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Ion energy spread of a universal high current ion source for ions from solids
NUCLEAR
INSTRUMENTS
ION
ENERGY
AND
60(1968)
METHODS
SPREAD
231-232; 0 NORTH-HOLLAND
OF A UNIVERSAL
HIGH
FOR
SOLIDS
IONS
FROM
CURRENT
ION
PUBLISHING
co.
SOURCE
E. F.KRIMMEL Institut fir
Angewandte
Physik,
Universitiit
Tiibingen, Germany
Received 19 December 1967 A recently developed universal ion source for ions produced from solids emits ion currents at the present up to 26 mA by applied acceleration potentials of 50 kV. The energy spread of the emitted ions was measured to have an upper limit of several eV only.
The communication presented contributes some new results about the properties of a high current ion source for ions produced from solids’s2) with respect to the energy spread of the ions. The operation of the ion gun shall be indicated briefly (fig. 1). An electron beam of Z, = 10 mA, accelerated by a potential U, in the range up to 50 kV, is focused on to a target of a solid T from which one intends to obtain ions. Positive ions emitted by the target T are accelerated and collected by the collector electrode S, if one applies to S a negative potential Uisr with regard to the grounded target T and housing H. The ion acceleration potential Uist between S and T has to be higher than several kV to avoid recombination of slow ions with electrons which are around the target in large quantities due to reflection, secondary and thermionic emission. The diagram, fig. 2, shows the time dependence of the ion current in arbitrary units. If the irradiation of the target T with the full electron current Z, starts at the time I,, a very small residual ion current ZistO is measured at the collector S. Simultaneously one observes a very weak Lilienfeld radiation3p4) of the area of T which is hit by the electrons. Continuing the high electron current irradiation the target T is heated up and starts to evaporate at the time t,. At the same time the ion current increases by several orders. Currents are measured up to 26 mA by potentials Uist = -50 kV. At the time t, (usually some seconds), the ion source
reaches equilibrium. Coincidently with the optimal ion current Uis, a plasma channel P of high plasma density is built up (fig. 3). The channel P is covered by a thin film F of molten target materia15). Probably a big fraction of ions originates in the plasma channel P. The source was tested with Pt, Au, Ag, Cu, Fe, W, C, Si, constantan, glass and mica. The energy spread of the emitted ions can be appraised in a very simple way avoiding several difficulties associated with other methods. If T is grounded as the housing H, all ions extracted from the immediate surrounding of the emitting spot have to run to the collecting plate S and hence can be measured by the ammeter I. Applying a small negative potential Uith at T with respect to the grounded housing H creates a saddle point of the potential distribution between T and S. The ions climb over this saddle point if their initial energy is sufficiently high. Increasing the height of the saddle point by increasing the potential Uiththe ion current collected at the collector S decreases according to the direction and energy distribution of the emitted ions. If the height of the saddle point Uispexceeds the initial energy of the ions, no ion will reach S. The Uisp at which the ion current becomes zero is a measure for the energy spread of the ions. The potential distribution of the ion gun depending on Uith was determined on a model of the source with the aid of an electrolytical potential trough. The accuracy was 0.3 ‘loo referring to thepotential Ui,,.Thusfor Uist = 35 kV, potential differ-
Lr t
Is,
IO
to 11 ‘2 Fig. 2. Time dependence of the ion current.
Fig. 1. Circuit diagram of the ion gun.
231
232
1~. F. KRIMMEL F
P
Fig. 3. Diagram of the plasma channel P inside of the target T.
ences of about 10 V can be estimated. To obtain a more accurate determination of the saddle point of the ion source itself, T was replaced by a thermionic electron source and all potentials were changed in their polarity. The results are shown in fig. 4. The electron current les t drops to zero if Ueth exceeds + 50 V. Hence the saddle point has a height of about U~sp = + 1 V. Ions produced by an initial current l~st = 56 yA show the same result for Uith = - 6 0 V and correspondingly for I~st = 150yA and [ U,h I < + 150 V. Thus the energy spread of the ions is in the range of several eV. This r~sult is supported by the model measurement where Usp was found to be smaller than 10 V for Uth = 100 V. The normalized curves of electrons and ions plotted in fig. 4 show that at least at low ion currents the energy distribution of the ions should not be too different from that of thermionic electrons. The energy distribution of the ions for ion currents higher than 150/~A cannot be determined by this method because the surface of the emitting target T does change too fast, thereby causing a fast change of the saddle point. Only the beginning and the end point of the Ui th I~st plot can be estimated. These points are in the same range as the points of the 150/~A run, indicating that also the energy spread of high ion currents has an upper limit of several eV only. These results indicate that the main part of the produced ions originates in the immediate surrounding -
-
of the surface of the target T. Not included are measurements of secondary reactions, e.g. of ions with neutrals in the space between target T and collector S. Such reactions depend on the pressure and the construction of the system and they enlarge the energy spread of the beam and hence are interesting for the application of the total system but do not give any indication of the mechanism of the production of primary ions.
Uth
Volt
150-
100
\
\ \ \ \
0
"
r
wA Ist
I 2 3 4 5 6 7 Electrons 10 20 3,0 40 50 60 70 Ions 4'0 80 120 160 Ions
(35kV) * (35kV) + (40kV) a
Fig. 4. Diagram of the normalized emission currents/st vs Uth; parameter is Ue~t = + 35 kV; Ulst = - 3 5 kV and - 4 0 kV.
The author wishes to express his gratitude to Professor G. M611enstedt for his support of this work and also to thank O. K/Shler for careful workmanship of the ion source.
References 1) E. F. Krimmel, Rev. Sci. Instr. 37 (1966) 678. 2) E. F. Krimmel and A. Gordon, Z. angew. Phys. 22 (1966) 1. 3) j. E. Lilienfeld, Phys. Z. 20 (1919) 280. 4) H. Boersch, C. Radeloffand G. Sauerbrey, Z. Phys. 165 (1961)
464. 5) H. Schwarz, J. appl. Phys. 35 (1964) 2020.
INSTRUMENTS
ION
ENERGY
AND
60(1968)
METHODS
SPREAD
231-232; 0 NORTH-HOLLAND
OF A UNIVERSAL
HIGH
FOR
SOLIDS
IONS
FROM
CURRENT
ION
PUBLISHING
co.
SOURCE
E. F.KRIMMEL Institut fir
Angewandte
Physik,
Universitiit
Tiibingen, Germany
Received 19 December 1967 A recently developed universal ion source for ions produced from solids emits ion currents at the present up to 26 mA by applied acceleration potentials of 50 kV. The energy spread of the emitted ions was measured to have an upper limit of several eV only.
The communication presented contributes some new results about the properties of a high current ion source for ions produced from solids’s2) with respect to the energy spread of the ions. The operation of the ion gun shall be indicated briefly (fig. 1). An electron beam of Z, = 10 mA, accelerated by a potential U, in the range up to 50 kV, is focused on to a target of a solid T from which one intends to obtain ions. Positive ions emitted by the target T are accelerated and collected by the collector electrode S, if one applies to S a negative potential Uisr with regard to the grounded target T and housing H. The ion acceleration potential Uist between S and T has to be higher than several kV to avoid recombination of slow ions with electrons which are around the target in large quantities due to reflection, secondary and thermionic emission. The diagram, fig. 2, shows the time dependence of the ion current in arbitrary units. If the irradiation of the target T with the full electron current Z, starts at the time I,, a very small residual ion current ZistO is measured at the collector S. Simultaneously one observes a very weak Lilienfeld radiation3p4) of the area of T which is hit by the electrons. Continuing the high electron current irradiation the target T is heated up and starts to evaporate at the time t,. At the same time the ion current increases by several orders. Currents are measured up to 26 mA by potentials Uist = -50 kV. At the time t, (usually some seconds), the ion source
reaches equilibrium. Coincidently with the optimal ion current Uis, a plasma channel P of high plasma density is built up (fig. 3). The channel P is covered by a thin film F of molten target materia15). Probably a big fraction of ions originates in the plasma channel P. The source was tested with Pt, Au, Ag, Cu, Fe, W, C, Si, constantan, glass and mica. The energy spread of the emitted ions can be appraised in a very simple way avoiding several difficulties associated with other methods. If T is grounded as the housing H, all ions extracted from the immediate surrounding of the emitting spot have to run to the collecting plate S and hence can be measured by the ammeter I. Applying a small negative potential Uith at T with respect to the grounded housing H creates a saddle point of the potential distribution between T and S. The ions climb over this saddle point if their initial energy is sufficiently high. Increasing the height of the saddle point by increasing the potential Uiththe ion current collected at the collector S decreases according to the direction and energy distribution of the emitted ions. If the height of the saddle point Uispexceeds the initial energy of the ions, no ion will reach S. The Uisp at which the ion current becomes zero is a measure for the energy spread of the ions. The potential distribution of the ion gun depending on Uith was determined on a model of the source with the aid of an electrolytical potential trough. The accuracy was 0.3 ‘loo referring to thepotential Ui,,.Thusfor Uist = 35 kV, potential differ-
Lr t
Is,
IO
to 11 ‘2 Fig. 2. Time dependence of the ion current.
Fig. 1. Circuit diagram of the ion gun.
231
232
1~. F. KRIMMEL F
P
Fig. 3. Diagram of the plasma channel P inside of the target T.
ences of about 10 V can be estimated. To obtain a more accurate determination of the saddle point of the ion source itself, T was replaced by a thermionic electron source and all potentials were changed in their polarity. The results are shown in fig. 4. The electron current les t drops to zero if Ueth exceeds + 50 V. Hence the saddle point has a height of about U~sp = + 1 V. Ions produced by an initial current l~st = 56 yA show the same result for Uith = - 6 0 V and correspondingly for I~st = 150yA and [ U,h I < + 150 V. Thus the energy spread of the ions is in the range of several eV. This r~sult is supported by the model measurement where Usp was found to be smaller than 10 V for Uth = 100 V. The normalized curves of electrons and ions plotted in fig. 4 show that at least at low ion currents the energy distribution of the ions should not be too different from that of thermionic electrons. The energy distribution of the ions for ion currents higher than 150/~A cannot be determined by this method because the surface of the emitting target T does change too fast, thereby causing a fast change of the saddle point. Only the beginning and the end point of the Ui th I~st plot can be estimated. These points are in the same range as the points of the 150/~A run, indicating that also the energy spread of high ion currents has an upper limit of several eV only. These results indicate that the main part of the produced ions originates in the immediate surrounding -
-
of the surface of the target T. Not included are measurements of secondary reactions, e.g. of ions with neutrals in the space between target T and collector S. Such reactions depend on the pressure and the construction of the system and they enlarge the energy spread of the beam and hence are interesting for the application of the total system but do not give any indication of the mechanism of the production of primary ions.
Uth
Volt
150-
100
\
\ \ \ \
0
"
r
wA Ist
I 2 3 4 5 6 7 Electrons 10 20 3,0 40 50 60 70 Ions 4'0 80 120 160 Ions
(35kV) * (35kV) + (40kV) a
Fig. 4. Diagram of the normalized emission currents/st vs Uth; parameter is Ue~t = + 35 kV; Ulst = - 3 5 kV and - 4 0 kV.
The author wishes to express his gratitude to Professor G. M611enstedt for his support of this work and also to thank O. K/Shler for careful workmanship of the ion source.
References 1) E. F. Krimmel, Rev. Sci. Instr. 37 (1966) 678. 2) E. F. Krimmel and A. Gordon, Z. angew. Phys. 22 (1966) 1. 3) j. E. Lilienfeld, Phys. Z. 20 (1919) 280. 4) H. Boersch, C. Radeloffand G. Sauerbrey, Z. Phys. 165 (1961)
464. 5) H. Schwarz, J. appl. Phys. 35 (1964) 2020.