- Email: [email protected]
CVD of silicon and silicides on iron
Applied Surface Science 140 Ž1999. 99–105
CVD of silicon and silicides on iron M. Rebhan ) , M. Rohwerder, M. Stratmann Lehrstuhl fur Institut fur Martensstrasse 7, ¨ Korrosion und Oberflachentechnik, ¨ ¨ Werkstoffwissenschaften 4, UniÕersitat ¨ Erlangen-Nurnberg, ¨ 91058 Erlangen, Germany Received 6 July 1998; accepted 15 September 1998
Abstract Silicon and iron silicides are promising candidates as adhesion promoters on iron. The formation of silicon and iron silicides films on polycrystalline iron is studied in a CVD process with monosilane. The process is performed between 4758C and 8008C with a silane partial pressure of 0.2 to 10 mbar to a total pressure of the silane–hydrogen mixture of 100 to 990 mbar. A model of the reaction mechanism is presented which includes the transport of the gases, the adsorption on the surface, the chemical decomposition of the silane and the interdiffusion between Si and Fe. q 1999 Elsevier Science B.V. All rights reserved. Keywords: CVD process; Silicon; Iron silicides
1. Introduction One of the most interesting fields of current research is to find ways to improve the adhesive strength and the stability of metal–polymer joints by a well defined chemical engineering of the interface. For example, the stability of the interface between aluminum based alloys substrates and organic coatings can be improved by well-organized self-assembled monolayers of phosphoric acid compounds w1,2x. For iron surfaces thiol self-organized films might help to improve the adhesion of organic coatings w3–7x. However, the adsorption of thiols requires an oxide free surface which requires electrochemical controlled conditions. An alternative are siliconorganic compounds, but the stability of the iron– oxygen–silicon bond is not sufficient for many prac)
Corresponding author. Tel.: q49-9131-8527596; Fax: q499131-8527582; E-mail: [email protected]
tical conditions. Therefore, industrial application processes like the silicoater process are used to deposit an inorganic silicate in a first stage onto the base material Žsteel. and only in a second step the silicon-organic molecule is chemisorbed onto the silicate layer. These molecules serve as molecular adhesion promoters for polymer films like lacquers, which are deposited onto the modified surface in a third step. However, the weakest point in this concept is the interface between the steel and the silicon. The formation of the silane film requires the decomposition of silicon-organic compounds at high temperatures. The formation of the inorganic silicate is not completely understood yet. To obtain more information about this process, the formation of iron silicides is investigated in this study under well-defined conditions by thermal decomposition of silane. The long-term aim of this project is to learn how to deposit ultrathin, well-defined iron silicide layers on polycrystalline steel substrates which are oxidized in
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 5 0 0 - 5
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M. Rebhan et al.r Applied Surface Science 140 (1999) 99–105
a second step in the outermost surface layer under formation of silicates. This layer, which should be homogeneous in thickness and composition over the surface, will serve as substrate for the adsorption of the silicon-organic adhesion promoters. Up to now very little work has been done on the deposition and formation of silicides on iron substrates. Klam et al. w8x used either a gas mixture of Ar, SiH 4 and H 2 or Ar, SiH 4 , SiCl 4 and H 2 for the chemical vapor deposition ŽCVD. in the temperature between 7508C and 11008C. The layer contains less than 11 at.% Si after the CVD with H 2rAr and SiH 4 . Fe 3 Si crystals are formed if SiCl 4 is present in the gas mixture. They state that the presence of chloride catalyses the formation of Fe 3 Si and also suppresses the diffusion of Si into the bulk Fe. The formation of a stable iron–silicon system in a CVD process would be impossible below 8508C. In contradiction to this Sanjurjo et al. w9x showed that the formation of silicides is possible for temperatures between 5008C and 7508C. They found at the surface Fe 3 Si, FeSi or crystalline Si with increasing temperature. This is consistent with the experiments by Cabrera et al. w10x. Many work has been done on the deposition of iron and iron silicides on silicon single crystal surfaces for the purpose of semiconductor devices. Iron or iron together with silicon was deposited on the silicon substrates and then annealed. Weiß et al. w11x and von Kanel ¨ et al. w12x analysed the formation of silicides as function of deposited iron thickness and annealing temperature. A good review on the electronic properties of silicides is given by Lange w13x and in the book of Maex and van Rossum w14x.
2. Experimental procedure Pure polycrystalline iron samples ŽFe content ) 99.9 at.%, typical lateral dimension of the crystallites ; 0.5–2 mm, size of the samples 21 = 14 = 2 mm3 . with low carbon and sulfur content Ž C - 0.001 at.%, S - 0.0001 at.%. were used for the chemical vapor deposition. The samples were polished with 1 mm diamond powder, cleaned in alcohol and acetone. Then they were annealed at 8008C in a normal pressure CVD-reactor in pure hydrogen ŽH 2 ) 99.999 vol.%. for more than 1 h to reduce the oxide layer on
the surface and to heal the scratches caused by the polishing. During the subsequent CVD the temperature was varied between 4758C and 8008C. The partial pressure of the monosilane SiH 4 gas Žpurity ) 99.9999 vol.%. was chosen between 0.2 mbar and 10 mbar, whereas the total pressure of the silane-hydrogen mixture was changed from 100 to 990 mbar. The total gas flow rate was chosen higher than 200 cm3 miny1 to get a deposition rate which is independent of the transport of the reacting gas. The reactor was a glass tube with a circular cross section of 31.5 mm. The duration of the CVD-process t was varied from 50 s to 20 h. Afterwards the samples were cooled down to room temperature within 5 min in vacuum and subsequently exposed to air. This led to the oxidation of the surface in which 2–3 monolayers of SiO 2 were formed. The samples were heated either thermally or by induction and the temperature was measured by a thermocouple or by a pyrometer respectively. When the thermal oven was used the change of mass of the samples was detected as a function of time by a Sartorius gravimetric system with a resolution of 1 mg. This corresponds to 0.7 nm deposited silicon. This heating was used up to 6008C since higher temperatures led to the deposition of silicon on the hot reactor walls. Temperatures above 6008C were reached by the induction oven which heated up just the samples and neither the surrounding gas nor the reactor wall. But in this case the in situ measurements of the change of mass could not be performed by gravimetry because of the interaction of the magnetic field with the sample. The reaction of the SiH 4 with the Fe causes as well a change of the mass of the sample as a change of the magnetic susceptibility at the surface. It is not possible to distinguish between these two effects in the gravimetric signal. A scanning electron microscope ŽSEM, Jeol JSM 6400. was used to visualize the morphology of the surface and the interface. An integrated electron diffraction X-ray system ŽEDX. was used for the chemical analysis. Depth sputter profiles were taken either by XPS or a high resolution scanning Auger microscope ŽXPS Phi 5600 and SAM Phi 670. to analyse the chemical structure up to 40 nm depth. The calibration of the sputter rate was performed with a SiO 2 sample. The orientation of the crystallites were analysed by Kikuchi patterns.
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3. Results 3.1. Reaction kinetics studied by thermo-graÕimetry 3.1.1. Temperature dependence The temperature T, the partial pressure of the silane gas p SiH 4 and the partial pressure of hydrogen p H 2 have a strong influence on the CVD-process of
Fig. 2. SEM-picture of a sample with the following CVD parameters: p SiH 4 s10 mbar, p H 2 s980 mbar, T s 4768C deposition time t s 2300 s: spherical structures appear at the grain boundaries and also occasionally on the grains. The points a and b refer to the Auger depth profiles in Fig. 5.
silicon and silicides on iron. If the temperature of the sample is less than 4758C the change of mass d m per area A and time d t. r[
Fig. 1. Ža. The change of mass is linear in time for p SiH 4 s10 mbar, p H 2 s980 mbar, T s 5358C. The exposition to SiH 4 starts at t s 0 min. Žb. The growth rate r increases exponentially with 1r T up to 5508C. r is constant above this temperature. The deposition rate of 10y3 mg cmy2 miny1 is equivalent to the growth rate of Si of 4.14 nm miny1 .
1 dm A dt
Ž 1.
is less than 0.01 mg cmy2 miny1 within 30 min SiH 4-exposition and therefore below the measurement accuracy. On the other hand r is constant in time for higher temperatures up to 6008C and total pressure of 990 mbar ŽFig. 1a.. The linear growth rate r is plotted as a function of temperature in Fig. 1b. If the temperature is higher than 4808C and lower than 5508C r increases exponentially with 1rT. The transport of SiH 4 to the surface limits r for T ) 5508C which leads to a nearly temperature independent r. A comparable temperature dependence of the growth rate is observed for homoepitactical CVD of Si w15–17x. But r is about 50 times smaller there than in the CVD-process with iron as substrate reported in this paper. This points to a catalytic decomposition of SiH 4 caused by the presence of iron at the surface. The growth rate also increases with the silane partial pressure ŽFig. 1b. but in a sublinear way.
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3.2. Morphology of the surface and the interface At the initial stages of the film growth for temperatures 4808C - T - 6008C small spherical structures can be observed. These balls appear first at the grain boundaries ŽFig. 2. and on the grains with a density of approximately 10 8 cmy2 . They cover the whole surface after longer deposition times ŽFig. 3a. as can be seen in the cross-section of Fig. 3b. If the CVDprocess takes place at higher temperatures the spherical structures are not formed. Then the surface has periodic structures and their shape depends on the grain orientation ŽFig. 4.. 3.3. Chemical analysis EDX measurements of the surface and of the cross-section proof that the layer mainly consists of the thermodynamical stable phase Fe 3 Si if the CVD is performed below 6008C. An Auger depth profile of a spherical structure and of a grain Žmarked as a and b in Fig. 2. can be seen in Fig. 5. The surface is silicon rich and the top 3 nm are oxidized due to the exposure of the samples to air. The iron content is below 20 at% at the surface but increases with depth. At about 40 nm the layer consists of the Fe 3 Si phase.
Fig. 3. SEM-pictures Ža. The balls cover the whole surface but the former grain boundaries can still be seen. Žb. Cross-section of the same sample at an angle of 158 to the surface normal. There are balls at the top of the samples Žbottom of the picture. whereas a homogenous layer of Fe 3 Si is below which is deduced from EDX-measurements Ždeposition parameters: p SiH 4 s1 mbar, p H 2 s989 mbar and T s 5828C, t s 2400 s..
3.1.2. Dependence on the total pressure If p SiH 4 is constant at 5808C in the transport limited temperature range and the partial pressure of hydrogen is varied between 100 and 990 mbar the time dependence of the change of mass alters from linear at high p H 2 to parabolic at low p H 2 . Also r Ž t . is larger during the measured 2000 s of deposition at p H 2 s 100 mbar than the corresponding constant r at 990 mbar hydrogen partial pressure. The parabolic growth at low total pressure points to a diffusion limited process.
Fig. 4. Surface after the reaction of SiH 4 with iron at p SiH 4 s 0.2 mbar, p H 2 s990 mbar, T s8008C for t s100 s. Periodic structures are formed on the grains whereas the pattern depends on the grain orientation. Grain number 1 has a low index surface in comparison to grain number 2 and 3. The orientation was identified by Kikuchi patterns.
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temperature, the diffusion of Si in Fe is dominant over the iron diffusion at the grain boundaries and defects.
4. Discussion In the following, a model ŽFig. 6. is presented which explains the CVD process of SiH 4 with the iron surface. This model has to consider the following facts: Ži. the flow rate of the SiH 4 and the diffusion of silane through the laminar boundary layer to the surface; Žii. adsorption processes at the surface; Žiii. the chemical decomposition of the SiH 4 at the surface; and Živ. the interdiffusion of Si and Fe. The growth rate r is dominated by the slowest step in the CVD-process starting from the gas flow till the interdiffusion and reaction of Si and Fe. The growth rate r is independent of the gas flow for rates exceeding 200 cm3 miny1 and temperature higher than 5508C. Therefore neither the transport of the gas nor the diffusion of the silane through the laminar boundary layer to the surface can limit the
Fig. 5. Auger-depth-profile of a sample with silicon deposited at p SiH 4 s10 mbar, pH 2 s980 mbar and T s 5288C for 600 s: Ža. depth-profile of a ball and Žb. the grain surface which is not covered by a ball in Fig. 2. The outer surface is silicon rich and partly oxidized due to the exposure of the sample to air. An underlying layer of Fe 3 Si is formed below. This is consistent with EDX-measurements. One minute sputtering is equivalent to 2.0 nm of SiO 2 .
The iron content at the surface is higher in the spherical structures than at the surface of the grain. This is assumed to be caused by the higher diffusion rate of iron at the grain boundaries and the defects. If the CVD-process is performed at 8008C for 100 s and p SiH 4 s 0.2 mbar then XPS depth profiles show that the silicon content is less than 40 at.% and decreases within 15 nm to less than 20 at.%. At this
Fig. 6. Sketch of a reaction model Žg: gaseous, ad: adsorbed..
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rate of the CVD-process since diffusion would cause a parabolic instead of a linear time dependence of the change of mass. So the chemical decomposition of the SiH 4 is the slowest process. On the other hand r is about 50 times higher than for homoepitaxy of silicon at the same deposition conditions. Also, the AES measurements show that there is about 10 at.% of iron in the surface. Combining these facts one can conclude that the iron atoms catalyse the silicon deposition. Iron atoms diffuse from the bulk into the surface layers leading to the formation of the Fe 3 Si which explains the existence of Fe atoms at the surface ŽFig. 5.. The growth rate increases sublinearly with the silane partial pressure ŽFig. 1b.. So the number of occupied adsorption sites for the SiH 4 molecules influences r. The growth rate seems to reach a saturation value for p SiH 4 larger than 1 mbar. Iron atoms diffuse faster along the grain boundaries and defects than within the bulk iron w18x. Therefore, the spherical clusters are formed on top of the fast diffusion paths first. The density of the balls on the grains is approximately the same as the density of defects there w19x. Along the surface diffusing iron atoms can form the nucleation sites for the growth of the balls. So the whole surface is covered with these structures after long deposition times ŽFig. 3a.. The diffusion rate through the laminar boundary layer is inversely proportional to the mean free path and therefore to the total pressure. This increases with decreasing hydrogen partial pressure at constant p SiH 4 . But since the deposition rate is independent of the gas flow rate the SiH 4 transport is not the limiting step. So the diffusion of the bulk iron to the surface must be the slowest process at low p H 2 for temperatures higher than 5508C. At high temperatures the diffusion of Si in Fe dominates over the grain boundary and defect diffusion of Fe w20x which leads to a low Si content at the surface. Therefore no balls appear at 8008C ŽFig. 4.. The periodic structure is assumed to result from the minimization of surface energy. 5. Conclusion The main focus of this paper is to report the mechanism of the chemical reaction between the
SiH 4 gas and the Fe substrate. This reaction strongly depends on the temperature. The diffusion of Fe along the grain boundaries and defects leads to the formation of spherical structures of Fe 3 Si at these sites for temperature below 6008C. At 8008C the Si diffusion into the bulk Fe is dominant and therefore no balls appear on the surface. Having understood the physical and chemical mechanism of these processes the next step of our work will be the production thin layers of some 10 nm thickness which can be used in a technical process as a substrate for silicon-organic adhesion promoters.
Acknowledgements We would like to thank the Deutsche Forschungsgemeinschaft which finances this project within the Sonderforschungsbereich 292 ‘Mehrkomponentige Schichtsysteme’ as project number D6.
References w1x I. Maege, E. Jaehne, A. Henke, H.-J.P. Adler, Ch. Bram, Ch. Jung, M. Stratmann, Macromol. Symp. 126 Ž1997. 7. w2x Ch. Bram, Ch. Jung, M. Stratmann, in: I. Olefjord, L. Nyberg, D. Briggs ŽEds.., 7th European Conference on Applications of Surface and Interface Analysis, Wiley, New York, 1997. w3x M. Stratmann, M. Rohwerder, C. Reinartz, Ch. Bram, Ch. Jung, in: I. Olefjord, L. Nyberg, D. Briggs ŽEds.., 7th European Conference on Applications of Surface and Interface Analysis, Wiley, New York, 1997. w4x M. Volmer, M. Stratmann, H. Viefhaus, Fresenius Zeitschrift fur ¨ Analytische Chemie 333 Ž1988. 545. w5x M. Volmer, M. Stratmann, H. Viefhaus, Surface and Interface Analysis 16 Ž1990. 278. w6x M. Volmer-Uebing, B. Reynders, M. Stratmann, Werkstoffe und Korrosion 42 Ž1991. 19. w7x M. Volmer-Uebing, M. Stratmann, Appl. Surf. Sci. 55 Ž1992. 19. w8x C. Klam, J.-P. Millet, H. Mazille, J.M. Gras, J. Mater. Sci. 26 Ž1991. 4945. w9x A. Sanjurjo, B. Wood, K. Lau, G. Krishnan, Scripta Metallurgica et Materialia 31 Ž8. Ž1994. 1019. w10x A.L. Cabrera, J.F. Kirner, R. Pierantozzi, J. Mater. Res. 5 Ž1. Ž1990. 74. w11x W. Weiß, M. Kutschera, U. Starke, M. Mozaffari, K. Reshoft, ¨ U. Kohler, K. Heinz, Surf. Sci. 377–379 Ž1996. 861. ¨ w12x H. von Kanel, N. Onda, H. Sirringhaus, E. Muller-Gubler, S. ¨ ¨
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w13x w14x w15x w16x
Gonvalves-Conto, C. Schwarz, Appl. Surf. Sci. 70–71 Ž1993. 559. H. Lange, Phys. Status Solidi B 201 Ž1997. 3. K. Maex, M. van Rossum ŽEds.., Properties of Metal Silicides, Inspec Publication, London, 1995. A. Sherman, Chemical Vapour Deposition for Microelectronics, Noyes Publications, Park Ridge, 1987. K.K. Schuegraf, Handbook of Thin-film Deposition Pro-
w17x w18x w19x w20x
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cesses and Techniques, Noyes Publications, Park Ridge, 1988. B.A. Scott, R.M. Plecenik, E.E. Simonyi, Appl. Phys. Lett. 39 Ž1. Ž1981. 73. L. Stratmann, Thesis, Dusseldorf, 1982. ¨ F. Gehrmann, Thesis, VDI-Verlag Reihe 5, Nr. 350, Dussel¨ dorf, 1994. H. de Rugy, H. Viefhaus, Surf. Sci. 173 Ž1986. 418.
CVD of silicon and silicides on iron M. Rebhan ) , M. Rohwerder, M. Stratmann Lehrstuhl fur Institut fur Martensstrasse 7, ¨ Korrosion und Oberflachentechnik, ¨ ¨ Werkstoffwissenschaften 4, UniÕersitat ¨ Erlangen-Nurnberg, ¨ 91058 Erlangen, Germany Received 6 July 1998; accepted 15 September 1998
Abstract Silicon and iron silicides are promising candidates as adhesion promoters on iron. The formation of silicon and iron silicides films on polycrystalline iron is studied in a CVD process with monosilane. The process is performed between 4758C and 8008C with a silane partial pressure of 0.2 to 10 mbar to a total pressure of the silane–hydrogen mixture of 100 to 990 mbar. A model of the reaction mechanism is presented which includes the transport of the gases, the adsorption on the surface, the chemical decomposition of the silane and the interdiffusion between Si and Fe. q 1999 Elsevier Science B.V. All rights reserved. Keywords: CVD process; Silicon; Iron silicides
1. Introduction One of the most interesting fields of current research is to find ways to improve the adhesive strength and the stability of metal–polymer joints by a well defined chemical engineering of the interface. For example, the stability of the interface between aluminum based alloys substrates and organic coatings can be improved by well-organized self-assembled monolayers of phosphoric acid compounds w1,2x. For iron surfaces thiol self-organized films might help to improve the adhesion of organic coatings w3–7x. However, the adsorption of thiols requires an oxide free surface which requires electrochemical controlled conditions. An alternative are siliconorganic compounds, but the stability of the iron– oxygen–silicon bond is not sufficient for many prac)
Corresponding author. Tel.: q49-9131-8527596; Fax: q499131-8527582; E-mail: [email protected]
tical conditions. Therefore, industrial application processes like the silicoater process are used to deposit an inorganic silicate in a first stage onto the base material Žsteel. and only in a second step the silicon-organic molecule is chemisorbed onto the silicate layer. These molecules serve as molecular adhesion promoters for polymer films like lacquers, which are deposited onto the modified surface in a third step. However, the weakest point in this concept is the interface between the steel and the silicon. The formation of the silane film requires the decomposition of silicon-organic compounds at high temperatures. The formation of the inorganic silicate is not completely understood yet. To obtain more information about this process, the formation of iron silicides is investigated in this study under well-defined conditions by thermal decomposition of silane. The long-term aim of this project is to learn how to deposit ultrathin, well-defined iron silicide layers on polycrystalline steel substrates which are oxidized in
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 5 0 0 - 5
100
M. Rebhan et al.r Applied Surface Science 140 (1999) 99–105
a second step in the outermost surface layer under formation of silicates. This layer, which should be homogeneous in thickness and composition over the surface, will serve as substrate for the adsorption of the silicon-organic adhesion promoters. Up to now very little work has been done on the deposition and formation of silicides on iron substrates. Klam et al. w8x used either a gas mixture of Ar, SiH 4 and H 2 or Ar, SiH 4 , SiCl 4 and H 2 for the chemical vapor deposition ŽCVD. in the temperature between 7508C and 11008C. The layer contains less than 11 at.% Si after the CVD with H 2rAr and SiH 4 . Fe 3 Si crystals are formed if SiCl 4 is present in the gas mixture. They state that the presence of chloride catalyses the formation of Fe 3 Si and also suppresses the diffusion of Si into the bulk Fe. The formation of a stable iron–silicon system in a CVD process would be impossible below 8508C. In contradiction to this Sanjurjo et al. w9x showed that the formation of silicides is possible for temperatures between 5008C and 7508C. They found at the surface Fe 3 Si, FeSi or crystalline Si with increasing temperature. This is consistent with the experiments by Cabrera et al. w10x. Many work has been done on the deposition of iron and iron silicides on silicon single crystal surfaces for the purpose of semiconductor devices. Iron or iron together with silicon was deposited on the silicon substrates and then annealed. Weiß et al. w11x and von Kanel ¨ et al. w12x analysed the formation of silicides as function of deposited iron thickness and annealing temperature. A good review on the electronic properties of silicides is given by Lange w13x and in the book of Maex and van Rossum w14x.
2. Experimental procedure Pure polycrystalline iron samples ŽFe content ) 99.9 at.%, typical lateral dimension of the crystallites ; 0.5–2 mm, size of the samples 21 = 14 = 2 mm3 . with low carbon and sulfur content Ž C - 0.001 at.%, S - 0.0001 at.%. were used for the chemical vapor deposition. The samples were polished with 1 mm diamond powder, cleaned in alcohol and acetone. Then they were annealed at 8008C in a normal pressure CVD-reactor in pure hydrogen ŽH 2 ) 99.999 vol.%. for more than 1 h to reduce the oxide layer on
the surface and to heal the scratches caused by the polishing. During the subsequent CVD the temperature was varied between 4758C and 8008C. The partial pressure of the monosilane SiH 4 gas Žpurity ) 99.9999 vol.%. was chosen between 0.2 mbar and 10 mbar, whereas the total pressure of the silane-hydrogen mixture was changed from 100 to 990 mbar. The total gas flow rate was chosen higher than 200 cm3 miny1 to get a deposition rate which is independent of the transport of the reacting gas. The reactor was a glass tube with a circular cross section of 31.5 mm. The duration of the CVD-process t was varied from 50 s to 20 h. Afterwards the samples were cooled down to room temperature within 5 min in vacuum and subsequently exposed to air. This led to the oxidation of the surface in which 2–3 monolayers of SiO 2 were formed. The samples were heated either thermally or by induction and the temperature was measured by a thermocouple or by a pyrometer respectively. When the thermal oven was used the change of mass of the samples was detected as a function of time by a Sartorius gravimetric system with a resolution of 1 mg. This corresponds to 0.7 nm deposited silicon. This heating was used up to 6008C since higher temperatures led to the deposition of silicon on the hot reactor walls. Temperatures above 6008C were reached by the induction oven which heated up just the samples and neither the surrounding gas nor the reactor wall. But in this case the in situ measurements of the change of mass could not be performed by gravimetry because of the interaction of the magnetic field with the sample. The reaction of the SiH 4 with the Fe causes as well a change of the mass of the sample as a change of the magnetic susceptibility at the surface. It is not possible to distinguish between these two effects in the gravimetric signal. A scanning electron microscope ŽSEM, Jeol JSM 6400. was used to visualize the morphology of the surface and the interface. An integrated electron diffraction X-ray system ŽEDX. was used for the chemical analysis. Depth sputter profiles were taken either by XPS or a high resolution scanning Auger microscope ŽXPS Phi 5600 and SAM Phi 670. to analyse the chemical structure up to 40 nm depth. The calibration of the sputter rate was performed with a SiO 2 sample. The orientation of the crystallites were analysed by Kikuchi patterns.
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3. Results 3.1. Reaction kinetics studied by thermo-graÕimetry 3.1.1. Temperature dependence The temperature T, the partial pressure of the silane gas p SiH 4 and the partial pressure of hydrogen p H 2 have a strong influence on the CVD-process of
Fig. 2. SEM-picture of a sample with the following CVD parameters: p SiH 4 s10 mbar, p H 2 s980 mbar, T s 4768C deposition time t s 2300 s: spherical structures appear at the grain boundaries and also occasionally on the grains. The points a and b refer to the Auger depth profiles in Fig. 5.
silicon and silicides on iron. If the temperature of the sample is less than 4758C the change of mass d m per area A and time d t. r[
Fig. 1. Ža. The change of mass is linear in time for p SiH 4 s10 mbar, p H 2 s980 mbar, T s 5358C. The exposition to SiH 4 starts at t s 0 min. Žb. The growth rate r increases exponentially with 1r T up to 5508C. r is constant above this temperature. The deposition rate of 10y3 mg cmy2 miny1 is equivalent to the growth rate of Si of 4.14 nm miny1 .
1 dm A dt
Ž 1.
is less than 0.01 mg cmy2 miny1 within 30 min SiH 4-exposition and therefore below the measurement accuracy. On the other hand r is constant in time for higher temperatures up to 6008C and total pressure of 990 mbar ŽFig. 1a.. The linear growth rate r is plotted as a function of temperature in Fig. 1b. If the temperature is higher than 4808C and lower than 5508C r increases exponentially with 1rT. The transport of SiH 4 to the surface limits r for T ) 5508C which leads to a nearly temperature independent r. A comparable temperature dependence of the growth rate is observed for homoepitactical CVD of Si w15–17x. But r is about 50 times smaller there than in the CVD-process with iron as substrate reported in this paper. This points to a catalytic decomposition of SiH 4 caused by the presence of iron at the surface. The growth rate also increases with the silane partial pressure ŽFig. 1b. but in a sublinear way.
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3.2. Morphology of the surface and the interface At the initial stages of the film growth for temperatures 4808C - T - 6008C small spherical structures can be observed. These balls appear first at the grain boundaries ŽFig. 2. and on the grains with a density of approximately 10 8 cmy2 . They cover the whole surface after longer deposition times ŽFig. 3a. as can be seen in the cross-section of Fig. 3b. If the CVDprocess takes place at higher temperatures the spherical structures are not formed. Then the surface has periodic structures and their shape depends on the grain orientation ŽFig. 4.. 3.3. Chemical analysis EDX measurements of the surface and of the cross-section proof that the layer mainly consists of the thermodynamical stable phase Fe 3 Si if the CVD is performed below 6008C. An Auger depth profile of a spherical structure and of a grain Žmarked as a and b in Fig. 2. can be seen in Fig. 5. The surface is silicon rich and the top 3 nm are oxidized due to the exposure of the samples to air. The iron content is below 20 at% at the surface but increases with depth. At about 40 nm the layer consists of the Fe 3 Si phase.
Fig. 3. SEM-pictures Ža. The balls cover the whole surface but the former grain boundaries can still be seen. Žb. Cross-section of the same sample at an angle of 158 to the surface normal. There are balls at the top of the samples Žbottom of the picture. whereas a homogenous layer of Fe 3 Si is below which is deduced from EDX-measurements Ždeposition parameters: p SiH 4 s1 mbar, p H 2 s989 mbar and T s 5828C, t s 2400 s..
3.1.2. Dependence on the total pressure If p SiH 4 is constant at 5808C in the transport limited temperature range and the partial pressure of hydrogen is varied between 100 and 990 mbar the time dependence of the change of mass alters from linear at high p H 2 to parabolic at low p H 2 . Also r Ž t . is larger during the measured 2000 s of deposition at p H 2 s 100 mbar than the corresponding constant r at 990 mbar hydrogen partial pressure. The parabolic growth at low total pressure points to a diffusion limited process.
Fig. 4. Surface after the reaction of SiH 4 with iron at p SiH 4 s 0.2 mbar, p H 2 s990 mbar, T s8008C for t s100 s. Periodic structures are formed on the grains whereas the pattern depends on the grain orientation. Grain number 1 has a low index surface in comparison to grain number 2 and 3. The orientation was identified by Kikuchi patterns.
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temperature, the diffusion of Si in Fe is dominant over the iron diffusion at the grain boundaries and defects.
4. Discussion In the following, a model ŽFig. 6. is presented which explains the CVD process of SiH 4 with the iron surface. This model has to consider the following facts: Ži. the flow rate of the SiH 4 and the diffusion of silane through the laminar boundary layer to the surface; Žii. adsorption processes at the surface; Žiii. the chemical decomposition of the SiH 4 at the surface; and Živ. the interdiffusion of Si and Fe. The growth rate r is dominated by the slowest step in the CVD-process starting from the gas flow till the interdiffusion and reaction of Si and Fe. The growth rate r is independent of the gas flow for rates exceeding 200 cm3 miny1 and temperature higher than 5508C. Therefore neither the transport of the gas nor the diffusion of the silane through the laminar boundary layer to the surface can limit the
Fig. 5. Auger-depth-profile of a sample with silicon deposited at p SiH 4 s10 mbar, pH 2 s980 mbar and T s 5288C for 600 s: Ža. depth-profile of a ball and Žb. the grain surface which is not covered by a ball in Fig. 2. The outer surface is silicon rich and partly oxidized due to the exposure of the sample to air. An underlying layer of Fe 3 Si is formed below. This is consistent with EDX-measurements. One minute sputtering is equivalent to 2.0 nm of SiO 2 .
The iron content at the surface is higher in the spherical structures than at the surface of the grain. This is assumed to be caused by the higher diffusion rate of iron at the grain boundaries and the defects. If the CVD-process is performed at 8008C for 100 s and p SiH 4 s 0.2 mbar then XPS depth profiles show that the silicon content is less than 40 at.% and decreases within 15 nm to less than 20 at.%. At this
Fig. 6. Sketch of a reaction model Žg: gaseous, ad: adsorbed..
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rate of the CVD-process since diffusion would cause a parabolic instead of a linear time dependence of the change of mass. So the chemical decomposition of the SiH 4 is the slowest process. On the other hand r is about 50 times higher than for homoepitaxy of silicon at the same deposition conditions. Also, the AES measurements show that there is about 10 at.% of iron in the surface. Combining these facts one can conclude that the iron atoms catalyse the silicon deposition. Iron atoms diffuse from the bulk into the surface layers leading to the formation of the Fe 3 Si which explains the existence of Fe atoms at the surface ŽFig. 5.. The growth rate increases sublinearly with the silane partial pressure ŽFig. 1b.. So the number of occupied adsorption sites for the SiH 4 molecules influences r. The growth rate seems to reach a saturation value for p SiH 4 larger than 1 mbar. Iron atoms diffuse faster along the grain boundaries and defects than within the bulk iron w18x. Therefore, the spherical clusters are formed on top of the fast diffusion paths first. The density of the balls on the grains is approximately the same as the density of defects there w19x. Along the surface diffusing iron atoms can form the nucleation sites for the growth of the balls. So the whole surface is covered with these structures after long deposition times ŽFig. 3a.. The diffusion rate through the laminar boundary layer is inversely proportional to the mean free path and therefore to the total pressure. This increases with decreasing hydrogen partial pressure at constant p SiH 4 . But since the deposition rate is independent of the gas flow rate the SiH 4 transport is not the limiting step. So the diffusion of the bulk iron to the surface must be the slowest process at low p H 2 for temperatures higher than 5508C. At high temperatures the diffusion of Si in Fe dominates over the grain boundary and defect diffusion of Fe w20x which leads to a low Si content at the surface. Therefore no balls appear at 8008C ŽFig. 4.. The periodic structure is assumed to result from the minimization of surface energy. 5. Conclusion The main focus of this paper is to report the mechanism of the chemical reaction between the
SiH 4 gas and the Fe substrate. This reaction strongly depends on the temperature. The diffusion of Fe along the grain boundaries and defects leads to the formation of spherical structures of Fe 3 Si at these sites for temperature below 6008C. At 8008C the Si diffusion into the bulk Fe is dominant and therefore no balls appear on the surface. Having understood the physical and chemical mechanism of these processes the next step of our work will be the production thin layers of some 10 nm thickness which can be used in a technical process as a substrate for silicon-organic adhesion promoters.
Acknowledgements We would like to thank the Deutsche Forschungsgemeinschaft which finances this project within the Sonderforschungsbereich 292 ‘Mehrkomponentige Schichtsysteme’ as project number D6.
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