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Effects of MeV ion implantation on metal films
Surface and Coatings Technology, 66 (1994) 305—309
305
Effects of MeV ion implantation on metal films M. Ikeyama, K. Saitoh, S. Nakao, H. Niwa, S. Tanemura, Y. Miyagawa and S. Miyagawa National Industrial Research Institute of Nagoya, 1-1, Hirate-cho, Kita-ku, Nagoya 462 (Japan)
Abstract Au films on optically flat Pyrex glasses or MgO single wereoftheir implanted withwere Si, Ni and Au ions with ion energies ofthe from surface 0.75 to 2 crystals The changes surfaces investigated from measurements 3 MeV and doses from of5 x 1013 to 5 x 1016 ions cm profiles and light reflection and/or scattering, and from observations by scanning electron microscopy(SEM). After ion implantation the surface showed a depression. The depression increased with increasing dose and mass of implanted ions. The ion-implanted surface became smoother than that of the as-deposited one. The intensity of light scattering was decreased and a metallic brilliance was observed after the implantation. The depression and themorphology changes were caused by sputtering and grain growth. In the SEM observations, the implanted regions were contrasted with as-deposited regions as blacker in secondary electron images and as whiter in composition images ofreflected electrons. The impurity level of thefilms appeared to diminish after the ion implantation.
1. Introduction The ion implantation technique has been applied in metals to improve several kinds of properties, such as hardness and wear resistance [1]. MeV ion implantation is expected to be able to modify deeper layers and to reduce the effects of sputtering because high-energy implantation results in greater penetration of the ion. Our group has studied the effects of MeV ion implantation on ceramics such as alumina, sapphire, silicon nitride and quartz, and glasses [2—6]. In our previous studies, surface profile measurements revealed that after the MeV ion implantation, mirror-polished ceramics surfaces were swollen and optically flat glass surfaces were depressed without changing their surface flatness and smoothness. Those materials consist of relatively light mass atoms and the effects of sputtering induced by MeV ion implantation were very small. However, the higher mass particles give larger sputtering yields, and the higher masses of the target atoms allow less penetration of implanted ions [7]. Therefore, for a heavy mass target such as gold, the effects of sputtering cannot be ignored, especially in heavy (such as gold) ion implantation. It is interesting to investigate how the surface morphologies of metals are affected by MeV ion implantation. Consequently, we have studied the effects of MeV i~ implantation on gold (Au) films by measuring the surface height changes and reflection and scattering of light and by observations by scanning electron microscopy (SEM). 2. Experimental details Au films were deposited on optically flat Pyrex glasses or MgO single crystals to a thickness of 1—3 ~m by
0257-8972/94/57.00 SSDI 0257-8972(94)07050-2
vacuum evaporation. Substrates were heated to approx. 600—700 K during deposition. Ion implantation was performed using a tandem type ion accelerator (NEC 5SDH-2). These films were implanted with Si, Ni and Au ions, changing the ion energies from 0.75 to 3 MeV in order to adjust the average projection ranges of implanted ions to about 200 nm. The dose was 5 x 1013_~5x 1016 ions cm2 and the ion current density was 2—20 j.tA cm2. The size of the ion implantation region was about 0.7 mm x 0.7 mm and 10 mm x 10 mm for the step height measurements and for light scattering measurements and surface observation, respectively. The temperature of the sample was either room temperature (300 K) or liquid nitrogen temperature (100 K) during implantation. The differences in surface height between implanted and unimplanted regions were evaluated with a surface profilemeter (WYKO TOPO-3D). The changes of surface morphology were estimated from spectroscopic measurement of reflected and/or scattered light from the surface and SEM observation. The spectroscopic light measurement was done in two ways, one by measuring the total light intensity of reflected and scattered light with the sample offset 12~from the normal to the incident light, and the other by measuring only the scattered light at the film surface with the sample set normal to the incident light. In the latter case the reflected light emerged through the same window as the incident light. The purity of metal films was examined by secondary ion mass spectrometry (SIMS), (Cameca IMS-3F). To compare the regions of implanted and unimplanted, secondary ion images for various ion species on a
© 1994
—
Elsevier Science S.A. All rights reserved
306
tI. lkct’aina
It al,
Lijects ()j MI’ t
channel plate/fluorescent screen were acquired with a CCD camera and processed by a computer [8].
3. Results and discussion The results of surface profile measurements for an Au film that was deposited on glass and then implanted with 0.75 MeV Si~,0.9 MeV Ni~ and 3 MeV Au2~at various doses are shown in Fig. I. It shows a clear depression of the implanted region. and the depression increased with the dose. This depression depends on the ion species, i.e. heavier ion implantation makes a deeper depression. For example, the depression of the Au films at a dose of 2x 1016 ions cm2 is about l70nm, 40 nm and 10 nm for 3 MeV Au, 0.9 MeV Ni and 0.75 MeV Si implantation, respectively. The results for implantation at 100 K (not shown) are almost the same as those at 300 K. In this temperature region, we could not find any temperature dependence of the surface depression. To estimate the effects of sputtering for the surface depression, we have performed TRIM [9] simulations. This yielded sputtering rates of Au for 0.75 MeV Si, 0.9 MeV Ni and 3 MeV Au irradiation of 2.4, 5.7 and 19, respectively. Comparing the data with the estimated depressions due to the sputtering (dashed lines), good agreement was observed for Si implantation, but not for Ni and Au implantation, where the observed values are considerably larger than the estimated ones.
1000
loll
unpiantation Oil
metal Il/HIS
simulation can give a rough estimation of the sputtering. In the higher dose region, an increase of the sample temperature may have occurred and the larger depression might be caused by the higher sputtering rate at high temperature. Figure 2 shows the dependency of the surface depression on the current density. In the range 4 16 gA cm 2 the difference in ion current densities did not affect the depression. It would appear that ion beam heating did not influence the depression, although this is not a definite conclusion and further studies are planned on this aspect. Next, we have investigated the changes of micromorphology of Au film surface. Figure 3 shows SEM images of unimplanted and implanted (3MeV Au2~. 2 x 1016 ions cm2 at 100 K) Au films of about 2 ttm in thickness on MgO substrates. SEM observation shows that the ion-implanted surface became smoother than that of the as-deposited surface. As-deposited Au films consisted of columnar microcrystals. However, after ion implantation these microcrystals disappeared. From the surface morphology changes due to ion implantation, it can be said that grain growth and a flow of surface atoms occurred. Grain growth of metals induced by ion irradiation has been studied, and it was revealed that the grain growth is proportional to the energy deposited in elastic collisions [10] and the grain growth kinetics are determined by the defect density in the cascade [11]. In this study, morphology of the surfaces implanted with Si and Ni was not changed in this way. Both the grain growth and sputtering correspond to the amount of the nuclear energy deposition or collision cascade. TRIM
I
100C
Au Film/h[ass 0
3MeVAu 2~
0
009MeV Ni’~
/ /13
~0.75MeVSi~
10
00
Au Film/Glass —3MeVAu2~
,.‘
°
100
I
°
100
0
10
4jJA/cm2
08
-
~16
10
300K 1
‘
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I
1015
‘
I
300K
I
I
io16
i017
Dose (ions/cm2)
1 1014
I
1015
I
i016
Dose (ions/cm2)
Fig. 1. Dose dependence of the surface depressions (—At) of an Au film on glass induced by 0.75 MeV Sit. 0.9 MeV Ni~ and 3 McV Au2~implantation at about 300K. Dashed lines show the estimated sputtering by TRIM simulation [9].
Fig. 2. Ion current density dependence of the surface dcpression (—Ar) of an Au film on glass induced by 3 McV Au2 implantation at about 300 K.
M. Ikeyama et al.
/
Effects of MeVion implantation on metal films 20C
307
‘‘‘‘I’’’’’’
Au Film (Reference)
~~Scat.
_
______
~:b00
5.
(a) w 2 w _____________________________
I
500
‘
Scat.
1000
1500
2000
Wave Length (nml
(a) 20C~
Au Film (b)
) Before Implantation)
5um
2~implanted Fig. 3. SEM images of(a) unimplanted and (b) 3 MeV Au Au film surfaces on MgO substrate with the dose of 2 x 1016 ions cm2 at about 100 K.
~
Reft ~Scaf
>.‘
~100
Observing the film surfaces visually, the as-deposited Au films did not show metallic brilliance and had a white—gold colour. In contrast, the ion-implanted surface exhibited metallic brilliance and had a general gold colour, so the changes of optical properties were exammed. The results of light reflection and/or scattering from the film surface (the same film as shown in Fig. 3) are shown in Fig. 4. The ordinate shows the light intensity relative to the reflectance of the reference alumina surface. The “reference Au film” in Fig. 4 was sold as a mirror and has been used as a standard material in optical measurements; it was made by physical vapour deposition and was 400 nm in thickness. The surface scattering of the “reference” (Fig. 4(a)) is very small, corresponding to the flatness and smoothness of its surface. In contrast, the as-deposited Au film (Fig. 4(b)) shows very large scattering and the scattering has a
of broad light, the peak scattering, colour at about ofthe the 700 total filmnm. was light Because white—gold. intensity this ofAsreflection scattered a result andthis scattering was reduced because theofscattered light could escape from the small windows of the light integration ball. After 3 MeV Au2~implantation at the dose of 2 x 1016 ions cm2 (Fig. 4(c)), the scattered light inten sity was reduced remarkably, and the total intensity was increased. The reduction of light scattering led to the metallic brilliance. Spectroscopy of the scattered light of as-deposited and ion-implanted films shows the peaks at around 700 nm (Fig. 4(b)) and 600 nm (Fig. 4(c)). However, the
~
~ ix ‘I’’’
500
(b)
1000
1500
2000
Wave Length (rim)
20C
Au Film
I After Implantation
I
~
~ ~> ~
rScaf.
~‘
0 (c)
-__________________________________________ 500
1000 Wa v e Len g t h In ml
1500
2000
Fig. 4. Relative intensities of total (reflection scattering) scattering light for (a) reference Au film (400 nm and in thickness), (b) and asdeposited (2 pm in thickness) Au film on MgO substrate and (C) after 3 MeV Au2~ implantation on the film of (b) at a dose of 2 x lOIs ions cm2 at about 100 K.
308
M. Ikevaina
ci
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of ~t’IeV ion imp/antation on metal tunis
Au film has a large absorption for light below 500 nm, as shown in Fig. 4(a). Hence, we have introduced the normalized scattering, which is evaluated by the ratio of the scattered light intensity of an Au film to the total light intensity of the “reference Au film”. The results are shown in Fig. 5. The spectra of scattered light for both as-deposited and ion-implanted films show a clear decrease with increasing wavelength and a broad peak around 600 nm for the as-deposited film. The wavelength of the peak corresponds to the size of the columnar crystals of the as-deposited film. This implies that the peak may be related to Mie scattering by the crystals [12]. The general feature of the scattered light can be explained by Rayleigh scattering at the surface [12]. In summary, the results of the optical measurements of Au films showed that the intensity of the scattered light decreased and the peak disappeared after ion implantation. They led to the acquisition of metallic brilliance and were closely related to the reductions of surface roughness and depressions of the surface. It can be said that all of them are due to sputtering and grain growth. The colour change of the Au film surfaces by ion implantation is a macroscopic manifestation of the microscopic change in the films. In the SEM observation, a similar macroscopic change was observed: the ion implanted regions looked blacker as contrasted with the unimplanted region in secondary electron images (SEI) at low magnification. The examples are shown in Fig. 6(a). The contrast seems to increase with the dose. It was also observed that the contrast increased with increasing mass of the implanted ions. The black contrast of SEI can be explained by the difference in surface 200
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Normalized
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~i°o V
I
.
1
0
.
I
________________________________________ 500
1000
Wave Length
1500
2000
nm)
Fig. 5. Normalized intensities of scattering light of as-deposited (before implantation) film and after 3 MeV Au2~implantation at a dose of 2 x 1016 ions cm2 at about 100 K.
3
(a)
1mm
(b(
2~implanted Au film surface on Fig. 6.substrate. MgO SEM Images (a) A of secondary 3 MeV Au electron image and (h( a composition image of reflected electron. The doses of the regions noted 1. 2 and 3 are I x l0’~,2 x lO’~and 5 x 1015 ions cm
2
respectively.
roughness or edge effect. As shown in Fig. 3. the as-deposited Au films consisted of columnar microcrystals and some of the crystals emitted more secondary electrons (shown as clear white images in Fig. 3) because the crystals have many edges and secondary electron yield becomes larger at the edges compared with the flat faces [13]. Moreover, in composition images of reflected electrons (COMP), the ion-implanted regions were contrasted in white with the unimplanted region (Fig. 6(b)). The contrast intensities seem to increase with the dose as SEI images. The white contrast of COMP usually means a large atomic number, on average. To investigate the differences of impurity level between as-deposited and ion-implanted regions. we have observed the images of the secondary ions at an interface of the as-deposited and ion-implanted regions by SIMS. The results are shown in Fig. 7. In these images, the points that gave a larger signal of the secondary ions of each element beyond some level are shown as black dots. The left side of each image is the ion-implanted
~rImp
2
region
and
the
right
side
is
an
as-deposited region in the same field. Although the amount of Au was the same, the in amounts of other elements werealmost considerably lower the ionimplanted region. It could be said that the impurities were removed by ion implantation. However, we must note that the large amount of Ag in as-deposited regions suggests contamination from the silver paste that was used in SEM observation before the SIMS measurement, and that this contamination resulted
M. Ikeyama et al. _______
~
_____
Mg
‘.~
/
Effects of MeV ion implantation on metal films
309
A,
~
~
.: Fe
1~~
~
~
Ag
______
Au~’~~ ~
Fig. 7. Images from SIMS observation at the interface region of unimplanted and ion-implanted Au film (the same film as shown in Fig. 3). The size of each image is about 540 nm in diameter. The left side is the ion-implanted region and the right side is the unimplanted region. The points where the intensity of secondary ions exceeds some suitable threshold value are shown as black dots. The threshold values depend on the elements and a comparison of the dot intensity between the figures has no meaning.
from the easy capture of the paste at the rough surface of the as-deposited region. In any case, it can be concluded that the whiter contrasts of ion-implanted regions in COMP images are due to this slight contamination 4. Conclusion The changes in the Au film surfaces induced by ion implantation were investigated from measurements of the surface profiles and light reflection and/or scattering, and from SEM observations. After ion implantation the surface showed a depression. The depression increased with increasing dose and mass of the implanted ions. The ion-implanted surface became smoother than that of the as-deposited one and grain growth occurred. The intensity of light scattering was decreased and a metallic brilliance appeared after implantation. In the SEM observations, the implanted regions contrasted with the as-deposited regions, as they appeared blacker in the SEI image and whiter in the COMP image. The depression and the morphology changes were caused by sputtering and the grain growth. Impurities in the films seems to diminish after ion implantation.
References 1 M. Iwaki, Mater. Sci. Eng., A115 (1989) 369. 2 M. Ikeyama, K. Saitoh, H. Niwa, Y. Miyagawa and S. Miyagawa, J. Jpn. Soc. Posvder Powder Metal., 38 (1991) 411. 3 M. Ikeyama, K. Saitoh, H. Niwa, Y. Miyagawa and S. Miyagawa, in T. Takagi (ed), Proc. 14th Symp. on ISIAT’91, Tokyo, June 3—5. 1991, The Ion Engineering Society of Japan, Tokyo, 1991, p. 405. 4 M. Ikeyama, K. Saitoh, H. Niwa, Y. Miyagawa and S. Miyagawa, 17 ()99lj 5 lonics, M. Ikeyama, K. 67. Saitoh, H. Niwa, S. Nakao, Y. Miyagawa and S. Miyagawa, in T. Takagi (ed), Proc. 1st Meeting on IESJ ‘92, Tokyo, June 1—3, 1992, The Ion Engineering Society of Japan, Tokyo, 1992, p. 161. 6 M. Ikeyama, K. Saitoh, H. Niwa S. Nakao, Y. Miyagawa and S. Miyagawa. in S-C. Zou ted.), Proc. 5th China—Japan Symp. on 1SOM 92, Yellosv Mountain, China, October 25—30, 1992, Association of Ion Surface Optimization of Materials, Chinese Academy of Science, Shanghai, p. t61. 7 R. Behrish (ed.) Sputtering by Particle Bombardment I, Springer, Berlin, 1981, p. 281. . . 8 H. Morikawa, Y. lida, Y. Uwamino and T. Ishizuka, Mass Spectrosc., 39 (1991) 337. 9 J. F. Ziegler, J. P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985, p. 321. 10 H. A. Atwater, C. V. Thompson and H. I. Smith, J. Appl. Phys., 64 ~ J.LLiu,J Li and J. W. Mayer, J Appi. Phys., 67 (1990) 2354. t2 H. C. Van de Hulst, Light Scattering by Small Particles, Wiley, New York, 1957, p. 119.
Acknowledgments
13 P. R. Thornton, Scanning Electron Microscopy, Chapman & Hall,
London, 1968, p. 368.
The authors thank Dr H. Morikawa for his kind measurement and analysis of SIMS and Dr M. Yamada for his useful discussion on the results of SEM observation.
305
Effects of MeV ion implantation on metal films M. Ikeyama, K. Saitoh, S. Nakao, H. Niwa, S. Tanemura, Y. Miyagawa and S. Miyagawa National Industrial Research Institute of Nagoya, 1-1, Hirate-cho, Kita-ku, Nagoya 462 (Japan)
Abstract Au films on optically flat Pyrex glasses or MgO single wereoftheir implanted withwere Si, Ni and Au ions with ion energies ofthe from surface 0.75 to 2 crystals The changes surfaces investigated from measurements 3 MeV and doses from of5 x 1013 to 5 x 1016 ions cm profiles and light reflection and/or scattering, and from observations by scanning electron microscopy(SEM). After ion implantation the surface showed a depression. The depression increased with increasing dose and mass of implanted ions. The ion-implanted surface became smoother than that of the as-deposited one. The intensity of light scattering was decreased and a metallic brilliance was observed after the implantation. The depression and themorphology changes were caused by sputtering and grain growth. In the SEM observations, the implanted regions were contrasted with as-deposited regions as blacker in secondary electron images and as whiter in composition images ofreflected electrons. The impurity level of thefilms appeared to diminish after the ion implantation.
1. Introduction The ion implantation technique has been applied in metals to improve several kinds of properties, such as hardness and wear resistance [1]. MeV ion implantation is expected to be able to modify deeper layers and to reduce the effects of sputtering because high-energy implantation results in greater penetration of the ion. Our group has studied the effects of MeV ion implantation on ceramics such as alumina, sapphire, silicon nitride and quartz, and glasses [2—6]. In our previous studies, surface profile measurements revealed that after the MeV ion implantation, mirror-polished ceramics surfaces were swollen and optically flat glass surfaces were depressed without changing their surface flatness and smoothness. Those materials consist of relatively light mass atoms and the effects of sputtering induced by MeV ion implantation were very small. However, the higher mass particles give larger sputtering yields, and the higher masses of the target atoms allow less penetration of implanted ions [7]. Therefore, for a heavy mass target such as gold, the effects of sputtering cannot be ignored, especially in heavy (such as gold) ion implantation. It is interesting to investigate how the surface morphologies of metals are affected by MeV ion implantation. Consequently, we have studied the effects of MeV i~ implantation on gold (Au) films by measuring the surface height changes and reflection and scattering of light and by observations by scanning electron microscopy (SEM). 2. Experimental details Au films were deposited on optically flat Pyrex glasses or MgO single crystals to a thickness of 1—3 ~m by
0257-8972/94/57.00 SSDI 0257-8972(94)07050-2
vacuum evaporation. Substrates were heated to approx. 600—700 K during deposition. Ion implantation was performed using a tandem type ion accelerator (NEC 5SDH-2). These films were implanted with Si, Ni and Au ions, changing the ion energies from 0.75 to 3 MeV in order to adjust the average projection ranges of implanted ions to about 200 nm. The dose was 5 x 1013_~5x 1016 ions cm2 and the ion current density was 2—20 j.tA cm2. The size of the ion implantation region was about 0.7 mm x 0.7 mm and 10 mm x 10 mm for the step height measurements and for light scattering measurements and surface observation, respectively. The temperature of the sample was either room temperature (300 K) or liquid nitrogen temperature (100 K) during implantation. The differences in surface height between implanted and unimplanted regions were evaluated with a surface profilemeter (WYKO TOPO-3D). The changes of surface morphology were estimated from spectroscopic measurement of reflected and/or scattered light from the surface and SEM observation. The spectroscopic light measurement was done in two ways, one by measuring the total light intensity of reflected and scattered light with the sample offset 12~from the normal to the incident light, and the other by measuring only the scattered light at the film surface with the sample set normal to the incident light. In the latter case the reflected light emerged through the same window as the incident light. The purity of metal films was examined by secondary ion mass spectrometry (SIMS), (Cameca IMS-3F). To compare the regions of implanted and unimplanted, secondary ion images for various ion species on a
© 1994
—
Elsevier Science S.A. All rights reserved
306
tI. lkct’aina
It al,
Lijects ()j MI’ t
channel plate/fluorescent screen were acquired with a CCD camera and processed by a computer [8].
3. Results and discussion The results of surface profile measurements for an Au film that was deposited on glass and then implanted with 0.75 MeV Si~,0.9 MeV Ni~ and 3 MeV Au2~at various doses are shown in Fig. I. It shows a clear depression of the implanted region. and the depression increased with the dose. This depression depends on the ion species, i.e. heavier ion implantation makes a deeper depression. For example, the depression of the Au films at a dose of 2x 1016 ions cm2 is about l70nm, 40 nm and 10 nm for 3 MeV Au, 0.9 MeV Ni and 0.75 MeV Si implantation, respectively. The results for implantation at 100 K (not shown) are almost the same as those at 300 K. In this temperature region, we could not find any temperature dependence of the surface depression. To estimate the effects of sputtering for the surface depression, we have performed TRIM [9] simulations. This yielded sputtering rates of Au for 0.75 MeV Si, 0.9 MeV Ni and 3 MeV Au irradiation of 2.4, 5.7 and 19, respectively. Comparing the data with the estimated depressions due to the sputtering (dashed lines), good agreement was observed for Si implantation, but not for Ni and Au implantation, where the observed values are considerably larger than the estimated ones.
1000
loll
unpiantation Oil
metal Il/HIS
simulation can give a rough estimation of the sputtering. In the higher dose region, an increase of the sample temperature may have occurred and the larger depression might be caused by the higher sputtering rate at high temperature. Figure 2 shows the dependency of the surface depression on the current density. In the range 4 16 gA cm 2 the difference in ion current densities did not affect the depression. It would appear that ion beam heating did not influence the depression, although this is not a definite conclusion and further studies are planned on this aspect. Next, we have investigated the changes of micromorphology of Au film surface. Figure 3 shows SEM images of unimplanted and implanted (3MeV Au2~. 2 x 1016 ions cm2 at 100 K) Au films of about 2 ttm in thickness on MgO substrates. SEM observation shows that the ion-implanted surface became smoother than that of the as-deposited surface. As-deposited Au films consisted of columnar microcrystals. However, after ion implantation these microcrystals disappeared. From the surface morphology changes due to ion implantation, it can be said that grain growth and a flow of surface atoms occurred. Grain growth of metals induced by ion irradiation has been studied, and it was revealed that the grain growth is proportional to the energy deposited in elastic collisions [10] and the grain growth kinetics are determined by the defect density in the cascade [11]. In this study, morphology of the surfaces implanted with Si and Ni was not changed in this way. Both the grain growth and sputtering correspond to the amount of the nuclear energy deposition or collision cascade. TRIM
I
100C
Au Film/h[ass 0
3MeVAu 2~
0
009MeV Ni’~
/ /13
~0.75MeVSi~
10
00
Au Film/Glass —3MeVAu2~
,.‘
°
100
I
°
100
0
10
4jJA/cm2
08
-
~16
10
300K 1
‘
1014
I
1015
‘
I
300K
I
I
io16
i017
Dose (ions/cm2)
1 1014
I
1015
I
i016
Dose (ions/cm2)
Fig. 1. Dose dependence of the surface depressions (—At) of an Au film on glass induced by 0.75 MeV Sit. 0.9 MeV Ni~ and 3 McV Au2~implantation at about 300K. Dashed lines show the estimated sputtering by TRIM simulation [9].
Fig. 2. Ion current density dependence of the surface dcpression (—Ar) of an Au film on glass induced by 3 McV Au2 implantation at about 300 K.
M. Ikeyama et al.
/
Effects of MeVion implantation on metal films 20C
307
‘‘‘‘I’’’’’’
Au Film (Reference)
~~Scat.
_
______
~:b00
5.
(a) w 2 w _____________________________
I
500
‘
Scat.
1000
1500
2000
Wave Length (nml
(a) 20C~
Au Film (b)
) Before Implantation)
5um
2~implanted Fig. 3. SEM images of(a) unimplanted and (b) 3 MeV Au Au film surfaces on MgO substrate with the dose of 2 x 1016 ions cm2 at about 100 K.
~
Reft ~Scaf
>.‘
~100
Observing the film surfaces visually, the as-deposited Au films did not show metallic brilliance and had a white—gold colour. In contrast, the ion-implanted surface exhibited metallic brilliance and had a general gold colour, so the changes of optical properties were exammed. The results of light reflection and/or scattering from the film surface (the same film as shown in Fig. 3) are shown in Fig. 4. The ordinate shows the light intensity relative to the reflectance of the reference alumina surface. The “reference Au film” in Fig. 4 was sold as a mirror and has been used as a standard material in optical measurements; it was made by physical vapour deposition and was 400 nm in thickness. The surface scattering of the “reference” (Fig. 4(a)) is very small, corresponding to the flatness and smoothness of its surface. In contrast, the as-deposited Au film (Fig. 4(b)) shows very large scattering and the scattering has a
of broad light, the peak scattering, colour at about ofthe the 700 total filmnm. was light Because white—gold. intensity this ofAsreflection scattered a result andthis scattering was reduced because theofscattered light could escape from the small windows of the light integration ball. After 3 MeV Au2~implantation at the dose of 2 x 1016 ions cm2 (Fig. 4(c)), the scattered light inten sity was reduced remarkably, and the total intensity was increased. The reduction of light scattering led to the metallic brilliance. Spectroscopy of the scattered light of as-deposited and ion-implanted films shows the peaks at around 700 nm (Fig. 4(b)) and 600 nm (Fig. 4(c)). However, the
~
~ ix ‘I’’’
500
(b)
1000
1500
2000
Wave Length (rim)
20C
Au Film
I After Implantation
I
~
~ ~> ~
rScaf.
~‘
0 (c)
-__________________________________________ 500
1000 Wa v e Len g t h In ml
1500
2000
Fig. 4. Relative intensities of total (reflection scattering) scattering light for (a) reference Au film (400 nm and in thickness), (b) and asdeposited (2 pm in thickness) Au film on MgO substrate and (C) after 3 MeV Au2~ implantation on the film of (b) at a dose of 2 x lOIs ions cm2 at about 100 K.
308
M. Ikevaina
ci
(11/
Ejjects
of ~t’IeV ion imp/antation on metal tunis
Au film has a large absorption for light below 500 nm, as shown in Fig. 4(a). Hence, we have introduced the normalized scattering, which is evaluated by the ratio of the scattered light intensity of an Au film to the total light intensity of the “reference Au film”. The results are shown in Fig. 5. The spectra of scattered light for both as-deposited and ion-implanted films show a clear decrease with increasing wavelength and a broad peak around 600 nm for the as-deposited film. The wavelength of the peak corresponds to the size of the columnar crystals of the as-deposited film. This implies that the peak may be related to Mie scattering by the crystals [12]. The general feature of the scattered light can be explained by Rayleigh scattering at the surface [12]. In summary, the results of the optical measurements of Au films showed that the intensity of the scattered light decreased and the peak disappeared after ion implantation. They led to the acquisition of metallic brilliance and were closely related to the reductions of surface roughness and depressions of the surface. It can be said that all of them are due to sputtering and grain growth. The colour change of the Au film surfaces by ion implantation is a macroscopic manifestation of the microscopic change in the films. In the SEM observation, a similar macroscopic change was observed: the ion implanted regions looked blacker as contrasted with the unimplanted region in secondary electron images (SEI) at low magnification. The examples are shown in Fig. 6(a). The contrast seems to increase with the dose. It was also observed that the contrast increased with increasing mass of the implanted ions. The black contrast of SEI can be explained by the difference in surface 200
.,.,,,,,,
‘I’’
Normalized
Scattering
>
~i°o V
I
.
1
0
.
I
________________________________________ 500
1000
Wave Length
1500
2000
nm)
Fig. 5. Normalized intensities of scattering light of as-deposited (before implantation) film and after 3 MeV Au2~implantation at a dose of 2 x 1016 ions cm2 at about 100 K.
3
(a)
1mm
(b(
2~implanted Au film surface on Fig. 6.substrate. MgO SEM Images (a) A of secondary 3 MeV Au electron image and (h( a composition image of reflected electron. The doses of the regions noted 1. 2 and 3 are I x l0’~,2 x lO’~and 5 x 1015 ions cm
2
respectively.
roughness or edge effect. As shown in Fig. 3. the as-deposited Au films consisted of columnar microcrystals and some of the crystals emitted more secondary electrons (shown as clear white images in Fig. 3) because the crystals have many edges and secondary electron yield becomes larger at the edges compared with the flat faces [13]. Moreover, in composition images of reflected electrons (COMP), the ion-implanted regions were contrasted in white with the unimplanted region (Fig. 6(b)). The contrast intensities seem to increase with the dose as SEI images. The white contrast of COMP usually means a large atomic number, on average. To investigate the differences of impurity level between as-deposited and ion-implanted regions. we have observed the images of the secondary ions at an interface of the as-deposited and ion-implanted regions by SIMS. The results are shown in Fig. 7. In these images, the points that gave a larger signal of the secondary ions of each element beyond some level are shown as black dots. The left side of each image is the ion-implanted
~rImp
2
region
and
the
right
side
is
an
as-deposited region in the same field. Although the amount of Au was the same, the in amounts of other elements werealmost considerably lower the ionimplanted region. It could be said that the impurities were removed by ion implantation. However, we must note that the large amount of Ag in as-deposited regions suggests contamination from the silver paste that was used in SEM observation before the SIMS measurement, and that this contamination resulted
M. Ikeyama et al. _______
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_____
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Effects of MeV ion implantation on metal films
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.: Fe
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Fig. 7. Images from SIMS observation at the interface region of unimplanted and ion-implanted Au film (the same film as shown in Fig. 3). The size of each image is about 540 nm in diameter. The left side is the ion-implanted region and the right side is the unimplanted region. The points where the intensity of secondary ions exceeds some suitable threshold value are shown as black dots. The threshold values depend on the elements and a comparison of the dot intensity between the figures has no meaning.
from the easy capture of the paste at the rough surface of the as-deposited region. In any case, it can be concluded that the whiter contrasts of ion-implanted regions in COMP images are due to this slight contamination 4. Conclusion The changes in the Au film surfaces induced by ion implantation were investigated from measurements of the surface profiles and light reflection and/or scattering, and from SEM observations. After ion implantation the surface showed a depression. The depression increased with increasing dose and mass of the implanted ions. The ion-implanted surface became smoother than that of the as-deposited one and grain growth occurred. The intensity of light scattering was decreased and a metallic brilliance appeared after implantation. In the SEM observations, the implanted regions contrasted with the as-deposited regions, as they appeared blacker in the SEI image and whiter in the COMP image. The depression and the morphology changes were caused by sputtering and the grain growth. Impurities in the films seems to diminish after ion implantation.
References 1 M. Iwaki, Mater. Sci. Eng., A115 (1989) 369. 2 M. Ikeyama, K. Saitoh, H. Niwa, Y. Miyagawa and S. Miyagawa, J. Jpn. Soc. Posvder Powder Metal., 38 (1991) 411. 3 M. Ikeyama, K. Saitoh, H. Niwa, Y. Miyagawa and S. Miyagawa, in T. Takagi (ed), Proc. 14th Symp. on ISIAT’91, Tokyo, June 3—5. 1991, The Ion Engineering Society of Japan, Tokyo, 1991, p. 405. 4 M. Ikeyama, K. Saitoh, H. Niwa, Y. Miyagawa and S. Miyagawa, 17 ()99lj 5 lonics, M. Ikeyama, K. 67. Saitoh, H. Niwa, S. Nakao, Y. Miyagawa and S. Miyagawa, in T. Takagi (ed), Proc. 1st Meeting on IESJ ‘92, Tokyo, June 1—3, 1992, The Ion Engineering Society of Japan, Tokyo, 1992, p. 161. 6 M. Ikeyama, K. Saitoh, H. Niwa S. Nakao, Y. Miyagawa and S. Miyagawa. in S-C. Zou ted.), Proc. 5th China—Japan Symp. on 1SOM 92, Yellosv Mountain, China, October 25—30, 1992, Association of Ion Surface Optimization of Materials, Chinese Academy of Science, Shanghai, p. t61. 7 R. Behrish (ed.) Sputtering by Particle Bombardment I, Springer, Berlin, 1981, p. 281. . . 8 H. Morikawa, Y. lida, Y. Uwamino and T. Ishizuka, Mass Spectrosc., 39 (1991) 337. 9 J. F. Ziegler, J. P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985, p. 321. 10 H. A. Atwater, C. V. Thompson and H. I. Smith, J. Appl. Phys., 64 ~ J.LLiu,J Li and J. W. Mayer, J Appi. Phys., 67 (1990) 2354. t2 H. C. Van de Hulst, Light Scattering by Small Particles, Wiley, New York, 1957, p. 119.
Acknowledgments
13 P. R. Thornton, Scanning Electron Microscopy, Chapman & Hall,
London, 1968, p. 368.
The authors thank Dr H. Morikawa for his kind measurement and analysis of SIMS and Dr M. Yamada for his useful discussion on the results of SEM observation.