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Electron emission characteristics of solidified gold alloy liquid metal ion sources
Applied Surface Science 146 Ž1999. 134–137
Electron emission characteristics of solidified gold alloy liquid metal ion sources Wolfram Knapp a
a,)
, Lothar Bischoff b, Jochen Teichert
b
Otto-Õon-Guericke UniÕersitat ¨ Magdeburg, IEP, PF 4120, D-39016 Magdeburg, Germany b Forschungszentrum Rossendorf e.V., PF 51 01 19, D-01314 Dresden, Germany
Abstract Solidified liquid metal ion sources ŽLMISs. operating with Au alloy wetted hair-pin emitters can be used as high-intensity electron point sources for application in the field of ultrahigh vacuum techniques. A nanotip emitter on a solidified LMIS emitter can be formed by quenching during ion emission mode. I–V characteristics and the performance of the electron emitting LMIS are presented. q 1999 Elsevier Science B.V. All rights reserved. PACS: 07.77.K; 85.45.F Keywords: Electron source; Field emitter; Solidified liquid metal ion source; Nanotip emitter
1. Introduction In the last decades, liquid metal ion sources ŽLMISs. have attracted wide interest as high-brightness point sources for focused ion beam applications such as micromachining and imaging, as well as objects of experimental and theoretical investigation. Some experiments were reported that describe the use of LMISs as electron sources. The electron emission behavior was investigated for liquid w1–4x and for solid source material coated emitters w5,6x. For our studies, solidified alloy LMISs were used as vacuum field emission electron point sources. The source material was chosen with respect to ultrahigh vacuum applications, operating at elevated temperatures, and also under air exposure conditions. Gold
)
Corresponding author. Tel.: q49-391-671-2505; Fax: q49391-671-8109; E-mail: [email protected]
alloys are well suited for this task since their melting temperatures at the eutectic concentration are high enough to form a stable tip at room temperature, compared to most-used LMIS material gallium with a melting point of 29.68C. Inertness against air is also important. Fig. 1 shows the experimental setup for the investigation of the dual ionrelectron source operation in a high vacuum chamber.
2. Solidified LMIS fabrication and temperature characteristics The LMISs used consist of directly heated hairpin-like tungsten emitters. The needle tips were prepared by electrochemical etching in 1 N NaOH to a final tip radius of about 3 mm. The emitters were cleaned by chemical treatment and by heating in a vacuum. Wetting and filling of the reservoir were
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 6 0 - 4
W. Knapp et al.r Applied Surface Science 146 (1999) 134–137
135
used for comparison shows a significantly different slope in the heating current–temperature characteristic. It must be noted here that the measured temperatures were that of the liquid drop, because it is not possible to determine the temperature at the emitter tip due to its small area. In our experiments, the heating current was varied between 1.7 A and 2.4 A corresponding to emitter temperatures of about 3508C to 7008C.
3. Ion emission mode and formation of a nanotip
Fig. 1. Experimental setup.
performed by dipping the heated needle tips in the molten alloy using an electrically heated crucible w7–9x. For our experiments, Au 73 Ge 27 and Au 77 Ge14 Si 9 alloy were chosen for their very good wetting properties, especially the ternary alloy. The melting point of the Au 73 Ge 27 alloy is 3658C and that of the Au 77 Ge14 Si 9 alloy is some degrees lower. The heating current at the melting point of the alloy was determined by an optical microscope. The relationship between heating current and the emitter temperature was measured using an optical pyrometer. Fig. 2 shows the results compared to LMISs applying other materials. Because of the very high thermal conductivity of the gold alloys, the temperature of the emitter drop is close to that of the tungsten filament. In contrast, a Co 36 Nd 64 emitter
Fig. 2. Temperature of different liquid metal ion source emitters as a function of heating current.
At the beginning of experiments, the LMIS was operated in the ion emission mode. Heating current, ion emission current, and extraction voltage were controlled and adjusted for stable field-induced ion emission. The typical ion emission current was in the range from 2 to 30 mA. The corresponding I–V characteristics for the AuGeSi LMIS depending on the heating current are shown in Fig. 3. Surprisingly, the ion current becomes lower with increasing temperature. In general, one would expect an increase of the current due to the lowering of the viscosity as observed for other LMIS materials at higher temperatures. The AuGeSi alloy is a very fluid material and moves to the hotter parts of the filament, so the tip becomes impoverished, causing the current to decrease. Points A, B, and C in Fig. 3 indicate emission currents for later quenching processes. Another important feature of an LMIS is the relationship between the electrical field strength when ion emission starts and the tip radius, which is investigated
Fig. 3. I–V characteristics for ion emission under different heating conditions.
136
W. Knapp et al.r Applied Surface Science 146 (1999) 134–137
switching off. The extraction voltage had to be applied up to the ion emission break off. After quenching, a nanoemitter tip was formed on the top of the emitter needle. The radii of the nanoemitter tips were measured by scanning electron microscopy imaging. After quenching at 10 mA ion emission current, the nanoemitter tip radius was determined to be about 10 nm, which is demonstrated in Fig. 4 for an Au 77 Ge14 Si 9 emitter.
4. Electron emission properties and emitter regeneration After a cooling time of about half an hour, the quenched emitter can be used as an electron source. Therefore, voltage potentials must be switched and the solidified LMIS works as an electron emitter in the cold cathode mode. A well-prepared LMIS and proper quenching are the key points for good electron emission performance. Experiments showed that stable ion emission results also in stable electron emission. The electron mode current–voltage characteristics are presented in Fig. 5 for the AuGeSi LMIS quenched at three different ion emission currents at a constant heating current. It was found that quenching at lower ion emission currents improves the electron field emission characteristic caused by smaller nanoemitter tip radii. Field strength E at the emitter tip can be expressed by the equation E s b V, where V is the
Fig. 4. Scanning electron microscopy ŽSEM. images of an Au 77 Ge14 Si 9 emitter. Ža. General view of solidified LMIS-emitter. Žb. Emitter-top nanotip with a radius of less than 10 nm.
and discussed in detail elsewhere w10–15x. This relationship depends on the source material used, especially on its viscosity in the liquid phase. To form a frozen nanotip for electron emission, the LMIS was operated some minutes in a stable ion emission mode. Then, it was abruptly quenched by switching off the heating current. Due to the heat capacity of the drop, the LMIS still operated some seconds after
Fig. 5. I–V characteristics for electron emission in dependence on quenching conditions. The three curves correspond to the parameters indicated in Fig. 3.
W. Knapp et al.r Applied Surface Science 146 (1999) 134–137
137
5. Conclusions
Fig. 6. Fowler–Nordheim plot of electron emission from a Au 73 Ge 27 emitter.
extraction voltage and b is a geometrical factor given in my1 . Assuming a field strength of E s 2 = 10 9 Vrm in the case of stable field emission, a b on the order of 10 6 my1 is obtained. In our setup, the distance between the emitter tip and the extraction electrode was held constant at 0.5 mm. The geometrical dimension of the nanotip and electron emission properties are comparable to the results of others w16,17x, but their preparation techniques are more difficult, especially in the case of asperity tips w18x. The Fowler–Nordheim plot for the AuGe LMIS in the electron emission mode is shown in Fig. 6. Stable electron field emission behavior was found in the current range of 0.2 to 3 mA. At higher electron emission currents, space charge effects limit the field emission. An increase of the electron emission current above about 15–25 mA can destroy the nanotips by overheating or explosive field emission w19x, which limits the lifetime of the source. At a pressure higher than 3 = 10y7 mbar in the vacuum chamber, microarc breakdowns may occur w20x. In situ regeneration of the electron emission properties under vacuum conditions is possible by repeating quenching in order to form a new nanotip. After the in situ regeneration of the nanotip, a gradual change of the emission characteristic to a higher voltage was found for ion emission as well as for electron emission. Of course, after a number of stress and quenching cycles, the electron emission characteristic deteriorates. Another advance in solidified LMIS is the possibility of ex situ regeneration by wetting needles again.
Solidified LMIS operating with tungsten emitters wetted with Au alloys can be used as high intensity electron point sources. Electron emission occurs from a nanotip formed on a solidified LMIS emitter during quenching. The arrangement provides field enhancement similar to that for asperity tips and other supertips. The I–V characteristics and the overall performance of the electron-emitting LMIS were determined. Applications of these sources can be found in the field of ultrahigh vacuum techniques. In further investigations, long term stability tests in a wide range of vacuum pressure and with different residual gas conditions will be performed and other source materials will be studied. References w1x L.W. Swanson, G.A. Schwind, J. Appl. Phys. 49 Ž1978. 5655. w2x K. Hata, S. Nishigaki, M. Watanabe, T. Noda, H. Tamura, H. Watanabe, J. Phys. 47 Ž1986. C7–375. w3x J. Mitterauer, J. Vac. Sci. Technol. B 13 Ž1995. 625. w4x J. Mitterauer, Appl. Surf. Sci. 87r88 Ž1995. 79. w5x K.A. Rao, A.E. Bell, G.A. Schwind, L.W. Swanson, J. Vac. Sci. Technol. B 7 Ž1989. 1793. w6x L.W. Chen, Y.L. Wang, Appl. Phys. Lett. 72 Ž1998. 389. w7x A. Wagner, T.M. Hall, J. Vac. Sci. Technol. 16 Ž1979. 1871. w8x P.D. Prewett, G.L.R. Mair, Focused Ion Beams From Liquid Metal Ion Sources, Research Studies Press, Taunton, 1991. w9x B. Praprotnik, Emissionsverhalten von Flussigmetall-Ionen¨ rElektronen-Emitter, Thesis Žin German., 1st edn., Koster¨ Verlag, Berlin, 1995. w10x R.G. Forbes, Appl. Surf. Sci. 67 Ž1993. 9. w11x W. Liu, R.G. Forbes, Appl. Surf. Sci. 87r88 Ž1995. 122. w12x E. Hesse, W. Driesel, Ch. Dietzsch, L. Bischoff, J. Teichert, Jpn. J. Appl. Phys. 35 Ž1996. 5564. w13x W. Driesel, Ch. Dietzsch, Appl. Surf. Sci. 93 Ž1996. 179. w14x W. Driesel, Ch. Dietzsch, E. Hesse, L. Bischoff, J. Teichert, J. Vac. Sci. Technol. B 14 Ž1996. 1621. w15x G.L.R. Mair, R.G. Forbes, Surf. Sci. 266 Ž1992. 180. w16x K. Jousten, K. Bohringer, R. Borret, S. Kalbitzer, Ultrami¨ ¨ croscopy Ž1988. 301. w17x H.W.P. Koops, Ch. Schossler, Proc. 9th Int. Vac. Micro¨ electr. Conf., St. Petersburg Ž1996. 458. w18x T.M. Mayer, D.P. Adams, B.M. Marder, J. Vac. Sci. Technol. B 14 Ž1996. 2438. w19x K. Hata, S. Nishigaki, M. Ionue, T. Noda, H. Tamura, J. Phys. 48 Ž1987. C6–177. w20x R.V. Latham ŽEd.., High Voltage Vacuum Insulation, Academic Press, London, 1995.
Electron emission characteristics of solidified gold alloy liquid metal ion sources Wolfram Knapp a
a,)
, Lothar Bischoff b, Jochen Teichert
b
Otto-Õon-Guericke UniÕersitat ¨ Magdeburg, IEP, PF 4120, D-39016 Magdeburg, Germany b Forschungszentrum Rossendorf e.V., PF 51 01 19, D-01314 Dresden, Germany
Abstract Solidified liquid metal ion sources ŽLMISs. operating with Au alloy wetted hair-pin emitters can be used as high-intensity electron point sources for application in the field of ultrahigh vacuum techniques. A nanotip emitter on a solidified LMIS emitter can be formed by quenching during ion emission mode. I–V characteristics and the performance of the electron emitting LMIS are presented. q 1999 Elsevier Science B.V. All rights reserved. PACS: 07.77.K; 85.45.F Keywords: Electron source; Field emitter; Solidified liquid metal ion source; Nanotip emitter
1. Introduction In the last decades, liquid metal ion sources ŽLMISs. have attracted wide interest as high-brightness point sources for focused ion beam applications such as micromachining and imaging, as well as objects of experimental and theoretical investigation. Some experiments were reported that describe the use of LMISs as electron sources. The electron emission behavior was investigated for liquid w1–4x and for solid source material coated emitters w5,6x. For our studies, solidified alloy LMISs were used as vacuum field emission electron point sources. The source material was chosen with respect to ultrahigh vacuum applications, operating at elevated temperatures, and also under air exposure conditions. Gold
)
Corresponding author. Tel.: q49-391-671-2505; Fax: q49391-671-8109; E-mail: [email protected]
alloys are well suited for this task since their melting temperatures at the eutectic concentration are high enough to form a stable tip at room temperature, compared to most-used LMIS material gallium with a melting point of 29.68C. Inertness against air is also important. Fig. 1 shows the experimental setup for the investigation of the dual ionrelectron source operation in a high vacuum chamber.
2. Solidified LMIS fabrication and temperature characteristics The LMISs used consist of directly heated hairpin-like tungsten emitters. The needle tips were prepared by electrochemical etching in 1 N NaOH to a final tip radius of about 3 mm. The emitters were cleaned by chemical treatment and by heating in a vacuum. Wetting and filling of the reservoir were
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 6 0 - 4
W. Knapp et al.r Applied Surface Science 146 (1999) 134–137
135
used for comparison shows a significantly different slope in the heating current–temperature characteristic. It must be noted here that the measured temperatures were that of the liquid drop, because it is not possible to determine the temperature at the emitter tip due to its small area. In our experiments, the heating current was varied between 1.7 A and 2.4 A corresponding to emitter temperatures of about 3508C to 7008C.
3. Ion emission mode and formation of a nanotip
Fig. 1. Experimental setup.
performed by dipping the heated needle tips in the molten alloy using an electrically heated crucible w7–9x. For our experiments, Au 73 Ge 27 and Au 77 Ge14 Si 9 alloy were chosen for their very good wetting properties, especially the ternary alloy. The melting point of the Au 73 Ge 27 alloy is 3658C and that of the Au 77 Ge14 Si 9 alloy is some degrees lower. The heating current at the melting point of the alloy was determined by an optical microscope. The relationship between heating current and the emitter temperature was measured using an optical pyrometer. Fig. 2 shows the results compared to LMISs applying other materials. Because of the very high thermal conductivity of the gold alloys, the temperature of the emitter drop is close to that of the tungsten filament. In contrast, a Co 36 Nd 64 emitter
Fig. 2. Temperature of different liquid metal ion source emitters as a function of heating current.
At the beginning of experiments, the LMIS was operated in the ion emission mode. Heating current, ion emission current, and extraction voltage were controlled and adjusted for stable field-induced ion emission. The typical ion emission current was in the range from 2 to 30 mA. The corresponding I–V characteristics for the AuGeSi LMIS depending on the heating current are shown in Fig. 3. Surprisingly, the ion current becomes lower with increasing temperature. In general, one would expect an increase of the current due to the lowering of the viscosity as observed for other LMIS materials at higher temperatures. The AuGeSi alloy is a very fluid material and moves to the hotter parts of the filament, so the tip becomes impoverished, causing the current to decrease. Points A, B, and C in Fig. 3 indicate emission currents for later quenching processes. Another important feature of an LMIS is the relationship between the electrical field strength when ion emission starts and the tip radius, which is investigated
Fig. 3. I–V characteristics for ion emission under different heating conditions.
136
W. Knapp et al.r Applied Surface Science 146 (1999) 134–137
switching off. The extraction voltage had to be applied up to the ion emission break off. After quenching, a nanoemitter tip was formed on the top of the emitter needle. The radii of the nanoemitter tips were measured by scanning electron microscopy imaging. After quenching at 10 mA ion emission current, the nanoemitter tip radius was determined to be about 10 nm, which is demonstrated in Fig. 4 for an Au 77 Ge14 Si 9 emitter.
4. Electron emission properties and emitter regeneration After a cooling time of about half an hour, the quenched emitter can be used as an electron source. Therefore, voltage potentials must be switched and the solidified LMIS works as an electron emitter in the cold cathode mode. A well-prepared LMIS and proper quenching are the key points for good electron emission performance. Experiments showed that stable ion emission results also in stable electron emission. The electron mode current–voltage characteristics are presented in Fig. 5 for the AuGeSi LMIS quenched at three different ion emission currents at a constant heating current. It was found that quenching at lower ion emission currents improves the electron field emission characteristic caused by smaller nanoemitter tip radii. Field strength E at the emitter tip can be expressed by the equation E s b V, where V is the
Fig. 4. Scanning electron microscopy ŽSEM. images of an Au 77 Ge14 Si 9 emitter. Ža. General view of solidified LMIS-emitter. Žb. Emitter-top nanotip with a radius of less than 10 nm.
and discussed in detail elsewhere w10–15x. This relationship depends on the source material used, especially on its viscosity in the liquid phase. To form a frozen nanotip for electron emission, the LMIS was operated some minutes in a stable ion emission mode. Then, it was abruptly quenched by switching off the heating current. Due to the heat capacity of the drop, the LMIS still operated some seconds after
Fig. 5. I–V characteristics for electron emission in dependence on quenching conditions. The three curves correspond to the parameters indicated in Fig. 3.
W. Knapp et al.r Applied Surface Science 146 (1999) 134–137
137
5. Conclusions
Fig. 6. Fowler–Nordheim plot of electron emission from a Au 73 Ge 27 emitter.
extraction voltage and b is a geometrical factor given in my1 . Assuming a field strength of E s 2 = 10 9 Vrm in the case of stable field emission, a b on the order of 10 6 my1 is obtained. In our setup, the distance between the emitter tip and the extraction electrode was held constant at 0.5 mm. The geometrical dimension of the nanotip and electron emission properties are comparable to the results of others w16,17x, but their preparation techniques are more difficult, especially in the case of asperity tips w18x. The Fowler–Nordheim plot for the AuGe LMIS in the electron emission mode is shown in Fig. 6. Stable electron field emission behavior was found in the current range of 0.2 to 3 mA. At higher electron emission currents, space charge effects limit the field emission. An increase of the electron emission current above about 15–25 mA can destroy the nanotips by overheating or explosive field emission w19x, which limits the lifetime of the source. At a pressure higher than 3 = 10y7 mbar in the vacuum chamber, microarc breakdowns may occur w20x. In situ regeneration of the electron emission properties under vacuum conditions is possible by repeating quenching in order to form a new nanotip. After the in situ regeneration of the nanotip, a gradual change of the emission characteristic to a higher voltage was found for ion emission as well as for electron emission. Of course, after a number of stress and quenching cycles, the electron emission characteristic deteriorates. Another advance in solidified LMIS is the possibility of ex situ regeneration by wetting needles again.
Solidified LMIS operating with tungsten emitters wetted with Au alloys can be used as high intensity electron point sources. Electron emission occurs from a nanotip formed on a solidified LMIS emitter during quenching. The arrangement provides field enhancement similar to that for asperity tips and other supertips. The I–V characteristics and the overall performance of the electron-emitting LMIS were determined. Applications of these sources can be found in the field of ultrahigh vacuum techniques. In further investigations, long term stability tests in a wide range of vacuum pressure and with different residual gas conditions will be performed and other source materials will be studied. References w1x L.W. Swanson, G.A. Schwind, J. Appl. Phys. 49 Ž1978. 5655. w2x K. Hata, S. Nishigaki, M. Watanabe, T. Noda, H. Tamura, H. Watanabe, J. Phys. 47 Ž1986. C7–375. w3x J. Mitterauer, J. Vac. Sci. Technol. B 13 Ž1995. 625. w4x J. Mitterauer, Appl. Surf. Sci. 87r88 Ž1995. 79. w5x K.A. Rao, A.E. Bell, G.A. Schwind, L.W. Swanson, J. Vac. Sci. Technol. B 7 Ž1989. 1793. w6x L.W. Chen, Y.L. Wang, Appl. Phys. Lett. 72 Ž1998. 389. w7x A. Wagner, T.M. Hall, J. Vac. Sci. Technol. 16 Ž1979. 1871. w8x P.D. Prewett, G.L.R. Mair, Focused Ion Beams From Liquid Metal Ion Sources, Research Studies Press, Taunton, 1991. w9x B. Praprotnik, Emissionsverhalten von Flussigmetall-Ionen¨ rElektronen-Emitter, Thesis Žin German., 1st edn., Koster¨ Verlag, Berlin, 1995. w10x R.G. Forbes, Appl. Surf. Sci. 67 Ž1993. 9. w11x W. Liu, R.G. Forbes, Appl. Surf. Sci. 87r88 Ž1995. 122. w12x E. Hesse, W. Driesel, Ch. Dietzsch, L. Bischoff, J. Teichert, Jpn. J. Appl. Phys. 35 Ž1996. 5564. w13x W. Driesel, Ch. Dietzsch, Appl. Surf. Sci. 93 Ž1996. 179. w14x W. Driesel, Ch. Dietzsch, E. Hesse, L. Bischoff, J. Teichert, J. Vac. Sci. Technol. B 14 Ž1996. 1621. w15x G.L.R. Mair, R.G. Forbes, Surf. Sci. 266 Ž1992. 180. w16x K. Jousten, K. Bohringer, R. Borret, S. Kalbitzer, Ultrami¨ ¨ croscopy Ž1988. 301. w17x H.W.P. Koops, Ch. Schossler, Proc. 9th Int. Vac. Micro¨ electr. Conf., St. Petersburg Ž1996. 458. w18x T.M. Mayer, D.P. Adams, B.M. Marder, J. Vac. Sci. Technol. B 14 Ž1996. 2438. w19x K. Hata, S. Nishigaki, M. Ionue, T. Noda, H. Tamura, J. Phys. 48 Ž1987. C6–177. w20x R.V. Latham ŽEd.., High Voltage Vacuum Insulation, Academic Press, London, 1995.