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CHARACTERISTICS OF LIQUID METAL ION SOURCE FOR BORON AND ARSENIC
H. Fukuda, K. Gamo, S. Namba
To cite this version:
H. Fukuda, K. Gamo, S. Namba. CHARACTERISTICS OF LIQUID METAL ION SOURCE FOR BORON AND ARSENIC. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-153-C6-158.
�10.1051/jphyscol:1987625�. �jpa-00226828�
CHARACTERISTICS OF LIQUID METAL ION SOURCE FOR BORON AND ARSENIC
*H. Fukuda, K. Gamo and S. Namba
Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
Abstract
-
We have fabricated a liquid metal ion (LMI) source using Pd-Ni-As-B alloy which is useful for a direct ion implantation for silicon devices. Basic characteristics such as current-voltage characteristics, mass spectra and energy distribution have been measured. The ion source showed a threshold voltage of 5.0 keV for ionization, and the source current increased from 1 to 50 pA with increasing the extraction voltage from 5 to 10 keV. The mass+spectra of this $MI source exhibjted the ion rat$? ; 1 2 % 10B,
2.3% 11B,
8.6%75As
,
14.2% 75As.
The full width at half muxjmum of the energy distributiot$+ were typically-
10, eV for 11B,
and-15 eV for 75As ions at the source current of 15 F A . The present Pd-Ni-As-B alloy source was operated for more than 20 hours without change in the emission current and the mass spectra.
1. Introduction
Recently, focused ion beam (FIB) technology has been widely known due to its potential to the microfabrication process of semiconductor devices. Microfocused ion beam from LMI source is available for an ion beam lithography /1,2/; a maskless etching /I/; direct doping / 3 , 4 / . In particular, FIB implantation is one of the most promising .application. In order to apply the FIB implantation to Si device fabrication, LMI sources for the dopant element such as boron (p-), arsenic and phosphorus (n-type) are required. There are a few reports for B and As alloy LMI sources /5-8/. However, it is difficult to obtain a high reliability as a LMI source, because boron has a high melting temperature, arsenic and phosphorus have a high vapour pressure.
*permanent address : Oki Electric Industry Co., Ltd, Hachioji Tokyo 193, Japan
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987625
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In the present work, the LMI sources including B and As have been fabricated and basic characteristics such as current-voltage (I-V) characteristics, mass spectra and energy distribution have been measured. Also, a composition of the alloy on emitter tip was determined by using of Auger electron spectroscopy (AES) analysis.
2. Experimental
0 Alloy used for LMI source was Pd30 Ni30 As20 B20 (m.p. 780 C). Pd75 As25 (m.p. 6 8 0 ~ ~ ) and Pd40 Ni40 As20 (m.p. 7 5 0 ~ ~ ) were also fabricated. These alloys were melted in an evaquated quartz ampoule by using a furnace at 9 0 0 ~ ~ about 50 hours. A LMI source was fabricated as follows; a 300 tungsten wire was used as an emitter, and the emitter was spot welded to a 300 pm@ tungsten coil heater which is used as a reservoir of liquid metals.. Next, the emitter tip was sharpened by electrochemical polishing for 30 min at an ac bias voltage of 4 V in the etchant (KOH:H20:NH40H
=4:5:1). The cleaning of the needle surface prior to loadlng was 6performed by flashing the emitter above 1 5 0 0 ~ ~ in vacuum of 1x10- Torr. A LMI source was constructed by locating the tip 1 mrn apart from the extractor plane. The loading temperature of the source was measured by an optical pyrometer. The LMI source was operated slightly above the melting point by adjusting the heater current. The energy distribution of the ion beam was measured using a filter type retarding potential analyzer.
3. Results
Fig. 1 shows the typical I-V characteristics for Pd-As and Pd-Ni-As alloy sources. The ion current for the Pd-As and Pd-Ni- As LMI sources increased similarly with increasing the extraction voltage. However, a lower threshold voltage Vth and a larger current of Pd-Ni-As for Pd-As source were found. The Vth may correspond to the voltage to form a Tayler cone which depends on the emitter tip radius and the surface tension of a molten alloy.
Source lifetime of Pd-As alloy was so short ( S 2 h ) compared with Pd-Ni-As alloy source (-15h). The I-V characteristics for Pd-Ni-As-B LMI source is shown in Fig. 2. A similar characteristics between Pd-Ni-As and Pd-Ni-As-B LMI sources were obtained. However, only Pd-Ni-As-B LMI source was stable in the I-V characteristics after a use for longer than 20 hours.
Mass spectrum for Pd-Ni-As-B alloy source was measured by using ExB mass seperator set in FIB column. Fig.3 shows the typical mass spectrum for Pd-Ni-As-B source at the source current Is of 10 pA and at a temperature of 1073 K. The mass spectrum exhibits a strong peak from Pd and Ni iqns. Arsenic i $ ~ show the follqwing ion flux fraction ; 8.6% 75As
,
14.2% 75As and 0.8%75As2
.
For B iqn, the intensity of 11~' ion is slightly higher than that of 10B ion according $0 the iaotope ratio of B ions. In Fig. 4, the intensity of Pd and Ni ions siightly+increas$d with increasing Is. Whereas, the intensity of As
,
As and B ionsare almost independent of the source current. The ion flux ratio of (B/As)ion is about 80% smaller than the atom flux ratio
(B/As)atom of the alloy. The observed ion flux ratio are summarized in Table I.
(FWHM) were typically .\- 10 eV for 11B and -15 eV for 75As ions. This may be due to a larger space charge broadening effect.
The liquid flow state on a needle surface was investigated by measuring AES spectra along the emitter needle. Fig. 6 shows the distribution of the Pd, Ni, As and B concentrations along needle surface after a 1 hour operation at 10 FA. In Fig. 6, Pd and As concentrations maintain a constant value along needle, and are nearly equal to the value of the material compositions. Ni enrichment on the needle apex was found. On the contrary, B concentration become gradually reduced from reservoir to the needle apex. This may be due to the reasons that the boron is not dissolved and/or has a corrosive effect on needle surface. Its cause is not yet well understood.
4 . Discussion
We have investigated the ion emission characteristics for Pd based alloy LMI source including As and B. For Pd-As alloy LMI system, arsenic was rapidly vapourized, and the sources exhibited a short lifetime. As shown in Fig. 1, addition of Ni to Pd-As alloy (Pd:Ni:As=2:2:1) produced an increase of Is and a reduction of Vth. The threshold voltage decreases if Ni reduces a flow impedance on needle surface. A similar I-V characteristics between Pd-Ni-As and Pd-Ni-As-B alloy sources were obtained. The mass spectrum of Pd-Ni-As-B LMI source shows that the ion flux ratio (B/As)ion is smaller than the ratio expected from the atom flux ratio (B/As)atom
.
A probable reason is a shortage of supply for B on needle apex, which is suggested by the AES analysis, as shown in Fig. 6. Thus, the ion emission is limited by a liquid flow on needle surface. However, it is not clear why only B shows an insufficient flow on the needle surface.Up to now, the ion emission from a LMI source have not been fully understood because of its complex ionization mechanism. A hypothesis is that the ion emission can be explained by the field emission, where the ~owler-~ordheim type ionization is realized / 9 / . It is also possible that the liquid flow to the emitter tip is the rate dete mining process for the ionization 0 The 1/V v.s. log(I/$ ) plots (i.e. F-N plots) for Pd based alloy LMI sources are shown in Fig. 7. Thus, it can be understood that the F-N plots is linear near the Vth. If the liquid flow at apex limited the emissjon rate, then the slope of d ~ / d V must be proportional to 1/v
.
Our results show a linearity in the range of a higher part of VEX,
as shown in Fig.8. Therefore, it should be probable that the Ion generation occur via several mechanism depending on the operating condition.
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5. Conclusion
We have fabricated a LMI source for B and As, which is useful for a direct doping to Si devices. A basic characteristics such as I-V characteristics, mass spectra and energy distribution have been measured. Also, using AES analysis, the relationship between the ion beam composition in the mass spectrum and the alloy concentrations on the needle apex have been investigated.
Pd-Ni-As LMI source produced a higher Is and a lower Vth than that of Pd-As alloy source. It is considered that addition of Ni improve the flow state. A similar I-V characteristics between Pd- Ni-As and Pd-Ni-As-B LMI sources was obtained. However, the intensity of B ion current in the mass spectrum was lower compared with that of As ion current. Probably, it may be related to a smaller supply of B on needle apex, as suggested by the AES measurements.
Acknowledgements
The authers would like tO thank Mr.K. Kawasaki for his help in performing the experiments and Mr. Z.Xu for Auger electron spectroscopy measurement.
References
/1/ Y. Ochiai, K. Gamo and S. Namba, J. Vac. Sci. Technol.
B3, 67 (1985)
/2/ L. Karapiperis and C.A. Lee, Appl. Phys. Lett. 35, 395 (1979) /3/ Y. Bamba, E. Miyauchi, H. Arimoto, K. Kuramoto, A. Takamori
and H. Hashimoto, Jpn. J. Appl. Phys. 22, L650 (1983) /4/ V. Wang, J. W. Ward and R.L. Seliger, J. Vac. Sci. Technol.
19, 1158 (1981) /5/ K. Gamo, T. Ukegawa, Y. Inornoto, Y. Ochiai and S. Namba
ibid., 19, 1182 (1981)
/6/ R.H. Higuchi-Rusli, K.C. Cadien, J.C. Corelli and A.J. Steckl J. Vac. Sci. Technol. B5, 190 (1987) /7/ T. Ishitani, A. Shimase and H. Tamura, Jpn. J. Appl. Phys.
21, L277 (1982) /8/ K. Gamo, T. Ukegawa and S. Namba, ibid., 19, L595 (1980) /9/ R.H. Fowler and L.W. Nordheim, Proc. Roy.Soc.Al19. 173 (1928) /lo/ A. Wagner, Appl. Phys. Lett. 40, 440 (1982)
Fig. 1 Current-voltage characteristics Fig. 2 Current-Voltage characteristics for Pd-As and Pd-Ni-As alloy sources. for Pd-Ni-As-B alloy source.
Pd' L
PdNiAsB LMI source
0 2
- -
oAs2'* .
A * 2 A B +
10 20 30
SOURCE CURRENT ( PA)
MAGNETIC FIELD (orb. units)
.
Fig. 4 Relative ion flux ratio of Pd-Ni-As-B ion source as a Fig. 3 Typical mass spectrum for function of the source current.for Pd-Ni-As-B alloy source.
Material
co.~osition (st. %)
Table I Alloy composition and ion flux fraction for Pd-Ni-As-B ion source.
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RETARDING W E N T I A L (eV)
DISTANCE FROM APEX {mm) Fig. 5 Energy distribution curves
for
llBf
and 7 5 ~ s + + ions. Fig. 6 Distribution of Pd,Ni,As and B on the needle surface, measured by using of AES analysis.Fig. 7 The Fowler-Nordheim plots for Pd-As, Pd-Ni-As and Pd-Ni-As-B ion sources.
Fig. 8 The dI/dV v.s. V plots for Pd-As, Pd-Ni-As and Pd-Ni-As-B ion sources.
