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Emission properties of AuSiSb liquid metal ion source
Vacuum/volume 44/numbers Printed in Great Britain
Emission source N Drandarov, Sciences,
11 /12/pages
1105 to 1107/l 993
properties
B Nikolov,
T Tsvetkov
0042-207X/93$6.00+.00 @ 1993 Pergamon Press Ltd
of AuSiSb and A Kebedgiev,
liquid
lnstitute
of Electronics,
metal Bulgarian
ion Academy
of
72 Blvd Trakia, 1784 Sofia, Bulgaria
A liquid metal ion source which produces Si and Sb ions from a AuSiSb alloy has been investigated. Intensive lines of Si2+ and Sb2’ ions, which are very suitable for use in focused ion beam systems, have been identified in the mass spectrum of the source. The full width at half maximum of the energy distribution is evaluated at 15 eV. Three emission regions and hysteresis behaviour have been observed in the current-voltage characteristics.
1. Introduction ‘The interest in liquid metal ion sources (LMIS) for obtaining hnely focused high-brightness ion beams has increased sub,;tantially in recent years. These sources have been successfully (applied in maskless implantation processes’, ion beam sputi.ering’, maskless lithography with self-developing resists’.3,4 and in systems including both focused ion beams and molecular beam epitaxy for production of nanometre-scale devices’.5. However, many of the important chemical elements used in electronic device fabrication have high vapour pressures or high melting points. One possible way to obtain fine beams of these elements is to develop field ionization sources using gaseous compounds which include the relevant elements6. However, field ionization !,ources require clean ultra-high vacuum to ensure stable operation which is not easy to achieve. Another possibility is to develop liquid metal ion sources using metal alloys which have a lower melting point than the individual elements. For practical purposes it is important to know certain parameters of the liquid metal ion sources such as the current-voltage characteristics, the angular distribution of the emitted particles, the mass spectrum and the energy distribution. Si and Sb ions are of particular interest for microfabrication, because they are widely used as doping elements. In the present paper, we describe a AuSiSb liquid metal ion source and present rneasurement results of some principal emission properties of this source. 2:. Experimental The liquid metal ion source is of a ribbon type with resistive bleating. The emitter needle is made of 0.3 mm tungsten wire, the tip of which is electrochemically formed and roughened. The current-voltage characteristics and the angular distribution are measured at a pressure of 4 x 10m4 Pa, the mass spectrum and the energy distribution are measured at a pressure of 7 x 10e5 Pa. The mass spectrum is measured using a quadrupole mass spectrometer MC7303, with a mass resolution equal to M (M being the mass number). The energy distribution ir; measured using a retarding potential. The Au,,Si,,Sb,, (at%) alloy is obtained by packing the necessary quantities of Si and Sb in Au foil and then melting them in a vacuum furnace at lop4 Pa.
The source has been operated continuously for more than 30 h at 7 x IO-’ Pa with only a slight rise of the working temperature and the threshold voltage. 3. Results A typical current-voltage characteristic for the AuSiSb LMIS is shown in Figure 1. The measured current is the emitter current. The emission of the source starts at a threshold voltage of 4.1 kV. Then the current increases smoothly with increasing voltage, its value being proportional to the square root of the voltage. At a voltage of 6.3 kV, the current sharply rises to 212 PA at 6.6 kV. When the voltage is decreased, the current-voltage characteristic does not follow the above-mentioned rising curve, but shows a noticeable hysteresis, remaining in the typical regions of the rising curve. The current smoothly decreases to zero at a voltage of 3.9 kV. Similar hysteresis in the current-voltage characteristics has also been observed in a AuSi LMI source’, where the existence of hysteresis is attributed to changes in the structure of the emitting surface. Another reason for the hysteresis may be the heating effect connected with the secondary electron bombardment
z
200
-
150
-
100
-
: ?
E ‘S .r( i
5o
0
2
3
4
5
6
U(kV) Figure 1. Current-voltage temperature 500°C.
characteristic for the AuSiSb LMIS at source 1105
et a/: Emission
N Drandarov
0
-40
-60
properties
-20
of AuSiSb
0
emission
20
angle
40
LMIS
60
(deg)
Figure 2. Angular distribution of the emitted ions at emission 25, 50, 75 and 100 PA, respectively.
currents
of
Figure 5. Energy
The angular distribution of the emitted ions is shown in Figure 2. The shape of the angular distribution did not change when the emission current was varied, only the intensity of the ion current increases when the emission current increases. The emission current is evenly distributed in the form of a cone with a half-angle of 40“, where the aperture half-angle is 56”. Figure 3 shows a typical AuSiSb source mass spectrum, measured by means of a quadrupole mass spectrometer. The mass spectrum is measured at an emission current of 4 PIA and a temperature of 500°C. It is clearly seen that the Au+ ions are dominating in this spectrum. Intense lines of Si*+, Si+, Sb’+, Sb+, Au2+, Au: and less intense lines of AuSb2+, Au,.%“+, AuSi+, AuSb*+, Au:+, Au,Si*+, AuSb+ are also present in the mass spectrum. Because of the limited range of the mass spectrometer, we could not register the Au: line, which would be
Si” !
spread
of Si’+ and Si+ ions
rather intense if we consider the presence and intensity of Au:*. As observed in the spectra of other similar alloys measured by other authors”-“, the intensity of the Si and Sb doubly charged ions is much greater than that of the corresponding singly charged ions. This agrees with the data concerning the probability of obtaining doubly charged ions during field ionization, reported in ref 11. In the mass spectrum of this alloy measured by Machlett et al “, at an emission current of 100 @A, Au:+ and Au$b*+ are absent, while in the spectrum measured by us the Si:, AuSi*+ , Sb:+ and Au,Si+ lines are absent. In the spectrum, there are also two lines at 381 and 385 M/q, respectively, which we have been unable to identify. In the observed spectrum, the doubly charged molecular Sb:+ ions, reported by Ishitani et al’ for a AuSb alloy are also missing. For the purpose of comparison, Figure 4 shows the mass spectrum of this source, measured by means of a Wien filter. Figures 5-7 present the energy distribution spectra of the principal lines in the mass spectrum, measured at an emission current of 4 PA and a temperature of 500°C. The spectrum intensities are normalized to the maximum intensity. The energy distribution measurements are qualitative and should be considered as preliminary and suitable only for acquir-
1
M/q Figure 3. A typical
mass spectrum
of the AuSiSb
_I-_/
;;++Sb++ \ iP’, _2
0
50
LMIS.
Si++ P\
Si+ ‘,_-V’
100
L
i 200
150 U(V)
Figure 4. Mass spectrum Wien filter. 1106
of the AuSiSb
LMIS measured
by means of a Figure 6. Energy
spread of Sb’+ and Sb+ ions
et al: Emission
,Y Dmndarov
properties
of AuSiSb
LMIS
4. Conclusion The principal emission properties of a liquid metal ion source of AuSiSb alloy such as current-voltage characteristics, angular distribution, mass spectrum and energy distribution of the emitted ions, have been measured. The obtained full width at half maximum is between 10 and 15 eV. This fact and the ability of the source to produce intensive Si2+ and Sb2+ lines along with the stable continuous operation of the source allows us to make the conclusion that such a source would be suitable for ion beam processing techniques. Acknowledgement
Figure 7.
The authors of this paper express their deep gratitude to V V Kavitsky and V Kaznacheev from the Scienific and Technological Corporation of the Russian Academy of Sciences in St Petersburg for the assistance rendered.
Energy spread of AU*+ and Au+ ions.
References ’ Y Yasuoka, K Hayakawa, K Gamo and S Namba, Microelectron Engng, ing an initial
notion of the source properties. From the energy distribution curve we established that the full width at half maximum is of the order of 10-15 eV. A low energy tail with an energy deficit related to the main peak of the order of 10 eV is observed in the energy distribution of the Si+ and Sb+ spectrum. It occurs probably because of the charge transfer collision’*: M+
+M + M,,,,+M,:,,
M2+ +M
-+ Mr;,,+M,:,,.
The absence of a high energy tirst process as dominating. In the Au’ energy distribution insignificant.
tail enables
us to consider
the low energy tail intensity
the is
5,335s340
(1986).
‘A Wagner, Nucl Instrum Meth, 218, 335 (1983). 3 H Kaneko, Y Yasuoka, K Gamo and S Namba, J Vuc Sci Technol, B6, 982 (1988). 4H Kaneko, Y Yasuoka, K Gamo and S Namba. Japan J Appl Phys, 27, 1764 (1988). ‘T Shiokawa, P HionKim, M Hamagaki, T Haras, Y Aoyagi, T Toyoda and S Namba, Japan J Appl Phys, 27, 1160 (1988). 6G Hanson and B M Siegel, J Vuc Sci Technol, 16, 1875 (1979). ‘N Drandarov, B Nikolov and T Valchovska, Vacuum, 42,95 (1991). ‘F Machlett, R Miihle, I Sticbritz and J Urban, Private communication. 9T Ishitani, K Umemura and K Kawanami, Surface Sci, 218,295 (1989). ‘“F Machlett, R Miihle and I Sticbritz, J Phys D: Appl Phys, 20, 1417 (1987). ” E W Mutter and T T ‘Tsong, Field Ion Microscopy. Pergamon Press, New York (1969). “M Komuro. H Arimoto and T Kato. J Vuc Sci Technol, B6,923 (1988).
1107
Emission source N Drandarov, Sciences,
11 /12/pages
1105 to 1107/l 993
properties
B Nikolov,
T Tsvetkov
0042-207X/93$6.00+.00 @ 1993 Pergamon Press Ltd
of AuSiSb and A Kebedgiev,
liquid
lnstitute
of Electronics,
metal Bulgarian
ion Academy
of
72 Blvd Trakia, 1784 Sofia, Bulgaria
A liquid metal ion source which produces Si and Sb ions from a AuSiSb alloy has been investigated. Intensive lines of Si2+ and Sb2’ ions, which are very suitable for use in focused ion beam systems, have been identified in the mass spectrum of the source. The full width at half maximum of the energy distribution is evaluated at 15 eV. Three emission regions and hysteresis behaviour have been observed in the current-voltage characteristics.
1. Introduction ‘The interest in liquid metal ion sources (LMIS) for obtaining hnely focused high-brightness ion beams has increased sub,;tantially in recent years. These sources have been successfully (applied in maskless implantation processes’, ion beam sputi.ering’, maskless lithography with self-developing resists’.3,4 and in systems including both focused ion beams and molecular beam epitaxy for production of nanometre-scale devices’.5. However, many of the important chemical elements used in electronic device fabrication have high vapour pressures or high melting points. One possible way to obtain fine beams of these elements is to develop field ionization sources using gaseous compounds which include the relevant elements6. However, field ionization !,ources require clean ultra-high vacuum to ensure stable operation which is not easy to achieve. Another possibility is to develop liquid metal ion sources using metal alloys which have a lower melting point than the individual elements. For practical purposes it is important to know certain parameters of the liquid metal ion sources such as the current-voltage characteristics, the angular distribution of the emitted particles, the mass spectrum and the energy distribution. Si and Sb ions are of particular interest for microfabrication, because they are widely used as doping elements. In the present paper, we describe a AuSiSb liquid metal ion source and present rneasurement results of some principal emission properties of this source. 2:. Experimental The liquid metal ion source is of a ribbon type with resistive bleating. The emitter needle is made of 0.3 mm tungsten wire, the tip of which is electrochemically formed and roughened. The current-voltage characteristics and the angular distribution are measured at a pressure of 4 x 10m4 Pa, the mass spectrum and the energy distribution are measured at a pressure of 7 x 10e5 Pa. The mass spectrum is measured using a quadrupole mass spectrometer MC7303, with a mass resolution equal to M (M being the mass number). The energy distribution ir; measured using a retarding potential. The Au,,Si,,Sb,, (at%) alloy is obtained by packing the necessary quantities of Si and Sb in Au foil and then melting them in a vacuum furnace at lop4 Pa.
The source has been operated continuously for more than 30 h at 7 x IO-’ Pa with only a slight rise of the working temperature and the threshold voltage. 3. Results A typical current-voltage characteristic for the AuSiSb LMIS is shown in Figure 1. The measured current is the emitter current. The emission of the source starts at a threshold voltage of 4.1 kV. Then the current increases smoothly with increasing voltage, its value being proportional to the square root of the voltage. At a voltage of 6.3 kV, the current sharply rises to 212 PA at 6.6 kV. When the voltage is decreased, the current-voltage characteristic does not follow the above-mentioned rising curve, but shows a noticeable hysteresis, remaining in the typical regions of the rising curve. The current smoothly decreases to zero at a voltage of 3.9 kV. Similar hysteresis in the current-voltage characteristics has also been observed in a AuSi LMI source’, where the existence of hysteresis is attributed to changes in the structure of the emitting surface. Another reason for the hysteresis may be the heating effect connected with the secondary electron bombardment
z
200
-
150
-
100
-
: ?
E ‘S .r( i
5o
0
2
3
4
5
6
U(kV) Figure 1. Current-voltage temperature 500°C.
characteristic for the AuSiSb LMIS at source 1105
et a/: Emission
N Drandarov
0
-40
-60
properties
-20
of AuSiSb
0
emission
20
angle
40
LMIS
60
(deg)
Figure 2. Angular distribution of the emitted ions at emission 25, 50, 75 and 100 PA, respectively.
currents
of
Figure 5. Energy
The angular distribution of the emitted ions is shown in Figure 2. The shape of the angular distribution did not change when the emission current was varied, only the intensity of the ion current increases when the emission current increases. The emission current is evenly distributed in the form of a cone with a half-angle of 40“, where the aperture half-angle is 56”. Figure 3 shows a typical AuSiSb source mass spectrum, measured by means of a quadrupole mass spectrometer. The mass spectrum is measured at an emission current of 4 PIA and a temperature of 500°C. It is clearly seen that the Au+ ions are dominating in this spectrum. Intense lines of Si*+, Si+, Sb’+, Sb+, Au2+, Au: and less intense lines of AuSb2+, Au,.%“+, AuSi+, AuSb*+, Au:+, Au,Si*+, AuSb+ are also present in the mass spectrum. Because of the limited range of the mass spectrometer, we could not register the Au: line, which would be
Si” !
spread
of Si’+ and Si+ ions
rather intense if we consider the presence and intensity of Au:*. As observed in the spectra of other similar alloys measured by other authors”-“, the intensity of the Si and Sb doubly charged ions is much greater than that of the corresponding singly charged ions. This agrees with the data concerning the probability of obtaining doubly charged ions during field ionization, reported in ref 11. In the mass spectrum of this alloy measured by Machlett et al “, at an emission current of 100 @A, Au:+ and Au$b*+ are absent, while in the spectrum measured by us the Si:, AuSi*+ , Sb:+ and Au,Si+ lines are absent. In the spectrum, there are also two lines at 381 and 385 M/q, respectively, which we have been unable to identify. In the observed spectrum, the doubly charged molecular Sb:+ ions, reported by Ishitani et al’ for a AuSb alloy are also missing. For the purpose of comparison, Figure 4 shows the mass spectrum of this source, measured by means of a Wien filter. Figures 5-7 present the energy distribution spectra of the principal lines in the mass spectrum, measured at an emission current of 4 PA and a temperature of 500°C. The spectrum intensities are normalized to the maximum intensity. The energy distribution measurements are qualitative and should be considered as preliminary and suitable only for acquir-
1
M/q Figure 3. A typical
mass spectrum
of the AuSiSb
_I-_/
;;++Sb++ \ iP’, _2
0
50
LMIS.
Si++ P\
Si+ ‘,_-V’
100
L
i 200
150 U(V)
Figure 4. Mass spectrum Wien filter. 1106
of the AuSiSb
LMIS measured
by means of a Figure 6. Energy
spread of Sb’+ and Sb+ ions
et al: Emission
,Y Dmndarov
properties
of AuSiSb
LMIS
4. Conclusion The principal emission properties of a liquid metal ion source of AuSiSb alloy such as current-voltage characteristics, angular distribution, mass spectrum and energy distribution of the emitted ions, have been measured. The obtained full width at half maximum is between 10 and 15 eV. This fact and the ability of the source to produce intensive Si2+ and Sb2+ lines along with the stable continuous operation of the source allows us to make the conclusion that such a source would be suitable for ion beam processing techniques. Acknowledgement
Figure 7.
The authors of this paper express their deep gratitude to V V Kavitsky and V Kaznacheev from the Scienific and Technological Corporation of the Russian Academy of Sciences in St Petersburg for the assistance rendered.
Energy spread of AU*+ and Au+ ions.
References ’ Y Yasuoka, K Hayakawa, K Gamo and S Namba, Microelectron Engng, ing an initial
notion of the source properties. From the energy distribution curve we established that the full width at half maximum is of the order of 10-15 eV. A low energy tail with an energy deficit related to the main peak of the order of 10 eV is observed in the energy distribution of the Si+ and Sb+ spectrum. It occurs probably because of the charge transfer collision’*: M+
+M + M,,,,+M,:,,
M2+ +M
-+ Mr;,,+M,:,,.
The absence of a high energy tirst process as dominating. In the Au’ energy distribution insignificant.
tail enables
us to consider
the low energy tail intensity
the is
5,335s340
(1986).
‘A Wagner, Nucl Instrum Meth, 218, 335 (1983). 3 H Kaneko, Y Yasuoka, K Gamo and S Namba, J Vuc Sci Technol, B6, 982 (1988). 4H Kaneko, Y Yasuoka, K Gamo and S Namba. Japan J Appl Phys, 27, 1764 (1988). ‘T Shiokawa, P HionKim, M Hamagaki, T Haras, Y Aoyagi, T Toyoda and S Namba, Japan J Appl Phys, 27, 1160 (1988). 6G Hanson and B M Siegel, J Vuc Sci Technol, 16, 1875 (1979). ‘N Drandarov, B Nikolov and T Valchovska, Vacuum, 42,95 (1991). ‘F Machlett, R Miihle, I Sticbritz and J Urban, Private communication. 9T Ishitani, K Umemura and K Kawanami, Surface Sci, 218,295 (1989). ‘“F Machlett, R Miihle and I Sticbritz, J Phys D: Appl Phys, 20, 1417 (1987). ” E W Mutter and T T ‘Tsong, Field Ion Microscopy. Pergamon Press, New York (1969). “M Komuro. H Arimoto and T Kato. J Vuc Sci Technol, B6,923 (1988).
1107