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Thermal stability of amorphous Ti3Si1O8 thin films
Microelectronic Engineering 55 (2001) 183–188 www.elsevier.nl / locate / mee
Thermal stability of amorphous Ti 3 Si 1 O 8 thin films a, b a P.H. Giauque *, H.B. Cherry , M.-A. Nicolet a
California Institute of Technology, Pasadena, CA 91125, USA b Jet Propulsion Laboratory, Pasadena, CA 91109, USA
Abstract Films (220 nm-thick) deposited by reactive rf sputtering from a Ti 3 Si target with an argon / oxygen gas mixture were annealed for 30 min in vacuum at temperatures between 400 and 9008C. The films were characterized by 2 MeV He 21 backscattering spectrometry and X-ray diffraction to monitor thermally induced changes. As-deposited, the films are X-ray-amorphous. First signs of crystallization appear at 6008C. Their composition remains constant and uniform throughout that temperature range, except for the loss of argon that is initially present in the film at a concentration of about 1 at.% and that fully escapes within 5 min at 6508C. Films without silicon obtained from a pure titanium target by reactive rf sputtering with oxygen and of a composition of Ti 1 O 2 are also X-ray-amorphous but crystallize much more readily. The significance of these results is discussed relative to other ternary films of analogous compositions that also tend to form highly stable amorphous or near-amorphous phases (‘mictamict’ alloys). 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Amorphous; Sputtering; Optical coatings; Ternary oxides
1. Introduction Various contributions have appeared in the literature beginning in 1988 that report on ternary nitride films with a strong propensity for an amorphous or near-amorphous microstructure that can be highly metastable, as demonstrated first by Asai et al. for W–Si–N films [1], Kolawa et al. for Ta–Si–N films [2], Reid et al. for Ti–Si–N [3] and Mo–Si–N [4]. Common to all these films is an atomic composition that can be viewed as a combination of a nitride of an early transition metal (metals of groups IVB, VB, and VIB, symbolized here by ‘TM’) and of a nitride of silicon, resulting in a generic composition TM x Si y N 100 2 x 2 y (subsequently abbreviated as TM–Si–N). Characteristic for these ternary systems is that in their fully nitrided state, the early transition metals and the silicon form a pair of quasi-binary compounds TM 1 N 1 and Si 3 N 4 , which have low mutual solubilities (Fig. 1). Ternary TM–Si–N films whose composition falls on, or near, the tie line between TM 1 N 1 and Si 3 N 4 are of particular interest. It is along that composition range that the metastability is highest, as a detailed study of Ti–Si–N has established [5]. The mononitrides of the early transition metals are *Corresponding author. Tel.: 11-626-395-6555; fax: 11-626-395-7564. E-mail address: [email protected] (P.H. Giauque). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 00 )00446-9
2001 Elsevier Science B.V. All rights reserved.
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Fig. 1. Sections of the ternary phase diagrams of the TM–Si–N systems (TM, transition metals of the groups IVB, VB, and VIB) (top), of the Ru–Si–O system (middle), and of the Ti–Si–O system (bottom), applicable for temperatures ranging from 700 to 10008C, depending on the system.
metallic interstitial compounds with simple cubic or hexagonal crystal structure. Si 3 N 4 has a complex unit cell and a predilection for an amorphous structure. The pair of constituting binary compounds thus differs in structure as well as in bonding character. RuO 2 is a metallic binary compound with a simple crystal structure, as are the TM 1 N 1 mononitrides. SiO 2 is a non-metallic covalently bonded material that has multiple polymorphs and tends to form a highly metastable amorphous phase, as Si 3 N 4 does. The two compounds RuO 2 and SiO 2 are mutually immiscible (Fig. 1), just as the TM 1 N 1 mononitrides and Si 3 N 4 are. A very recent paper has now shown that ternary Ru–Si–O films have properties that are analogous to those of TM–Si–N films [6]. The high metastability of amorphous or near-amorphous ternary films is thus a property shared by other ternary systems than the group of TM–Si–N compounds alone. TiO 2 and SiO 2 have little mutual solubility as well (Fig. 1) [7–9]. We report here on some properties of reactively sputtered Ti 3 Si 1 O 8 films as another representative of TM–Si–O compounds. 2. Experimental procedures The synthesis of an amorphous or near-amorphous film from immiscible components requires a deposition technique that suppresses the nucleation and growth of the constituent phases. In the
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experimental studies cited so far reactive sputtering has been the preferred technique to accomplish this aim. The same method has been applied in the present investigation. The substrates were 131 cm 2 carbon and silicon (100), the latter both bare and covered with a 170-nm-thick amorphous SiO 2 layer. Films were deposited by reactive rf sputtering from a Ti 3 Si target 3 inches in diameter, without bias applied to the substrate, in a dry-pumped vacuum system of (3–5)310 27 Torr base pressure. A power of 300 W was applied during the deposition. The argon flow was 56 sccm and the oxygen content of the film was controlled by adjusting the flow ratio of argon and oxygen while maintaining the total pressure constant at 10 mT. The thermal stability of the 220 nm-thick films on silicon and oxidized silicon substrates was investigated by annealing individual samples for 30 min each up to 9008C in an oil pumped tube furnace at a vacuum of 7310 27 Torr; 2.0 MeV 4 He 21 backscattering spectrometry and X-ray diffraction were used to measure, respectively, the atomic depth profiles and compositions of the films, and to characterize the phases and grain sizes present in the films. The bulk resistivity was extracted from four-point-probe measurements of the sheet resistance of the films and their thickness measured by Dektak, a stylus-type device.
3. Results and discussion As-deposited films with a composition Ti 25 Si 8 O 67 were found to be X-ray amorphous. These films were produced with an Ar / O 2 flow ratio of 13.5. They have about 1% of argon homogeneously distributed in the film. Fig. 2 shows backscattering spectra made from a kSil / Ti–Si–O sample before and after annealing at 8008C. The position of the signals from elements at the surface are indicated by arrows. After annealing, the atomic depth distributions in the film remain unchanged, except for the argon that has escaped from the layer. This loss of material reduces the energy width of the titanium and silicon signals slightly, as can be seen at the back edges of these two signals near 0.59 and 0.98 MeV, and is accompanied by a corresponding small increase in the position of the high-energy edge of
Fig. 2. 2 MeV He 21 backscattering spectra of a Ti 3 Si 1 O 8 film about 220 nm thick on Si(100) substrates, as-deposited and after annealing in vacuum for 30 min at 8008C. The observed changes are due to the loss of the argon impurity of about 1 at.% initially contained in the film (angle of beam incidence: 78 from sample normal; scattering angle of detected particles: 1658).
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the silicon signal from the substrate interface near 0.48 MeV. The composition of a film was derived from the number of backscattered particles in each elemental signal of a spectrum of that film on a carbon substrate. The ratio 25 / 8 / 67 obtained that way is within experimental error equal to Ti 3 Si 1 O 8 and corresponds to a mixture of 3 TiO 2 and 1 SiO 2 . The film is fully oxidized. The titanium to silicon ratio in film and in target are equal. There is no preferential sputtering due to the relatively small difference in the masses of titanium and silicon. Fig. 3 shows X-ray spectra of kSil / SiO 2 / Ti–Si–O samples as deposited and annealed for 30 min at temperatures from 500 to 9008C. The films remain X-ray amorphous for annealing temperature up to 5008C. At 6008C a peak appears at 29.3 degrees, corresponding to a diffraction by a nanocrystalline phase of anatase (TiO 2 ). The grain size, as derived by the Scherrer’s formula, is |6.5 nm. The same peak, with the same width, is also present in the spectrum of the sample annealed at 7008C. At 8008C the grain size is 7.6 nm and grows to 8.3 nm at 9008C. At this temperature two additional peaks appear at 32 and 64.5 degrees. They correspond to rutile (TiO 2 ) with a grain size of 5 nm. These diffraction peaks are still weak, which suggests that the material remains mainly amorphous, with a beginning of crystallization. The density of the films, determined from the areal atomic composition measured with backscattering spectrometry and the metric thickness of the film measured with a stylus-type device, is 8.5310 22 at. / cm 3 , compared to 6.9310 22 at. / cm 3 for the film produced with the Ti 3 Si target without oxygen.
Fig. 3. Co K a X-ray diffraction spectra of Ti 3 Si 1 O 8 films about 220 nm thick on oxidized silicon substrates, as-deposited and after annealing in vacuum for 30 min at 500, 600, 700, 800, and 9008C. The spectra are vertically displaced from each other for clarity.
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Four point probe measurements show that all films discussed here are insulating. Their appearance under an optical microscope remains unchanged even after annealing at 9008C. For comparison, films of TiO 2 of 80 nm thickness were reactively deposited from a pure titanium target. The films were X-ray amorphous as deposited and remained so up to 2008C upon annealing in argon. At 3008C, diffraction lines of rutile appeared. The addition of 25% SiO 2 to TiO 2 thus raises the stability of the amorphous phase by 3008C, which is consistent with the formation of a mictamict alloy. Pure TiO 2 films 300-nm-thick yielded diffraction spectra that contained lines of rutile and anatase even before annealing. The crystalline phase was presumably caused by the heating of the film during the extended process of deposition. Applications as hard coatings and particularly as optical coatings have motivated a number of earlier investigations of Ti–Si–O films [10–13]. Chao et al. [10] report results obtained on Ti–Si–O films formed through rapidly alternating depositions of titanium and silicon by reactive sputtering under conditions fairly similar to those applied here. They find that as-deposited films are X-rayamorphous for all silicon concentrations investigated (28 to 75%). Subsequent studies showed that this result applies also for films produced by reactive co-evaporation of titanium and silicon [11] and by reactive ion-beam-sputtering of a titanium / silicon target [13]. The latter investigation also establishes that as the silicon content rises from zero to 17% the crystallization temperature for 24 h of annealing in air increases monotonically from 200 [12] to 5008C. 4. Conclusions Amorphous films combining TiO 2 and SiO 2 remain amorphous up to significantly higher annealing temperatures than do pure amorphous TiO 2 films. This result is similar to that obtained for films that combine RuO 2 and SiO 2 and supports the idea that numerous mictamict alloys must exist for TM–Si–O as well as for TM–Si–N systems. Acknowledgements The technical assistance of Rob Gorris is thankfully acknowledged. References [1] K. Asai, H. Sugahara, Y. Matsuoka, M. Tokumitsu, Reactively sputtered WSiN film suppresses As and Ga outdiffusion, J. Vac. Sci. Technol. B6 (1988) 1526–1529. [2] E. Kolawa, J.M. Molarius, C.W. Nieh, M.-A. Nicolet, Amorphous Ta–Si–N thin film alloys as diffusion barrier in Al / Si metallizations, J. Vac. Sci. Technol. A8 (3) (1990) 3006–3010. [3] J.S. Reid, X. Sun, E. Kolawa, M.-A. Nicolet, Ti–Si–N diffusion barriers between silicon and copper, IEEE Electron Device Lett. 15 (8) (1994) 298–300. [4] J.S. Reid, E. Kolawa, R.P. Ruiz, M.-A. Nicolet, Evaluation of amorphous (Mo,Ta,W)–Si–N diffusion barriers for Si / Cu metallizations, Thin Solid Films 236 (1993) 319–324. [5] X. Sun, J.S. Reid, E. Kolawa, M.-A. Nicolet, Reactively sputtered Ti–Si–N films. I. Physical properties, J. Appl. Phys. 81 (2) (1997) 656–663. [6] S. Gasser, E. Kolawa, M.-A. Nicolet, Reactively sputtered Ru–Si–O films, J. Appl. Phys. 86 (4) (1999) 1974–1981.
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[7] I.P. Nikitina, Inorg. Mater. (USSR) (Engl. Transl.) 11 (11) (1975) 1798–1799. [8] R. Beyers, Thermodynamic considerations in refractory metal–silicon–oxygen systems, J. Appl. Phys. 56 (1) (1984) 147–152. [9] J.I. Goldstein, S.K. Choi, F.J.J. Van Loo, G.F. Bastin, R. Metselaar, Solid-state reactions and phase relations in the Ti–Si–O system at 1373 K, J. Am. Ceram. Soc. 78 (2) (1995) 313–322. [10] S. Chao, C.-K. Chang, J.-S. Chen, TiO 2 –SiO 2 mixed films prepared by the fast alternating sputter method, Appl. Opt. 30 (22) (1991) 3233–3237. [11] J.-S. Chen, S. Chao, J.-S. Kao, H. Niu, C.-H. Chen, Mixed films of TiO 2 –SiO 2 deposited by double electron-beam coevaporation, Appl. Opt. 35 (1) (1996) 90–96. [12] W.-H. Wang, S. Chao, Annealing effect on ion-beam-sputtered titanium dioxide film, Optics Lett. 23 (18) (1998) 1417–1419. [13] S. Chao, W.-H. Wang, M.-Y. Hsu, L.-C. Wang, Characteristics of ion-beam-sputtered high-refractive-index TiO 2 –SiO 2 mixed films, J. Opt. Soc. Am. A16 (6) (1999) 1477–1483.
Thermal stability of amorphous Ti 3 Si 1 O 8 thin films a, b a P.H. Giauque *, H.B. Cherry , M.-A. Nicolet a
California Institute of Technology, Pasadena, CA 91125, USA b Jet Propulsion Laboratory, Pasadena, CA 91109, USA
Abstract Films (220 nm-thick) deposited by reactive rf sputtering from a Ti 3 Si target with an argon / oxygen gas mixture were annealed for 30 min in vacuum at temperatures between 400 and 9008C. The films were characterized by 2 MeV He 21 backscattering spectrometry and X-ray diffraction to monitor thermally induced changes. As-deposited, the films are X-ray-amorphous. First signs of crystallization appear at 6008C. Their composition remains constant and uniform throughout that temperature range, except for the loss of argon that is initially present in the film at a concentration of about 1 at.% and that fully escapes within 5 min at 6508C. Films without silicon obtained from a pure titanium target by reactive rf sputtering with oxygen and of a composition of Ti 1 O 2 are also X-ray-amorphous but crystallize much more readily. The significance of these results is discussed relative to other ternary films of analogous compositions that also tend to form highly stable amorphous or near-amorphous phases (‘mictamict’ alloys). 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Amorphous; Sputtering; Optical coatings; Ternary oxides
1. Introduction Various contributions have appeared in the literature beginning in 1988 that report on ternary nitride films with a strong propensity for an amorphous or near-amorphous microstructure that can be highly metastable, as demonstrated first by Asai et al. for W–Si–N films [1], Kolawa et al. for Ta–Si–N films [2], Reid et al. for Ti–Si–N [3] and Mo–Si–N [4]. Common to all these films is an atomic composition that can be viewed as a combination of a nitride of an early transition metal (metals of groups IVB, VB, and VIB, symbolized here by ‘TM’) and of a nitride of silicon, resulting in a generic composition TM x Si y N 100 2 x 2 y (subsequently abbreviated as TM–Si–N). Characteristic for these ternary systems is that in their fully nitrided state, the early transition metals and the silicon form a pair of quasi-binary compounds TM 1 N 1 and Si 3 N 4 , which have low mutual solubilities (Fig. 1). Ternary TM–Si–N films whose composition falls on, or near, the tie line between TM 1 N 1 and Si 3 N 4 are of particular interest. It is along that composition range that the metastability is highest, as a detailed study of Ti–Si–N has established [5]. The mononitrides of the early transition metals are *Corresponding author. Tel.: 11-626-395-6555; fax: 11-626-395-7564. E-mail address: [email protected] (P.H. Giauque). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 00 )00446-9
2001 Elsevier Science B.V. All rights reserved.
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Fig. 1. Sections of the ternary phase diagrams of the TM–Si–N systems (TM, transition metals of the groups IVB, VB, and VIB) (top), of the Ru–Si–O system (middle), and of the Ti–Si–O system (bottom), applicable for temperatures ranging from 700 to 10008C, depending on the system.
metallic interstitial compounds with simple cubic or hexagonal crystal structure. Si 3 N 4 has a complex unit cell and a predilection for an amorphous structure. The pair of constituting binary compounds thus differs in structure as well as in bonding character. RuO 2 is a metallic binary compound with a simple crystal structure, as are the TM 1 N 1 mononitrides. SiO 2 is a non-metallic covalently bonded material that has multiple polymorphs and tends to form a highly metastable amorphous phase, as Si 3 N 4 does. The two compounds RuO 2 and SiO 2 are mutually immiscible (Fig. 1), just as the TM 1 N 1 mononitrides and Si 3 N 4 are. A very recent paper has now shown that ternary Ru–Si–O films have properties that are analogous to those of TM–Si–N films [6]. The high metastability of amorphous or near-amorphous ternary films is thus a property shared by other ternary systems than the group of TM–Si–N compounds alone. TiO 2 and SiO 2 have little mutual solubility as well (Fig. 1) [7–9]. We report here on some properties of reactively sputtered Ti 3 Si 1 O 8 films as another representative of TM–Si–O compounds. 2. Experimental procedures The synthesis of an amorphous or near-amorphous film from immiscible components requires a deposition technique that suppresses the nucleation and growth of the constituent phases. In the
P.H. Giauque et al. / Microelectronic Engineering 55 (2001) 183 – 188
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experimental studies cited so far reactive sputtering has been the preferred technique to accomplish this aim. The same method has been applied in the present investigation. The substrates were 131 cm 2 carbon and silicon (100), the latter both bare and covered with a 170-nm-thick amorphous SiO 2 layer. Films were deposited by reactive rf sputtering from a Ti 3 Si target 3 inches in diameter, without bias applied to the substrate, in a dry-pumped vacuum system of (3–5)310 27 Torr base pressure. A power of 300 W was applied during the deposition. The argon flow was 56 sccm and the oxygen content of the film was controlled by adjusting the flow ratio of argon and oxygen while maintaining the total pressure constant at 10 mT. The thermal stability of the 220 nm-thick films on silicon and oxidized silicon substrates was investigated by annealing individual samples for 30 min each up to 9008C in an oil pumped tube furnace at a vacuum of 7310 27 Torr; 2.0 MeV 4 He 21 backscattering spectrometry and X-ray diffraction were used to measure, respectively, the atomic depth profiles and compositions of the films, and to characterize the phases and grain sizes present in the films. The bulk resistivity was extracted from four-point-probe measurements of the sheet resistance of the films and their thickness measured by Dektak, a stylus-type device.
3. Results and discussion As-deposited films with a composition Ti 25 Si 8 O 67 were found to be X-ray amorphous. These films were produced with an Ar / O 2 flow ratio of 13.5. They have about 1% of argon homogeneously distributed in the film. Fig. 2 shows backscattering spectra made from a kSil / Ti–Si–O sample before and after annealing at 8008C. The position of the signals from elements at the surface are indicated by arrows. After annealing, the atomic depth distributions in the film remain unchanged, except for the argon that has escaped from the layer. This loss of material reduces the energy width of the titanium and silicon signals slightly, as can be seen at the back edges of these two signals near 0.59 and 0.98 MeV, and is accompanied by a corresponding small increase in the position of the high-energy edge of
Fig. 2. 2 MeV He 21 backscattering spectra of a Ti 3 Si 1 O 8 film about 220 nm thick on Si(100) substrates, as-deposited and after annealing in vacuum for 30 min at 8008C. The observed changes are due to the loss of the argon impurity of about 1 at.% initially contained in the film (angle of beam incidence: 78 from sample normal; scattering angle of detected particles: 1658).
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the silicon signal from the substrate interface near 0.48 MeV. The composition of a film was derived from the number of backscattered particles in each elemental signal of a spectrum of that film on a carbon substrate. The ratio 25 / 8 / 67 obtained that way is within experimental error equal to Ti 3 Si 1 O 8 and corresponds to a mixture of 3 TiO 2 and 1 SiO 2 . The film is fully oxidized. The titanium to silicon ratio in film and in target are equal. There is no preferential sputtering due to the relatively small difference in the masses of titanium and silicon. Fig. 3 shows X-ray spectra of kSil / SiO 2 / Ti–Si–O samples as deposited and annealed for 30 min at temperatures from 500 to 9008C. The films remain X-ray amorphous for annealing temperature up to 5008C. At 6008C a peak appears at 29.3 degrees, corresponding to a diffraction by a nanocrystalline phase of anatase (TiO 2 ). The grain size, as derived by the Scherrer’s formula, is |6.5 nm. The same peak, with the same width, is also present in the spectrum of the sample annealed at 7008C. At 8008C the grain size is 7.6 nm and grows to 8.3 nm at 9008C. At this temperature two additional peaks appear at 32 and 64.5 degrees. They correspond to rutile (TiO 2 ) with a grain size of 5 nm. These diffraction peaks are still weak, which suggests that the material remains mainly amorphous, with a beginning of crystallization. The density of the films, determined from the areal atomic composition measured with backscattering spectrometry and the metric thickness of the film measured with a stylus-type device, is 8.5310 22 at. / cm 3 , compared to 6.9310 22 at. / cm 3 for the film produced with the Ti 3 Si target without oxygen.
Fig. 3. Co K a X-ray diffraction spectra of Ti 3 Si 1 O 8 films about 220 nm thick on oxidized silicon substrates, as-deposited and after annealing in vacuum for 30 min at 500, 600, 700, 800, and 9008C. The spectra are vertically displaced from each other for clarity.
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Four point probe measurements show that all films discussed here are insulating. Their appearance under an optical microscope remains unchanged even after annealing at 9008C. For comparison, films of TiO 2 of 80 nm thickness were reactively deposited from a pure titanium target. The films were X-ray amorphous as deposited and remained so up to 2008C upon annealing in argon. At 3008C, diffraction lines of rutile appeared. The addition of 25% SiO 2 to TiO 2 thus raises the stability of the amorphous phase by 3008C, which is consistent with the formation of a mictamict alloy. Pure TiO 2 films 300-nm-thick yielded diffraction spectra that contained lines of rutile and anatase even before annealing. The crystalline phase was presumably caused by the heating of the film during the extended process of deposition. Applications as hard coatings and particularly as optical coatings have motivated a number of earlier investigations of Ti–Si–O films [10–13]. Chao et al. [10] report results obtained on Ti–Si–O films formed through rapidly alternating depositions of titanium and silicon by reactive sputtering under conditions fairly similar to those applied here. They find that as-deposited films are X-rayamorphous for all silicon concentrations investigated (28 to 75%). Subsequent studies showed that this result applies also for films produced by reactive co-evaporation of titanium and silicon [11] and by reactive ion-beam-sputtering of a titanium / silicon target [13]. The latter investigation also establishes that as the silicon content rises from zero to 17% the crystallization temperature for 24 h of annealing in air increases monotonically from 200 [12] to 5008C. 4. Conclusions Amorphous films combining TiO 2 and SiO 2 remain amorphous up to significantly higher annealing temperatures than do pure amorphous TiO 2 films. This result is similar to that obtained for films that combine RuO 2 and SiO 2 and supports the idea that numerous mictamict alloys must exist for TM–Si–O as well as for TM–Si–N systems. Acknowledgements The technical assistance of Rob Gorris is thankfully acknowledged. References [1] K. Asai, H. Sugahara, Y. Matsuoka, M. Tokumitsu, Reactively sputtered WSiN film suppresses As and Ga outdiffusion, J. Vac. Sci. Technol. B6 (1988) 1526–1529. [2] E. Kolawa, J.M. Molarius, C.W. Nieh, M.-A. Nicolet, Amorphous Ta–Si–N thin film alloys as diffusion barrier in Al / Si metallizations, J. Vac. Sci. Technol. A8 (3) (1990) 3006–3010. [3] J.S. Reid, X. Sun, E. Kolawa, M.-A. Nicolet, Ti–Si–N diffusion barriers between silicon and copper, IEEE Electron Device Lett. 15 (8) (1994) 298–300. [4] J.S. Reid, E. Kolawa, R.P. Ruiz, M.-A. Nicolet, Evaluation of amorphous (Mo,Ta,W)–Si–N diffusion barriers for Si / Cu metallizations, Thin Solid Films 236 (1993) 319–324. [5] X. Sun, J.S. Reid, E. Kolawa, M.-A. Nicolet, Reactively sputtered Ti–Si–N films. I. Physical properties, J. Appl. Phys. 81 (2) (1997) 656–663. [6] S. Gasser, E. Kolawa, M.-A. Nicolet, Reactively sputtered Ru–Si–O films, J. Appl. Phys. 86 (4) (1999) 1974–1981.
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[7] I.P. Nikitina, Inorg. Mater. (USSR) (Engl. Transl.) 11 (11) (1975) 1798–1799. [8] R. Beyers, Thermodynamic considerations in refractory metal–silicon–oxygen systems, J. Appl. Phys. 56 (1) (1984) 147–152. [9] J.I. Goldstein, S.K. Choi, F.J.J. Van Loo, G.F. Bastin, R. Metselaar, Solid-state reactions and phase relations in the Ti–Si–O system at 1373 K, J. Am. Ceram. Soc. 78 (2) (1995) 313–322. [10] S. Chao, C.-K. Chang, J.-S. Chen, TiO 2 –SiO 2 mixed films prepared by the fast alternating sputter method, Appl. Opt. 30 (22) (1991) 3233–3237. [11] J.-S. Chen, S. Chao, J.-S. Kao, H. Niu, C.-H. Chen, Mixed films of TiO 2 –SiO 2 deposited by double electron-beam coevaporation, Appl. Opt. 35 (1) (1996) 90–96. [12] W.-H. Wang, S. Chao, Annealing effect on ion-beam-sputtered titanium dioxide film, Optics Lett. 23 (18) (1998) 1417–1419. [13] S. Chao, W.-H. Wang, M.-Y. Hsu, L.-C. Wang, Characteristics of ion-beam-sputtered high-refractive-index TiO 2 –SiO 2 mixed films, J. Opt. Soc. Am. A16 (6) (1999) 1477–1483.