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Characteristics of erbium-ions-producing liquid metal ions sources
ARTICLE IN PRESS
Physica B 340–342 (2003) 1166–1170
Characteristics of erbium-ions-producing liquid metal ions sources Th. Ganetsosa,*, G.L.R. Maira, C.J. Aidinisb, L. Bischoffc a
Section of Solid State Physics, Department of Physics, University of Athens, Panepistimiopolis, Zographos, GR-15784 Athens, Greece b Section of Applied Physics, Department of Physics, University of Athens, Athens, Greece c Institute of Ion Beams and Materials Research, Research Centre Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany
Abstract Focused ion beams (FIBs) are of importance as direct write tools in the microelectronics industry, and beams containing Er ions are of importance for optoelectronics applications. Microstructures doped with Er are attractive because of their intra-4f transition of Er3+ at a wavelength of 1.54 mm, which corresponds to the minimum of attenuation of optical wave-guides. We describe the electric characteristics and mass spectra of two liquid metal alloy ion sources; namely Er70Fe22Cr3Ni5 and Er69Ni31. For the first time in the literature the energy spread of triply charged ions (Er 3+) is reported. r 2003 Elsevier B.V. All rights reserved. Keywords: Liquid metal ion sources; Microelectronics; Electric characteristics
1. Introduction Focused ion beams (FIBs) containing Er ions are potentially applicable in the field of optoelectronics. Er-doped microstructures are of interest, because of their intra-4f transition of Er3+ at a wavelength of 1.54 mm; this corresponds to the minimum of attenuation of optical wave guides [2]. In addition, transition metal ions are optically active wave-guide dopands [3]. FIB systems employ a liquid metal ion source (LMIS) [1]. When the metal of interest is not amenable for LMIS manufacture (e.g., high melting point, reaction of the liquid metal with the solid substrate, etc.), then a suitable alloy is *Corresponding author. Tel.: +30-210-727-6784. E-mail address: [email protected] (Th. Ganetsos).
produced and the desired species is mass-separated from the beam (by, e.g., ExB filter). Er has a melting point of 1529 C. Er ions from a liquid metal alloy ion source (LMAIS) have been produced by Machalet et al. [4], Chao and Steckl [5], and Bischoff and Teichert [6]. This, however, is the first systematic investigation into the physics of two Er-containing LMAISs, namely of Er70Fe22Cr3Ni5 (m.p. 860oC) and Er69Ni31 (m.p. 765oC). Detailed mass spectra were obtained, whereby the Er3+ ion could be identified, massseparated and its energy-spread measured.
2. Results and discussion We first start with the Er70Fe22Cr3Ni5 source. We can see from Fig. 1 that the current–voltage
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.093
ARTICLE IN PRESS Th. Ganetsos et al. / Physica B 340–342 (2003) 1166–1170
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Fig. 1. Current–voltage (i–Vo) characteristics of Er70Fe22Cr3Ni5 LMAIS; emitter temperature 920oC. Inset: Vo versus emitter temperature for i=10 mA.
Fig. 2. The mass spectrum of the LMAIS Er70Fe22Cr3Ni5 LMAIS.
(i–Vo) characteristics of the source are of the familiar linear and relatively steeply rising form, indicating a source of relatively low flow impedance. The inset shows that for a given current, the ion extraction voltage, Vo, diminishes with increasing emitter temperature, but not linearly, as usually is the case and as predicted by theory for liquid metals whose surface tension coefficient (g) decreases linearly with temperature [7]. This behaviour is most probably associated with the
nonlinear decrease of g with temperature in the present case [7]. Fig. 2 shows the mass spectrum of the Er70Fe22Cr3Ni5 source, whereas Fig. 3 shows a portion of the spectrum, where the presence of Er3+ ions in the beam is clearly visible. From Fig. 2 it is clear that Er2+ is the dominant species in the beam and it also dominates, by far, over Er+ and Er3+. As we shall see below, this is in accord with theory. The evaporation field E(n), n being the charge state
ARTICLE IN PRESS Th. Ganetsos et al. / Physica B 340–342 (2003) 1166–1170
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Fig. 3. Portion of the spectrum of Fig. 2 showing the existence of Er3+ ions in the beam.
Table 1 ( for the Values of the evaporation field E(n), n=1,2,3,y in V/A, species emitted by the Er70Fe22Cr3Ni5 source. Values of L, I1, I2, I3 and f (Eq. (1)) are from Refs. [9,15]. E(n) values in parentheses are derived using values of f given in Ref. [16], instead of Ref . [9]. For Er, L , In values are from Ref. [17]; f was estimated at B3 eV since it could not be found in the literature. For rare earth elements for which f could be found, f B3 eV
E(1) E(2) E(3)
Er
Fe
Cr
Ni
2.81 2.03 3.16
4.31 (3.82) 3.55 (3.23) 5.36 (5.04)
2.86 3.01 5.39
3.48 (3.59) 3.56 (3.64) 6.81 (6.9)
of a field-evaporated [8] ion, is defined as the field for which the field-reduced potential energy barrier ðQÞ seen by an escaping ion is equal to zero. It is given as [9] (see also Ref. [10]) o2 X 4peo n E ð nÞ ¼ 3 L þ In nf ; ð1Þ n where L is the heat of evaporation (binding energy) of the bound atom (subsequently ion); P In is the sum of the ionization potentials, if the ion is n-fold ionized; f is the work function of the metal (emitter); and eo is the electric constant. For an ion of charge state n, Brandon’s criterion [9] states that the value of E(n) determines which ionic species will dominate in the beam. If, for
example, E(2) o E(1), then the surface atom is likely to be field evaporated as doubly charged and vice versa. In Table 1, we have calculated E(1), E(2) and E(3) for all the elements constituting the alloy under consideration. It can be seen from this table that Er2+ should be, by far, the dominant species, and so it is (Fig. 2). The same holds true although to a lesser extent, for Fe, again, in agreement with the observation. For Ni and Cr the proportion of singly and doubly charged ions should be comparable, the singly charged species dominating. This is seen to be the case (Figs. 2 and 3) from a comparison of the heights of Ni58 and Ni60 for the singly and doubly charged state and, similarly, from a comparison of the heights of singly and doubly charged Cr ions. Now, given that the values calculated using Eq. (1) are not 100% accurate, the field acting at ( , enough, the emitter appears to be B3 V/A according to Table 1, to cause the direct field evaporation of Er 3+ (Fig. 3), although for such a highly charged ions post-ionization [11, 12] cannot be ruled out. Finally, Fig. 4 shows the energy spread, DE1/2, measured as the full-width at half-maximum (FWHM) of the energy distribution for Er2+, Er3+, Fe+ and Fe2+. For the Er3+ results the Er166 isotope was used (Fig. 3). The spread arises from coulomb interactions within the beam. All the curves, including the upper part of the Er2+
ARTICLE IN PRESS Th. Ganetsos et al. / Physica B 340–342 (2003) 1166–1170
curve obey a 0.3 power law, with regard to the dependence of DE1/2 on emission current. The tail seen at the lower currents in the case of Er+2 has been observed with many singly charged ions, even doubly charged ions, drawn from elemental LMISs (Ref. [13] and references therein). Now, turning our attention to the Er69Ni31 source, we start with its electric characteristics. Fig. 5 shows the i–Vo curve of the source. It is seen that the curve is extremely flat at the beginning, indicating that up to B10 mA, or so, the needle presents a very high impedance to flow; above this value of current there is a sudden rise, the slope of
1169
the curve rising by an order of magnitude. This behaviour is explained as follows. The initially depleted longitudinal grooves on the shank of the needle, at a sufficiently high voltage are replenished with fresh material coming from the reservoir, and this reduces the needle’s flow impedance [14]. The i–Vo characteristics of a LMIS of flow impedance Z are given as [14] " # 1=2 3p 2q=m rt g cos f Vo 2 1 i¼ 1=2 Vox 2Vo ! 3pr2t Z 1þ ð2Þ 1=2 ; 1=2 4rVo q=2m where q/m is the charge-to-mass ratio of the ions; rt is the needle apex radius-of-curvature, or, strictly speaking, the cone base radius; g is the surface tension coefficient of the liquid metal; f is 90 minus the cone base angle; Vo is the ion extraction voltage; Vox is the source extinction voltage, i.e., the value of Vo for which the cone collapses and i- 0 and r is the density of the liquid metal. It is seen that for sufficiently high Z 2 2 Vo Vox ip : ð3Þ Vo
Fig. 4. The energy spread, DE1/2, versus emission current, I, for various ionic species emitted by the Er70Fe22Cr3Ni5 LMAIS.
The ‘‘x’’ symbols in Fig. 5 are a fit of Eq. (3) to the experimental curve, the matching point being at
Fig. 5. Current–voltage characteristics of Er69Ni31 LMAIS; emitter temperature 1200 C. Symbols ‘‘x’’ are a theoretical fit to the experimental curve (Eq. (2)); matching point is at Vo=4 kV, Vox=3.63 kV. Inset: Vo versus emitter temperature, for i=10 mA.
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Th. Ganetsos et al. / Physica B 340–342 (2003) 1166–1170
Fig. 6. The mass spectrum of the Er69Ni31 source.
Vo=4 kV; Vox= 3.63 kV. The inset of Fig. 5 shows the familiar monotonic decrease of the ion extraction voltage (for fixed current) with emitter temperature and, as already mentioned, is due to the reduction of the surface tension coefficient of the liquid with temperature [7]. Fig. 6 shows the mass spectrum of the Er69Ni31 source. Again Er2+ is the dominant species in the beam, the abundance of Er2+ being, as with the previous source, about an order of magnitude larger than that of Er+. In the case of Ni the proportion of the singly and doubly charged species are comparable, as before, and as per the calculations presented in Table 1.
3. Conclusions Concluding this paper, we have tried to describe and understand some fundamental properties of LMISs that emit erbium ions. Improving source understanding contributes to better source construction and, therefore, source operation. An important property of any ion source is the measurement of its beam energy spread. This is crucial for the eventual focusing of the beam, which, in turn, determines the beam spot size. To this end, we (for the first time) succeeded in determining the energy spread of triply charged ions produced by an LMIS.
Acknowledgements Th. G., G.L.R. M. and C.J. A. are grateful to the Rossendorf Research Center for extended visits, where this work was carried out. Special thanks are due to Dr. L. Bischoff, Dr. J. Teichert and Prof. W. Moeller. Th. G. also wishes to extend his thanks to IKY for financial support.
References [1] P.D. Prewet, et al., Focused Ion Beams from Liquid Metal Ion Sources, Research Studies Press, Taunton, UK, 1991. [2] A. Polman, J. Appl. Phys. 82 (1997) 1. [3] A. King, et al., Nucl. Instrum. Methods B 148 (1999) 1044. [4] F. Machalet, et al., Rev. Sci. Instrum. 69 (1998) 1336. [5] L.C. Chao, et al., J. Vac. Sci. Technol. B 17 (1999) 1056. [6] L. Bischoff, et al., J. Phys. D: Appl. Phys. 33 (2000) L69. [7] L. Bischoff, et al., J. Phys. D: Appl. Phys. 33 (2000) 692. [8] E.W. Mueller, J. Appl. Phys. 27 (1956) 474. [9] D.G. Brandon, Surf. Sci. 3 (1964) 1. [10] G.L.R. Mair, et al., J. Phys. D: Appl. Phys. 16 (1983) L209. [11] N. Ernst, Surf. Sci. 61 (1979) 469. [12] R. Haydock, et al., Phys. Rev. Lett. 44 (1980) 1520. [13] G.L.R. Mair, J. Phys. D: Appl. Phys. 29 (1996) 2186. [14] G.L.R. Mair, J. Phys. D: Appl. Phys. 30 (1997) 1945. [15] Smithell’s Metals Reference Book, 5th Edition, Butterworth, London, 1976. [16] M. Von Ardenne, Tabellen zur angewandten Physik, Berlin; 1Bd, 1979, p. 75. [17] R.H. Schum, et al., National Bureau of Standards (USA) Technical Notes, 1973, pp. 270-1–270-8.
Physica B 340–342 (2003) 1166–1170
Characteristics of erbium-ions-producing liquid metal ions sources Th. Ganetsosa,*, G.L.R. Maira, C.J. Aidinisb, L. Bischoffc a
Section of Solid State Physics, Department of Physics, University of Athens, Panepistimiopolis, Zographos, GR-15784 Athens, Greece b Section of Applied Physics, Department of Physics, University of Athens, Athens, Greece c Institute of Ion Beams and Materials Research, Research Centre Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany
Abstract Focused ion beams (FIBs) are of importance as direct write tools in the microelectronics industry, and beams containing Er ions are of importance for optoelectronics applications. Microstructures doped with Er are attractive because of their intra-4f transition of Er3+ at a wavelength of 1.54 mm, which corresponds to the minimum of attenuation of optical wave-guides. We describe the electric characteristics and mass spectra of two liquid metal alloy ion sources; namely Er70Fe22Cr3Ni5 and Er69Ni31. For the first time in the literature the energy spread of triply charged ions (Er 3+) is reported. r 2003 Elsevier B.V. All rights reserved. Keywords: Liquid metal ion sources; Microelectronics; Electric characteristics
1. Introduction Focused ion beams (FIBs) containing Er ions are potentially applicable in the field of optoelectronics. Er-doped microstructures are of interest, because of their intra-4f transition of Er3+ at a wavelength of 1.54 mm; this corresponds to the minimum of attenuation of optical wave guides [2]. In addition, transition metal ions are optically active wave-guide dopands [3]. FIB systems employ a liquid metal ion source (LMIS) [1]. When the metal of interest is not amenable for LMIS manufacture (e.g., high melting point, reaction of the liquid metal with the solid substrate, etc.), then a suitable alloy is *Corresponding author. Tel.: +30-210-727-6784. E-mail address: [email protected] (Th. Ganetsos).
produced and the desired species is mass-separated from the beam (by, e.g., ExB filter). Er has a melting point of 1529 C. Er ions from a liquid metal alloy ion source (LMAIS) have been produced by Machalet et al. [4], Chao and Steckl [5], and Bischoff and Teichert [6]. This, however, is the first systematic investigation into the physics of two Er-containing LMAISs, namely of Er70Fe22Cr3Ni5 (m.p. 860oC) and Er69Ni31 (m.p. 765oC). Detailed mass spectra were obtained, whereby the Er3+ ion could be identified, massseparated and its energy-spread measured.
2. Results and discussion We first start with the Er70Fe22Cr3Ni5 source. We can see from Fig. 1 that the current–voltage
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.093
ARTICLE IN PRESS Th. Ganetsos et al. / Physica B 340–342 (2003) 1166–1170
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Fig. 1. Current–voltage (i–Vo) characteristics of Er70Fe22Cr3Ni5 LMAIS; emitter temperature 920oC. Inset: Vo versus emitter temperature for i=10 mA.
Fig. 2. The mass spectrum of the LMAIS Er70Fe22Cr3Ni5 LMAIS.
(i–Vo) characteristics of the source are of the familiar linear and relatively steeply rising form, indicating a source of relatively low flow impedance. The inset shows that for a given current, the ion extraction voltage, Vo, diminishes with increasing emitter temperature, but not linearly, as usually is the case and as predicted by theory for liquid metals whose surface tension coefficient (g) decreases linearly with temperature [7]. This behaviour is most probably associated with the
nonlinear decrease of g with temperature in the present case [7]. Fig. 2 shows the mass spectrum of the Er70Fe22Cr3Ni5 source, whereas Fig. 3 shows a portion of the spectrum, where the presence of Er3+ ions in the beam is clearly visible. From Fig. 2 it is clear that Er2+ is the dominant species in the beam and it also dominates, by far, over Er+ and Er3+. As we shall see below, this is in accord with theory. The evaporation field E(n), n being the charge state
ARTICLE IN PRESS Th. Ganetsos et al. / Physica B 340–342 (2003) 1166–1170
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Fig. 3. Portion of the spectrum of Fig. 2 showing the existence of Er3+ ions in the beam.
Table 1 ( for the Values of the evaporation field E(n), n=1,2,3,y in V/A, species emitted by the Er70Fe22Cr3Ni5 source. Values of L, I1, I2, I3 and f (Eq. (1)) are from Refs. [9,15]. E(n) values in parentheses are derived using values of f given in Ref. [16], instead of Ref . [9]. For Er, L , In values are from Ref. [17]; f was estimated at B3 eV since it could not be found in the literature. For rare earth elements for which f could be found, f B3 eV
E(1) E(2) E(3)
Er
Fe
Cr
Ni
2.81 2.03 3.16
4.31 (3.82) 3.55 (3.23) 5.36 (5.04)
2.86 3.01 5.39
3.48 (3.59) 3.56 (3.64) 6.81 (6.9)
of a field-evaporated [8] ion, is defined as the field for which the field-reduced potential energy barrier ðQÞ seen by an escaping ion is equal to zero. It is given as [9] (see also Ref. [10]) o2 X 4peo n E ð nÞ ¼ 3 L þ In nf ; ð1Þ n where L is the heat of evaporation (binding energy) of the bound atom (subsequently ion); P In is the sum of the ionization potentials, if the ion is n-fold ionized; f is the work function of the metal (emitter); and eo is the electric constant. For an ion of charge state n, Brandon’s criterion [9] states that the value of E(n) determines which ionic species will dominate in the beam. If, for
example, E(2) o E(1), then the surface atom is likely to be field evaporated as doubly charged and vice versa. In Table 1, we have calculated E(1), E(2) and E(3) for all the elements constituting the alloy under consideration. It can be seen from this table that Er2+ should be, by far, the dominant species, and so it is (Fig. 2). The same holds true although to a lesser extent, for Fe, again, in agreement with the observation. For Ni and Cr the proportion of singly and doubly charged ions should be comparable, the singly charged species dominating. This is seen to be the case (Figs. 2 and 3) from a comparison of the heights of Ni58 and Ni60 for the singly and doubly charged state and, similarly, from a comparison of the heights of singly and doubly charged Cr ions. Now, given that the values calculated using Eq. (1) are not 100% accurate, the field acting at ( , enough, the emitter appears to be B3 V/A according to Table 1, to cause the direct field evaporation of Er 3+ (Fig. 3), although for such a highly charged ions post-ionization [11, 12] cannot be ruled out. Finally, Fig. 4 shows the energy spread, DE1/2, measured as the full-width at half-maximum (FWHM) of the energy distribution for Er2+, Er3+, Fe+ and Fe2+. For the Er3+ results the Er166 isotope was used (Fig. 3). The spread arises from coulomb interactions within the beam. All the curves, including the upper part of the Er2+
ARTICLE IN PRESS Th. Ganetsos et al. / Physica B 340–342 (2003) 1166–1170
curve obey a 0.3 power law, with regard to the dependence of DE1/2 on emission current. The tail seen at the lower currents in the case of Er+2 has been observed with many singly charged ions, even doubly charged ions, drawn from elemental LMISs (Ref. [13] and references therein). Now, turning our attention to the Er69Ni31 source, we start with its electric characteristics. Fig. 5 shows the i–Vo curve of the source. It is seen that the curve is extremely flat at the beginning, indicating that up to B10 mA, or so, the needle presents a very high impedance to flow; above this value of current there is a sudden rise, the slope of
1169
the curve rising by an order of magnitude. This behaviour is explained as follows. The initially depleted longitudinal grooves on the shank of the needle, at a sufficiently high voltage are replenished with fresh material coming from the reservoir, and this reduces the needle’s flow impedance [14]. The i–Vo characteristics of a LMIS of flow impedance Z are given as [14] " # 1=2 3p 2q=m rt g cos f Vo 2 1 i¼ 1=2 Vox 2Vo ! 3pr2t Z 1þ ð2Þ 1=2 ; 1=2 4rVo q=2m where q/m is the charge-to-mass ratio of the ions; rt is the needle apex radius-of-curvature, or, strictly speaking, the cone base radius; g is the surface tension coefficient of the liquid metal; f is 90 minus the cone base angle; Vo is the ion extraction voltage; Vox is the source extinction voltage, i.e., the value of Vo for which the cone collapses and i- 0 and r is the density of the liquid metal. It is seen that for sufficiently high Z 2 2 Vo Vox ip : ð3Þ Vo
Fig. 4. The energy spread, DE1/2, versus emission current, I, for various ionic species emitted by the Er70Fe22Cr3Ni5 LMAIS.
The ‘‘x’’ symbols in Fig. 5 are a fit of Eq. (3) to the experimental curve, the matching point being at
Fig. 5. Current–voltage characteristics of Er69Ni31 LMAIS; emitter temperature 1200 C. Symbols ‘‘x’’ are a theoretical fit to the experimental curve (Eq. (2)); matching point is at Vo=4 kV, Vox=3.63 kV. Inset: Vo versus emitter temperature, for i=10 mA.
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Fig. 6. The mass spectrum of the Er69Ni31 source.
Vo=4 kV; Vox= 3.63 kV. The inset of Fig. 5 shows the familiar monotonic decrease of the ion extraction voltage (for fixed current) with emitter temperature and, as already mentioned, is due to the reduction of the surface tension coefficient of the liquid with temperature [7]. Fig. 6 shows the mass spectrum of the Er69Ni31 source. Again Er2+ is the dominant species in the beam, the abundance of Er2+ being, as with the previous source, about an order of magnitude larger than that of Er+. In the case of Ni the proportion of the singly and doubly charged species are comparable, as before, and as per the calculations presented in Table 1.
3. Conclusions Concluding this paper, we have tried to describe and understand some fundamental properties of LMISs that emit erbium ions. Improving source understanding contributes to better source construction and, therefore, source operation. An important property of any ion source is the measurement of its beam energy spread. This is crucial for the eventual focusing of the beam, which, in turn, determines the beam spot size. To this end, we (for the first time) succeeded in determining the energy spread of triply charged ions produced by an LMIS.
Acknowledgements Th. G., G.L.R. M. and C.J. A. are grateful to the Rossendorf Research Center for extended visits, where this work was carried out. Special thanks are due to Dr. L. Bischoff, Dr. J. Teichert and Prof. W. Moeller. Th. G. also wishes to extend his thanks to IKY for financial support.
References [1] P.D. Prewet, et al., Focused Ion Beams from Liquid Metal Ion Sources, Research Studies Press, Taunton, UK, 1991. [2] A. Polman, J. Appl. Phys. 82 (1997) 1. [3] A. King, et al., Nucl. Instrum. Methods B 148 (1999) 1044. [4] F. Machalet, et al., Rev. Sci. Instrum. 69 (1998) 1336. [5] L.C. Chao, et al., J. Vac. Sci. Technol. B 17 (1999) 1056. [6] L. Bischoff, et al., J. Phys. D: Appl. Phys. 33 (2000) L69. [7] L. Bischoff, et al., J. Phys. D: Appl. Phys. 33 (2000) 692. [8] E.W. Mueller, J. Appl. Phys. 27 (1956) 474. [9] D.G. Brandon, Surf. Sci. 3 (1964) 1. [10] G.L.R. Mair, et al., J. Phys. D: Appl. Phys. 16 (1983) L209. [11] N. Ernst, Surf. Sci. 61 (1979) 469. [12] R. Haydock, et al., Phys. Rev. Lett. 44 (1980) 1520. [13] G.L.R. Mair, J. Phys. D: Appl. Phys. 29 (1996) 2186. [14] G.L.R. Mair, J. Phys. D: Appl. Phys. 30 (1997) 1945. [15] Smithell’s Metals Reference Book, 5th Edition, Butterworth, London, 1976. [16] M. Von Ardenne, Tabellen zur angewandten Physik, Berlin; 1Bd, 1979, p. 75. [17] R.H. Schum, et al., National Bureau of Standards (USA) Technical Notes, 1973, pp. 270-1–270-8.