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Epitaxial growth of copper silicides by “bilayer” technique on monocrystalline silicon with and without native SiOx
Materials Science and Engineering B 132 (2006) 283–287
Epitaxial growth of copper silicides by “bilayer” technique on monocrystalline silicon with and without native SiOx N. Benouattas a,∗ , L. Osmani a , L. Salik a , C. Benazzouz c , M. Benkerri a , A. Bouabellou b , R. Halimi b a
Surface and interface laboratory, Physic Department, Sciences faculty, Ferhat Abbas University, 19000-Setif, Algeria b Interfaces and thin films laboratory, Mentouri University, Constantine-25000, Algeria c CRNA, 2 Bd Frantz Fanon, Alger, Algeria Received 2 October 2005; received in revised form 4 February 2006; accepted 27 April 2006
Abstract Cu/Au and Au/Cu multilayered films were thermally evaporated alternatively on (1 0 0) and (1 1 1) monocrystal silicon substrates with and without native silicon oxide. After heat treatment in situ either at 400 or at 600 ◦ C, the interfacial transformations were analyzed by Rutherford backscattering spectrometry, θ–2θ X-ray diffraction and scanning electron microscopy. It was found, that the samples surface was covered with Cu3 Si and Cu4 Si crystallites of square, rectangular and hexagonal basis shapes well-oriented on Si(1 0 0) and of triangular shape on Si(1 1 1) owing to the strong intermixing between the different elements. The dilution of the entire gold deposited layer in the Cu–Au–Si formed mixture suggests that gold atoms have a high limit solubility in the formed polycrystalline Cu3 Si and Cu4 Si silicides. © 2006 Published by Elsevier B.V. Keywords: Gold; Copper silicides; Silicon suboxide; Epitaxy growth
1. Introduction Epitaxial silicides films grown on Si have attracted special interest during the last decade because of the opportunities they provide for fundamental scientific studies in the area of semiconductor devices, detectors [1] and optical electronics [2]. It is well-known that the epitaxial silicide layers have an increasing advantage and are preferred over their polycrystalline counterparts because of their smoother interfaces, excellent layer uniformity and superior thermal stability, due to the favourable interface energetic and the absence of grain boundaries [3,4]. In microelectronics, epitaxial silicides such as CoSi2 , TiSi2 , NiSi2 and FeSi2 , are commonly used because of their best lattice match to silicon [5] and their low resistivity [6]. There are various growth techniques to produce epitaxial silicides with a high degree of perfection. Some of the more commun are: either by MBE [7] or SPE [8] metallic deposition on monocrystalline silicon followed by an annealing, either by heat treatment of
∗
Corresponding author. Tel.: +213 36 91 28 59; fax: +213 36 91 28 59. E-mail address: benouattas [email protected] (N. Benouattas).
0921-5107/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.mseb.2006.04.032
a buried silicide underneath the silicon surface produced by a high dose ion implantation (IBS), either again by the M1 /M2 /Si annealed “bilayer” technique. This last technique is an attractive and easy way to form epitaxial silicides [9]. It consists to interpose an appropriate metal layer M2 through which the M1 atoms of the layer in surface must initially diffuse before reacting with the silicon. The M2 underlayer plays the role of an epitaxy promoter for M1x Siy silicide formation and, delays the silicidation to high temperatures where the formation of the epitaxial phase is preferred [9,10] and is characterized by a higher activation energy than that of standard M1 /Si structure [11,12]. In previous works [13,14], we have reported a detailled study of the Cu and Au competitive interdiffusion and the reaction in Cu/Au/Si and Au/Cu/Si systems. From the results described hereinafter, we report a possibility for the epitaxial growth of copper silicides, with some certain easiness, on monocrystalline silicon in presence of gold layer and native suboxide barrier. This is the reason why only micrographies and spectra corresponding to the thresholds temperatures corresponding to the epitaxial formation of copper silicide are reported, i.e. 400 and 600 ◦ C for Cu/Au and Au/Cu bilayers deposited on Si and/or SiOx /Si substrates. Even though Cu3 Si is metallic (∼53 cm)
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[15], however our intention is merely to mention how easy its epitaxial formation is.
3. Experimental results
2. Experimental procedures
After heat treatment of Cu/Au/Si(1 0 0) sample at 400 ◦ C, the surface morphology becomes not uniform with a very spread out crystallites formation of square (15 m × 15 m) and rectangular (25 m × 13 m) shapes (Fig. 1(a)). All crystallite sides are oriented along the same direction and thus, it is concluded that they are epitaxially grown on the Si(1 0 0) substrate. The X-ray diffraction pattern (Fig. 2(a)) reports that only the reflection peak corresponding to the Cu4 Si silicide persists. The disappearance of Cu3 Si phase indicates that, at high temperature, Cu4 Si is more stable. These crystallites are separated by a gray bottom corresponding to the nearly pure silicon (96 at.% Si). On the other hand, the dendrites (56 at.% Si) are less rich in silicon and it is seen that the substrate is less uncovered and that copper and gold did not have sufficient time for a complete coalescence.
Si wafers (1 0 0) and (1 1 1) crystal oriented are degreased in acetone, trichloethylene and methanol baths, and plunged into diluted hydrofluoric acid (HF) solution. Half of each Si wafer was only cleaned in solvent baths but have not been etched with HF. Then, the silicon surface remains covered with a thin native silicon oxide layer of about 1.5 nm, which is in fact a mixture of suboxide SiOx [16]. Without breaking vacuum, 2 × 10−7 mbar, Cu/Au and Au/Cu bilayers were evaporated, with low rate deposition of about 0.7 nm/s. An in situ quartz crystal oscillator allows to limit the thicknesses of copper and gold layers to 120 and 100 nm, respectively. Silicidation annealing, in a vacuum of 2 × 10−7 mbar, was performed using a quartz crucible that one closed with a stumatite cap to make the temperature homogeneous. The morphology of the sample surface and the shape of formed crystallites are examined by scanning microscopy (SEM). The identification of elements composition in contrasting areas is performed by an X-ray dispersive energy analyzer (EDX). The primary energy of electron beam is chosen equal to 9 keV in order to limit the analyzed depth to ∼200 nm. The XRD scans were performed with a SIEMENS 5000 diffractometer instrument, using Cu K␣1 radiation, for new compounds identification. RBS analysis was undertaken using a 2 MeV 4 He+ ions beam and a backscattering angle of 165◦ in order to follow the compositional changes as a function of depth. RUMP computer program has especially been developed for the forward simulation of RBS spectra. This program, based on the algorithm of Doolittle [17], is well suited to compare the calculated and measured spectra from a sample of known composition.
3.1. Cu/Au/Si(1 0 0) sample annealed at 400 ◦ C
3.2. Cu/Au/Si(1 1 1) sample annealed at 400 ◦ C Fig. 1(b) illustrates the SEM image obtained on Cu/Au/Si(1 1 1) sample annealed at 400 ◦ C. Heating caused the development of great crystallites under the shape of equilateral triangles, of a side between 12.5 and 22.3 m. These epitaxied islands have well delimited edges by a mass-thickness image contrast mechanism. The X-ray energy analysis taken using 9 keV accelerating potentiel gives 27 at.% Si and 19 at.% Au atoms on the triangular crystallites, whereas, the gray bottom corresponds to sligtly diffused silicon of 4 at.% Cu and 3.5 at.% Au. To investigate the redistribution of each element, RBS depth profiling was performed (Fig. 3). In order to facilitate the task of analyzing data, the fitting of the corresponding experimental RBS spectrum permits to say that the metal signal
Fig. 1. SEM micrographs from the surfaces of (a) Cu/Au/Si(1 0 0) annealed at 400 ◦ C, (b) Cu/Au/Si(1 1 1) annealed at 400 ◦ C, (c) Cu/Au/SiOx /Si(1 0 0) annealed at 600 ◦ C and (d) Au/Cu/SiOx /Si(1 1 1) annealed at 600 ◦ C.
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Fig. 2. XRD scans of (a) Cu/Au/Si(1 0 0) annealed at 400 ◦ C, (b) Cu/Au/Si(1 1 1) annealed at 400 ◦ C, (c) Cu/Au/SiOx /Si(1 0 0) annealed at 600 ◦ C and (d) Au/Cu/SiOx /Si(1 1 1) annealed at 600 ◦ C.
Fig. 4. The theoretical profiles generated by fitting the experimental RBS spectra of (a) Cu/Au/Si(1 1 1) annealed at 400 ◦ C, (b) Cu/Au/SiOx /Si(1 0 0) annealed at 600 ◦ C and (c) Au/Cu/SiOx /Si(1 1 1) annealed at 600 ◦ C.
can be decomposed into two signals. The first part of the signal is Gaussian shape and corresponds to a copper layer in surface, of about 100 nm, resulting in a complete movement of 62 at.% Si and 13 at.% Au (Fig. 4(a)). The second part of the signal, of less concentration in metal, is spread out deeper (11 m) into the silicon substrate. According to SEM and RBS figures, one can say that the surface is uncovered over 80%. Cu3 Si and Cu4 Si copper silicides are identified by X-ray diffraction pattern (Fig. 2(b)). Compared to the case of the previous sample with Si(1 0 0) substrate, the persistence of the Cu3 Si phase suggests that the reaction kinetic is slower. 3.3. Cu/Au/SiOx /Si(1 0 0) sample annealed at 600 ◦ C
Fig. 3. RBS (4 He+ , 2 MeV) spectra of (a) Cu/Au/Si(1 1 1) not annealed and annealed at 400 ◦ C, (b) Cu/Au/SiOx /Si(1 0 0) annealed at 600 ◦ C and (c) Au/Cu/SiOx /Si(1 1 1) not annealed and annealed at 600 ◦ C.
Fig. 1(c) shows the SEM picture of the Cu/Au/SiOx /Si(1 0 0) sample heat treated at 600 ◦ C. The micrography presents a heterogeneous surface, sprinkled of crystallites of rectangular (10 m × 15 m) and irregular hexagonal basis with size in range 10–20 m. EDX shows that crystallites, independently of their shapes, are composed of copper mixed with 21–23% silicon and 18–19% gold atoms. The corresponding RBS spectrum (Fig. 3(b)) is very similar to the one seen in Fig. 3(a), although the copper and gold signals heights are lower. This means that the reaction is more pronounced in the sample of HF-etched substrate. It can be noticed that the mixing layer in surface has a thickness of about 30 nm (Fig. 4(b)). These crystallites are sep-
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arated by dark zones corresponding to silicon substrate which is slightly diffused in copper and gold atoms (EDX). From the silicon RBS signal, one can remark that the substrate is completely disturbed in-depth on about 12.6 m which coincides with the formation of Cu3 Si and Cu4 Si silicide according to the XRD pattern shown in Fig. 2(c). No trace of pure copper and gold peaks is signaled suggesting the total consumption of metal layers by the interfacial reaction. 3.4. Au/Cu/SiOx /Si(1 1 1) sample annealed at 600 ◦ C The SEM micrograph of annealed Au/Cu/SiOx /Si(1 1 1) sample exhibits uniformly growth of large great triangular crystallites, with sides in 12–15 m range (Fig. 1(d)). Even if the density of these crystallites is greater than in the case of Cu/Au/Si(1 1 1) sample annealed at the same conditions, their dimension is less. Localized EDX analysis on crystallites allowed to the identification of a copper layer (65 at.%) interdiffused with 27 at.% silicon, while the gray bottom corresponds to the silicon substrate covered by copper (10 at.%) and gold (1 at.%). The black stains, irregularly dispersed, are assigned to the pure silicon of the substrate. From the RBS spectrum presented in Fig. 3(c), as indicated by the arrows at surface positions, silicon and copper atoms have greatly segregated at sample surface. It is possible to calculate, by RUMP simulation, that 88 at.% Si are present in surface in the 138 nm formed copper–gold mixture (Fig. 4(c)). Similarly to the previous studied systems, copper and gold atoms disturb the substrate bulk over 12 m, and the XRD analysis shows the formation of polycrystalline Cu3 Si and Cu4 Si silicides (Fig. 2(d)). 4. Discussion After annealing, EDX and RBS examinations show that all elements mixed themselves mutually independently of the substrate orientation, the nature of diffusion barrier and the sequence of gold and copper evaporation. Much amount of these elements concentrated in surface under a thin layer of few hundreds of nanometers, although the remainder of metal atoms moved deeply in the substrate beyond 10 m. This in-depth penetration of copper atoms is not surprising since copper is recognized like fastest diffusant with a very high solubility in the silicon matrix [18]. From Istratov et al. [19], the copper supersaturation at the level of 1010 at./cm3 will be achieved at only 275 ◦ C temperature for 30 min and it will diffuse about 2000 m. The island morphology still appears to be the dominant growth mechanism with a crystallites growth, corresponding to Cu3 Si and/or Cu4 Si copper silicides in agreement with the results reported by our previous works [13,14]. Thus, even with gold and/or SiOx layers interposition, the morphological stability of the Cu/Si interface is rather poor since it is prone to form islands upon annealing at 400 or 600 ◦ C. These crystallites are separated by a gray bottom corresponding to the silicon of the substrate, slightly diffused with copper and gold showing that they did not completely coalesce on the substrate. After annealing, in all treated systems, no Cu XRD peaks corresponding to the deposited copper layer is detected, showing
its complete consumption and its full transformation in copper silicides. In the same way, no evidence of the presence of any remaining Au is signaled. This absence of pure gold layer suggests that the three elements are mixed themselves to form a Cu–Au–Si solution (RBS analysis). So, the 100 nm deposited gold layer nearly dissolved completely itself in Cu–Au–Si solution of crystallites, suggesting a high solid solubility of gold atoms in the formed of copper silicides. The binary phase diagrams of Au–Cu [20] and Cu/Si [21] systems show high miscibilities with the formation of some ordered phases, such as Au3 Cu, AuCu3 , AuCu and Cu3 Si, Cu4 Si, Cu5 Si, respectively. The absence of Cu–Au phases is in good agreement with the thermodynamic data. Indeed, the Cu–Au–Si liquidus surface plot [22] revealed the presence of a ternary eutectic at 74.7 at.% Au, 6.1 at.%Cu and 19.2 at.%Si for 337 ◦ C, with the formation of three solid phases: the solid solutions (Au, Cu) and (Si), and the intermediate phase -Cu3 Si without any sign of ternary phase presence. In other words, this Cu3 Si phase cannot coexist with Cu–Au compounds. If one considers that theoretically ˚ of formed silicides (Cu3 Si + Cu4 Si) consume 1 A ˚ of cop3.5 A per, the 120 nm deposited copper layer should be transfomed into 420 nm thickness of mixed layer, whereas the one that is detected exceeds 10 m. Thus, gold can be only dissolved in copper silicide and in a least measure in silicon substrate. Thus, one can say that dissolution of gold atoms, as a consequence, has the expansion of the crystallites sizes. These crystallites of Cu3 Si and Cu4 Si copper silicides have the shape of equilateral triangles, exceeding 10 m in thickness and in side, on Si(1 1 1) substrate. According to Solberg [23], the Cu3 Si compound possesses three allotropic phases: one is orthorhombic (pseudohexagonal) and both others trigonal. That is why two crystallites shapes, hexagonal and rectangular, are observed on Si(1 0 0) in this work. Such a growth of epitaxial islands, but assigned to Cu3 Si phase, on Si(1 0 0) has been observed by Benouattas et al. [24] after heat treatment of Cu/SiO2 (native)/Si(1 0 0) at 750 ◦ C, whereas Weber et al. [25] have reported the epitaxial growth of Cu3 Si silicide on both Si(1 0 0) and Si(1 1 1), with a mismatch of 5%, in the case of CVD copper deposition on silicon atomically clean and heat treated at 400 ◦ C. One notices that the epitaxied growth on the oxidized substrate takes place at higher temperature (600 ◦ C) than that on the etched silicon (400 ◦ C). It is owed to the fact that the presence of the silicon oxide, at the interface, delays the interdiffusion and therefore also the interfacial reaction [26]. The recorded excess in silicon concentration, detected by RBS, suggests that the growth of copper silicides takes place by nucleation and that the surface of the samples did not stay planar. This fact will be well corroborated thereafter by microscopic analysis (SEM) of the surface morphology. From this type of spectra, it is very difficult to obtain more information about the reaction taking place, the interpretation of RBS spectrum was not unambiguous. Now it is clear that the mechanism growth of Cu–Si crystallites can be explained as follows: copper and gold layers coalesce while being diffused to silicon; thus giving rise to copper silicide crystallites dissolved in gold atoms. This metallic coalescence increases the crystallites thickness in the ratio of uncovered to crystallite covered surface, which
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explains the great thicknesses of silicide crystallites recorded by RBS. In this study, we have shown the possibility of epitaxial copper silicide growth on both Si(1 0 0) and Si(1 1 1), even in presence of a gold layer and/or an interfacial silicon suboxide. Nevertheless, more detailed analysis, for example by TEM and RHEED, would be a great help to surround better the domains and epitaxial relationships between Cu3 Si or Cu4 Si, and the onaxis (1 0 0) or Si(1 1 1) substrate. A limited number of works tried with success to grow an epitaxied silicide layer, in particular CoSi2 , using bilayer technique (Co/Ti, Co/Hf, Co/Nb, Co/silicon nitrite) [27–29]. In this last case, it is reported that it is much easier to grow epitaxial CoSi2 films on Si substrates. Furthermore, in our case, the presence of the gold underlayer and the interfacial SiOx delay the copper–silicon reaction to high temperatures where the formation of the epitaxial phase is preferred, as it has been noted in Co-basis bilayer. On the other hand, the use of a titanium as interlayer in CoSi2 formation has led to the titanium interlayer-mediated epitaxy (TIME) method [30] in contrast to oxide-mediated epitaxy (OME), in which SiO2 controls the silicide reaction [31]. The epitaxial growth, close to the eutectic temperature, is preferred because it produces a more stable layer. This technique of bilayer evaporation is interesting for its simplicity, compared to MBE and CVD expensive elaboration methods, as well as for the thermal stability of the compounds formed at high temperatures. In our case, the dissolution of the gold layer extends the silicide lattice and causes the formation of very large silicide crystallites in thickness and laterally. 5. Conclusion In the present paper, we have showed that the “bilayer” approch is one of the easiest techniques to form epitaxial films on monocrystalline silicon independently of its orientation and surface state. After annealing, crystallites of Cu3 Si and/or Cu4 Si compounds of square, rectangular and hexagonal basis shapes well-oriented on Si(1 0 0) and of triangular shape on Si(1 1 1) are grown at the interface of the different structures. During the reaction, the gold intermediate layer may contribute to the epitaxial growth of copper silicide(s) by retarding the interdiffusion between Si and Cu and thereby promoting its epitaxial growth. Both metallic coalescence and gold dissolution in the formed silicides contributes to large crystallites formation, beyond 10 m in thickness and laterally. Furthermore, one can
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say that dissolution of gold atoms has as consequence only the expansion of the crystallites sizes. References [1] J.C. Hensel, A.F.J. Levi, R.T. Tung, d.J.M. Gibson, Appl. Phys. Lett. 47 (1985) 15. [2] D.R. Peal, R. Haight, H. Ott, Appl. Phys. Lett. 62 (1993) 149. [3] R.T. Tung, Mater. Chem. Phys. 32 (1993) 107. [4] L.J. Chen, K.N. Tu, Mater. Sci. Rep. 6 (1991) 53. [5] R. Sinclair, in: K. Maex, M.V. Rossen (Eds.), Properties of Metal Silicides, INSPEC publication, Leuven Belgium, 1995, p. 118. [6] S.L. Hsia, T.Y. Tan, P. Smith, G.E. Mc Guire, J. Appl. Phys. 72 (5) (1992) 1864. [7] J.Y. Duboz, P.A. Badoz, A. Perio, J.C. Oberlin, A. d’Avitaya, Y. Campidelli, J.A. Chroboczek, Appl. Surf. Sci. 38 (1989) 171. [8] C. Polop, C. Rogero, J.L. Saced, J.A. Martın Gago, Appl. Surf. Sci. 482 (2001) 1337. [9] F. Hong, G.A. Rozgonyl, Appl. Phys. Lett. 84 (17) (1994) 2241. [10] M. Lawrance, A. Dass, D.B. Fraser, C.S. Wei, Appl. Phys. Lett. 58 (12) (1991) 1308. [11] A. Vantomme, M.-A. Nicholet, N.D. Theodore, J. Appl. Phys. 75 (1994) 3882. [12] H. Miura, E. Ma, C.V. Thompson, J. Appl. Phys. 70 (1991) 4287. [13] C. Benazzouz, N. Benouattas, S. Iaiche, A. Bouabellou, Nucl. Instrum. Methods Phys. Res. Sect. B 213 (2004) 519. [14] C. Benazzouz, N. Benouattas, A. Bouabellou, Nucl. Instrum. Methods Phys. Res. Sect. B 230 (2005) 571. [15] M.O. Aboelfotoh, L. Krussin Elbaum, J. Appl. Phys. 70 (1991) 3382. [16] M. Grundner, H. Jacob, Appl. Phys. A 39 (1986) 73. [17] L.R. Doolittle, Nucl. Instrum. Methods Phys. Res. Sect. B 9 (1985) 291. [18] E.R. Weber, Appl. Phys. Lett. A 30 (1983) 1. [19] A. Istratov, H. Hieslmair, E.R. Weber, MRS Bull. (2000) 33. [20] H. Okamoto, D.J. Chakrabarti, D.E. Laughlin, T.B. Massalski, in: V. Raghavan (Ed.), Phase Diagrams of Ternary Alloys, Indian Institute of Metals, Calcutta, 1992, p. 359. [21] M. Hansen, K. Anderko, Constition of Binary Alloys, McGraw Hill, New York, 1958, p.629. [22] C. Baetzner, N. Lebrun, in: V. Raghavan (Ed.), Phase Diagrams of Ternary Alloys, Indian Institute of Metals, Calcutta, 1992, p. 398. [23] J.K. Solberg, Acta Cryst. A34 (1978) 684. [24] N. Benouattas, A. Mosser, A. Bouabellou, Appl. Surf. Sci. 153 (2000) 79. [25] G. Weber, B. Gillot, P. Barret, Phys. Status Solidi (a) 75 (1983) 567. [26] T. Nakahara, S. Ohkura, F. Shoji, T. Hnawa, K. Oura, Nucl. Instrum. Methods Phys. Res. Sect. B 45 (1990) 467. [27] M. Falke, B. Gebhardt, G. Beddies, S. Teichert, H.-J. Hinneberg, Microelectron. Eng. 55 (2001) 171. [28] Y. Kwon, C. Lee, Mater. Chem. Phys. 63 (2000) 202. [29] G.B. Kim, J.S. Kwak, H.K. Baik, S.M. Lee, J. Appl. Phys. 82 (1997) 2323. [30] M.L.A. Dass, D.B. Fraser, C.S. Wei, Appl. Phys. Lett. 58 (1991) 1308. [31] R.T. Tung, Appl. Phys. Lett. 68 (1996) 3461.
Epitaxial growth of copper silicides by “bilayer” technique on monocrystalline silicon with and without native SiOx N. Benouattas a,∗ , L. Osmani a , L. Salik a , C. Benazzouz c , M. Benkerri a , A. Bouabellou b , R. Halimi b a
Surface and interface laboratory, Physic Department, Sciences faculty, Ferhat Abbas University, 19000-Setif, Algeria b Interfaces and thin films laboratory, Mentouri University, Constantine-25000, Algeria c CRNA, 2 Bd Frantz Fanon, Alger, Algeria Received 2 October 2005; received in revised form 4 February 2006; accepted 27 April 2006
Abstract Cu/Au and Au/Cu multilayered films were thermally evaporated alternatively on (1 0 0) and (1 1 1) monocrystal silicon substrates with and without native silicon oxide. After heat treatment in situ either at 400 or at 600 ◦ C, the interfacial transformations were analyzed by Rutherford backscattering spectrometry, θ–2θ X-ray diffraction and scanning electron microscopy. It was found, that the samples surface was covered with Cu3 Si and Cu4 Si crystallites of square, rectangular and hexagonal basis shapes well-oriented on Si(1 0 0) and of triangular shape on Si(1 1 1) owing to the strong intermixing between the different elements. The dilution of the entire gold deposited layer in the Cu–Au–Si formed mixture suggests that gold atoms have a high limit solubility in the formed polycrystalline Cu3 Si and Cu4 Si silicides. © 2006 Published by Elsevier B.V. Keywords: Gold; Copper silicides; Silicon suboxide; Epitaxy growth
1. Introduction Epitaxial silicides films grown on Si have attracted special interest during the last decade because of the opportunities they provide for fundamental scientific studies in the area of semiconductor devices, detectors [1] and optical electronics [2]. It is well-known that the epitaxial silicide layers have an increasing advantage and are preferred over their polycrystalline counterparts because of their smoother interfaces, excellent layer uniformity and superior thermal stability, due to the favourable interface energetic and the absence of grain boundaries [3,4]. In microelectronics, epitaxial silicides such as CoSi2 , TiSi2 , NiSi2 and FeSi2 , are commonly used because of their best lattice match to silicon [5] and their low resistivity [6]. There are various growth techniques to produce epitaxial silicides with a high degree of perfection. Some of the more commun are: either by MBE [7] or SPE [8] metallic deposition on monocrystalline silicon followed by an annealing, either by heat treatment of
∗
Corresponding author. Tel.: +213 36 91 28 59; fax: +213 36 91 28 59. E-mail address: benouattas [email protected] (N. Benouattas).
0921-5107/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.mseb.2006.04.032
a buried silicide underneath the silicon surface produced by a high dose ion implantation (IBS), either again by the M1 /M2 /Si annealed “bilayer” technique. This last technique is an attractive and easy way to form epitaxial silicides [9]. It consists to interpose an appropriate metal layer M2 through which the M1 atoms of the layer in surface must initially diffuse before reacting with the silicon. The M2 underlayer plays the role of an epitaxy promoter for M1x Siy silicide formation and, delays the silicidation to high temperatures where the formation of the epitaxial phase is preferred [9,10] and is characterized by a higher activation energy than that of standard M1 /Si structure [11,12]. In previous works [13,14], we have reported a detailled study of the Cu and Au competitive interdiffusion and the reaction in Cu/Au/Si and Au/Cu/Si systems. From the results described hereinafter, we report a possibility for the epitaxial growth of copper silicides, with some certain easiness, on monocrystalline silicon in presence of gold layer and native suboxide barrier. This is the reason why only micrographies and spectra corresponding to the thresholds temperatures corresponding to the epitaxial formation of copper silicide are reported, i.e. 400 and 600 ◦ C for Cu/Au and Au/Cu bilayers deposited on Si and/or SiOx /Si substrates. Even though Cu3 Si is metallic (∼53 cm)
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[15], however our intention is merely to mention how easy its epitaxial formation is.
3. Experimental results
2. Experimental procedures
After heat treatment of Cu/Au/Si(1 0 0) sample at 400 ◦ C, the surface morphology becomes not uniform with a very spread out crystallites formation of square (15 m × 15 m) and rectangular (25 m × 13 m) shapes (Fig. 1(a)). All crystallite sides are oriented along the same direction and thus, it is concluded that they are epitaxially grown on the Si(1 0 0) substrate. The X-ray diffraction pattern (Fig. 2(a)) reports that only the reflection peak corresponding to the Cu4 Si silicide persists. The disappearance of Cu3 Si phase indicates that, at high temperature, Cu4 Si is more stable. These crystallites are separated by a gray bottom corresponding to the nearly pure silicon (96 at.% Si). On the other hand, the dendrites (56 at.% Si) are less rich in silicon and it is seen that the substrate is less uncovered and that copper and gold did not have sufficient time for a complete coalescence.
Si wafers (1 0 0) and (1 1 1) crystal oriented are degreased in acetone, trichloethylene and methanol baths, and plunged into diluted hydrofluoric acid (HF) solution. Half of each Si wafer was only cleaned in solvent baths but have not been etched with HF. Then, the silicon surface remains covered with a thin native silicon oxide layer of about 1.5 nm, which is in fact a mixture of suboxide SiOx [16]. Without breaking vacuum, 2 × 10−7 mbar, Cu/Au and Au/Cu bilayers were evaporated, with low rate deposition of about 0.7 nm/s. An in situ quartz crystal oscillator allows to limit the thicknesses of copper and gold layers to 120 and 100 nm, respectively. Silicidation annealing, in a vacuum of 2 × 10−7 mbar, was performed using a quartz crucible that one closed with a stumatite cap to make the temperature homogeneous. The morphology of the sample surface and the shape of formed crystallites are examined by scanning microscopy (SEM). The identification of elements composition in contrasting areas is performed by an X-ray dispersive energy analyzer (EDX). The primary energy of electron beam is chosen equal to 9 keV in order to limit the analyzed depth to ∼200 nm. The XRD scans were performed with a SIEMENS 5000 diffractometer instrument, using Cu K␣1 radiation, for new compounds identification. RBS analysis was undertaken using a 2 MeV 4 He+ ions beam and a backscattering angle of 165◦ in order to follow the compositional changes as a function of depth. RUMP computer program has especially been developed for the forward simulation of RBS spectra. This program, based on the algorithm of Doolittle [17], is well suited to compare the calculated and measured spectra from a sample of known composition.
3.1. Cu/Au/Si(1 0 0) sample annealed at 400 ◦ C
3.2. Cu/Au/Si(1 1 1) sample annealed at 400 ◦ C Fig. 1(b) illustrates the SEM image obtained on Cu/Au/Si(1 1 1) sample annealed at 400 ◦ C. Heating caused the development of great crystallites under the shape of equilateral triangles, of a side between 12.5 and 22.3 m. These epitaxied islands have well delimited edges by a mass-thickness image contrast mechanism. The X-ray energy analysis taken using 9 keV accelerating potentiel gives 27 at.% Si and 19 at.% Au atoms on the triangular crystallites, whereas, the gray bottom corresponds to sligtly diffused silicon of 4 at.% Cu and 3.5 at.% Au. To investigate the redistribution of each element, RBS depth profiling was performed (Fig. 3). In order to facilitate the task of analyzing data, the fitting of the corresponding experimental RBS spectrum permits to say that the metal signal
Fig. 1. SEM micrographs from the surfaces of (a) Cu/Au/Si(1 0 0) annealed at 400 ◦ C, (b) Cu/Au/Si(1 1 1) annealed at 400 ◦ C, (c) Cu/Au/SiOx /Si(1 0 0) annealed at 600 ◦ C and (d) Au/Cu/SiOx /Si(1 1 1) annealed at 600 ◦ C.
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Fig. 2. XRD scans of (a) Cu/Au/Si(1 0 0) annealed at 400 ◦ C, (b) Cu/Au/Si(1 1 1) annealed at 400 ◦ C, (c) Cu/Au/SiOx /Si(1 0 0) annealed at 600 ◦ C and (d) Au/Cu/SiOx /Si(1 1 1) annealed at 600 ◦ C.
Fig. 4. The theoretical profiles generated by fitting the experimental RBS spectra of (a) Cu/Au/Si(1 1 1) annealed at 400 ◦ C, (b) Cu/Au/SiOx /Si(1 0 0) annealed at 600 ◦ C and (c) Au/Cu/SiOx /Si(1 1 1) annealed at 600 ◦ C.
can be decomposed into two signals. The first part of the signal is Gaussian shape and corresponds to a copper layer in surface, of about 100 nm, resulting in a complete movement of 62 at.% Si and 13 at.% Au (Fig. 4(a)). The second part of the signal, of less concentration in metal, is spread out deeper (11 m) into the silicon substrate. According to SEM and RBS figures, one can say that the surface is uncovered over 80%. Cu3 Si and Cu4 Si copper silicides are identified by X-ray diffraction pattern (Fig. 2(b)). Compared to the case of the previous sample with Si(1 0 0) substrate, the persistence of the Cu3 Si phase suggests that the reaction kinetic is slower. 3.3. Cu/Au/SiOx /Si(1 0 0) sample annealed at 600 ◦ C
Fig. 3. RBS (4 He+ , 2 MeV) spectra of (a) Cu/Au/Si(1 1 1) not annealed and annealed at 400 ◦ C, (b) Cu/Au/SiOx /Si(1 0 0) annealed at 600 ◦ C and (c) Au/Cu/SiOx /Si(1 1 1) not annealed and annealed at 600 ◦ C.
Fig. 1(c) shows the SEM picture of the Cu/Au/SiOx /Si(1 0 0) sample heat treated at 600 ◦ C. The micrography presents a heterogeneous surface, sprinkled of crystallites of rectangular (10 m × 15 m) and irregular hexagonal basis with size in range 10–20 m. EDX shows that crystallites, independently of their shapes, are composed of copper mixed with 21–23% silicon and 18–19% gold atoms. The corresponding RBS spectrum (Fig. 3(b)) is very similar to the one seen in Fig. 3(a), although the copper and gold signals heights are lower. This means that the reaction is more pronounced in the sample of HF-etched substrate. It can be noticed that the mixing layer in surface has a thickness of about 30 nm (Fig. 4(b)). These crystallites are sep-
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arated by dark zones corresponding to silicon substrate which is slightly diffused in copper and gold atoms (EDX). From the silicon RBS signal, one can remark that the substrate is completely disturbed in-depth on about 12.6 m which coincides with the formation of Cu3 Si and Cu4 Si silicide according to the XRD pattern shown in Fig. 2(c). No trace of pure copper and gold peaks is signaled suggesting the total consumption of metal layers by the interfacial reaction. 3.4. Au/Cu/SiOx /Si(1 1 1) sample annealed at 600 ◦ C The SEM micrograph of annealed Au/Cu/SiOx /Si(1 1 1) sample exhibits uniformly growth of large great triangular crystallites, with sides in 12–15 m range (Fig. 1(d)). Even if the density of these crystallites is greater than in the case of Cu/Au/Si(1 1 1) sample annealed at the same conditions, their dimension is less. Localized EDX analysis on crystallites allowed to the identification of a copper layer (65 at.%) interdiffused with 27 at.% silicon, while the gray bottom corresponds to the silicon substrate covered by copper (10 at.%) and gold (1 at.%). The black stains, irregularly dispersed, are assigned to the pure silicon of the substrate. From the RBS spectrum presented in Fig. 3(c), as indicated by the arrows at surface positions, silicon and copper atoms have greatly segregated at sample surface. It is possible to calculate, by RUMP simulation, that 88 at.% Si are present in surface in the 138 nm formed copper–gold mixture (Fig. 4(c)). Similarly to the previous studied systems, copper and gold atoms disturb the substrate bulk over 12 m, and the XRD analysis shows the formation of polycrystalline Cu3 Si and Cu4 Si silicides (Fig. 2(d)). 4. Discussion After annealing, EDX and RBS examinations show that all elements mixed themselves mutually independently of the substrate orientation, the nature of diffusion barrier and the sequence of gold and copper evaporation. Much amount of these elements concentrated in surface under a thin layer of few hundreds of nanometers, although the remainder of metal atoms moved deeply in the substrate beyond 10 m. This in-depth penetration of copper atoms is not surprising since copper is recognized like fastest diffusant with a very high solubility in the silicon matrix [18]. From Istratov et al. [19], the copper supersaturation at the level of 1010 at./cm3 will be achieved at only 275 ◦ C temperature for 30 min and it will diffuse about 2000 m. The island morphology still appears to be the dominant growth mechanism with a crystallites growth, corresponding to Cu3 Si and/or Cu4 Si copper silicides in agreement with the results reported by our previous works [13,14]. Thus, even with gold and/or SiOx layers interposition, the morphological stability of the Cu/Si interface is rather poor since it is prone to form islands upon annealing at 400 or 600 ◦ C. These crystallites are separated by a gray bottom corresponding to the silicon of the substrate, slightly diffused with copper and gold showing that they did not completely coalesce on the substrate. After annealing, in all treated systems, no Cu XRD peaks corresponding to the deposited copper layer is detected, showing
its complete consumption and its full transformation in copper silicides. In the same way, no evidence of the presence of any remaining Au is signaled. This absence of pure gold layer suggests that the three elements are mixed themselves to form a Cu–Au–Si solution (RBS analysis). So, the 100 nm deposited gold layer nearly dissolved completely itself in Cu–Au–Si solution of crystallites, suggesting a high solid solubility of gold atoms in the formed of copper silicides. The binary phase diagrams of Au–Cu [20] and Cu/Si [21] systems show high miscibilities with the formation of some ordered phases, such as Au3 Cu, AuCu3 , AuCu and Cu3 Si, Cu4 Si, Cu5 Si, respectively. The absence of Cu–Au phases is in good agreement with the thermodynamic data. Indeed, the Cu–Au–Si liquidus surface plot [22] revealed the presence of a ternary eutectic at 74.7 at.% Au, 6.1 at.%Cu and 19.2 at.%Si for 337 ◦ C, with the formation of three solid phases: the solid solutions (Au, Cu) and (Si), and the intermediate phase -Cu3 Si without any sign of ternary phase presence. In other words, this Cu3 Si phase cannot coexist with Cu–Au compounds. If one considers that theoretically ˚ of formed silicides (Cu3 Si + Cu4 Si) consume 1 A ˚ of cop3.5 A per, the 120 nm deposited copper layer should be transfomed into 420 nm thickness of mixed layer, whereas the one that is detected exceeds 10 m. Thus, gold can be only dissolved in copper silicide and in a least measure in silicon substrate. Thus, one can say that dissolution of gold atoms, as a consequence, has the expansion of the crystallites sizes. These crystallites of Cu3 Si and Cu4 Si copper silicides have the shape of equilateral triangles, exceeding 10 m in thickness and in side, on Si(1 1 1) substrate. According to Solberg [23], the Cu3 Si compound possesses three allotropic phases: one is orthorhombic (pseudohexagonal) and both others trigonal. That is why two crystallites shapes, hexagonal and rectangular, are observed on Si(1 0 0) in this work. Such a growth of epitaxial islands, but assigned to Cu3 Si phase, on Si(1 0 0) has been observed by Benouattas et al. [24] after heat treatment of Cu/SiO2 (native)/Si(1 0 0) at 750 ◦ C, whereas Weber et al. [25] have reported the epitaxial growth of Cu3 Si silicide on both Si(1 0 0) and Si(1 1 1), with a mismatch of 5%, in the case of CVD copper deposition on silicon atomically clean and heat treated at 400 ◦ C. One notices that the epitaxied growth on the oxidized substrate takes place at higher temperature (600 ◦ C) than that on the etched silicon (400 ◦ C). It is owed to the fact that the presence of the silicon oxide, at the interface, delays the interdiffusion and therefore also the interfacial reaction [26]. The recorded excess in silicon concentration, detected by RBS, suggests that the growth of copper silicides takes place by nucleation and that the surface of the samples did not stay planar. This fact will be well corroborated thereafter by microscopic analysis (SEM) of the surface morphology. From this type of spectra, it is very difficult to obtain more information about the reaction taking place, the interpretation of RBS spectrum was not unambiguous. Now it is clear that the mechanism growth of Cu–Si crystallites can be explained as follows: copper and gold layers coalesce while being diffused to silicon; thus giving rise to copper silicide crystallites dissolved in gold atoms. This metallic coalescence increases the crystallites thickness in the ratio of uncovered to crystallite covered surface, which
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explains the great thicknesses of silicide crystallites recorded by RBS. In this study, we have shown the possibility of epitaxial copper silicide growth on both Si(1 0 0) and Si(1 1 1), even in presence of a gold layer and/or an interfacial silicon suboxide. Nevertheless, more detailed analysis, for example by TEM and RHEED, would be a great help to surround better the domains and epitaxial relationships between Cu3 Si or Cu4 Si, and the onaxis (1 0 0) or Si(1 1 1) substrate. A limited number of works tried with success to grow an epitaxied silicide layer, in particular CoSi2 , using bilayer technique (Co/Ti, Co/Hf, Co/Nb, Co/silicon nitrite) [27–29]. In this last case, it is reported that it is much easier to grow epitaxial CoSi2 films on Si substrates. Furthermore, in our case, the presence of the gold underlayer and the interfacial SiOx delay the copper–silicon reaction to high temperatures where the formation of the epitaxial phase is preferred, as it has been noted in Co-basis bilayer. On the other hand, the use of a titanium as interlayer in CoSi2 formation has led to the titanium interlayer-mediated epitaxy (TIME) method [30] in contrast to oxide-mediated epitaxy (OME), in which SiO2 controls the silicide reaction [31]. The epitaxial growth, close to the eutectic temperature, is preferred because it produces a more stable layer. This technique of bilayer evaporation is interesting for its simplicity, compared to MBE and CVD expensive elaboration methods, as well as for the thermal stability of the compounds formed at high temperatures. In our case, the dissolution of the gold layer extends the silicide lattice and causes the formation of very large silicide crystallites in thickness and laterally. 5. Conclusion In the present paper, we have showed that the “bilayer” approch is one of the easiest techniques to form epitaxial films on monocrystalline silicon independently of its orientation and surface state. After annealing, crystallites of Cu3 Si and/or Cu4 Si compounds of square, rectangular and hexagonal basis shapes well-oriented on Si(1 0 0) and of triangular shape on Si(1 1 1) are grown at the interface of the different structures. During the reaction, the gold intermediate layer may contribute to the epitaxial growth of copper silicide(s) by retarding the interdiffusion between Si and Cu and thereby promoting its epitaxial growth. Both metallic coalescence and gold dissolution in the formed silicides contributes to large crystallites formation, beyond 10 m in thickness and laterally. Furthermore, one can
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