total beam current increases furtherb. Deviations from this behaviour occur at very low plasma density at which the curvature of the plasma boundary may change from convex to concaveb. To account for this change the extraction voltage has to be increased (dashed line in figs 4 and 5). 200
I
I
j, 30 3
,
a = 2ram
~
",~
/-
.~
,12
180
v~ = 5 ~
160 ~3
3
140
v~ : ~ k v
~.
z
120
v. :3ov
V2 = 4 k V £I
0 L 0
I 20 Magnettc field
t 40 strength
60 [mT]
Fig. 3. M a x i m u m target current as a function of the axial magnetic field in the ion source. Parameters are the acceleration voltage V2 and the anode (discharge) voltage Va.
100 0
f 1 Target
= 2mtT~
I 2 current
3 [HA]
Fig. 4. O p t i m u m extraction voltage vs target current. Parameter is the axial magnetic field in the ion source.
4
K. W I T T M A A C K
2O0
I
I /2
180
/--
........ 30 4
160
3
c 140
2
t~ 120 12 = 2mro 100 0
1
I
20 40 Emission current [mA]
60
Fig. 5. O p t i m u m extraction voltage v e r s u s e m i s s i o n (discharge) current. P a r a m e t e r is the axial magnetic field in the ion
source. At very high plasma densities the extraction voltage has to lowered to achieve optimum ion transport through the beam defining aperture (dashed-dotted line in figs. 4 and 5). Using the data of ref. 1 the current density at the ion source exit can be estimated to be about 5 0 / i A / m m 2 for an emission current of 3 0 m A and B = 3 0 mT ( = 3 0 0 G ) . This is about ten times the space charge limited current density between plane electrodes in case that the. electrode separation is equal to the extraction gap distance and the voltage between the electrodes is equal to the extraction voltage ( ~ 180 V, see fig. 5). Accordingly one must expect that the plasma markedly diffuses out of the source under the above conditions. The resuiting plasma boundary will be characterized by a strongly curved meniscus which in turn will lead to a large angle of divergence of the extracted ion beaml0. Pronounced profile distortions have been observed under such conditions2). It is not astonishing, therefore, that at high plasma densities the extraction voltage has to be set in an unexpected way. It is interesting to realize (fig. 4) that for not too large source magnetic fields (B<20 roT) the extraction voltage required to transport a certain ion current through the beam defining aperture depends only little upon B [for i
ly to indicate that under the above conditions the beam profiles are similar. Measurements as those presented in figs. 2-5 have been carried out for acceleration voltages between 0.1 and 15 keV and diameters of the beam defining aperture between 2 and 6 mm. The maximum currents observed under the respective conditions are plotted in fig. 6 as a function of the acceleration voltage with the aperture size as a parameter. The data represent optimum values at low magnetic field. As can be seen from fig. 6, Im increases rapidly with increasing ion energy e V 2. Within certain energy intervals the energy dependence can be approximated by power laws, I m o~ V~, where ~ _
a[mm]. •
4
At÷ / / /
J
I°-6~
•~ tO-7
E
Ol Acceleration
1 I0 voltage ~ (RV]
Fig. 6. M a x i m u m target current as a function of the acceleration voltage. Parameter is the diameter of the beam defining aperture.
LOW E N E R G Y
500
I
I
a[mm]: 400 -
ION BEAM T R A N S P O R T
•
2
•
4
that for not too small apertures the upper limit of current is governed by space charge expansion of the beam in the drift region outside the acceleration field. This idea is supported by a quantitative estimate. Suppose the ion beam converges after having passed through the immersion lens made up by cup extractor and acceleration tube, the waist being formed at the position of the beam defining aperture. To determine the contour of the beam in the drift region we reverse the point of view and calculate the expansion of a beam emerging from the aperture and travelling towards the ion source, a procedure which is justified because of symmetry~S). The relative beam expansion due to space charge effects, r/ro, is given by~5):
/
"~ 300
°2oo
0 0
[
I
5
10
Acceleration
voltage
5
15 V2 [kV]
Fig. 7. Extraction voltage required for transport of the maximum target current vs acceleration voltage. Parameter is the diameter of the beam defining aperture.
ture had been covered with the same uniform current density. The results of fig. 6 may thus indicate that profile distortions are produced under the condition of maximum current transport through (small) apertures. In such a case the distorted beam profiles are characterized by radial current density distributions with a strong central peak and much less intense shoulders~). Information about the ion source plasma can be obtained from fig. 7 in which the extraction voltages V1 required to feed the maximum beam current through the respective aperture are plotted as a function of the acceleration voltage I/2. One can see that V2/V ~ is nearly constant through two orders of magnitude in ion energy. At a given ion energy eV 2 the extraction voltage has to be increased with increasing aperture size. This is due to the fact that for maximum target current the plasma density had to be increased with increasing aperture size. 4. Discussion The experimental set-up used in this study represents a very simple configuration not only for investigating formation and transport of ion beams but also for determining characteristics of the source plasma and the ion beam. The main results are contained in figs. 6 and 7. From the V~ dependence in fig. 6 we conclude
f
r/,o dp = 6.4x .)1 (lnp)~
10 - 3
z
1/}
(4)
r o'
where r0 is the minimum beam radius (in this case r0 = ½a), r is the beam radius at a distance z from the position of the minimum radius, l[/tA] is the beam current, eV~[keV] is the ion energy, M[u] is the ion mass and n ~s the charge state of the ion. With the help of the tabulated integral in eq. (4) one can plot a universal curve for the space-charge spreading of an ion beam in field-free space~5). As an example we apply the results of fig. 6 in the region where I m oc 1/~. For a = 6 mm we have lm/l~ = 0.76 ~ A / k e V i. Moreover M = 40, n = 1 and z-~ 650 mm (i.e. z/ro ~-220). Using these data the right hand side (rh) in eq. (4) yields rh = 3.1. From the universal curve ~5) the respective space charge expansion is found to be r/ro ~-2.9 so that r = 8.7 ram. Similarly the results for a = 2 m m are Ira/V~ = 0.38/zA/keVL rh = 6.5, r/ro = 7.1, r = 7 . 1 mm. According to the above estimates the converging beam starts with an initial diameter of at least 14-18 mm. This corresponds to a filling factor of the immersion lens of up to 20%. Under such conditions pronounced aberrations must be expected. The calculations thus demonstrate that for the geometry used in this study the maximum target currents represent an upper limit set by space charge effects and lens aberrations. Gaussian optics or waist-to-waist transfer 16) are less important under the condition of maximum beam current. Although, for example, the magnification could be calculated from the aperture position and by use of the focal properties of twotube electrostatic lenses ~7) this is of little use be3
6
K. W I T T M A A C K
cause the object size is not known. Image size or waist diameter must be taken into account merely to realize that the above estimates for the space charge effects represent a lower limit. Attributing a certain part of the beam diameter to particle optics would lead to a stronger space charge broadening and, therefore, to a larger initial beam diameter. On the other hand, the energy spread of the ions is likely to play a role, in particular at low energies and. with small apertures. This idea is supported by the results of fig. 6. Quantitative estimates cannot be given on the basis of the present data. One can expect, however, that more pronounced effects due to the energy spread would have been observed if the energy dependence of the maximum current had not been measured under optimum conditions (as deduced from figs. 2 and 3). Finally we discuss the results of fig. 7 which showed that V 2 / V ~ is constant under the respective conditions. This proportionality indicates that the plasma potential is equal to or very close to the anode potential. Moreover it supports the idea that the beam contours were very similar irrespective of the ion energy. 5. Conclusions The present study has shown that a lot of information about ion transport, beam quality and plasma properties can be obtained by means of a very simple experimental set-up. Maximum current transport through a given aperture can be achieved only after optimizing the operation parameters of the ion source. The upper limit of the current through the aperture is then set by space charge effects.
Further optimization of ion gun performance will require a quantitative investigation of source properties such as energy spread and emittance as a function of source pressure, magnetic field, discharge current and discharge voltage.
References I) K. Wittmaack, Nucl. Instr. and Meth. 118 (1974) 99. 2) K. Wittmaack and F. Schulz, J. Vac. Sci. Technol. 10 (1973) 918. 3) K. Wittmaack and F. Schulz, in Proc. 2nd Int. Conf. on Ion sources (eds. F. Viehb6ck, H. Winter and M. Bruck; SGAE, Vienna, 1972) p. 863. 4) K. Wittmaack, J. Maul and F. Schulz, in Proc. 6th lnt. Conf. on Electron and ion beam science and technology (ed. R. Bakish; The Electrochemical Society, Princeton, 1974) p. 164. 5) F. Schulz, K. Wittmaack and J. Maul, Rad. Effects 18 (1973) 211. 6) j. A. McHugh, Rad. Effects 21 (1974) 209. 7) K. Wittmaack and G. Staudewha, ier,. Appl. Phys. Lett.. 27 (1975) 318. 8) K. Wittmaack, Surface Sci. 53 (1975) 626. 9) K. Wittmaack, in Ion beam su(/ace layer analysis (eds. O. Meyer, G. Linker and F. KO_ppeler; Plenum Press, New York, 1976) p. 649. 10) O. AIm6n and K. O. Nielsen, Nucl. Instr. and Meth. ! (1957) 302. ll) C. E. Carlston and G. D. Magnuson, Rev. Sci. Instr. 33 (1962) 905. 12) K. Wittmaack and W. Wach, Nucl. Instr. and Meth., this issue, p. 7. 13) M. V. Ardenne, Tabellen zur angewandten Physik, vol. 2 (VEB Deutscher Verlag,der Wissenschaften, Berlin, 1964). 14) I. Chavet and R. Bernas~ Nucl. Instr. and Meth. 47 (1967) 77. 15) R. Hutter, in Focusing of charged particles, vol. 2 (ed. A. Septier; Academic Press, New York, 1967) p. 3. 16) H. F. Glavish, Nucl. Instr. and Meth. 99 (1972) 109. 27) D. DiChio, S. V. Natali and C. E. Kuyatt, Rev. Sci. lnstr. 45 (1974) 559.