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The properties of aluminum oxide and nitride films prepared by d.c. sputter-deposition from metallic targets
Surface and Coatings Technology 163 – 164 (2003) 164–168
The properties of aluminum oxide and nitride films prepared by d.c. sputter-deposition from metallic targets ¨ Tilo P. Drusedau*, Thomas Neubert, Andreas N. Panckow ¨ Experimentelle Physik, Otto-von-Guericke-Universitat, ¨ PF 4120, D-39016 Magdeburg, Germany Institut fur
Abstract Thin films of aluminum nitride and aluminum oxide were prepared at 540 K substrate temperature by d.c. magnetron sputtering from a metallic Al-target in pure nitrogen and oxygen atmosphere, respectively. Characterization of the films was performed by X-ray diffraction, X-ray reflection and by optical transmission spectroscopy. The stoichiometric composition of the films was ascertained by X-ray photoelectron spectroscopy. As a function of reactive gas pressure, the deposition rate showed, in general, a logarithmic decrease with increasing pressure. Oxide films were deposited at about a fourth the rate of nitride films as consequence of the lower sputtering yield. For the oxide films, specific gravity and static refractive index showed weak changes with pressure and amounted to 2.7 gycm3 , which is only approximately 68% of the bulk value, and 1.65, respectively. In contrast, low-pressure sputter-deposition of nitride films resulted in high-density films of approximately 97% the bulk value and a refractive index close to 2.1. Nitride films show a strong (0 0 2) texture (c-axis orientation) with large nanograins up to 90-nm length. Oxide films are in general amorphous. The very different structure of nitride and oxide films is related to the more complicated crystal lattice of the oxide and to higher-atomic Al–O complexes ejected from the target. It is concluded that stronger thermalization of these species compared to those ejected by nitride sputtering is responsible for insufficient enhancement of the mobility of the adatoms. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminum nitride; Alumina; Reactive magnetron sputtering; Specific gravity; RMS-roughness
1. Introduction Reactive magnetron sputtering (MSP) of metal oxides and nitrides has become one of the mostly investigated topics in thin film technology over the past decade. The interest in aluminum nitride (AlN) and alumina (Al2O3) thin films has rapidly increased because of their specific properties, which make them suitable for various applications. These properties are high chemical stability, mechanical strength and hardness, optical transparency and electrical insulation w1,2x. In addition, AlN shows a high thermal conductivity, a low thermal expansion coefficient, the high velocity of acoustic waves and a large piezoelectric coupling constant w1x. Therefore, AlN *Corresponding author. Tel.: q49-391-67-12470; fax: q49-39167-11130. E-mail address: [email protected] ¨ (T.P. Drusedau).
films are used for surface passivation of thin films, optical sensors in the UV-spectral range, acousto-optic devices w3x, strain gages w4x and surface acoustic wave devices w5x. Al2O3 films are mainly used for wear and corrosion protection, diffusion and thermal barrier applications w2,6,7x. Reactive radio frequency MSP w3–6,8– 10x has been established as one technique for the low-cost, large-area deposition of these films. A promising alternative is the mid-frequency or d.c. MSP w6,7,11x. For most applications of AlN films, the growth of (0 0 2) textured crystallites is desired w3–5,8,9,12,13x. Sputtering an Al-target in pure reactive gas atmosphere results in stoichiometric films w4,5,13x. Changing the system pressure in AryN2 and AryO2 mixtures, respectively, affects the surface coverage of the Al-target by a reactive layer, and therefore, influences the stoichiometry of the films w3,11x. In a pure reactive gas atmosphere,
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one expects a constant, pressure-independent coverage of the Al-target by AlN and Al2O3, respectively. Therefore, changing the pressure for a pure reactive gas discharge influences only the thermalization of the sputtered species. The aim of the present work is a comparative investigation of the influence of reactive gas pressure— namely of N2 and O2—on the properties of d.c.-sputtered Al2O3 and AlN films, respectively. 2. Experimental The Al2O3 and AlN films were prepared by reactive MSP in pure reactive gas atmosphere of N2 and O2, respectively, (5-N purity) using a commercial LA 440S sputtering apparatus w10,14x. The magnetron source with a polycrystalline aluminum target of 125-mm diameter and 5-N purity was operated with d.c. power. The rotatable substrate holder with six substrate positions was isolated vs. ground and operated at floating potential. The substrate holder was completely covered by a shutter table with a 125-mm diameter opening permitting deposition of one of the six substrates. The target to substrate distance was dtss80 mm. The reader is referred to Table 1 for the main process parameters of Al2O3 and AlN film deposition. The film thickness was adjusted to approximately 140 and 500 nm for the oxide and nitride films, respectively. Polished silica substrates with an X-ray root mean square (rms) surface roughness of 0.7 nm are used for the optical and X-ray investigations. UV–NIR transmission spectra in the 200–2500-nm wavelength region were taken for optical characterization. The determination of the film thickness, the refractive index and the gap was performed according to the method described in the literature w15x. Wide angle X-ray scattering (WAXS) was performed by a D5000 X-ray goniometer in Bragg–Brentano geometry w16,17x. A Cu Ka source was used and the scattered intensity was measured by a Soller-collimator (divergence 0.48), secondary monochromator and scintillation counter equipment. The scattering spectra derived were corrected for the signal from the silica substrates. From the diffraction patterns the homogenous strain and the grain size were determined (Scherrer single-peak method, see literature w16,17x). XTable 1 Main process parameters of the Al2O3 and AlN films prepared: reactive gas pressure p, d.c.-power P and substrate temperature T Set of films
Variable parameter
Fixed parameter
Al2O3 Al2O3 AlN AlN
ps0.4–5.3 Pa Ps250–1500 W ps0.2–4.0 Pa Ps200–1500 W
Ps500 W, Ts540 K ps0.77 Pa, Ts540 K Ps500 W, Ts540 K ps1.0 Pa, Ts540 K
Fig. 1. Deposition rate (a) as a function of reactive gas partial pressure and (b) as a function of sputtering d.c.-power for AlN and oxide (see legend), respectively. The fit curves in (a) are according to Eq. (1) and due to empirical formulae given in the inset of (b), respectively.
ray reflectivity was measured in a four-slit geometry (resolution 0.048 symmetric set-up), secondary monochromator (URD6 goniometer, Co Ka radiation) and scintillation counting tube w16,17x. From the reflection spectra the film density and rms-surface roughness were determined w16,17x. The stoichiometry of the films was checked by X-ray photoelectron spectroscopy (XPS). 3. Results Fig. 1a shows the deposition rate as a function of sputtering gas pressure. The dependence of rate r on
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pressure p is well described by a logarithmic dependence according to rsr0yr1log(pyPa)
(1)
The fit parameters for r0 are 0.087 and 0.322 nmys for oxygen and nitrogen sputtering, respectively. The ratio of 3.7 between both values of r0 results from the different sputtering yields of nitrogen and oxygen projectiles for the nitride and the oxide, respectively. (Note that sputtering of an Al-target in reactive gas atmosphere results in the formation of a nitride or oxide surface layer of a thickness of approximately 2 nm w11x.) It is very interesting to note that the decay of rate with pressure, which is expressed in terms of r1 yr0 is much stronger for oxide deposition, where it amounts to 0.59 compared to 0.30. The deposition rate as a function of sputtering power shows a strict linear behavior for the case of Al2O3 as can be seen from Fig. 1b. For AlN, there is a sublinear (parabolic) dependence. This can be rationalized by the fact that because the target bias changes weakly with power, approximately 310 V w11x, increasing power decreases the sputtering yield of the projectiles. This could be, for example, due to a ratio of Nq to N2q ions decreasing with power (see Ref. w11x). The next striking contrast between AlN and Al2O3 deposition is evident from Fig. 2a. Low-pressure sputterdeposition of AlN results in high-density films of a maximum specific gravity of 3.16 gycm3, which equals nearly 97% of the value of the bulk crystal. Increasing pressure results in decreasing density due to thermalization and related reduced bombardment. Quite in contrast, Al2O3 films are highly porous with about only 65% of the bulk density independent of pressure. A similar low density was also obtained for Al2O3 films prepared under variation of power. The only exception was the film deposited at Ps1500 W with a specific gravity of 2.93 gycm3. The dependence of roughness on sputtering gas pressure is very similar for both types of films—see Fig. 2b. It seems to be a general effect observed for magnetron sputter-deposition that increasing pressure results in an increased roughness of the films (see, e.g. the results obtained for Mo and Al2O2 w2,17x). This could be due to increased thermalization of sputtered atoms and related lowered ad-atom mobility. The difference in the absolute roughness values of the AlN and Al2O3 films is related to the different film thickness of the films (see above). The stoichiometric composition of AlN films is not influenced by sputtering pressure. In contrast, there is a significant effect of pressure on the composition of the Al2O3 films as shown in Fig. 3a. It must be noted that the atomic composition of the as-deposited films at the surface suffers from a significant contamination especially with carbon. Therefore, sputter cleaning was performed. After sputter cleaning, low-pressure deposited films are stoichiometric within experimental uncer-
Fig. 2. Specific gravity (a) and rms-surface roughness (b) measured by X-ray reflectometry as a function of the reactive gas sputtering pressure—see legend—for the AlN and Al2O3 films, respectively.
tainty. In contrast, high-pressure films show the existence of excess oxygen. Films prepared under variation of power in the low-pressure regime (Fig. 3b) are in general stoichiometric within experimental uncertainty. (The film with maximum density has a composition of exactly xs0.66.) There are two rationalizations of the effect: (1) Conditions resulting in low energetic bombardment lead to the incorporation of excess oxygen. (2) The microstructure of films prepared under these conditions results in a preferred sputtering of Al atoms from the films during sputter cleaning. Then the changes in composition are due to the specific method of
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measurements reveal that Al2O3 films are in general amorphous, whereas the AlN films are nanocrystalline. From Fig. 4b, it can be seen that energetic bombardment inherent to low-pressure sputtering results in relatively big grains. These films have a complete (0 0 2) texture (also referred to as c-axis orientation of the hexagonal lattice). This is in contrast with r.f. sputter-deposited films (see, e.g. Ref. w10x and works cited therein), which show always a fraction of additional grain orientations. 4. Discussion In the following, the very different nature of AlN and oxide films with respect to their crystallinity and density
Fig. 3. Composition of the Al2O3 films in terms of the atomic ratio of xsAlyO as a function of (a) oxygen pressure and (b) sputtering d.c.-power measured by XPS. Data were taken at the surface of the films and after sputter cleaning—see legend. The dashed lines indicate stoichiometric composition according to xs0.67.
quantitative analysis. The static refractive index of the films is shown in Fig. 4a. The gradual decrease in the quantity for AlN is directly related to the decrease in the films’ density shown in Fig. 2a. For the low-density Al2O3 films, one should then expect negligible effect of pressure on the refractive index, what indeed can be seen in Fig. 4a. It should be noted that the AlN films shown in Fig. 4a have an optical Tauc gap, which increases proportional to the logarithm of pressure from 5.2 to 5.4 eV. Measurements of the optical gap of the Al2O3 films could not be performed because of the limited spectral range of the spectrometer used. WAXS
Fig. 4. (a) Static refractive index of the sputtered AlN and Al2O3 films, respectively, as a function of the reactive gas sputtering pressure—see legend. (b) Grain size of the AlN films determined by WAXS from the (0 0 2) and (1 0 1) diffraction peaks.
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is discussed. The decay of deposition rate with pressure is an expression for the thermalization length (or throw distance) of sputtered atoms and molecules, respectively w14,18x. From the results of Fig. 1a it becomes clear that the throw distance of sputtered species in O2atmosphere is significantly smaller than for those sputtered in N2. One has to take into consideration that sputtering nitride results in a higher target bias compared to the oxide. (For the films from Fig. 1a the target bias decreases with pressure from 369 to 322 V w11x for the nitride and it increases with increasing pressure from 238 to 287 V for the oxide.) On the other hand, the higher heat of sublimation of the oxide should increase the average kinetic energy of sputtered atoms compensating the latter effect. Therefore, it can be concluded that the reduced throw distance for oxygen sputtering results from an increased particle diameter. It is well known that AlN molecules form a significant fraction of sputtered species w9x in case of the nitride. Reactive sputtering of various metal oxides in contrast results in the formation of higher-atomic molecules w19x. Probably, this also occurs during sputtering of Al2O3. The sputterejection of higher-atomic Al and O containing complexes is also a rationalization for the reduced density of the Al2O3 films. The pre-formed molecules consisting of Al and O are stronger thermalized than AlN molecules. They carry less energy to enhance the mobility of the ad-atoms and the crystal formation is suppressed by the arrival of the precursors. (It is worth noting that in general the more complicated Al2O3 lattice is more difficult to grow than the relative simple AlN lattice.) 5. Summary and conclusions Transparent films of Al2O3 and AlN were deposited by d.c. magnetron-MSP in pure reactive gas atmosphere without significant problems regarding arcing at the Altargets. The structure and the density of these films show very strong differences. This effect is supported by results of the pressure-dependent deposition rate, rationalized by the sputter-ejection of higher-atomic aluminum–oxygen complexes from the oxidized Al-
target. In contrast, sputtering from the nitrided target results mainly in the ejection of atomic Al and N and AlN dimers, which are responsible for the formation of dense, crystalline AlN films. Acknowledgments ¨ The authors would like to acknowledge Dr J. Blasing for X-ray investigations and Mrs U. Lehmann for performing XPS. References w1x J.H. Edgar (Ed.), Properties of Group III Nitrides, EMIS Datareviews, vol. 11, INSPEC, London, 1994. w2x K. Koski, J. Holsa, ¨ ¨ P. Juliet, Thin Solid Films 339 (1999) 240. w3x C.C. Cheng, Y.C. Chen, H.J. Wang, W.R. Chen, J. Vac. Sci. Technol. A 14 (1996) 2238. w4x O.J. Gregory, A.B. Slot, P.S. Amons, E.E. Crisman, Surf. Coat. Technol. 88 (1995) 79. w5x M. Penza, M.F. De Riccardis, L. Mirenghi, M.A. Tagliente, E. Verona, Thin Solid Films 259 (1994) 154. w6x R. Cremer, M. Witthaut, D. Neuschutz, ¨ G. Erkens, T. Leyendecker, M. Feldhege, Surf. Coat. Technol. 120 (1999) 213. w7x M. Kharrazi Olsson, K. Macak, W. Graf, Surf. Coat. Technol. 122 (1999) 202. w8x D.-Y. Wang, Y. Nagahata, M. Masuda, Y. Hayashi, J. Vac. Sci. Technol. A 14 (1996) 3092. w9x M. Ishihara, S.J. Li, K. Akashi, Y. Ide, Thin Solid Films 316 (1998) 152. w10x T.P. Drusedau, ¨ ¨ J. Blasing, Thin Solid Films 377–378 (2000) 27. w11x T. Drusedau, ¨ K. Koppenhagen, Surf. Coat. Technol. 153 (2002) 155. w12x T. Takahashi, F. Takeda, M. Naoe, Mat. Res. Soc. Symp. Proc. 167 (1990) 277. w13x R.S. Naik, R. Reif, J.J. Lutsky, C.G. Sodini, J. Electrochem. Soc. 146 (1999) 691. w14x T. Drusedau, ¨ T. Bock, T.-M. John, F. Klabunde, W. Eckstein, J. Vac. Sci. Technol. A 17 (1999) 2896. w15x T. Drusedau, ¨ J. Non-Cryst. Solids 135 (1991) 204. w16x H. Freistedt, F. Stolze, M. Zacharias, J. Blasing, ¨ ¨ T. Drusedau, Phys. Stat. Sol. (b) 193 (1996) 375. w17x F. Klabunde, M. Lohmann, ¨ ¨ ¨ J. Blasing, T. Drusedau, J. Appl. Phys. 80 (1996) 6266. w18x R.E. Somekh, J. Vac. Sci. Technol. A 2 (1984) 1285. w19x G. Kienel (Ed.), Vakuumbeschichtung, vol. 1–5, VDI-Verlag, ¨ Dusseldorf, 1994, in German.
The properties of aluminum oxide and nitride films prepared by d.c. sputter-deposition from metallic targets ¨ Tilo P. Drusedau*, Thomas Neubert, Andreas N. Panckow ¨ Experimentelle Physik, Otto-von-Guericke-Universitat, ¨ PF 4120, D-39016 Magdeburg, Germany Institut fur
Abstract Thin films of aluminum nitride and aluminum oxide were prepared at 540 K substrate temperature by d.c. magnetron sputtering from a metallic Al-target in pure nitrogen and oxygen atmosphere, respectively. Characterization of the films was performed by X-ray diffraction, X-ray reflection and by optical transmission spectroscopy. The stoichiometric composition of the films was ascertained by X-ray photoelectron spectroscopy. As a function of reactive gas pressure, the deposition rate showed, in general, a logarithmic decrease with increasing pressure. Oxide films were deposited at about a fourth the rate of nitride films as consequence of the lower sputtering yield. For the oxide films, specific gravity and static refractive index showed weak changes with pressure and amounted to 2.7 gycm3 , which is only approximately 68% of the bulk value, and 1.65, respectively. In contrast, low-pressure sputter-deposition of nitride films resulted in high-density films of approximately 97% the bulk value and a refractive index close to 2.1. Nitride films show a strong (0 0 2) texture (c-axis orientation) with large nanograins up to 90-nm length. Oxide films are in general amorphous. The very different structure of nitride and oxide films is related to the more complicated crystal lattice of the oxide and to higher-atomic Al–O complexes ejected from the target. It is concluded that stronger thermalization of these species compared to those ejected by nitride sputtering is responsible for insufficient enhancement of the mobility of the adatoms. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminum nitride; Alumina; Reactive magnetron sputtering; Specific gravity; RMS-roughness
1. Introduction Reactive magnetron sputtering (MSP) of metal oxides and nitrides has become one of the mostly investigated topics in thin film technology over the past decade. The interest in aluminum nitride (AlN) and alumina (Al2O3) thin films has rapidly increased because of their specific properties, which make them suitable for various applications. These properties are high chemical stability, mechanical strength and hardness, optical transparency and electrical insulation w1,2x. In addition, AlN shows a high thermal conductivity, a low thermal expansion coefficient, the high velocity of acoustic waves and a large piezoelectric coupling constant w1x. Therefore, AlN *Corresponding author. Tel.: q49-391-67-12470; fax: q49-39167-11130. E-mail address: [email protected] ¨ (T.P. Drusedau).
films are used for surface passivation of thin films, optical sensors in the UV-spectral range, acousto-optic devices w3x, strain gages w4x and surface acoustic wave devices w5x. Al2O3 films are mainly used for wear and corrosion protection, diffusion and thermal barrier applications w2,6,7x. Reactive radio frequency MSP w3–6,8– 10x has been established as one technique for the low-cost, large-area deposition of these films. A promising alternative is the mid-frequency or d.c. MSP w6,7,11x. For most applications of AlN films, the growth of (0 0 2) textured crystallites is desired w3–5,8,9,12,13x. Sputtering an Al-target in pure reactive gas atmosphere results in stoichiometric films w4,5,13x. Changing the system pressure in AryN2 and AryO2 mixtures, respectively, affects the surface coverage of the Al-target by a reactive layer, and therefore, influences the stoichiometry of the films w3,11x. In a pure reactive gas atmosphere,
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 4 8 4 - X
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one expects a constant, pressure-independent coverage of the Al-target by AlN and Al2O3, respectively. Therefore, changing the pressure for a pure reactive gas discharge influences only the thermalization of the sputtered species. The aim of the present work is a comparative investigation of the influence of reactive gas pressure— namely of N2 and O2—on the properties of d.c.-sputtered Al2O3 and AlN films, respectively. 2. Experimental The Al2O3 and AlN films were prepared by reactive MSP in pure reactive gas atmosphere of N2 and O2, respectively, (5-N purity) using a commercial LA 440S sputtering apparatus w10,14x. The magnetron source with a polycrystalline aluminum target of 125-mm diameter and 5-N purity was operated with d.c. power. The rotatable substrate holder with six substrate positions was isolated vs. ground and operated at floating potential. The substrate holder was completely covered by a shutter table with a 125-mm diameter opening permitting deposition of one of the six substrates. The target to substrate distance was dtss80 mm. The reader is referred to Table 1 for the main process parameters of Al2O3 and AlN film deposition. The film thickness was adjusted to approximately 140 and 500 nm for the oxide and nitride films, respectively. Polished silica substrates with an X-ray root mean square (rms) surface roughness of 0.7 nm are used for the optical and X-ray investigations. UV–NIR transmission spectra in the 200–2500-nm wavelength region were taken for optical characterization. The determination of the film thickness, the refractive index and the gap was performed according to the method described in the literature w15x. Wide angle X-ray scattering (WAXS) was performed by a D5000 X-ray goniometer in Bragg–Brentano geometry w16,17x. A Cu Ka source was used and the scattered intensity was measured by a Soller-collimator (divergence 0.48), secondary monochromator and scintillation counter equipment. The scattering spectra derived were corrected for the signal from the silica substrates. From the diffraction patterns the homogenous strain and the grain size were determined (Scherrer single-peak method, see literature w16,17x). XTable 1 Main process parameters of the Al2O3 and AlN films prepared: reactive gas pressure p, d.c.-power P and substrate temperature T Set of films
Variable parameter
Fixed parameter
Al2O3 Al2O3 AlN AlN
ps0.4–5.3 Pa Ps250–1500 W ps0.2–4.0 Pa Ps200–1500 W
Ps500 W, Ts540 K ps0.77 Pa, Ts540 K Ps500 W, Ts540 K ps1.0 Pa, Ts540 K
Fig. 1. Deposition rate (a) as a function of reactive gas partial pressure and (b) as a function of sputtering d.c.-power for AlN and oxide (see legend), respectively. The fit curves in (a) are according to Eq. (1) and due to empirical formulae given in the inset of (b), respectively.
ray reflectivity was measured in a four-slit geometry (resolution 0.048 symmetric set-up), secondary monochromator (URD6 goniometer, Co Ka radiation) and scintillation counting tube w16,17x. From the reflection spectra the film density and rms-surface roughness were determined w16,17x. The stoichiometry of the films was checked by X-ray photoelectron spectroscopy (XPS). 3. Results Fig. 1a shows the deposition rate as a function of sputtering gas pressure. The dependence of rate r on
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pressure p is well described by a logarithmic dependence according to rsr0yr1log(pyPa)
(1)
The fit parameters for r0 are 0.087 and 0.322 nmys for oxygen and nitrogen sputtering, respectively. The ratio of 3.7 between both values of r0 results from the different sputtering yields of nitrogen and oxygen projectiles for the nitride and the oxide, respectively. (Note that sputtering of an Al-target in reactive gas atmosphere results in the formation of a nitride or oxide surface layer of a thickness of approximately 2 nm w11x.) It is very interesting to note that the decay of rate with pressure, which is expressed in terms of r1 yr0 is much stronger for oxide deposition, where it amounts to 0.59 compared to 0.30. The deposition rate as a function of sputtering power shows a strict linear behavior for the case of Al2O3 as can be seen from Fig. 1b. For AlN, there is a sublinear (parabolic) dependence. This can be rationalized by the fact that because the target bias changes weakly with power, approximately 310 V w11x, increasing power decreases the sputtering yield of the projectiles. This could be, for example, due to a ratio of Nq to N2q ions decreasing with power (see Ref. w11x). The next striking contrast between AlN and Al2O3 deposition is evident from Fig. 2a. Low-pressure sputterdeposition of AlN results in high-density films of a maximum specific gravity of 3.16 gycm3, which equals nearly 97% of the value of the bulk crystal. Increasing pressure results in decreasing density due to thermalization and related reduced bombardment. Quite in contrast, Al2O3 films are highly porous with about only 65% of the bulk density independent of pressure. A similar low density was also obtained for Al2O3 films prepared under variation of power. The only exception was the film deposited at Ps1500 W with a specific gravity of 2.93 gycm3. The dependence of roughness on sputtering gas pressure is very similar for both types of films—see Fig. 2b. It seems to be a general effect observed for magnetron sputter-deposition that increasing pressure results in an increased roughness of the films (see, e.g. the results obtained for Mo and Al2O2 w2,17x). This could be due to increased thermalization of sputtered atoms and related lowered ad-atom mobility. The difference in the absolute roughness values of the AlN and Al2O3 films is related to the different film thickness of the films (see above). The stoichiometric composition of AlN films is not influenced by sputtering pressure. In contrast, there is a significant effect of pressure on the composition of the Al2O3 films as shown in Fig. 3a. It must be noted that the atomic composition of the as-deposited films at the surface suffers from a significant contamination especially with carbon. Therefore, sputter cleaning was performed. After sputter cleaning, low-pressure deposited films are stoichiometric within experimental uncer-
Fig. 2. Specific gravity (a) and rms-surface roughness (b) measured by X-ray reflectometry as a function of the reactive gas sputtering pressure—see legend—for the AlN and Al2O3 films, respectively.
tainty. In contrast, high-pressure films show the existence of excess oxygen. Films prepared under variation of power in the low-pressure regime (Fig. 3b) are in general stoichiometric within experimental uncertainty. (The film with maximum density has a composition of exactly xs0.66.) There are two rationalizations of the effect: (1) Conditions resulting in low energetic bombardment lead to the incorporation of excess oxygen. (2) The microstructure of films prepared under these conditions results in a preferred sputtering of Al atoms from the films during sputter cleaning. Then the changes in composition are due to the specific method of
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measurements reveal that Al2O3 films are in general amorphous, whereas the AlN films are nanocrystalline. From Fig. 4b, it can be seen that energetic bombardment inherent to low-pressure sputtering results in relatively big grains. These films have a complete (0 0 2) texture (also referred to as c-axis orientation of the hexagonal lattice). This is in contrast with r.f. sputter-deposited films (see, e.g. Ref. w10x and works cited therein), which show always a fraction of additional grain orientations. 4. Discussion In the following, the very different nature of AlN and oxide films with respect to their crystallinity and density
Fig. 3. Composition of the Al2O3 films in terms of the atomic ratio of xsAlyO as a function of (a) oxygen pressure and (b) sputtering d.c.-power measured by XPS. Data were taken at the surface of the films and after sputter cleaning—see legend. The dashed lines indicate stoichiometric composition according to xs0.67.
quantitative analysis. The static refractive index of the films is shown in Fig. 4a. The gradual decrease in the quantity for AlN is directly related to the decrease in the films’ density shown in Fig. 2a. For the low-density Al2O3 films, one should then expect negligible effect of pressure on the refractive index, what indeed can be seen in Fig. 4a. It should be noted that the AlN films shown in Fig. 4a have an optical Tauc gap, which increases proportional to the logarithm of pressure from 5.2 to 5.4 eV. Measurements of the optical gap of the Al2O3 films could not be performed because of the limited spectral range of the spectrometer used. WAXS
Fig. 4. (a) Static refractive index of the sputtered AlN and Al2O3 films, respectively, as a function of the reactive gas sputtering pressure—see legend. (b) Grain size of the AlN films determined by WAXS from the (0 0 2) and (1 0 1) diffraction peaks.
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is discussed. The decay of deposition rate with pressure is an expression for the thermalization length (or throw distance) of sputtered atoms and molecules, respectively w14,18x. From the results of Fig. 1a it becomes clear that the throw distance of sputtered species in O2atmosphere is significantly smaller than for those sputtered in N2. One has to take into consideration that sputtering nitride results in a higher target bias compared to the oxide. (For the films from Fig. 1a the target bias decreases with pressure from 369 to 322 V w11x for the nitride and it increases with increasing pressure from 238 to 287 V for the oxide.) On the other hand, the higher heat of sublimation of the oxide should increase the average kinetic energy of sputtered atoms compensating the latter effect. Therefore, it can be concluded that the reduced throw distance for oxygen sputtering results from an increased particle diameter. It is well known that AlN molecules form a significant fraction of sputtered species w9x in case of the nitride. Reactive sputtering of various metal oxides in contrast results in the formation of higher-atomic molecules w19x. Probably, this also occurs during sputtering of Al2O3. The sputterejection of higher-atomic Al and O containing complexes is also a rationalization for the reduced density of the Al2O3 films. The pre-formed molecules consisting of Al and O are stronger thermalized than AlN molecules. They carry less energy to enhance the mobility of the ad-atoms and the crystal formation is suppressed by the arrival of the precursors. (It is worth noting that in general the more complicated Al2O3 lattice is more difficult to grow than the relative simple AlN lattice.) 5. Summary and conclusions Transparent films of Al2O3 and AlN were deposited by d.c. magnetron-MSP in pure reactive gas atmosphere without significant problems regarding arcing at the Altargets. The structure and the density of these films show very strong differences. This effect is supported by results of the pressure-dependent deposition rate, rationalized by the sputter-ejection of higher-atomic aluminum–oxygen complexes from the oxidized Al-
target. In contrast, sputtering from the nitrided target results mainly in the ejection of atomic Al and N and AlN dimers, which are responsible for the formation of dense, crystalline AlN films. Acknowledgments ¨ The authors would like to acknowledge Dr J. Blasing for X-ray investigations and Mrs U. Lehmann for performing XPS. References w1x J.H. Edgar (Ed.), Properties of Group III Nitrides, EMIS Datareviews, vol. 11, INSPEC, London, 1994. w2x K. Koski, J. Holsa, ¨ ¨ P. Juliet, Thin Solid Films 339 (1999) 240. w3x C.C. Cheng, Y.C. Chen, H.J. Wang, W.R. Chen, J. Vac. Sci. Technol. A 14 (1996) 2238. w4x O.J. Gregory, A.B. Slot, P.S. Amons, E.E. Crisman, Surf. Coat. Technol. 88 (1995) 79. w5x M. Penza, M.F. De Riccardis, L. Mirenghi, M.A. Tagliente, E. Verona, Thin Solid Films 259 (1994) 154. w6x R. Cremer, M. Witthaut, D. Neuschutz, ¨ G. Erkens, T. Leyendecker, M. Feldhege, Surf. Coat. Technol. 120 (1999) 213. w7x M. Kharrazi Olsson, K. Macak, W. Graf, Surf. Coat. Technol. 122 (1999) 202. w8x D.-Y. Wang, Y. Nagahata, M. Masuda, Y. Hayashi, J. Vac. Sci. Technol. A 14 (1996) 3092. w9x M. Ishihara, S.J. Li, K. Akashi, Y. Ide, Thin Solid Films 316 (1998) 152. w10x T.P. Drusedau, ¨ ¨ J. Blasing, Thin Solid Films 377–378 (2000) 27. w11x T. Drusedau, ¨ K. Koppenhagen, Surf. Coat. Technol. 153 (2002) 155. w12x T. Takahashi, F. Takeda, M. Naoe, Mat. Res. Soc. Symp. Proc. 167 (1990) 277. w13x R.S. Naik, R. Reif, J.J. Lutsky, C.G. Sodini, J. Electrochem. Soc. 146 (1999) 691. w14x T. Drusedau, ¨ T. Bock, T.-M. John, F. Klabunde, W. Eckstein, J. Vac. Sci. Technol. A 17 (1999) 2896. w15x T. Drusedau, ¨ J. Non-Cryst. Solids 135 (1991) 204. w16x H. Freistedt, F. Stolze, M. Zacharias, J. Blasing, ¨ ¨ T. Drusedau, Phys. Stat. Sol. (b) 193 (1996) 375. w17x F. Klabunde, M. Lohmann, ¨ ¨ ¨ J. Blasing, T. Drusedau, J. Appl. Phys. 80 (1996) 6266. w18x R.E. Somekh, J. Vac. Sci. Technol. A 2 (1984) 1285. w19x G. Kienel (Ed.), Vakuumbeschichtung, vol. 1–5, VDI-Verlag, ¨ Dusseldorf, 1994, in German.