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Effect of oxidized Al prelayer for the growth of polycrystalline Al2O3 films on Si using ionized beam deposition
Thin Solid Films 388 Ž2001. 290᎐294
Effect of oxidized Al prelayer for the growth of polycrystalline Al 2 O 3 films on Si using ionized beam deposition S.W. Whangbo, Y.K. Choi, H.K. Jang, Y.D. Chung, I.W. Lyo, C.N. WhangU Atomic-scale Surface Science Research Center and Institute of Physics and Applied Physics, Yonsei Uni¨ ersity, Seoul, 120-749, South Korea Received 23 June 2000; received in revised form 30 November 2000; accepted 18 January 2001
Abstract Polycrystalline Al 2 O 3 thin films have been grown on Si substrates by ionized beam deposition using an aluminum solid source in O 2 environments. To prevent the aluminum interdiffusion to Si at high substrate temperature ŽTs ) 500⬚C., the oxidized Al prelayer with a thickness of 3 nm was deposited on the Si substrate at room temperature before Al 2 O 3 deposition, and subsequently polycrystalline Al 2 O 3 films were grown at 700⬚C. Although Al interdiffusion increased at the interface with increasing Ts , the oxidized Al prelayer was effective as a buffer layer on which to grow stoichiometric and crystalline Al 2 O 3 films up to Ts s 800⬚C. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Aluminum oxide; Ionized beam deposition; Interfaces; X-Ray photoelectron spectroscopy ŽXPS.
1. Introduction Aluminum oxide ŽAl 2 O 3 . thin films are very useful for many applications such as microelectronic devices, wear resistive coatings, corrosion protective coating material, and catalysts etc. w1᎐3x. Its wide usefulness is primarily dependent on its hardness, high melting point and low electrical conductivity maintained at high temperature. Al 2 O 3 films have been commonly produced by chemical vapor deposition ŽCVD. using various kinds of aluminum sources such as trimethylaluminum wTMA, AlŽCH 3 . 3 x and aluminum borohydride wAlŽBH 4 . 3 x w4x. However, the films grown by the CVD process have some disadvantages: carbon contamination cannot be avoided due to the organic byproducts of its dissociation, and an excessively high substrate temperature ŽTs ) 800⬚C. or post annealing process’ are needed to U
Corresponding author. Tel.: q82-2-361-3841r2; fax: q82-23127090. E-mail address: [email protected] ŽC.N. Whang..
obtain the crystalline aluminum oxide w5,6x. Therefore, it would be highly desirable to have the alumina thin film growth at a relatively low substrate temperature with no carbon contamination. In physical vapor deposition ŽPVD., ion beam assisted deposition and magnetron sputtering techniques, using an Al target, have been applied to fabricate aluminum oxide films. However, the formation of a highly insulating coating for the Al target led to a charge buildup and arcing w7x. Pulsed sputtering and rapid responding automatically controlled oxygen partial pressure techniques were employed to avoid the complete oxidation of a target. On the other hand, the reports on aluminum oxide film growth with an Al solid source are seldom found, because molten aluminum is extremely reactive to most metals, and thus a practical crucible for an aluminum evaporation with long emission times and sufficient stability has not been realized. In this paper, we report the successful growth of stoichiometric polycrystalline Al 2 O 3 films on Si substrates by ionized beam deposition ŽIBD. using an aluminum solid source in O 2 environments. It is well
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 0 8 1 2 - 4
S.W. Whangbo et al. r Thin Solid Films 388 (2001) 290᎐294
known that the direct aluminum᎐silicon contact exhibits various poor contact characteristics, such as junction spiking at high temperature, therefore a diffusion barrier is required to deposit the film at high substrate temperatures w8x. In the present work, the oxidized Al prelayer was used for the prevention of Al interdiffusion at high substrate temperatures and the oxidation of Si substrate in the initial growth stage of A 2 O 3 on Si. The interdiffuison phenomena of Al at the interface region was investigated at the substrate temperatures of 500᎐800⬚C. 2. Experimental Al 2 O 3 films were prepared on p-SiŽ111. substrate by the ionized beam deposition system. The IBD growth chamber is equipped with in situ reflection high-energy electron diffraction ŽRHEED. and cold-cathode type oxygen ion gun. The growth chamber was evacuated to 3 = 10y1 0 torr and the oxygen partial pressure during Al 2 O 3 film growth was 1.5= 10y5 torr. To generate ionized Al beam, the TiB 2 based ceramic crucible was filled with aluminum solid source Ž99.999% purity.. The crucible is composed of TiB 2 Ž; 50%., BN and AlN composites, which is a relatively resistant material to molten Al w9x. The temperature of the crucible was kept at approximately 1460⬚C during evaporation, as measured by an optical pyrometer. The evaporated aluminum vapor was ionized by electron bombardment at the ionization region located at above the crucible, then the ionized Al beam was accelerated by the electric field. Oxygen gas with a purity of 99.995% was used as an oxygen ion source for the oxidation process of Al prelayer and for a reactive oxygen atmosphere during the Al 2 O 3 deposition. The p-type SiŽ111. wafers with a resistivity of 1᎐3 ⍀ cm were used as substrates. They were cleaned and dipped in a 5% dilute HF acid solution to remove a native oxide and then, rinsed in deionized water. The substrate was heated up to 1000⬚C to obtain clean 7 = 7 reconstructed surface, checked by RHEED, prior to deposition. In this experiment, the film growth was performed through the three stages. First, the epitaxial Al prelayer with 3-nm thickness was deposited at room temperature. Secondly, the Al prelayer was oxidized with oxygen ion irradiation, by a cold-cathode type oxygen ion gun. Finally, the Al 2 O 3 films were grown at a substrate temperature range of 500᎐800⬚C. The detailed experimental conditions are summarized in Table 1. The thickness of the grown films was measured by a quartz crystal oscillator and calibrated by Rutherford backscattering spectroscopy. The crystallinity of the Al 2 O 3 films were measured by RHEED. X-Ray photoelectron spectroscopy ŽXPS. measurements were carried out to investigate the atomic composition and
291
Table 1 Experimental conditions of Al 2 O 3 film growth Al prelayer deposition Substrate Substrate temperature Ionization current Ž II . Acceleration voltage Ž Va . Deposition rate, Film thickness
clean 7 = 7᎐Si Ž111. Room temperature 80 mA 500 V 0.6 nmrmin, 3 nm
Oxidation of Al prelayer Substrate temperature Oxygen current density Oxygen ion energy Irradiation time
Room temperature 2 Arcm2 1 kV 20 min
Al2 O3 film deposition Substrate temperature Ionization current Ž Ii . Acceleration voltage Ž Va . O2 partial pressure
500᎐800⬚C 200 mA 500᎐1000 V 1.5= 10y5 torr
depth distribution of the films. The XPS data were obtained with a PHI 5700 ESCA spectrometer using an Al K ␣ Ž h s 1486.6 eV., with an energy resolution of 0.89 eV. The XPS spectra were aligned with respect to the binding energy of the surface carbon peak Ž284.5 eV. obtained at an initial survey scan. The atomic composition of the films was estimated from the XPS peak areas using relative sensitivity factors obtained from single crystalline Al 2 O 3 as a reference. 3. Results and discussion Fig. 1a᎐c shows the RHEED patterns of the SiŽ111. ᎐ Ž7 = 7. surface, as-deposited Al prelayer, and polycrystalline Al 2 O 3 film deposited on the oxidized Al prelayer, respectively. Fig. 1a shows a typical Ž7 = 7. reconstructed SiŽ111. surface obtained before deposition. Upon depositing the Al prelayer, as shown in Fig. 1b, the RHEED pattern develops spots and streaks depending on the incident angle of electron beam, indicating that the Al prelayer is grown epitaxially on the SiŽ111. surface. Crystalline orientation of the Al prelayer on the SiŽ111. substrate is determined as AlŽ111.rrSiŽ111., Alw110xrrSiw110x. The epitaxial Al prelayer has an effect of reducing the void density and the grain boundaries. As described in Table 1 the epitaxial Al prelayer was obtained under the deposition conditions of very low deposition rates and acceleration voltages of 500 V. However, in the case of the Al prelayer formed with a high deposition rate and no acceleration voltage, it showed a polycrystalline phase. Furthermore, the Al 2 O 3 films grown on polycrystalline Al prelayer showed an amorphous phase and a small Si 2p peak was observed at surface in XPS measurements. It is likely that the grain boundaries of the polycrystalline Al prelayer provided fast diffusion paths for the
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Fig. 1. The RHEED patterns of the Ž7 = 7. ᎐Si Ž111. substrate Ža., epitaxially grown Al layer Žb., and Al 2 O 3 film grown at 700⬚C.
Si at the substrate temperatures above 400⬚C w8x. Thus, to obtain stoichiometric and crystalline Al 2 O 3 films on Si substrates, a uniform and flat epitaxial Al prelayer without voids and grain boundaries is required. The Al 2 O 3 films grown on an epitaxial Al prelayer show ring patterns as shown in Fig. 1c, this means that the grown film is a polycrystal. Fig. 2Ia᎐c shows the XPS survey spectra obtained from the oxidized Al prelayer, Al 2 O 3 film, and single crystalline Al 2 O 3 Žsapphire. as a reference, respectively. A 3-keV Arq sputtering was carried out for 10 s to eliminate the surface contamination which might have been accumulated during the transfer through air to the XPS chamber. Fig. 2Ia was obtained from the oxidized Al prelayer before Al 2 O 3 deposition, where Si signals as well as Al and O signals were observed, because the thickness of the layer was approximately 3 nm and the expanded view is presented at Fig. 2II to see the accurate peak positions over the Al and Si peaks. However, the Al 2 O 3 film is composed only of Al and O as shown in Fig. 2Ib, and the ratio of signal intensities between the Al 2p and O 1s spectra is similar to that of sapphire ŽO 1srAl 2ps 4.0. shown in Fig. 2Ic. Usually, in the case of the films deposited by CVD and metalorganic molecular beam epitaxy ŽMOMBE., carbon contamination cannot be avoided, because the C concentration at the interface of Al 2 O 3rSi increases with increasing Ts , due to the further dissociation of the source gases w5,10x. However, as shown in these spectra, the C signal is not observed within the sensitivity of XPS measurements. Therefore, these results clearly indicate that the Al 2 O 3 film as well as the oxidized Al prelayer is grown without carbon contamination. Fig. 3a,b shows the XPS spectra of Al 2p and Si 2p peak obtained after the oxidation of epitaxially grown Al prelayer. The binding energy of the Al 2p peak from a metallic Al is 72.9 eV w11x and that of the Al 2p peak from an Al in Al 2 O 3 film is 2.5᎐3.0 eV higher than 72.9 eV w12,13x. As shown in Fig. 3a, the Al prelayer was totally oxidized by oxygen ion irradiation, and the binding energy of Al 2p Ž75.7 eV. corresponds to that of Al 3q in Al 2 O 3 . The Si 2p spectrum in Fig. 3b was obtained from the Si substrate and the binding energy
of 98.9 eV indicates the bonding between the Si atoms, and the Si᎐O bonding Ž102᎐104 eV. was not detected. Therefore, the oxidized Al prelayer was well formed with Al 2 O 3rSi interface without the formation of SiO 2 at the Al 2 O 3rSi interface. Fig. 4a᎐c displays the XPS_ depth profiles of the Al 2 O 3 films grown at different substrate temperatures of 600⬚C, 700⬚C and 800⬚C, respectively. We will use the term interface region to describe the region where
Fig. 2. ŽI. XPS survey spectra from Ža. the oxidized Al layer, Žb. the Al 2 O 3 film, and Žc. bulk-sapphire as a reference. The small Ar 2p peaks are due to the Arq ion sputtering for 10 s to eliminate the surface contamination. ŽII. Expanded view of the I-Ža..
S.W. Whangbo et al. r Thin Solid Films 388 (2001) 290᎐294
293
Fig. 3. The XPS spectra of Ža. Al 2p and Žb. Si 2p from thin oxidized Al layer.
the concentration of Si 2p changes from an appearance point to maximum values in our depth profile. The ratios of oxygen concentration Ž Co . to aluminum conFig. 5. The XPS depth spectra obtained from the Al 2 O 3 film grown at Ts s 600⬚C: Ža. Al 2p; Žb. Si 2p. The spectra show the origin of Al rich state at the interface region.
Fig. 4. The XPS depth profile spectra of the Al 2 O 3 films as a function of a substrate temperature: Ža. Ts s 600⬚C, Žb. 700⬚C, and Žc. 800⬚C.
centration Ž CAl . were evaluated using the sensitivity factors obtained from a single crystal Al 2 O 3 . The ratios of oxygen to aluminum of the Al 2 O 3 films Ž CorCAl . in Fig. 4a,b,c are 1.52, 1.49 and 1.53, respectively. Therefore, it suggests that the stoichiometric Al 2 O 3 films were obtained. As the substrate temperature increases, the interface regions are widened and simultaneously the decrease rates of CAl and Co become slower. In addition, the decrease rate of CAl is lower than that of Co at near substrate region for all samples. Thus the non-stoichiometric alumina are formed at the interface. To have a closer look the interface state, the XPS spectra of the Al 2 O 3 film grown at 600⬚C are depicted in Fig. 5. Fig. 5a,b shows the Al 2p and Si 2p spectra of near substrate region, respectively. The sputtering time corresponds to the time scale of Fig. 4a, and each sputtering was carried out with the time interval of 0.5 min. As shown in Fig. 5a, the aluminum atoms in the Al 2 O 3 film were totally oxidized, which is indicated by a solid line and the binding energy of Al 3q is 75.7 eV. The Al 3q peaks continuously decrease as the exposed surface approaches the interface, and the small metallic aluminum ŽAl 0 . peak appears near the interface, which has a binding energy of 72.9 eV. This result was reproducible for the films grown at a substrate temperature ) 500⬚C. There are two possibilities on this excess metallic aluminum, one being residual metallic Al in the process of Al prelayer oxidation, and the other is the interdiffusion of Al during deposition at high substrate temperatures. The first possibility can be ignored from the result of the abrupt interface forma-
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4. Conclusions
Fig. 6. Dependence of the Al 2 O 3 growth rate on the substrate temperature.
tion as shown in Fig. 3. To test the second possibility, we grew the Al 2 O 3 films on 2-nm thick SiO 2 on Si substrates at the same temperature range without an oxidized Al prelayer, and investigated the interface by the same technique. We found similar results: the Al 0 peak also appears and as the Ts increases, the Al 0 peak also increases. Thus we conclude that the origin of metallic aluminum at the interface is the interdiffusion of Al during the deposition at high substrate temperatures, leading to the aluminum rich state at the interface as shown in Fig. 4. In addition, although the deposition process was executed in oxygen environments at high substrate temperatures, the Si᎐O bond was not detected within the XPS_ resolution limit at the interface as shown in Fig. 5b. This might be partly due to the difference of the heat of formation between AlŽy1677 kJrkmol. and Si Žy912 kJrkmol. with oxygen w14x. Fig. 6 shows the dependence of the growth rate of the Al 2 O 3 films on the substrate temperatures. The polycrystalline Al 2 O 3 films are grown at temperatures ) 700⬚C. The growth rate slightly decreases with increasing substrate temperatures up to 700⬚C, and in the temperature range of Ts ) 700⬚C, the growth rate rapidly drops. Here the desorption of a volatile gas phase AlO is believed to be responsible for the abrupt fall in the growth rate.
The polycrystalline Al 2 O 3 films have been grown on an Si substrate by ionized beam deposition using Al solid source in O 2 environments. The oxidized Al prelayer was used for the prevention of Al interdiffusion at high substrate temperatures. Although the Al interdiffusion increased and the CAl exceeded the CO near the interface with increasing Ts , the oxidized Al prelayer was effective in maintaining the growth of the polycrystalline Al 2 O 3 film up to 800⬚C. The stoichiometry of the Al 2 O 3 films were found to be nearly identical to that of sapphire, and the initially nearly constant growth rate fell sharply beyond Ts s 700⬚C possibly due to the desorption of AlO phase. The carbon contamination and the formation of the Si᎐O bonds at the interface were not found in this growth method. References w1x Z. Jin, H.S. Kwok, M. Wong, IEEE Electron Devices Lett. 11 Ž1998. 502. w2x I.Y. Konyashin, Surf. Coat. Technol 85 Ž1996. 131. w3x J.-E. Sundgren, H.T.G. Hentzell, J. Vac. Sci. Technol. A 4 Ž1986. 2259. w4x J.A. Glassjr, S.S. Kher, J.T. Spencer, Chem. Mater. 4 Ž1992. 530. w5x M. Ishida, I. Katakabe, T. Nakamura, N. Ohtake, Appl. Phys. Lett. 52 Ž1988. 1326. w6x W. Koh, S.J. Ku, Y. Kim, Thin Solid Films 304 Ž1997. 222. w7x O. Zywitzki, G. Hoetzsch, Surf. Coat. Technol. 86᎐87 Ž1996. 640. w8x S. Wolf, Silicon processing for the VLSI Era, 2, Lattice Press, 1986, pp. 111᎐134. w9x Y. Torii, H. Yamada, Proceedings of the International Ion Engineering Congress, Ž1983. 363. w10x H. Iizuka, K. Yokoo, S. Ono, Appl. Phys. Lett. 61 Ž1992. 2978. w11x J.F. Moulder, W.F. Stickle, P.E. Sobal, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, Perkin᎐Elmer, Eden Prairie, Minesota, 1992. w12x G. Faraci, S. La Rosa, A.R. Pennisi, Y. Hwu, G. Margaritondo, Phys. Rev. B 47 Ž1993. 4052. w13x R.Z. Bachrach, S.B.M. Hagstrom, S.A. Flodrstom, Phys. Rev. B 19 Ž1979. 2837. w14x G.V. Samsonov, The oxide handbook, IFIrPlenum, 1982, pp. 20᎐31.
Effect of oxidized Al prelayer for the growth of polycrystalline Al 2 O 3 films on Si using ionized beam deposition S.W. Whangbo, Y.K. Choi, H.K. Jang, Y.D. Chung, I.W. Lyo, C.N. WhangU Atomic-scale Surface Science Research Center and Institute of Physics and Applied Physics, Yonsei Uni¨ ersity, Seoul, 120-749, South Korea Received 23 June 2000; received in revised form 30 November 2000; accepted 18 January 2001
Abstract Polycrystalline Al 2 O 3 thin films have been grown on Si substrates by ionized beam deposition using an aluminum solid source in O 2 environments. To prevent the aluminum interdiffusion to Si at high substrate temperature ŽTs ) 500⬚C., the oxidized Al prelayer with a thickness of 3 nm was deposited on the Si substrate at room temperature before Al 2 O 3 deposition, and subsequently polycrystalline Al 2 O 3 films were grown at 700⬚C. Although Al interdiffusion increased at the interface with increasing Ts , the oxidized Al prelayer was effective as a buffer layer on which to grow stoichiometric and crystalline Al 2 O 3 films up to Ts s 800⬚C. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Aluminum oxide; Ionized beam deposition; Interfaces; X-Ray photoelectron spectroscopy ŽXPS.
1. Introduction Aluminum oxide ŽAl 2 O 3 . thin films are very useful for many applications such as microelectronic devices, wear resistive coatings, corrosion protective coating material, and catalysts etc. w1᎐3x. Its wide usefulness is primarily dependent on its hardness, high melting point and low electrical conductivity maintained at high temperature. Al 2 O 3 films have been commonly produced by chemical vapor deposition ŽCVD. using various kinds of aluminum sources such as trimethylaluminum wTMA, AlŽCH 3 . 3 x and aluminum borohydride wAlŽBH 4 . 3 x w4x. However, the films grown by the CVD process have some disadvantages: carbon contamination cannot be avoided due to the organic byproducts of its dissociation, and an excessively high substrate temperature ŽTs ) 800⬚C. or post annealing process’ are needed to U
Corresponding author. Tel.: q82-2-361-3841r2; fax: q82-23127090. E-mail address: [email protected] ŽC.N. Whang..
obtain the crystalline aluminum oxide w5,6x. Therefore, it would be highly desirable to have the alumina thin film growth at a relatively low substrate temperature with no carbon contamination. In physical vapor deposition ŽPVD., ion beam assisted deposition and magnetron sputtering techniques, using an Al target, have been applied to fabricate aluminum oxide films. However, the formation of a highly insulating coating for the Al target led to a charge buildup and arcing w7x. Pulsed sputtering and rapid responding automatically controlled oxygen partial pressure techniques were employed to avoid the complete oxidation of a target. On the other hand, the reports on aluminum oxide film growth with an Al solid source are seldom found, because molten aluminum is extremely reactive to most metals, and thus a practical crucible for an aluminum evaporation with long emission times and sufficient stability has not been realized. In this paper, we report the successful growth of stoichiometric polycrystalline Al 2 O 3 films on Si substrates by ionized beam deposition ŽIBD. using an aluminum solid source in O 2 environments. It is well
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 0 8 1 2 - 4
S.W. Whangbo et al. r Thin Solid Films 388 (2001) 290᎐294
known that the direct aluminum᎐silicon contact exhibits various poor contact characteristics, such as junction spiking at high temperature, therefore a diffusion barrier is required to deposit the film at high substrate temperatures w8x. In the present work, the oxidized Al prelayer was used for the prevention of Al interdiffusion at high substrate temperatures and the oxidation of Si substrate in the initial growth stage of A 2 O 3 on Si. The interdiffuison phenomena of Al at the interface region was investigated at the substrate temperatures of 500᎐800⬚C. 2. Experimental Al 2 O 3 films were prepared on p-SiŽ111. substrate by the ionized beam deposition system. The IBD growth chamber is equipped with in situ reflection high-energy electron diffraction ŽRHEED. and cold-cathode type oxygen ion gun. The growth chamber was evacuated to 3 = 10y1 0 torr and the oxygen partial pressure during Al 2 O 3 film growth was 1.5= 10y5 torr. To generate ionized Al beam, the TiB 2 based ceramic crucible was filled with aluminum solid source Ž99.999% purity.. The crucible is composed of TiB 2 Ž; 50%., BN and AlN composites, which is a relatively resistant material to molten Al w9x. The temperature of the crucible was kept at approximately 1460⬚C during evaporation, as measured by an optical pyrometer. The evaporated aluminum vapor was ionized by electron bombardment at the ionization region located at above the crucible, then the ionized Al beam was accelerated by the electric field. Oxygen gas with a purity of 99.995% was used as an oxygen ion source for the oxidation process of Al prelayer and for a reactive oxygen atmosphere during the Al 2 O 3 deposition. The p-type SiŽ111. wafers with a resistivity of 1᎐3 ⍀ cm were used as substrates. They were cleaned and dipped in a 5% dilute HF acid solution to remove a native oxide and then, rinsed in deionized water. The substrate was heated up to 1000⬚C to obtain clean 7 = 7 reconstructed surface, checked by RHEED, prior to deposition. In this experiment, the film growth was performed through the three stages. First, the epitaxial Al prelayer with 3-nm thickness was deposited at room temperature. Secondly, the Al prelayer was oxidized with oxygen ion irradiation, by a cold-cathode type oxygen ion gun. Finally, the Al 2 O 3 films were grown at a substrate temperature range of 500᎐800⬚C. The detailed experimental conditions are summarized in Table 1. The thickness of the grown films was measured by a quartz crystal oscillator and calibrated by Rutherford backscattering spectroscopy. The crystallinity of the Al 2 O 3 films were measured by RHEED. X-Ray photoelectron spectroscopy ŽXPS. measurements were carried out to investigate the atomic composition and
291
Table 1 Experimental conditions of Al 2 O 3 film growth Al prelayer deposition Substrate Substrate temperature Ionization current Ž II . Acceleration voltage Ž Va . Deposition rate, Film thickness
clean 7 = 7᎐Si Ž111. Room temperature 80 mA 500 V 0.6 nmrmin, 3 nm
Oxidation of Al prelayer Substrate temperature Oxygen current density Oxygen ion energy Irradiation time
Room temperature 2 Arcm2 1 kV 20 min
Al2 O3 film deposition Substrate temperature Ionization current Ž Ii . Acceleration voltage Ž Va . O2 partial pressure
500᎐800⬚C 200 mA 500᎐1000 V 1.5= 10y5 torr
depth distribution of the films. The XPS data were obtained with a PHI 5700 ESCA spectrometer using an Al K ␣ Ž h s 1486.6 eV., with an energy resolution of 0.89 eV. The XPS spectra were aligned with respect to the binding energy of the surface carbon peak Ž284.5 eV. obtained at an initial survey scan. The atomic composition of the films was estimated from the XPS peak areas using relative sensitivity factors obtained from single crystalline Al 2 O 3 as a reference. 3. Results and discussion Fig. 1a᎐c shows the RHEED patterns of the SiŽ111. ᎐ Ž7 = 7. surface, as-deposited Al prelayer, and polycrystalline Al 2 O 3 film deposited on the oxidized Al prelayer, respectively. Fig. 1a shows a typical Ž7 = 7. reconstructed SiŽ111. surface obtained before deposition. Upon depositing the Al prelayer, as shown in Fig. 1b, the RHEED pattern develops spots and streaks depending on the incident angle of electron beam, indicating that the Al prelayer is grown epitaxially on the SiŽ111. surface. Crystalline orientation of the Al prelayer on the SiŽ111. substrate is determined as AlŽ111.rrSiŽ111., Alw110xrrSiw110x. The epitaxial Al prelayer has an effect of reducing the void density and the grain boundaries. As described in Table 1 the epitaxial Al prelayer was obtained under the deposition conditions of very low deposition rates and acceleration voltages of 500 V. However, in the case of the Al prelayer formed with a high deposition rate and no acceleration voltage, it showed a polycrystalline phase. Furthermore, the Al 2 O 3 films grown on polycrystalline Al prelayer showed an amorphous phase and a small Si 2p peak was observed at surface in XPS measurements. It is likely that the grain boundaries of the polycrystalline Al prelayer provided fast diffusion paths for the
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Fig. 1. The RHEED patterns of the Ž7 = 7. ᎐Si Ž111. substrate Ža., epitaxially grown Al layer Žb., and Al 2 O 3 film grown at 700⬚C.
Si at the substrate temperatures above 400⬚C w8x. Thus, to obtain stoichiometric and crystalline Al 2 O 3 films on Si substrates, a uniform and flat epitaxial Al prelayer without voids and grain boundaries is required. The Al 2 O 3 films grown on an epitaxial Al prelayer show ring patterns as shown in Fig. 1c, this means that the grown film is a polycrystal. Fig. 2Ia᎐c shows the XPS survey spectra obtained from the oxidized Al prelayer, Al 2 O 3 film, and single crystalline Al 2 O 3 Žsapphire. as a reference, respectively. A 3-keV Arq sputtering was carried out for 10 s to eliminate the surface contamination which might have been accumulated during the transfer through air to the XPS chamber. Fig. 2Ia was obtained from the oxidized Al prelayer before Al 2 O 3 deposition, where Si signals as well as Al and O signals were observed, because the thickness of the layer was approximately 3 nm and the expanded view is presented at Fig. 2II to see the accurate peak positions over the Al and Si peaks. However, the Al 2 O 3 film is composed only of Al and O as shown in Fig. 2Ib, and the ratio of signal intensities between the Al 2p and O 1s spectra is similar to that of sapphire ŽO 1srAl 2ps 4.0. shown in Fig. 2Ic. Usually, in the case of the films deposited by CVD and metalorganic molecular beam epitaxy ŽMOMBE., carbon contamination cannot be avoided, because the C concentration at the interface of Al 2 O 3rSi increases with increasing Ts , due to the further dissociation of the source gases w5,10x. However, as shown in these spectra, the C signal is not observed within the sensitivity of XPS measurements. Therefore, these results clearly indicate that the Al 2 O 3 film as well as the oxidized Al prelayer is grown without carbon contamination. Fig. 3a,b shows the XPS spectra of Al 2p and Si 2p peak obtained after the oxidation of epitaxially grown Al prelayer. The binding energy of the Al 2p peak from a metallic Al is 72.9 eV w11x and that of the Al 2p peak from an Al in Al 2 O 3 film is 2.5᎐3.0 eV higher than 72.9 eV w12,13x. As shown in Fig. 3a, the Al prelayer was totally oxidized by oxygen ion irradiation, and the binding energy of Al 2p Ž75.7 eV. corresponds to that of Al 3q in Al 2 O 3 . The Si 2p spectrum in Fig. 3b was obtained from the Si substrate and the binding energy
of 98.9 eV indicates the bonding between the Si atoms, and the Si᎐O bonding Ž102᎐104 eV. was not detected. Therefore, the oxidized Al prelayer was well formed with Al 2 O 3rSi interface without the formation of SiO 2 at the Al 2 O 3rSi interface. Fig. 4a᎐c displays the XPS_ depth profiles of the Al 2 O 3 films grown at different substrate temperatures of 600⬚C, 700⬚C and 800⬚C, respectively. We will use the term interface region to describe the region where
Fig. 2. ŽI. XPS survey spectra from Ža. the oxidized Al layer, Žb. the Al 2 O 3 film, and Žc. bulk-sapphire as a reference. The small Ar 2p peaks are due to the Arq ion sputtering for 10 s to eliminate the surface contamination. ŽII. Expanded view of the I-Ža..
S.W. Whangbo et al. r Thin Solid Films 388 (2001) 290᎐294
293
Fig. 3. The XPS spectra of Ža. Al 2p and Žb. Si 2p from thin oxidized Al layer.
the concentration of Si 2p changes from an appearance point to maximum values in our depth profile. The ratios of oxygen concentration Ž Co . to aluminum conFig. 5. The XPS depth spectra obtained from the Al 2 O 3 film grown at Ts s 600⬚C: Ža. Al 2p; Žb. Si 2p. The spectra show the origin of Al rich state at the interface region.
Fig. 4. The XPS depth profile spectra of the Al 2 O 3 films as a function of a substrate temperature: Ža. Ts s 600⬚C, Žb. 700⬚C, and Žc. 800⬚C.
centration Ž CAl . were evaluated using the sensitivity factors obtained from a single crystal Al 2 O 3 . The ratios of oxygen to aluminum of the Al 2 O 3 films Ž CorCAl . in Fig. 4a,b,c are 1.52, 1.49 and 1.53, respectively. Therefore, it suggests that the stoichiometric Al 2 O 3 films were obtained. As the substrate temperature increases, the interface regions are widened and simultaneously the decrease rates of CAl and Co become slower. In addition, the decrease rate of CAl is lower than that of Co at near substrate region for all samples. Thus the non-stoichiometric alumina are formed at the interface. To have a closer look the interface state, the XPS spectra of the Al 2 O 3 film grown at 600⬚C are depicted in Fig. 5. Fig. 5a,b shows the Al 2p and Si 2p spectra of near substrate region, respectively. The sputtering time corresponds to the time scale of Fig. 4a, and each sputtering was carried out with the time interval of 0.5 min. As shown in Fig. 5a, the aluminum atoms in the Al 2 O 3 film were totally oxidized, which is indicated by a solid line and the binding energy of Al 3q is 75.7 eV. The Al 3q peaks continuously decrease as the exposed surface approaches the interface, and the small metallic aluminum ŽAl 0 . peak appears near the interface, which has a binding energy of 72.9 eV. This result was reproducible for the films grown at a substrate temperature ) 500⬚C. There are two possibilities on this excess metallic aluminum, one being residual metallic Al in the process of Al prelayer oxidation, and the other is the interdiffusion of Al during deposition at high substrate temperatures. The first possibility can be ignored from the result of the abrupt interface forma-
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4. Conclusions
Fig. 6. Dependence of the Al 2 O 3 growth rate on the substrate temperature.
tion as shown in Fig. 3. To test the second possibility, we grew the Al 2 O 3 films on 2-nm thick SiO 2 on Si substrates at the same temperature range without an oxidized Al prelayer, and investigated the interface by the same technique. We found similar results: the Al 0 peak also appears and as the Ts increases, the Al 0 peak also increases. Thus we conclude that the origin of metallic aluminum at the interface is the interdiffusion of Al during the deposition at high substrate temperatures, leading to the aluminum rich state at the interface as shown in Fig. 4. In addition, although the deposition process was executed in oxygen environments at high substrate temperatures, the Si᎐O bond was not detected within the XPS_ resolution limit at the interface as shown in Fig. 5b. This might be partly due to the difference of the heat of formation between AlŽy1677 kJrkmol. and Si Žy912 kJrkmol. with oxygen w14x. Fig. 6 shows the dependence of the growth rate of the Al 2 O 3 films on the substrate temperatures. The polycrystalline Al 2 O 3 films are grown at temperatures ) 700⬚C. The growth rate slightly decreases with increasing substrate temperatures up to 700⬚C, and in the temperature range of Ts ) 700⬚C, the growth rate rapidly drops. Here the desorption of a volatile gas phase AlO is believed to be responsible for the abrupt fall in the growth rate.
The polycrystalline Al 2 O 3 films have been grown on an Si substrate by ionized beam deposition using Al solid source in O 2 environments. The oxidized Al prelayer was used for the prevention of Al interdiffusion at high substrate temperatures. Although the Al interdiffusion increased and the CAl exceeded the CO near the interface with increasing Ts , the oxidized Al prelayer was effective in maintaining the growth of the polycrystalline Al 2 O 3 film up to 800⬚C. The stoichiometry of the Al 2 O 3 films were found to be nearly identical to that of sapphire, and the initially nearly constant growth rate fell sharply beyond Ts s 700⬚C possibly due to the desorption of AlO phase. The carbon contamination and the formation of the Si᎐O bonds at the interface were not found in this growth method. References w1x Z. Jin, H.S. Kwok, M. Wong, IEEE Electron Devices Lett. 11 Ž1998. 502. w2x I.Y. Konyashin, Surf. Coat. Technol 85 Ž1996. 131. w3x J.-E. Sundgren, H.T.G. Hentzell, J. Vac. Sci. Technol. A 4 Ž1986. 2259. w4x J.A. Glassjr, S.S. Kher, J.T. Spencer, Chem. Mater. 4 Ž1992. 530. w5x M. Ishida, I. Katakabe, T. Nakamura, N. Ohtake, Appl. Phys. Lett. 52 Ž1988. 1326. w6x W. Koh, S.J. Ku, Y. Kim, Thin Solid Films 304 Ž1997. 222. w7x O. Zywitzki, G. Hoetzsch, Surf. Coat. Technol. 86᎐87 Ž1996. 640. w8x S. Wolf, Silicon processing for the VLSI Era, 2, Lattice Press, 1986, pp. 111᎐134. w9x Y. Torii, H. Yamada, Proceedings of the International Ion Engineering Congress, Ž1983. 363. w10x H. Iizuka, K. Yokoo, S. Ono, Appl. Phys. Lett. 61 Ž1992. 2978. w11x J.F. Moulder, W.F. Stickle, P.E. Sobal, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, Perkin᎐Elmer, Eden Prairie, Minesota, 1992. w12x G. Faraci, S. La Rosa, A.R. Pennisi, Y. Hwu, G. Margaritondo, Phys. Rev. B 47 Ž1993. 4052. w13x R.Z. Bachrach, S.B.M. Hagstrom, S.A. Flodrstom, Phys. Rev. B 19 Ž1979. 2837. w14x G.V. Samsonov, The oxide handbook, IFIrPlenum, 1982, pp. 20᎐31.