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Microanalysis of masklessly MeV-ion-implanted area by MeV heavy-ion microprobe
Nuclear Instruments North-Holland
and Methods
in Physics Research
Nuclear Instruments 8 Methods in Physics Research
B64 (1992) 358-361
SectionB
Microanalysis of masklessly MeV-ion-implanted heavy-ion microprobe
area by MeV
Yuji Horino ‘, Yoshiaki Mokuno a, Akiyoshi Chayahara a, Masato Kiuchi a, Kanenaga Fujii a, Mamoru Satou a and Mikio Takai b a Gocernment Industrial Research Institute Osaka, Ikeda, Osaka 563, Japan h Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560, Japan
Maskless
MeV
ion implantation
into
a silicon
substrate
and
in-situ
microanalysis
before
and
Rutherford backscattering spectrometry (RBS) or secondary electron (SE) detection using a heavy-ion The usefulness
of the heavy-ion
microprobe
for microanalysis
was shown.
1. Introduction Focused high-energy ion beams have been actively used for materials analysis of micro-sized areas, e.g., elemental analysis using particle-induced X-ray emission (PIXE) [l-4]. Recently, the combination of scanning focused ion beams and Rutherford backscattering spectrometry (RBS) has realized nondestructive hvoor three-dimensional microanalysis, i.e., RBS-mapping (multidimensional ion microscopy) [5,6]. RBS using heavy ions turns out to have some characteristic advantages over light ions: (1) higher sensitivity, (2) better mass separation, (3) better depth resolution, etc. Furthermore, hydrogen can be detected by means of some nuclear reactions such as ‘H(‘“F, (~~1~~0. So high-energy heavy ion microprobes are considered to possess good possibilities for the microanalysis of materials. In order to apply the heavy-ion beams to microanalysis, a focused beam line of MeV heavy ion has been developed [7,8]. At present, micro-sized ion beams of boron, carbon, oxygen, silicon, nickel and gold ions with an energy range of 0.5-4 MeV were obtained. This system has realized both microanalysis with heavy ions and maskless MeV ion implantation (or ion beam processing) in materials processes. In this paper, masklessly MeV-ion-implanted areas were analyzed in-situ by RBS or secondary electron (SE) detection using scanning focused heavy ion beams. It is shown that advantageous microanalyses are possible with MeV heavy-ion microprobes.
2. Experimental
0168-583X/92/$05.00
source, consists of objective slits, subsidiary slits and a magnetic quadrupole doublet. Thus MeV microbeams of almost any element are available, with some exceptions such as noble gases. A minimum beam spot size of about 4 pm x 4 pm has been achieved for some ions with a typical current density of about 1 pA/(pm)’
[71. The target was a silicon substrate with gold pads, which were deposited by thermal evaporation. The dimensions of a pad are 25 pm x 25 km with a period of 50 pm and a thickness of 20 nm. Focused 3 MeV C ‘+ or 0.7 MeV Ni+ ions were masklessly implanted into the target and then the implanted areas were insitu analyzed. For microanalysis of the implanted area, focused 3 or 3.9 MeV C *+ beams were employed. Fig. 1 shows a schematic diagram of the analyzing system. There are two detectors: one is an annular-type
SCANNING
PLATE MOD ION
-BEAM
POWER SUPPLY x
ifi
setup
c
The ion focusing system, combined with a tandemtype accelerator with a cesium sputtering-type ion 0 1992 - Elsevier
Science
Publishers
after ion implantation by microprobe were performed.
I
Fig. 1. Block diagram
B.V. All rights reserved
P
u
I I
of the data acquisition
system.
Y Horino et al. / Maskless MeV ion implantation
surface-barrier silicon detector (SSD) for RBS measurement and the other is a photomultiplier tube coupled with a scintillator for SE measurement. A 32-bit microcomputer controls the beam scanning and collects the signal from the detectors through a multichannel analyzer (MCA) or analog-digital converter (ADC). During scanning of the microprobe, RBS or SE yields are stored in the computer memory at each microprobe position. Each of the yields is displayed on
a monitor screen real time.
359
by different
colors in 8 or 16 steps, in
3. Results and discussion First, maskless 3 MeV CZt ion implantation and analysis have been performed. In order to determine the implantation site, a SE-mapping image of the tar-
25 pm Fig. 2. Secondary-electron-mapping focused 3 MeV
CL+
images of the target (a) before maskless ion implantation
ions. They were obtained
by a 3 MeV
C’+
and (b) after maskless implantation
of
ion beam. The SE yields are shown by the output voltage of the
photomultiplier
tube. VII.
ACCELERATORS/BEAMS
6V
I
t
25 pm Fig. 3. Secorrdary-electron-mappitl~ obtained by a 3 MeV
C”
image of the target
after
maskiess
implantation
of focused 0.7 MeV
Ni’
ions. Thi? wa.c
ion beam. The SE yields are shown by the output voltage of the photomultiplier
by the same ions used for implantation, as in fig. 2a. The clear square images are the SE-images of gold pads. Then the focused carbon ions were implanted into the target by scanning the beam about IS pm x 13 km in such a way that there was an overlap between the implanted arca and the gold pad. The average ion current was 18 PA. get was obtained
tube.
Fig. 2b shows a SE-mapping image of the target after in~p~antation to a fluence of 3.1 X 10” ions/cm’, indicating clearly that half of the implanted area is on the gold pad and the other half is on the silicon (compare to fig. 2a). Since the probe spot size was of the order of a micrometer. the resolution was much worse than that of conventional SEM using an electron
200
100
Ni ~
Fig. 4. RBS-tomography direction
image
0 0
of the same target
jn the figure (see text). The
respectively.
horizontal
The arrow on the vertical
as in fig. 3. This WLS obtained and vertical
axis indicates
by scanning a 3.9 McV
axes stand for beam
the channel
position
and channel
numhcr of signals from the implanted
CL+
beam in verticai
number
of the MCA,
nickel atoms.
361
Y. Horino et al. / Maskless MeV ion implantation probe. However, it should be noted that the heavyion-induced SE can provide a clear image with high contrast [9]. In fact, besides tilting the sample up to a few tens of degrees, it was very hard to obtain a clear SE image of the implanted area by conventional SEM. The SE contrast of the implanted area is ascribed to some changes in surface conditions such as damages, contamination from residual gas in the vacuum chamber, etc. Since the range of the carbon ions is estimated to be about 4.3 pm by the TRIM simulation code [lo], RBS measurement with the energy range used in this study is not possible. So the high-contrast SE-mapping imaging using a heavy-ion microprobe will be one of the more powerful methods for in-situ analysis of the area before and after implantation. Next, maskless 0.7 MeV Ni+ ion implantation and analysis have been performed. After determination of the implantation site by SE-imaging, the focused nickel ions were implanted into the target by scanning the beam by about 8 km X 13 km without overlap with a gold pad. The fluence was 1.8 X 10” ions/cm*. Fig. 3 shows a SE-mapping image of the target after implantation obtained by a focused 3 MeV C2+ ion probe. It indicates that the implantation site is directly on the silicon substrate between two gold pads. The change of contrast at the corner of the gold pad image is not attributed to nickel ion implantation. Fig. 4 shows the RBS-mapping image of the implanted area by a focused 3.9 MeV C*+ beam. By scanning the microprobe along the vertical direction in fig. 3 (from the top to the bottom across the implanted area), full RBS spectra were stored in the memory at each microprobe position, which provide us with information on depth (RBS-tomography) [ll]. In the figure, the horizontal and vertical axes stand for beam position and channel number of the MCA which corresponds to the energies of backscattered carbon ions. The signals near channel 32 at around 25 km correspond to implanted nickel atoms and near channel 24 to the silicon substrate. The signals near channel 200 correspond to the lower energy part of gold pads; the higher energy parts were missing because of memory limitation. Though there is a little interference with the silicon, the distribution of implanted nickel atoms has been detected because of high sensitivity and good mass separation of the heavy ion probe. It indicates that the lateral dimension of the implanted area is about 13 pm, though the statistics for nickel signals is not enough. As seen in fig. 4, there are noticeable minimum level noises which make the S/N ratio poor. It is
possible to consider some reasons for the noise, for instance, scattered ions at the objective slit edge or neutralization after the slit, but it is not known where it comes from. The backscattering cross section is proportional to (Z/E)‘, where Z and E are atomic number and energy of incident ions, respectively. With light ions like helium, an increase in incident energy is necessary for good mass separation, which results in a decrease in cross section, i.e., less sensitivity. In fact, a computer simulation of the RBS spectrum shows an overlap of the signals between the silicon substrate and implanted nickel using 1.5 MeV He ions. Thus it is considered that the heavy ion microprobe is useful to analyze a small amount of species in materials such as masklessly implanted atoms.
4. Conclusion Maskless MeV ion implantation into a silicon substrate has been demonstrated by utilizing focused 3 MeV carbon and 0.7 MeV nickel ions. The focused MeV carbon beam was applied as a microprobe to analyze the implanted area by SE or RBS. It was found that high-contrast SE investigation and high-sensitivity RBS measurements were possible by heavy-ion microprobes. The usefulness of the heavy-ion microprobe has been shown.
References [l] J.A. Cookson, Nucl. Instr. and Meth. 165 (1979) 477. [Z] F. Watt, G.W. Grime, G.D. Blower, J. Takacs and D.J.T. Vaux, Nucl. Instr. and Meth. 197 (1982) 65. [3] G.J.F. Legge, Nucl. Instr. and Meth. 197 (1982) 243. [4] R. Nobihng, Nucl. Instr. and Meth. 218 (19831 197. [5] B.L. Doyle, Nucl. Instr. and Meth. B15 (1986) 654. [6] A. Kinomura, M. Takai, K. Inoue, K. Matsunaga, M. Izumi, T. Matsuo, K. Gamo, S. Namba and M. Satou, Nucl. Instr. and Meth. B33 (1988) 862. [7] Y. Horino, A. Chayahara, M. Kiuchi, K. Fujii, M. Satou and M. Takai, Jpn. J. Appl. Phys. 29 (1990) 2680. [8] Y. Horino, A. Chayahara, M. Satou and M. Takai, Nucl. Instr. and Meth. B54 (1991) 269. [9] S. Nomura and H. Shichi, Jpn. J. Appl. Phys. 30 (1991) L1059. [lo] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [ll] A. Kinomura, M. Takai, S. Namba, M. Satou and A. Chayahara, Nucl. Instr. and Meth. B45 (1990) 523.
VII. ACCELERATORS/BEAMS
and Methods
in Physics Research
Nuclear Instruments 8 Methods in Physics Research
B64 (1992) 358-361
SectionB
Microanalysis of masklessly MeV-ion-implanted heavy-ion microprobe
area by MeV
Yuji Horino ‘, Yoshiaki Mokuno a, Akiyoshi Chayahara a, Masato Kiuchi a, Kanenaga Fujii a, Mamoru Satou a and Mikio Takai b a Gocernment Industrial Research Institute Osaka, Ikeda, Osaka 563, Japan h Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560, Japan
Maskless
MeV
ion implantation
into
a silicon
substrate
and
in-situ
microanalysis
before
and
Rutherford backscattering spectrometry (RBS) or secondary electron (SE) detection using a heavy-ion The usefulness
of the heavy-ion
microprobe
for microanalysis
was shown.
1. Introduction Focused high-energy ion beams have been actively used for materials analysis of micro-sized areas, e.g., elemental analysis using particle-induced X-ray emission (PIXE) [l-4]. Recently, the combination of scanning focused ion beams and Rutherford backscattering spectrometry (RBS) has realized nondestructive hvoor three-dimensional microanalysis, i.e., RBS-mapping (multidimensional ion microscopy) [5,6]. RBS using heavy ions turns out to have some characteristic advantages over light ions: (1) higher sensitivity, (2) better mass separation, (3) better depth resolution, etc. Furthermore, hydrogen can be detected by means of some nuclear reactions such as ‘H(‘“F, (~~1~~0. So high-energy heavy ion microprobes are considered to possess good possibilities for the microanalysis of materials. In order to apply the heavy-ion beams to microanalysis, a focused beam line of MeV heavy ion has been developed [7,8]. At present, micro-sized ion beams of boron, carbon, oxygen, silicon, nickel and gold ions with an energy range of 0.5-4 MeV were obtained. This system has realized both microanalysis with heavy ions and maskless MeV ion implantation (or ion beam processing) in materials processes. In this paper, masklessly MeV-ion-implanted areas were analyzed in-situ by RBS or secondary electron (SE) detection using scanning focused heavy ion beams. It is shown that advantageous microanalyses are possible with MeV heavy-ion microprobes.
2. Experimental
0168-583X/92/$05.00
source, consists of objective slits, subsidiary slits and a magnetic quadrupole doublet. Thus MeV microbeams of almost any element are available, with some exceptions such as noble gases. A minimum beam spot size of about 4 pm x 4 pm has been achieved for some ions with a typical current density of about 1 pA/(pm)’
[71. The target was a silicon substrate with gold pads, which were deposited by thermal evaporation. The dimensions of a pad are 25 pm x 25 km with a period of 50 pm and a thickness of 20 nm. Focused 3 MeV C ‘+ or 0.7 MeV Ni+ ions were masklessly implanted into the target and then the implanted areas were insitu analyzed. For microanalysis of the implanted area, focused 3 or 3.9 MeV C *+ beams were employed. Fig. 1 shows a schematic diagram of the analyzing system. There are two detectors: one is an annular-type
SCANNING
PLATE MOD ION
-BEAM
POWER SUPPLY x
ifi
setup
c
The ion focusing system, combined with a tandemtype accelerator with a cesium sputtering-type ion 0 1992 - Elsevier
Science
Publishers
after ion implantation by microprobe were performed.
I
Fig. 1. Block diagram
B.V. All rights reserved
P
u
I I
of the data acquisition
system.
Y Horino et al. / Maskless MeV ion implantation
surface-barrier silicon detector (SSD) for RBS measurement and the other is a photomultiplier tube coupled with a scintillator for SE measurement. A 32-bit microcomputer controls the beam scanning and collects the signal from the detectors through a multichannel analyzer (MCA) or analog-digital converter (ADC). During scanning of the microprobe, RBS or SE yields are stored in the computer memory at each microprobe position. Each of the yields is displayed on
a monitor screen real time.
359
by different
colors in 8 or 16 steps, in
3. Results and discussion First, maskless 3 MeV CZt ion implantation and analysis have been performed. In order to determine the implantation site, a SE-mapping image of the tar-
25 pm Fig. 2. Secondary-electron-mapping focused 3 MeV
CL+
images of the target (a) before maskless ion implantation
ions. They were obtained
by a 3 MeV
C’+
and (b) after maskless implantation
of
ion beam. The SE yields are shown by the output voltage of the
photomultiplier
tube. VII.
ACCELERATORS/BEAMS
6V
I
t
25 pm Fig. 3. Secorrdary-electron-mappitl~ obtained by a 3 MeV
C”
image of the target
after
maskiess
implantation
of focused 0.7 MeV
Ni’
ions. Thi? wa.c
ion beam. The SE yields are shown by the output voltage of the photomultiplier
by the same ions used for implantation, as in fig. 2a. The clear square images are the SE-images of gold pads. Then the focused carbon ions were implanted into the target by scanning the beam about IS pm x 13 km in such a way that there was an overlap between the implanted arca and the gold pad. The average ion current was 18 PA. get was obtained
tube.
Fig. 2b shows a SE-mapping image of the target after in~p~antation to a fluence of 3.1 X 10” ions/cm’, indicating clearly that half of the implanted area is on the gold pad and the other half is on the silicon (compare to fig. 2a). Since the probe spot size was of the order of a micrometer. the resolution was much worse than that of conventional SEM using an electron
200
100
Ni ~
Fig. 4. RBS-tomography direction
image
0 0
of the same target
jn the figure (see text). The
respectively.
horizontal
The arrow on the vertical
as in fig. 3. This WLS obtained and vertical
axis indicates
by scanning a 3.9 McV
axes stand for beam
the channel
position
and channel
numhcr of signals from the implanted
CL+
beam in verticai
number
of the MCA,
nickel atoms.
361
Y. Horino et al. / Maskless MeV ion implantation probe. However, it should be noted that the heavyion-induced SE can provide a clear image with high contrast [9]. In fact, besides tilting the sample up to a few tens of degrees, it was very hard to obtain a clear SE image of the implanted area by conventional SEM. The SE contrast of the implanted area is ascribed to some changes in surface conditions such as damages, contamination from residual gas in the vacuum chamber, etc. Since the range of the carbon ions is estimated to be about 4.3 pm by the TRIM simulation code [lo], RBS measurement with the energy range used in this study is not possible. So the high-contrast SE-mapping imaging using a heavy-ion microprobe will be one of the more powerful methods for in-situ analysis of the area before and after implantation. Next, maskless 0.7 MeV Ni+ ion implantation and analysis have been performed. After determination of the implantation site by SE-imaging, the focused nickel ions were implanted into the target by scanning the beam by about 8 km X 13 km without overlap with a gold pad. The fluence was 1.8 X 10” ions/cm*. Fig. 3 shows a SE-mapping image of the target after implantation obtained by a focused 3 MeV C2+ ion probe. It indicates that the implantation site is directly on the silicon substrate between two gold pads. The change of contrast at the corner of the gold pad image is not attributed to nickel ion implantation. Fig. 4 shows the RBS-mapping image of the implanted area by a focused 3.9 MeV C*+ beam. By scanning the microprobe along the vertical direction in fig. 3 (from the top to the bottom across the implanted area), full RBS spectra were stored in the memory at each microprobe position, which provide us with information on depth (RBS-tomography) [ll]. In the figure, the horizontal and vertical axes stand for beam position and channel number of the MCA which corresponds to the energies of backscattered carbon ions. The signals near channel 32 at around 25 km correspond to implanted nickel atoms and near channel 24 to the silicon substrate. The signals near channel 200 correspond to the lower energy part of gold pads; the higher energy parts were missing because of memory limitation. Though there is a little interference with the silicon, the distribution of implanted nickel atoms has been detected because of high sensitivity and good mass separation of the heavy ion probe. It indicates that the lateral dimension of the implanted area is about 13 pm, though the statistics for nickel signals is not enough. As seen in fig. 4, there are noticeable minimum level noises which make the S/N ratio poor. It is
possible to consider some reasons for the noise, for instance, scattered ions at the objective slit edge or neutralization after the slit, but it is not known where it comes from. The backscattering cross section is proportional to (Z/E)‘, where Z and E are atomic number and energy of incident ions, respectively. With light ions like helium, an increase in incident energy is necessary for good mass separation, which results in a decrease in cross section, i.e., less sensitivity. In fact, a computer simulation of the RBS spectrum shows an overlap of the signals between the silicon substrate and implanted nickel using 1.5 MeV He ions. Thus it is considered that the heavy ion microprobe is useful to analyze a small amount of species in materials such as masklessly implanted atoms.
4. Conclusion Maskless MeV ion implantation into a silicon substrate has been demonstrated by utilizing focused 3 MeV carbon and 0.7 MeV nickel ions. The focused MeV carbon beam was applied as a microprobe to analyze the implanted area by SE or RBS. It was found that high-contrast SE investigation and high-sensitivity RBS measurements were possible by heavy-ion microprobes. The usefulness of the heavy-ion microprobe has been shown.
References [l] J.A. Cookson, Nucl. Instr. and Meth. 165 (1979) 477. [Z] F. Watt, G.W. Grime, G.D. Blower, J. Takacs and D.J.T. Vaux, Nucl. Instr. and Meth. 197 (1982) 65. [3] G.J.F. Legge, Nucl. Instr. and Meth. 197 (1982) 243. [4] R. Nobihng, Nucl. Instr. and Meth. 218 (19831 197. [5] B.L. Doyle, Nucl. Instr. and Meth. B15 (1986) 654. [6] A. Kinomura, M. Takai, K. Inoue, K. Matsunaga, M. Izumi, T. Matsuo, K. Gamo, S. Namba and M. Satou, Nucl. Instr. and Meth. B33 (1988) 862. [7] Y. Horino, A. Chayahara, M. Kiuchi, K. Fujii, M. Satou and M. Takai, Jpn. J. Appl. Phys. 29 (1990) 2680. [8] Y. Horino, A. Chayahara, M. Satou and M. Takai, Nucl. Instr. and Meth. B54 (1991) 269. [9] S. Nomura and H. Shichi, Jpn. J. Appl. Phys. 30 (1991) L1059. [lo] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [ll] A. Kinomura, M. Takai, S. Namba, M. Satou and A. Chayahara, Nucl. Instr. and Meth. B45 (1990) 523.
VII. ACCELERATORS/BEAMS