- Email: [email protected]
Si(100) films
Surface and Coatings Technology 158 – 159 (2002) 334–338
Irradiation-induced improvement of crystalline quality of epitaxial Cuy Si(100) films K. Takahiroa,*, N. Takeshimaa, K. Kawatsuraa, S. Nagatab, S. Yamamotoc, H. Naramotoc a
Department of Chemistry and Materials Engineering, Kyoto Institute of Engineering, Kyoto 606-8585, Japan b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan c Japan Atomic Energy Institute, Takasaki 370-1292, Japan
Abstract We have demonstrated that ion-irradiation greatly improves the crystalline quality of epitaxial grown Cu films on Si(100) substrates. For the CuySi system, it is well known that thermal annealing is inapplicable to the improvement of the crystalline quality of the Cu films because inter-diffusion easily occurs at the CuySi interfaces at temperatures as low as 470 K. Accordingly, ion-irradiation is used to anneal the epitaxial Cu films. Irradiation with 0.5 MeV 28 Si ions was carried out to doses ranging from 1=1016 to 4=1016 cmy2 at temperatures of 323 and 123 K. The quality of the Cu crystalline films (;80 nm thick) was analyzed by Rutherford backscattering spectrometryychanneling (RBSyC) before and after irradiation. In the RBSyC spectra, the minimum yield at the Cu surface decreases from 90% to 42% after irradiation to 3=1016 cmy2 at 323 K. On the other hand, the minimum yield at the CuySi interface decreases up to 2=1016 cmy2 , but increases above that dose due to the reaction between the Cu film and the Si substrate. It is found that irradiation at 123 K effectively prevents the interfacial reaction. The RBSyC analysis reveals that the improvement of the crystalline quality of the Cu film is brought about by the decrease in mosaic spread in the Cu film. Also, it is suggested that collision-induced atomic rearrangements reduce the mosaic spread in the Cu film. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion irradiation; Epitaxial film; Nuclear collision; Ion channeling; Copper; Silicon
1. Introduction In the past decade thin film epitaxy has been of great interest from scientific and technological points of view. In particular, there have been a number of studies of epitaxial copper (Cu) thin films on silicon (Si) substrates w1–5x, because Cu metallization is an essential process to fabrication of microelectronic devices. For the CuySi system, it is well known that rapid silicide (Cu3Si) formation w6,7x at temperatures of 420–470 K destroys the epitaxial Cu films. Therefore, post-deposition thermal annealing is inapplicable to the improvement of their quality. Alternatively, ion-irradiation will be an effective method for the low temperature processing. The atomic rearrangements induced by ion-irradiation make it possible to modify materials at temperatures well below those required for thermally activated processes. Typical examples of low-temperature processing *Corresponding author. Tel.: q81-75-724-7507; fax: q81-75-7247507. E-mail address: [email protected] (K. Takahiro).
using ion beams are ion beam-induced epitaxial crystallization (IBIEC) w8–11x, ion bombardment enhanced grain growth (IBEGG) w12–15x, and ion beam smoothing w16–19x. In our previous work w20x, we found that the crystalline quality of epitaxial silver (Ag) thin films on Si(100) substrates was improved remarkably by irradiation with fast ions. Our results indicate that the improvement, which is originated from the decrease of mosaic spread in the films, is attributed to nuclear collisions. Furthermore, we speculate that the irradiation-induced improvement is a phenomenon similar to IBEGG w12–15x, in which atomic rearrangements occur at grain boundaries. In this paper, we examine effects of ion-irradiation on the crystalline quality of epitaxial Cu films deposited on Si(100) substrates. The primary purpose of the present work is to demonstrate the ability of ionirradiation to improve the crystalline quality of the epitaxial CuySi films. Firstly, we report that irradiation with 0.5 MeV 28Si ions at 323 K anneals the epitaxial CuySi films. In some cases, irradiation temperature was
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 2 7 3 - 6
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kept at 123 K to prevent silicide formation at the Cuy Si interface. Then to study the mechanism of the irradiation-induced improvement, a variety of ion species, 0.5 MeV 19F ions and 3.0 MeV 32S ions as well as 0.5–2 MeV 28Si ions, was used to irradiate the samples. Rutherford backscattering combined with channeling (RBSyC) technique was utilized to characterize the ion-irradiated CuySi samples. 2. Experimental Thin Cu films of approximately 80 nm in thickness were prepared using an electron beam evaporation method in a vacuum system at a base pressure of 6=10y7 Pa or less. Single-crystal Si of w100x orientation was used as a substrate, which was etched in a dilute HF (;10%) solution prior to the deposition. The etched Si surfaces are known to be hydrogen terminated w21x and inert for several hours in ultra high vacuum at room temperature w22x. The deposition rate of the film was kept at 0.1–0.15 nm sy1. The experiments using ion beams were performed on a 1.7 MV tandem accelerator at the Institute for Materials Research, Tohoku University. Irradiation with 0.5 MeV 28Si ions was performed to doses ranging from 1=1016 to 4=1016 cmy2 keeping the current density at 0.6 mA cmy2. A variety of ion species, 0.5 MeV 19Fq, 1.9 MeV 28Si2q and 3.0 MeV 32S2q, was also used to irradiate the samples. The projected ranges of the respective ions, predicted by the TRIM w23x, were much larger than the film thickness. Therefore, collision-induced defects are expected to distribute almost uniformly in depth. The sample was mounted on a water-cooled aluminum plate. The temperature of the sample during irradiation was measured to be 323"5 K with a thermocouple. In the case of irradiation at 123 K, the sample was cooled by liquid nitrogen during irradiation, and the current density was reduced to 0.2 mA cmy2 in order to avoid excess heating by irradiation. Rutherford backscattering spectrometry combined with channeling technique (RBSyC) with 2 MeV 4He ions was made in order to characterize the samples before and after irradiation. The samples were mounted on a two-axis goniometer, which allows both tilt and azimuthal rotation of the samples. Backscattered 4He ions were detected with an SSD placed at an angle of 1708. The divergence of the analyzing beam was approximately 0.018. All measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows the w100x aligned spectra taken from the CuySi sample before and after irradiation with 0.5 MeV Si ions at 323 K. The random and aligned spectra for the non-irradiated sample indicate that the Cu film is
Fig. 1. w100x axial aligned spectra for 2 MeV 4He ions incident on samples before (dotted line) and after 0.5 MeV 28Siq irradiation to doses of 1=1016 cmy2 (broken line) and 3=1016 cmy2 (solid line) at 323 K. An inset shows enlarged graphs at energies of 1.02–1.08 MeV.
grown epitaxially onto the Si (100) substrate, although its crystalline quality is rather poor. The minimum yield xmin at the Cu surface for w100x axial channeling is 90% before irradiation, and is decreased to 62% and 42% after irradiation at 1=1016 and 3=1016 cmy2, respectively, indicating that the irradiation greatly improves the crystalline quality of the epitaxial Cu film. For the signal scattered from the Si substrate, the aligned yield in the ion-implanted region, which extends from the CuySi interface to the depth of ;0.9 mm, reaches the random level. This result shows that the irradiation turns the Si substrate into an amorphous state. Thus, the improvement of the Cu crystalline quality does not result from the layer-by-layer mechanism as observed in IBIEC w8–11x, but from the mechanism that occurs inside the Cu film. It should be noted that in the case of 3=1016 cmy2, the Cu yields at the inner region of the film are much higher than those at the surface region. That is, the crystalline quality in the vicinity of the CuySi interface is much poorer than that at the Cu surface. Judging from an enlarged spectra insetted in Fig. 1, a small amount of Si atoms diffuses from the substrate into the Cu film. It is considered, therefore, that the diffusion of the Si atoms degrades the crystalline quality of the Cu film near the interface. As mentioned above, the Si substrate at the interface is transformed into amorphous by irradiation. This amorphization would stimulate the reaction between the Cu film and the Si substrate. If the interfacial reaction deteriorates the crystalline quality of
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Fig. 3. Angular scans of normalized Cu yields around the w100x axis for 2 MeV 4He ions incident on the CuySi sample irradiated with 0.5 MeV 28Si ions to doses of 1=1016 cmy2 (circles) and 3=1016 cmy2 (filled circles) at 323 K. Fig. 2. The change in crystalline quality a after irradiation at 323 K (a), and 123 K (b) at surface region (filled circles) and interface region (circles) as a function of irradiation-dose. The a value is defined as (xnon-irr. yxirr.)yxnon-irr., where xnon-irr. and xirr. are minimum yields in aligned spectra for the sample before and after irradiation, respectively.
the irradiated Cu film, low-temperature irradiation may lead to significant improvement at the interface region. Fig. 2a and b show the change in crystalline quality a of the CuySi films irradiated at 323 and 123 K as a function of irradiation-dose. The a is defined as (xnon-irr. yxirr.)yxnon-irr., where xnon-irr. and xirr. are minimum yields in aligned spectra for the sample before and after irradiation, respectively. A positive value of a is an indication of the improvement of crystallinity. In addition, the a is close to unity (a™1) when the film becomes a perfect crystal (xirr.™0). In these figures, the a values at the surface are always higher than those at the interface, implying that the crystalline quality at the CuySi interface is hard to improve. At the surface region, the a value increases with increasing dose both for the high-temperature (323 K) irradiation and for the low-temperature (123 K) irradiation. At the interface region, however, the high-temperature irradiation reduces the a value at doses above 2=1016 cmy2. In contrast to the high-temperature irradiation, the a value increases as dose increases for the low-temperature irradiation even at the interface region. Accordingly, lowering temperature during irradiation is effective against the interfacial reaction between Cu and Si as seen in Fig. 1. Next, we characterize the ion-irradiated CuySi films using ion channeling. The w100x axial angular scans of Cu yields for the sample after irradiation with 28Si ions to 1=1016 and 3=1016 cmy2 at 323 K are shown in
Fig. 3, where the yields are evaluated from total area of the Cu peak in the RBS spectra. As for the Cu film without irradiation, it was impossible to present the angular scan curve because its crystalline quality was too poor. The channeling half angle c1y2 for the highdose sample is 0.898, smaller than that for the low-dose sample (1.018), indicating that ion-irradiation reduces the angular spread of crystallite orientation in the Cuy Si film with a mosaic structure. Also, the c1y2 value of 0.898 is considerably larger than that for a calculated value (0.678) w24x for a perfect Cu crystal. This implies the presence of a mosaic structure in the irradiated Cuy Si films. Fig. 4 represents azimuthal angular scans of the Cu film and the Si substrate. Such scans measure the
Fig. 4. Azimuthal angular scans for the Cu irradiated with 0.5 MeV 28 Si ions to doses of 1=1016 cmy2 and 3=1016 cmy2 at 323 K, together with a scan for the underlying Si without irradiation. {100} planes (asterisks) and {110} planes (circles) are indicated.
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varying ion energy and ion species at an irradiation temperature of 123 K, and is shown in Fig. 5a, where the a value is plotted as a function of the number of displaced atoms in the unit of displacements per atom (dpa) calculated by the TRIM w23x. The a gradually increases as the number of displaced atoms increases up to ;28 dpa. Fig. 5b shows the correlation between the a and the energy deposited by electronic excitation for the Cu film irradiated at 2.1 dpa, where the deposited energy is estimated by using the TRIM w23x. The a is found to be constant within the range of 1–3.7 keV nmy1, indicating that electronic excitation does not contribute to the increase of a as shown in Fig. 5a. Thus, we concluded that only atomic displacements due to nuclear collisions improve the crystalline quality of the epitaxial Cu film. Also, it is suggested that collisioninduced atomic rearrangements reduce the mosaic spread in the Cu film as can be seen in Fig. 3. 4. Conclusions
Fig. 5. The change in crystalline quality a after irradiation at 123 K as functions of the number of displaced atoms (a) and the electronic energy deposition (b). Both the number of displaced atoms and the electronic energy deposition are calculated by the TRIM w23x.
crystalline quality of the Cu film and determine the epitaxial relationship between the Cu film and the Si substrate. An azimuthal angular scan for the non-irradiated Cu film is not shown here because no significant planar dips could be observed. As for the irradiated Cu film, the dips for the high-dose Cu are larger and sharper than those for the low-dose Cu. This result is also indicative of irradiation-induced improvement of the crystalline quality of the Cu film. In Fig. 4, the planar dips are indexed referring to the minimum yields. In the scan curve for the Si, the four largest dips and the four next largest dips correspond to {100} planes and {110} planes, respectively. On the other hand, the opposite correspondence is valid for the Cu because of the difference in crystal structure between Cu (f.c.c.) and Si (diamond structure). The position of Cu {100} dips coincides with that of Si {110} dips as can be seen in Fig. 4. Thus, the azimuthal scans together with the w100x axial angular scans reveal the epitaxial relationship of (100)Cucc(100)Si and w011xCuccw010xSi. This relationship shows that the Cu lattice is rotated by 458 relative to the Si lattice, as has been observed previously by Chang et al. w4x for a 2-mm-thick Cu film on Si(100). Finally, we shall discuss the mechanism of the irradiation-induced improvement of crystalline quality. The change in crystalline quality a has been examined by
Irradiation with 28Siq at 323 and 123 K has been applied to annealing of the epitaxial CuySi films to improve their crystalline quality. The RBSyC analysis on the ion-irradiated CuySi reveals that the crystalline quality at the interface is much poorer than that at the Cu surface due to the reaction between the Cu film and the Si substrate. Lowering irradiation-temperature is effective against the interfacial reaction. The channeling half angle c1y2 decreases with increasing irradiation-dose, indicating that ion-irradiation reduces the angular spread of crystallite orientation in the CuySi film with a mosaic structure. Also, we have examined the change in crystalline quality by varying ion energy and ion species at 123 K. The change in crystalline quality depends only on the number of atoms displaced by nuclear collisions. It is concluded, therefore, that collision-induced atomic rearrangements reduce the mosaic spread in the Cu film, resulting in the improvement of its crystalline quality. References w1x E.T. Krastev, L.D. Voice, R.G. Tobin, J. Appl. Phys. 79 (1996) 6865. w2x B.W. Karr, Y.W. Kim, I. Petrov, et al., J. Appl. Phys. 80 (1996) 6699. w3x T. Ohmi, T. Saito, T. Shibata, T. Nitta, Appl. Phys. Lett. 52 (1988) 2236. w4x C. Chang, J.C. Liu, J. Angilello, Appl. Phys. Lett. 57 (1990) 2239. w5x B.G. Demezyk, R. Naik, G. Auner, C. Kota, U. Rao, J. Appl. Phys. 75 (1990) 1956. w6x S.Q. Hong, C. Comrie, S.W. Russell, J.W. Mayer, J. Appl. Phys. 70 (1990) 3655. w7x T.L. Alford, D. Adams, J. Li, B. Cao, S.W. Russell, S.Q. Hong, R. Spreitzer, J.W. Mayer, in: D.P. Favreau, Y. Shacham-
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w8 x w9 x w10x w11x w12x w13x w14x w15x
K. Takahiro et al. / Surface and Coatings Technology 158 – 159 (2002) 334–338 Diamand, Y. Horiike (Eds.), Advanced Metallization for ULSI Applications 1993, MRS, 1994, p. 101. I. Golecki, G.E. Champman, S.S. Lau, B.Y. Tsaur, J.W. Mayer, Phys. Lett. 71A (1979) 267. J. Nakata, K. Kajiyama, Appl. Phys. Lett. 40 (1982) 686. J.S. Williams, R.G. Elliman, W.L. Brown, T.E. Seidel, Phys. Rev. Lett. 55 (1985) 1482. ¨ V. Heera, T. Henkel, R. Kogler, W. Skorupa, Phys. Rev. B 52 (1995) 15776. P. Wang, D.A. Thompson, W.W. Smeltzer, Nucl. Instrum. Methods B 7y8 (1986) 97. J.C. Liu, M. Nastasi, J.W. Mayer, J. Appl. Phys. 62 (1987) 423. H.A. Atwater, C.V. Thompson, H.I. Smith, Phys. Rev. Lett. 60 (1988) 112. H.A. Atwater, C.V. Thompson, H.I. Smith, J. Appl. Phys. 64 (1988) 2337.
w16x G.H. bu-Abbud, D.L. Mathine, P. Snyder, et al., J. Appl. Phys. 59 (1986) 257. w17x D.C. Ingram, A.W. McCormick, P.P. Pronko, J.A. Woollam, P.G. Snyder, D.B. Poker, Mater. Sci. Eng. 90 (1987) 417. w18x E. Spiller, Appl. Phys. Lett. 54 (1989) 2293. w19x K. Oyoshi, T. Tagami, S. Tanaka, Jpn. J. Appl. Phys. 30 (1854) 1991. w20x K. Takahiro, S. Nagata, S. Yamaguchi, Appl. Phys. Lett. 69 (1996) 2828. w21x E. Yablonovitch, D.L. Allara, C.C. Chang, T. Gmitter, T.B. Bright, Phys. Rev. Lett. 57 (1986) 249. w22x S.S. Iyer, M. Arienzo, E. de Fresart, Appl. Phys. Lett. 57 (1990) 893. w23x J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon, New York, 1985. w24x J.S. Williams, R.G. Elliman, in: J.R. Bird, J.S. Williams (Eds.), Ion Beams for Materials Analysis, Academic Press, Sydney, 1989, pp. 327–330.
Irradiation-induced improvement of crystalline quality of epitaxial Cuy Si(100) films K. Takahiroa,*, N. Takeshimaa, K. Kawatsuraa, S. Nagatab, S. Yamamotoc, H. Naramotoc a
Department of Chemistry and Materials Engineering, Kyoto Institute of Engineering, Kyoto 606-8585, Japan b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan c Japan Atomic Energy Institute, Takasaki 370-1292, Japan
Abstract We have demonstrated that ion-irradiation greatly improves the crystalline quality of epitaxial grown Cu films on Si(100) substrates. For the CuySi system, it is well known that thermal annealing is inapplicable to the improvement of the crystalline quality of the Cu films because inter-diffusion easily occurs at the CuySi interfaces at temperatures as low as 470 K. Accordingly, ion-irradiation is used to anneal the epitaxial Cu films. Irradiation with 0.5 MeV 28 Si ions was carried out to doses ranging from 1=1016 to 4=1016 cmy2 at temperatures of 323 and 123 K. The quality of the Cu crystalline films (;80 nm thick) was analyzed by Rutherford backscattering spectrometryychanneling (RBSyC) before and after irradiation. In the RBSyC spectra, the minimum yield at the Cu surface decreases from 90% to 42% after irradiation to 3=1016 cmy2 at 323 K. On the other hand, the minimum yield at the CuySi interface decreases up to 2=1016 cmy2 , but increases above that dose due to the reaction between the Cu film and the Si substrate. It is found that irradiation at 123 K effectively prevents the interfacial reaction. The RBSyC analysis reveals that the improvement of the crystalline quality of the Cu film is brought about by the decrease in mosaic spread in the Cu film. Also, it is suggested that collision-induced atomic rearrangements reduce the mosaic spread in the Cu film. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion irradiation; Epitaxial film; Nuclear collision; Ion channeling; Copper; Silicon
1. Introduction In the past decade thin film epitaxy has been of great interest from scientific and technological points of view. In particular, there have been a number of studies of epitaxial copper (Cu) thin films on silicon (Si) substrates w1–5x, because Cu metallization is an essential process to fabrication of microelectronic devices. For the CuySi system, it is well known that rapid silicide (Cu3Si) formation w6,7x at temperatures of 420–470 K destroys the epitaxial Cu films. Therefore, post-deposition thermal annealing is inapplicable to the improvement of their quality. Alternatively, ion-irradiation will be an effective method for the low temperature processing. The atomic rearrangements induced by ion-irradiation make it possible to modify materials at temperatures well below those required for thermally activated processes. Typical examples of low-temperature processing *Corresponding author. Tel.: q81-75-724-7507; fax: q81-75-7247507. E-mail address: [email protected] (K. Takahiro).
using ion beams are ion beam-induced epitaxial crystallization (IBIEC) w8–11x, ion bombardment enhanced grain growth (IBEGG) w12–15x, and ion beam smoothing w16–19x. In our previous work w20x, we found that the crystalline quality of epitaxial silver (Ag) thin films on Si(100) substrates was improved remarkably by irradiation with fast ions. Our results indicate that the improvement, which is originated from the decrease of mosaic spread in the films, is attributed to nuclear collisions. Furthermore, we speculate that the irradiation-induced improvement is a phenomenon similar to IBEGG w12–15x, in which atomic rearrangements occur at grain boundaries. In this paper, we examine effects of ion-irradiation on the crystalline quality of epitaxial Cu films deposited on Si(100) substrates. The primary purpose of the present work is to demonstrate the ability of ionirradiation to improve the crystalline quality of the epitaxial CuySi films. Firstly, we report that irradiation with 0.5 MeV 28Si ions at 323 K anneals the epitaxial CuySi films. In some cases, irradiation temperature was
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 2 7 3 - 6
K. Takahiro et al. / Surface and Coatings Technology 158 – 159 (2002) 334–338
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kept at 123 K to prevent silicide formation at the Cuy Si interface. Then to study the mechanism of the irradiation-induced improvement, a variety of ion species, 0.5 MeV 19F ions and 3.0 MeV 32S ions as well as 0.5–2 MeV 28Si ions, was used to irradiate the samples. Rutherford backscattering combined with channeling (RBSyC) technique was utilized to characterize the ion-irradiated CuySi samples. 2. Experimental Thin Cu films of approximately 80 nm in thickness were prepared using an electron beam evaporation method in a vacuum system at a base pressure of 6=10y7 Pa or less. Single-crystal Si of w100x orientation was used as a substrate, which was etched in a dilute HF (;10%) solution prior to the deposition. The etched Si surfaces are known to be hydrogen terminated w21x and inert for several hours in ultra high vacuum at room temperature w22x. The deposition rate of the film was kept at 0.1–0.15 nm sy1. The experiments using ion beams were performed on a 1.7 MV tandem accelerator at the Institute for Materials Research, Tohoku University. Irradiation with 0.5 MeV 28Si ions was performed to doses ranging from 1=1016 to 4=1016 cmy2 keeping the current density at 0.6 mA cmy2. A variety of ion species, 0.5 MeV 19Fq, 1.9 MeV 28Si2q and 3.0 MeV 32S2q, was also used to irradiate the samples. The projected ranges of the respective ions, predicted by the TRIM w23x, were much larger than the film thickness. Therefore, collision-induced defects are expected to distribute almost uniformly in depth. The sample was mounted on a water-cooled aluminum plate. The temperature of the sample during irradiation was measured to be 323"5 K with a thermocouple. In the case of irradiation at 123 K, the sample was cooled by liquid nitrogen during irradiation, and the current density was reduced to 0.2 mA cmy2 in order to avoid excess heating by irradiation. Rutherford backscattering spectrometry combined with channeling technique (RBSyC) with 2 MeV 4He ions was made in order to characterize the samples before and after irradiation. The samples were mounted on a two-axis goniometer, which allows both tilt and azimuthal rotation of the samples. Backscattered 4He ions were detected with an SSD placed at an angle of 1708. The divergence of the analyzing beam was approximately 0.018. All measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows the w100x aligned spectra taken from the CuySi sample before and after irradiation with 0.5 MeV Si ions at 323 K. The random and aligned spectra for the non-irradiated sample indicate that the Cu film is
Fig. 1. w100x axial aligned spectra for 2 MeV 4He ions incident on samples before (dotted line) and after 0.5 MeV 28Siq irradiation to doses of 1=1016 cmy2 (broken line) and 3=1016 cmy2 (solid line) at 323 K. An inset shows enlarged graphs at energies of 1.02–1.08 MeV.
grown epitaxially onto the Si (100) substrate, although its crystalline quality is rather poor. The minimum yield xmin at the Cu surface for w100x axial channeling is 90% before irradiation, and is decreased to 62% and 42% after irradiation at 1=1016 and 3=1016 cmy2, respectively, indicating that the irradiation greatly improves the crystalline quality of the epitaxial Cu film. For the signal scattered from the Si substrate, the aligned yield in the ion-implanted region, which extends from the CuySi interface to the depth of ;0.9 mm, reaches the random level. This result shows that the irradiation turns the Si substrate into an amorphous state. Thus, the improvement of the Cu crystalline quality does not result from the layer-by-layer mechanism as observed in IBIEC w8–11x, but from the mechanism that occurs inside the Cu film. It should be noted that in the case of 3=1016 cmy2, the Cu yields at the inner region of the film are much higher than those at the surface region. That is, the crystalline quality in the vicinity of the CuySi interface is much poorer than that at the Cu surface. Judging from an enlarged spectra insetted in Fig. 1, a small amount of Si atoms diffuses from the substrate into the Cu film. It is considered, therefore, that the diffusion of the Si atoms degrades the crystalline quality of the Cu film near the interface. As mentioned above, the Si substrate at the interface is transformed into amorphous by irradiation. This amorphization would stimulate the reaction between the Cu film and the Si substrate. If the interfacial reaction deteriorates the crystalline quality of
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Fig. 3. Angular scans of normalized Cu yields around the w100x axis for 2 MeV 4He ions incident on the CuySi sample irradiated with 0.5 MeV 28Si ions to doses of 1=1016 cmy2 (circles) and 3=1016 cmy2 (filled circles) at 323 K. Fig. 2. The change in crystalline quality a after irradiation at 323 K (a), and 123 K (b) at surface region (filled circles) and interface region (circles) as a function of irradiation-dose. The a value is defined as (xnon-irr. yxirr.)yxnon-irr., where xnon-irr. and xirr. are minimum yields in aligned spectra for the sample before and after irradiation, respectively.
the irradiated Cu film, low-temperature irradiation may lead to significant improvement at the interface region. Fig. 2a and b show the change in crystalline quality a of the CuySi films irradiated at 323 and 123 K as a function of irradiation-dose. The a is defined as (xnon-irr. yxirr.)yxnon-irr., where xnon-irr. and xirr. are minimum yields in aligned spectra for the sample before and after irradiation, respectively. A positive value of a is an indication of the improvement of crystallinity. In addition, the a is close to unity (a™1) when the film becomes a perfect crystal (xirr.™0). In these figures, the a values at the surface are always higher than those at the interface, implying that the crystalline quality at the CuySi interface is hard to improve. At the surface region, the a value increases with increasing dose both for the high-temperature (323 K) irradiation and for the low-temperature (123 K) irradiation. At the interface region, however, the high-temperature irradiation reduces the a value at doses above 2=1016 cmy2. In contrast to the high-temperature irradiation, the a value increases as dose increases for the low-temperature irradiation even at the interface region. Accordingly, lowering temperature during irradiation is effective against the interfacial reaction between Cu and Si as seen in Fig. 1. Next, we characterize the ion-irradiated CuySi films using ion channeling. The w100x axial angular scans of Cu yields for the sample after irradiation with 28Si ions to 1=1016 and 3=1016 cmy2 at 323 K are shown in
Fig. 3, where the yields are evaluated from total area of the Cu peak in the RBS spectra. As for the Cu film without irradiation, it was impossible to present the angular scan curve because its crystalline quality was too poor. The channeling half angle c1y2 for the highdose sample is 0.898, smaller than that for the low-dose sample (1.018), indicating that ion-irradiation reduces the angular spread of crystallite orientation in the Cuy Si film with a mosaic structure. Also, the c1y2 value of 0.898 is considerably larger than that for a calculated value (0.678) w24x for a perfect Cu crystal. This implies the presence of a mosaic structure in the irradiated Cuy Si films. Fig. 4 represents azimuthal angular scans of the Cu film and the Si substrate. Such scans measure the
Fig. 4. Azimuthal angular scans for the Cu irradiated with 0.5 MeV 28 Si ions to doses of 1=1016 cmy2 and 3=1016 cmy2 at 323 K, together with a scan for the underlying Si without irradiation. {100} planes (asterisks) and {110} planes (circles) are indicated.
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varying ion energy and ion species at an irradiation temperature of 123 K, and is shown in Fig. 5a, where the a value is plotted as a function of the number of displaced atoms in the unit of displacements per atom (dpa) calculated by the TRIM w23x. The a gradually increases as the number of displaced atoms increases up to ;28 dpa. Fig. 5b shows the correlation between the a and the energy deposited by electronic excitation for the Cu film irradiated at 2.1 dpa, where the deposited energy is estimated by using the TRIM w23x. The a is found to be constant within the range of 1–3.7 keV nmy1, indicating that electronic excitation does not contribute to the increase of a as shown in Fig. 5a. Thus, we concluded that only atomic displacements due to nuclear collisions improve the crystalline quality of the epitaxial Cu film. Also, it is suggested that collisioninduced atomic rearrangements reduce the mosaic spread in the Cu film as can be seen in Fig. 3. 4. Conclusions
Fig. 5. The change in crystalline quality a after irradiation at 123 K as functions of the number of displaced atoms (a) and the electronic energy deposition (b). Both the number of displaced atoms and the electronic energy deposition are calculated by the TRIM w23x.
crystalline quality of the Cu film and determine the epitaxial relationship between the Cu film and the Si substrate. An azimuthal angular scan for the non-irradiated Cu film is not shown here because no significant planar dips could be observed. As for the irradiated Cu film, the dips for the high-dose Cu are larger and sharper than those for the low-dose Cu. This result is also indicative of irradiation-induced improvement of the crystalline quality of the Cu film. In Fig. 4, the planar dips are indexed referring to the minimum yields. In the scan curve for the Si, the four largest dips and the four next largest dips correspond to {100} planes and {110} planes, respectively. On the other hand, the opposite correspondence is valid for the Cu because of the difference in crystal structure between Cu (f.c.c.) and Si (diamond structure). The position of Cu {100} dips coincides with that of Si {110} dips as can be seen in Fig. 4. Thus, the azimuthal scans together with the w100x axial angular scans reveal the epitaxial relationship of (100)Cucc(100)Si and w011xCuccw010xSi. This relationship shows that the Cu lattice is rotated by 458 relative to the Si lattice, as has been observed previously by Chang et al. w4x for a 2-mm-thick Cu film on Si(100). Finally, we shall discuss the mechanism of the irradiation-induced improvement of crystalline quality. The change in crystalline quality a has been examined by
Irradiation with 28Siq at 323 and 123 K has been applied to annealing of the epitaxial CuySi films to improve their crystalline quality. The RBSyC analysis on the ion-irradiated CuySi reveals that the crystalline quality at the interface is much poorer than that at the Cu surface due to the reaction between the Cu film and the Si substrate. Lowering irradiation-temperature is effective against the interfacial reaction. The channeling half angle c1y2 decreases with increasing irradiation-dose, indicating that ion-irradiation reduces the angular spread of crystallite orientation in the CuySi film with a mosaic structure. Also, we have examined the change in crystalline quality by varying ion energy and ion species at 123 K. The change in crystalline quality depends only on the number of atoms displaced by nuclear collisions. It is concluded, therefore, that collision-induced atomic rearrangements reduce the mosaic spread in the Cu film, resulting in the improvement of its crystalline quality. References w1x E.T. Krastev, L.D. Voice, R.G. Tobin, J. Appl. Phys. 79 (1996) 6865. w2x B.W. Karr, Y.W. Kim, I. Petrov, et al., J. Appl. Phys. 80 (1996) 6699. w3x T. Ohmi, T. Saito, T. Shibata, T. Nitta, Appl. Phys. Lett. 52 (1988) 2236. w4x C. Chang, J.C. Liu, J. Angilello, Appl. Phys. Lett. 57 (1990) 2239. w5x B.G. Demezyk, R. Naik, G. Auner, C. Kota, U. Rao, J. Appl. Phys. 75 (1990) 1956. w6x S.Q. Hong, C. Comrie, S.W. Russell, J.W. Mayer, J. Appl. Phys. 70 (1990) 3655. w7x T.L. Alford, D. Adams, J. Li, B. Cao, S.W. Russell, S.Q. Hong, R. Spreitzer, J.W. Mayer, in: D.P. Favreau, Y. Shacham-
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