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Influence of substrate properties on the growth of titanium films: part IV
Thin Solid Films 434 (2003) 24–29
Influence of substrate properties on the growth of titanium films: part IV P. Oberhauser, R. Abermann* Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria Received 4 July 2002; received in revised form 20 February 2003; accepted 4 March 2003
Abstract The influence of substrate properties on the growth of thin titanium films was investigated by in situ ultra high vacuum internal stress measurements and by transmission electron microscopy. The substrates were highly ordered TiO2 -films. Depositing an increasing number of aluminium monolayers changed the surface properties of the TiO2 -substrate. The resulting variation in the substrate properties was monitored via changes in the growth stress of a titanium film deposited on this substrate. The respective stress vs. thickness curves of the titanium film indicate three different growth modes, as a consequence of changes in the properties of the substrate surface. By way of an oxidationyreduction reaction the highly ordered TiO2 surface is transformed to amorphous aluminium oxide and eventually covered by metallic aluminium. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Titanium films; Amorphous aluminium oxide; Aluminium monolayers; Thin film stress
1. Introduction In a number of earlier papers w1–3x we have been able to demonstrate that the physical-chemical properties of the substrate have a strong influence on the evolution of the internal stress of a growing film and its microstructure. In order to further investigate the influence of substrate properties on thin film growth, we have recently started a series of experiments w4–6x, in which we prepared thin film substrates with different properties by reactive evaporation of titanium films in different atmospheres (water, oxygen) at 120 8C substrate temperature. Under these deposition conditions amorphous oxide films are formed. Annealing of the as-deposited oxide films gives rise to the formation of highly crystalline TiO2 (rutile) in the case of the TiyO2-films, while Tiy H2O-films remain amorphous. The amount of crystalline TiO2 formed during annealing is strongly dependent on the oxygen partial pressure and on the substrate temperature during TiyO2-film deposition. Using these oxide films as substrates for the deposition of a thin titanium film we were able to demonstrate that the growth stress of the titanium film indicates the *Corresponding author. Tel.: q43-512-507-5051; fax: q43-512507-2925. E-mail address: [email protected] (R. Abermann).
growth of a polycrystalline titanium film with island formation during the initial growth stage on the amorphous oxide substrate, while epitaxial growth of a highly ordered titanium film occurs on the crystalline TiO2substrate w6x. All the experiments performed to date w7–10x have shown that the growth stress of a polycrystalline film on an amorphous substrate is tensile during the initial nucleation of individual islands with varying crystallographic orientations and reaches a maximum at the end of the coalescence stage. After film coalescence the growth stress turns compressive and increases with film thickness. When the film deposition is terminated, the stress change with time is always tensile and increases with thickness, too. Such a stress vs. thicknessytime curve is generally characteristic of the growth of mobile film materials on amorphous substrates and a fingerprint for the growth of polycrystalline films. In view of these and other experimental results stress models have been proposed in which primarily the origin of the compressive stress during and of the tensile stress after deposition of the film is discussed. Chason et al. propose that this compressive stress is associated with a flow of adatoms on the film surface in and out of grain boundaries due to changes in the chemical surface potential w11,12x. Friesen and Thompson suggest that the com-
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00529-7
P. Oberhauser, R. Abermann / Thin Solid Films 434 (2003) 24–29
pressive stress is due to an elastic displacement field on the surface as the adatom population on the film surface increases during growth w13x. A model proposed by us, assumes that this stress contribution is generated by a compressive strain locked in at the substrateyfilm interface, which originates from surface tension effects in the precoalescence growth stage. As the thickness of the continuous film is increased, this compressive interface strain is transmitted to the film surface and the film continues to grow by strained layer epitaxy w14–18x. In view of the experimental evidence it is assumed that this compressive stress dominates all other stress contributions as long as the compressive strain is transmitted from the substrate interface to the film surface as the film deposition proceeds. This interface strain can be reduced by the formation of dislocations, andyor its transmission to the film surface can be inhibited by grain boundaries, defects, etc. This assumption is supported by stress curves in which the film stress once again turns tensile at higher film thickness w19–21x. This feature in the stress vs. thickness curve is not compatible with the stress models mentioned above. We have demonstrated earlier that the tensile stress change after deposition can be ascribed to film recrystallization (elimination of defects, grain boundaries, etc.) resulting in a volume reduction w22x. The stress vs. thickness curve measured during the deposition of titanium on crystalline TiO2 shows novel features which have to date not been found for mobile film materials. Analysis of the film structure by electron microscopy has demonstrated that this new type of stress vs. thickness curve is characteristic of epitaxial film growth w6x. The compressive stress measured during the deposition of the first titanium monolayer is assigned to an oxidationyreduction reaction between the arriving titanium atoms and oxygen at the highly ordered TiO2interface. A detailed investigation of this interface reaction is presently under way. Preliminary experiments have shown that the high degree of order in the TiO2 substrate is essential for this interface stress to be compressive, since it is tensile during the initial stages of growth on the amorphous TiOx-substrate. The tensile growth stress measured after deposition of the first monolayer is interpreted to be due to a mismatch between the larger oxide and the smaller titanium lattice resulting in a tensile interface strain. The titanium film continues to grow by strained layer epitaxy. Thus, the concept of this stress contribution is comparable with that used in our stress model to interpret the compressive stress in a continuous polycrystalline film mentioned above, where we assume the film stress to originate from a compressive strain at the substrateyfilm interface. Stress changes after deposition of the highly crystalline titanium film are negligible. The main objective of the present paper is to present further experimental evidence for our working hypoth-
25
esis that the stress generating mechanism can be located at the substrateyfilm interface. Thus, it is expected that the properties of the interface strongly affect the growth stress and evolution of the microstructure of a thin film growing on it. In view of the results of Dake and Lad w23,24x we have changed the properties of the topmost surface layer of the crystalline TiO2 substrate step by step by depositing an increasing number of aluminium monolayers prior to the deposition of the titanium film. Thus only the substrate surface is affected in the present experiments, while in earlier investigations the properties of the substrate film as a whole were changed by varying the deposition conditions. Changes in the properties of the substrate interphase should again be reflected in changes in the growth stress of a titanium film deposited onto it. 2. Experimental The experiments have been performed in an oil diffusion pumped ultra high vacuum system with two additional liquid nitrogen cooled titanium sublimation pumps producing a base pressure of 1–2=10y10 mbar. The term ‘film stress’ used in this paper, refers to the film forces generated in the film plane normalized to substrate width (N my1 ). This film stress was measured in situ as a function of film thickness during and as a function of time after film deposition, using a cantilever beam technique. Details of the experimental set-up have been published earlier w25x. The bending beam used in this experimental set-up is a quartz plate (Synsil, Min¨ Goslar, FRG) of length 80 mm, width 10 mm nahutte and thickness 0.5 mm. With this glass beam, the sensitivity of the apparatus is approximately 0.02 N my1. Positive values are assigned to tensile and negative values to compressive film stresses. The elevated substrate temperature is achieved by radiation from a heater surrounding the bending beam. To avoid the possible influence of temperature gradients along the bending beam on the growth of a film, the respective film system is deposited only onto part of the bending beam (20=10 mm2). Oxygen was introduced into the system via a leak valve and its purity was controlled by a quadrupole mass analyser. The film thickness and evaporation rate was measured with a quartz microbalance. The titanium (99.99%, Goodfellow) evaporation rate was 0.079"0.002 nm sy1. In order to eliminate the influence of possible contamination layers covering the bending beam after bake out of the vacuum system, a 10 nm thick Al2O3-film was deposited by reactive evaporation of aluminium (99.999%, Goodfellow) in an oxygen atmosphere (pO2s 4=10y5 mbar, evaporation rates0.03 nm sy1) prior to the start of each experiment w26x. Subsequently, a 50 nm thick TiOx-film was formed by reactive evaporation of titanium at 2=10y4 mbar oxygen and 105 8C
26
P. Oberhauser, R. Abermann / Thin Solid Films 434 (2003) 24–29
substrate temperature. After again establishing UHVconditions, this oxide film was heated to 400 8C for 20 min to initiate the formation of highly crystalline TiO2 w6x. Aluminium andyor titanium were then deposited at a substrate temperature of 120 8C. In view of results published earlier, a predetermined time schedule was always observed w5x. For the investigation of the film microstructure in the transmission electron microscope (TEM) the respective thin film system together with the alumina substrate was deposited in separate experiments onto carbon precoated gold grids. The gold grids were positioned in small holes in a bending quartz beam. Thus, identical substrates and evaporation conditions were used for the deposition of the respective film system in the stress measurement and the TEM-experiment. The micrographs were taken in a Zeiss TEM (EM10C) at an accelerating voltage of 100 kV. 3. Results and discussion 3.1. Influence of the substrate temperature on the growth stress of aluminium films on TiO2 In order to investigate the stress generated in the interaction between TiO2 and aluminium, in a first series of experiments highly crystalline TiO2 (rutile) films were used as substrate films for the deposition of aluminium films at selected substrate temperatures. The respective stress curves, measured in situ under UHV conditions as a function of thickness during and as a function of time after deposition, are shown in Fig. 1. During deposition of the first few monolayers a large compressive stress is built up, which is the larger the higher the substrate temperature. With further thickness increase the changes in the growth stress are fairly small. In the case of the film deposition at 27 8C substrate temperature a small tensile stress contribution (positive slope) is indicated, which reaches it’s maximum at a thickness of approximately 20 nm. Above this thickness the film stress is again compressive (negative slope). The stress changes with time after deposition are significant in the case of the room temperature film and decrease with increasing substrate temperature. The compressive stress built up during the deposition of the first few monolayers of aluminium is comparable in magnitude with that found in the case of the deposition of titanium on the same substrate w6x, indicating a comparable interaction with the substrate surface. In view of the results of Dake and Lad w23,24x we interpret this growth stress to be due to an oxidationyreduction interaction between the crystalline TiO2 substrate and the arriving aluminium atoms, and to the formation of a thin oxide layer. However, in view of the results which will be shown later, an amorphous oxide is formed in the case of the aluminiumyTiO2 interaction, while a
Fig. 1. Internal stress of aluminium films generated during and after their evaporation onto TiO2 substrate films at different substrate temperatures: 27 8C, curve a; 120 8C, curve b; 300 8C, curve c.
highly ordered oxide is formed in the case of the titaniumyTiO2 interaction. This interpretation confirms a scanning tunnelling microscopy study by Lai et al., which shows that oxidized aluminium clusters disorder the TiO2 substrate w27x. The stress features (tensile and compressive) found during further thickness increase of the aluminium film at 27 8C substrate temperature are interpreted to indicate the growth of a polycrystalline aluminium film on alumina. Finally, the larger initial compressive stress found at higher substrate temperature (Fig. 1, curves b and c) is interpreted to indicate the formation of a thicker oxide layer. 3.2. Influence of aluminium interlayer on TiO2 films on the growth of titanium on these bilayer substrates The stress vs. thickness curves of aluminium on TiO2 shown above have been interpreted to indicate the formation of a thin aluminium oxide layer at the interface. This interface reaction between TiO2 and aluminium was now used to change the surface properties of the TiO2 substrate in a controlled manner. By depositing an increasing number of aluminium monolayers it is expected that the nature of the substrate surface is changed from the highly ordered TiO2 to aluminium oxide and finally metallic aluminium. The variation of
P. Oberhauser, R. Abermann / Thin Solid Films 434 (2003) 24–29
Fig. 2. Internal stress of titanium films generated during and after evaporation at 120 8C substrate temperature onto: TiO2yAl bilayer substrates: Thickness of aluminium interlayer: 0 nm, curve a; 0.4 nm, curve b; 0.6 nm, curve c; 0.8 nm, curve e; 2.3 nm, curve e. Alumina substrate: curve f.
the properties of this interface will again be monitored via the growth stress of a titanium film evaporated onto these substrates. Thus, changes in the growth stress of the titanium film would again directly be related to changes in the properties of the substrate interface, since all other deposition parameters for the titanium film are kept the same. 3.2.1. Aluminium-interlayer with 0–2.5 nm thickness Fig. 2 summarizes the stress vs. thickness curves of titanium growing on these bilayer substrate surfaces at a substrate temperature of 120 8C. For comparison the growth stress of titanium on a bare TiO2 substrate, discussed earlier, is shown in curve a. In the case of curves b–e the surface of the TiO2 substrate film was modified by depositing 0.4, 0.6, 0.8 and 2.3 nm thick aluminium layers, respectively. Curve f represents the growth stress of titanium on an amorphous alumina substrate film (10 nm), prepared by reactive evaporation of aluminium in oxygen w26x. From the large reduction of the initial compressive stress (compare curves a and b) built up during the deposition of the first titanium monolayer it is seen that already the deposition of approximately 0.4 nm aluminium on TiO2 significantly alters the chemical properties
27
of the interface and reduces the interaction of the arriving titanium atoms with the substrate interphase. However, the titanium growth stress at higher thickness is again comparable to that on the bare TiO2 (curve a). This is interpreted to show that the high degree of order of the TiO2 interface is preserved and consequently epitaxial growth is still representative for the growth of the titanium film (tensile lattice mismatch at the interface) on this surface. The small increase in the thickness of the aluminium interlayer from 0.4 to 0.6 and 0.8 nm completely changes the general shape of the stress vs. thickness curve of titanium on this substrate. First, the initial compressive stress is further reduced and secondly, a tensile stress is built up only to a film thickness of approximately 10 nm, and changes in the incremental film stress (slope) indicate the development of compressive stress contributions. At an aluminium interlayer thickness of 2.3 nm even the initial growth stress is tensile and reaches a maximum at approximately 5 nm. Above this thickness the stress built up in the titanium film is compressive. After deposition a tensile stress change is measured. This stress vs. thickness curve is identical with that shown in curve f and a fingerprint for the growth of a polycrystalline titanium film on an amorphous alumina substrate. The transition from epitaxial film growth on the crystalline TiO2 to the growth of a polycrystalline titanium film on the amorphous alumina is also indicated by the increase in the tensile stress changes with time after the end of the titanium deposition, interpreted to indicate an increasing stress contribution due to recrystallization in the polycrystalline titanium film formed. When the aluminium interlayer is deposited at the higher substrate temperature of 300 8C this transition in the growth stress of the titanium film, discussed above, is extended to significantly higher aluminium interlayer thickness (more than 4 nm), which is interpreted to indicate that the thickness of the amorphous oxide formed is higher, which agrees well with the larger compressive stress measured during the deposition of the first few monolayers aluminium (Fig. 1). In summary, the trends seen in the stress vs. thickness curves of titanium, shown in Fig. 2, are interpreted to indicate a gradual transition from epitaxial growth (tensile interface strain) of the titanium film on the highly crystalline TiO2 substrate to the growth of a polycrystalline film on the amorphous alumina substrate formed in the oxidationyreduction reaction between TiO2 and aluminium at the interface. In view of these and earlier experimental results, we believe that the substrateyfilm interface and the film strain built up in this interphase region primarily determine the growth stress of thin films with high adatom mobility at higher film thickness. As discussed above, this stress model is at variance with models proposed recently by Chason et al. w11x, Friesen and Thompson w13x and Nix and Clemens w28x, in which
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P. Oberhauser, R. Abermann / Thin Solid Films 434 (2003) 24–29
Fig. 3. Internal stress of titanium films generated during and after evaporation at 120 8C substrate temperature onto TiO2 yAl bilayer substrates: Thickness of aluminium interlayer: 3.4 nm, curve a; 7 nm, curve b; 15 nm, curve c; 50 nm, curve d; 100 nm, curve e.
it is assumed that the compressive stress in the continuous film originates from processes occurring on the growing film surface. 3.2.2. Aluminium interlayer with 2.5–100 nm thickness Increasing the thickness of the aluminium interlayer to higher values once again gives rise to characteristic changes in the stress vs. thickness curve of a titanium film deposited onto this bilayer substrate. These changes are due to the fact that with increasing interlayer thickness the aluminium is only partially oxidized and the surface of the substrate onto which the titanium film is deposited eventually becomes metallic. The results of this series of experiments are summarized in Fig. 3. The dominating new feature is a tensile stress built up during deposition of the first titanium monolayer. This tensile stress contribution increases with the thickness of the aluminium interlayer. The magnitude of the tensile stress is strongly influenced by differences in the surface topography of the aluminium substrate film but limited to the first few titanium monolayers w29x. The observation that the interaction between the depositing titanium atoms and the aluminium interface is limited to the interface is also confirmed by a number of spectroscopic investigations w30–33x. We assume that the sudden change in slope after deposition of the first
few monolayers may be due to the formation and glide of dislocations, which reduces this giant interface strain. The stress vs. thickness curve above monolayer thickness in curve a and b is still comparable with that discussed above for the aluminium oxide substrate. We take this to indicate island growth on portions of remnant amorphous oxide and the formation of a polycrystalline film on these substrate areas. Consequently, the decreasing tensile stress in the thickness range up to approximately 5 nm, as well as the smaller compressive stress at even higher titanium thickness and finally the decreasing tensile stress change after deposition are the expected trends in terms of our stress model and indicate a decreasing amount of polycrystalline titanium growing on the respective substrate interface. On substrates with an aluminium interlayer thickness above approximately 15 nm (curve c) all the characteristic features of the growth stress of the polycrystalline film on an amorphous substrate have disappeared. Further increasing the thickness of the aluminium interlayer only affects the tensile stress built up during deposition of the first few titanium monolayers. The subsequent growth stress during the titanium deposition as well as the stress change with time after deposition is compressive. On the basis of preliminary experiments at higher substrate temperature, performed in this laboratory, we interpret this compressive stress to be due to interdiffusion of titanium into the aluminium substrate andyor alloy formation, which is slow at this substrate temperature and continues for some time after the titanium deposition has been completed. The film microstructure deduced from the stress vs. thickness curves on the basis of the proposed stress model is clearly confirmed by the plan view micrographs of the respective multilayer film system shown in Fig. 4. Fig. 4a shows the microstructure of the bare TiO2substrate film, which contains large crystalline domains. These domains are still visible after the deposition of a 30 nm thick titanium film on this substrate (Fig. 4b), confirming the formation of a highly ordered titanium film. As is seen in Fig. 4c, the highly crystalline character of the TiO2 substrate is eliminated after the deposition of a 2.3 nm thick aluminium interlayer, resulting in the growth of a polycrystalline titanium film. This change in microstructure of the respective film is also reflected in the electron diffraction patterns. 4. Conclusion In this work we studied the influence of substrate properties on the growth of titanium films by in situ internal stress measurements under UHV-conditions. When an increasing number of aluminium monolayers are deposited on top of a highly ordered TiO2-substrate, the latter is transformed to amorphous aluminium oxide in an oxidationyreduction reaction at the substrate inter-
P. Oberhauser, R. Abermann / Thin Solid Films 434 (2003) 24–29
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As soon as metallic aluminium is present on the substrate surface the dominating new feature in the growth stress is a large tensile stress, which is built up during the deposition of the first titanium monolayer, which increases with the thickness of the aluminium layer. References
Fig. 4. Electron micrographs of (a) TiO2 substrate (ds10 nm); (b) a titanium film (ds30 nm) on TiO2 substrate shown in (a); (c) a titanium film (ds30 nm) on a TiO2yAl bilayer substrate. Aluminium interlayer (ds2.3 nm) and titanium film were evaporated at 120 8C.
face, and finally gets covered by metallic aluminium. The variation in the substrate properties is reflected in significant changes in the growth stress of a titanium film deposited on the respective substrate. This result is interpreted to show that a transition from epitaxial film growth on the crystalline TiO2 to the growth of a polycrystalline titanium film on the amorphous alumina.
w1x R. Abermann, R. Koch, Thin Solid Films 142 (1986) 65. w2x R. Abermann, Thin Solid Films 188 (1990) 385. w3x H.J. Schneeweiß, R. Abermann, Thin Solid Films 228 (1993) 40. w4x M. Poppeller, R. Abermann, Thin Solid Films 295 (1997) 60. w5x M. Poppeller, R. Abermann, Thin Solid Films 320 (1998) 331. w6x P. Oberhauser, R. Abermann, Thin Solid Films 350 (1999) 59. w7x R. Abermann, R. Koch, Thin Solid Films 66 (1980) 217. w8x R. Abermann, R. Koch, Thin Solid Films 129 (1985) 71. w9x R. Abermann, Thin Solid Films 186 (1990) 233. w10x A.L. Shull, F. Spaepen, J. Appl. Phys. 80 (1996) 6243. w11x E. Chason, B.W. Sheldon, L.B. Freund, Phys. Rev. Lett. 88 (2002) 156103. w12x J.A. Floro, E. Chason, R.C. Cammarata, D.J. Srolovitz, Mater. Res. Soc. Bull. 27 (2002) 19. w13x C. Friesen, C.V. Thompson, Phys. Rev. Lett. 89 (2002) 126103. w14x R. Abermann, R. Kramer, J. Maser, ¨ Thin Solid Films 52 (1978) 215. w15x R. Abermann, R. Koch, H.P. Martinz, Vacuum 33 (1983) 871. w16x R. Abermann, Vacuum 41 (1990) 1279. w17x R. Abermann, Mater. Res. Soc. Symp. Proc. 239 (1992) 25. w18x R. Koch, J. Phys. Condens. Matter 6 (1994) 9519. w19x R. Abermann, H.P. Martinz, Thin Solid Films 115 (1984) 185. w20x G. Thurner, R. Abermann, Thin Solid Films 192 (1990) 277. w21x R. Abermann, Thin Solid Films 186 (1990) 233. w22x R. Koch, R. Abermann, Thin Solid Films 140 (1986) 217. w23x L.S. Dake, R.J. Lad, Surf. Sci. 289 (1993) 297. w24x L.S. Dake, R.J. Lad, J. Vac. Sci. Technol. A 13 (1995) 122. w25x R. Koch, H. Leonhard, G. Thurner, R. Abermann, Rev. Sci. Instrum. 61 (1990) 3859. w26x R. Abermann, Thin Solid Films 186 (1990) 233. w27x X. Lai, C. Xu, D.W. Goodman, J. Vac. Sci. Technol. A 16 (1998) 2562. w28x W.D. Nix, B.M. Clemens, J. Mater. Res. 14 (1999) 3467. w29x P. Oberhauser, M. Poppeller, R. Abermann, Mater. Res. Soc. Symp. Proc. 648 (2001) P3.19.1. w30x C. Palacio, A. Arranz, Surf. Interface Anal. 27 (1999) 871. w31x A.A. Saleh, V. Shutthanandan, R.J. Smith, Phys. Rev. B 49 (1994) 4908. w32x R.J. Smith, Y.W. Kim, N.R. Shivaparan, G.A. White, M.A. Teter, Surf. Interface Anal. 27 (1999) 185. w33x S.K. Kim, F. Jona, P.M. Marcus, J. Phys. Condens. Matter 8 (1996) 25.
Influence of substrate properties on the growth of titanium films: part IV P. Oberhauser, R. Abermann* Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria Received 4 July 2002; received in revised form 20 February 2003; accepted 4 March 2003
Abstract The influence of substrate properties on the growth of thin titanium films was investigated by in situ ultra high vacuum internal stress measurements and by transmission electron microscopy. The substrates were highly ordered TiO2 -films. Depositing an increasing number of aluminium monolayers changed the surface properties of the TiO2 -substrate. The resulting variation in the substrate properties was monitored via changes in the growth stress of a titanium film deposited on this substrate. The respective stress vs. thickness curves of the titanium film indicate three different growth modes, as a consequence of changes in the properties of the substrate surface. By way of an oxidationyreduction reaction the highly ordered TiO2 surface is transformed to amorphous aluminium oxide and eventually covered by metallic aluminium. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Titanium films; Amorphous aluminium oxide; Aluminium monolayers; Thin film stress
1. Introduction In a number of earlier papers w1–3x we have been able to demonstrate that the physical-chemical properties of the substrate have a strong influence on the evolution of the internal stress of a growing film and its microstructure. In order to further investigate the influence of substrate properties on thin film growth, we have recently started a series of experiments w4–6x, in which we prepared thin film substrates with different properties by reactive evaporation of titanium films in different atmospheres (water, oxygen) at 120 8C substrate temperature. Under these deposition conditions amorphous oxide films are formed. Annealing of the as-deposited oxide films gives rise to the formation of highly crystalline TiO2 (rutile) in the case of the TiyO2-films, while Tiy H2O-films remain amorphous. The amount of crystalline TiO2 formed during annealing is strongly dependent on the oxygen partial pressure and on the substrate temperature during TiyO2-film deposition. Using these oxide films as substrates for the deposition of a thin titanium film we were able to demonstrate that the growth stress of the titanium film indicates the *Corresponding author. Tel.: q43-512-507-5051; fax: q43-512507-2925. E-mail address: [email protected] (R. Abermann).
growth of a polycrystalline titanium film with island formation during the initial growth stage on the amorphous oxide substrate, while epitaxial growth of a highly ordered titanium film occurs on the crystalline TiO2substrate w6x. All the experiments performed to date w7–10x have shown that the growth stress of a polycrystalline film on an amorphous substrate is tensile during the initial nucleation of individual islands with varying crystallographic orientations and reaches a maximum at the end of the coalescence stage. After film coalescence the growth stress turns compressive and increases with film thickness. When the film deposition is terminated, the stress change with time is always tensile and increases with thickness, too. Such a stress vs. thicknessytime curve is generally characteristic of the growth of mobile film materials on amorphous substrates and a fingerprint for the growth of polycrystalline films. In view of these and other experimental results stress models have been proposed in which primarily the origin of the compressive stress during and of the tensile stress after deposition of the film is discussed. Chason et al. propose that this compressive stress is associated with a flow of adatoms on the film surface in and out of grain boundaries due to changes in the chemical surface potential w11,12x. Friesen and Thompson suggest that the com-
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00529-7
P. Oberhauser, R. Abermann / Thin Solid Films 434 (2003) 24–29
pressive stress is due to an elastic displacement field on the surface as the adatom population on the film surface increases during growth w13x. A model proposed by us, assumes that this stress contribution is generated by a compressive strain locked in at the substrateyfilm interface, which originates from surface tension effects in the precoalescence growth stage. As the thickness of the continuous film is increased, this compressive interface strain is transmitted to the film surface and the film continues to grow by strained layer epitaxy w14–18x. In view of the experimental evidence it is assumed that this compressive stress dominates all other stress contributions as long as the compressive strain is transmitted from the substrate interface to the film surface as the film deposition proceeds. This interface strain can be reduced by the formation of dislocations, andyor its transmission to the film surface can be inhibited by grain boundaries, defects, etc. This assumption is supported by stress curves in which the film stress once again turns tensile at higher film thickness w19–21x. This feature in the stress vs. thickness curve is not compatible with the stress models mentioned above. We have demonstrated earlier that the tensile stress change after deposition can be ascribed to film recrystallization (elimination of defects, grain boundaries, etc.) resulting in a volume reduction w22x. The stress vs. thickness curve measured during the deposition of titanium on crystalline TiO2 shows novel features which have to date not been found for mobile film materials. Analysis of the film structure by electron microscopy has demonstrated that this new type of stress vs. thickness curve is characteristic of epitaxial film growth w6x. The compressive stress measured during the deposition of the first titanium monolayer is assigned to an oxidationyreduction reaction between the arriving titanium atoms and oxygen at the highly ordered TiO2interface. A detailed investigation of this interface reaction is presently under way. Preliminary experiments have shown that the high degree of order in the TiO2 substrate is essential for this interface stress to be compressive, since it is tensile during the initial stages of growth on the amorphous TiOx-substrate. The tensile growth stress measured after deposition of the first monolayer is interpreted to be due to a mismatch between the larger oxide and the smaller titanium lattice resulting in a tensile interface strain. The titanium film continues to grow by strained layer epitaxy. Thus, the concept of this stress contribution is comparable with that used in our stress model to interpret the compressive stress in a continuous polycrystalline film mentioned above, where we assume the film stress to originate from a compressive strain at the substrateyfilm interface. Stress changes after deposition of the highly crystalline titanium film are negligible. The main objective of the present paper is to present further experimental evidence for our working hypoth-
25
esis that the stress generating mechanism can be located at the substrateyfilm interface. Thus, it is expected that the properties of the interface strongly affect the growth stress and evolution of the microstructure of a thin film growing on it. In view of the results of Dake and Lad w23,24x we have changed the properties of the topmost surface layer of the crystalline TiO2 substrate step by step by depositing an increasing number of aluminium monolayers prior to the deposition of the titanium film. Thus only the substrate surface is affected in the present experiments, while in earlier investigations the properties of the substrate film as a whole were changed by varying the deposition conditions. Changes in the properties of the substrate interphase should again be reflected in changes in the growth stress of a titanium film deposited onto it. 2. Experimental The experiments have been performed in an oil diffusion pumped ultra high vacuum system with two additional liquid nitrogen cooled titanium sublimation pumps producing a base pressure of 1–2=10y10 mbar. The term ‘film stress’ used in this paper, refers to the film forces generated in the film plane normalized to substrate width (N my1 ). This film stress was measured in situ as a function of film thickness during and as a function of time after film deposition, using a cantilever beam technique. Details of the experimental set-up have been published earlier w25x. The bending beam used in this experimental set-up is a quartz plate (Synsil, Min¨ Goslar, FRG) of length 80 mm, width 10 mm nahutte and thickness 0.5 mm. With this glass beam, the sensitivity of the apparatus is approximately 0.02 N my1. Positive values are assigned to tensile and negative values to compressive film stresses. The elevated substrate temperature is achieved by radiation from a heater surrounding the bending beam. To avoid the possible influence of temperature gradients along the bending beam on the growth of a film, the respective film system is deposited only onto part of the bending beam (20=10 mm2). Oxygen was introduced into the system via a leak valve and its purity was controlled by a quadrupole mass analyser. The film thickness and evaporation rate was measured with a quartz microbalance. The titanium (99.99%, Goodfellow) evaporation rate was 0.079"0.002 nm sy1. In order to eliminate the influence of possible contamination layers covering the bending beam after bake out of the vacuum system, a 10 nm thick Al2O3-film was deposited by reactive evaporation of aluminium (99.999%, Goodfellow) in an oxygen atmosphere (pO2s 4=10y5 mbar, evaporation rates0.03 nm sy1) prior to the start of each experiment w26x. Subsequently, a 50 nm thick TiOx-film was formed by reactive evaporation of titanium at 2=10y4 mbar oxygen and 105 8C
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substrate temperature. After again establishing UHVconditions, this oxide film was heated to 400 8C for 20 min to initiate the formation of highly crystalline TiO2 w6x. Aluminium andyor titanium were then deposited at a substrate temperature of 120 8C. In view of results published earlier, a predetermined time schedule was always observed w5x. For the investigation of the film microstructure in the transmission electron microscope (TEM) the respective thin film system together with the alumina substrate was deposited in separate experiments onto carbon precoated gold grids. The gold grids were positioned in small holes in a bending quartz beam. Thus, identical substrates and evaporation conditions were used for the deposition of the respective film system in the stress measurement and the TEM-experiment. The micrographs were taken in a Zeiss TEM (EM10C) at an accelerating voltage of 100 kV. 3. Results and discussion 3.1. Influence of the substrate temperature on the growth stress of aluminium films on TiO2 In order to investigate the stress generated in the interaction between TiO2 and aluminium, in a first series of experiments highly crystalline TiO2 (rutile) films were used as substrate films for the deposition of aluminium films at selected substrate temperatures. The respective stress curves, measured in situ under UHV conditions as a function of thickness during and as a function of time after deposition, are shown in Fig. 1. During deposition of the first few monolayers a large compressive stress is built up, which is the larger the higher the substrate temperature. With further thickness increase the changes in the growth stress are fairly small. In the case of the film deposition at 27 8C substrate temperature a small tensile stress contribution (positive slope) is indicated, which reaches it’s maximum at a thickness of approximately 20 nm. Above this thickness the film stress is again compressive (negative slope). The stress changes with time after deposition are significant in the case of the room temperature film and decrease with increasing substrate temperature. The compressive stress built up during the deposition of the first few monolayers of aluminium is comparable in magnitude with that found in the case of the deposition of titanium on the same substrate w6x, indicating a comparable interaction with the substrate surface. In view of the results of Dake and Lad w23,24x we interpret this growth stress to be due to an oxidationyreduction interaction between the crystalline TiO2 substrate and the arriving aluminium atoms, and to the formation of a thin oxide layer. However, in view of the results which will be shown later, an amorphous oxide is formed in the case of the aluminiumyTiO2 interaction, while a
Fig. 1. Internal stress of aluminium films generated during and after their evaporation onto TiO2 substrate films at different substrate temperatures: 27 8C, curve a; 120 8C, curve b; 300 8C, curve c.
highly ordered oxide is formed in the case of the titaniumyTiO2 interaction. This interpretation confirms a scanning tunnelling microscopy study by Lai et al., which shows that oxidized aluminium clusters disorder the TiO2 substrate w27x. The stress features (tensile and compressive) found during further thickness increase of the aluminium film at 27 8C substrate temperature are interpreted to indicate the growth of a polycrystalline aluminium film on alumina. Finally, the larger initial compressive stress found at higher substrate temperature (Fig. 1, curves b and c) is interpreted to indicate the formation of a thicker oxide layer. 3.2. Influence of aluminium interlayer on TiO2 films on the growth of titanium on these bilayer substrates The stress vs. thickness curves of aluminium on TiO2 shown above have been interpreted to indicate the formation of a thin aluminium oxide layer at the interface. This interface reaction between TiO2 and aluminium was now used to change the surface properties of the TiO2 substrate in a controlled manner. By depositing an increasing number of aluminium monolayers it is expected that the nature of the substrate surface is changed from the highly ordered TiO2 to aluminium oxide and finally metallic aluminium. The variation of
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Fig. 2. Internal stress of titanium films generated during and after evaporation at 120 8C substrate temperature onto: TiO2yAl bilayer substrates: Thickness of aluminium interlayer: 0 nm, curve a; 0.4 nm, curve b; 0.6 nm, curve c; 0.8 nm, curve e; 2.3 nm, curve e. Alumina substrate: curve f.
the properties of this interface will again be monitored via the growth stress of a titanium film evaporated onto these substrates. Thus, changes in the growth stress of the titanium film would again directly be related to changes in the properties of the substrate interface, since all other deposition parameters for the titanium film are kept the same. 3.2.1. Aluminium-interlayer with 0–2.5 nm thickness Fig. 2 summarizes the stress vs. thickness curves of titanium growing on these bilayer substrate surfaces at a substrate temperature of 120 8C. For comparison the growth stress of titanium on a bare TiO2 substrate, discussed earlier, is shown in curve a. In the case of curves b–e the surface of the TiO2 substrate film was modified by depositing 0.4, 0.6, 0.8 and 2.3 nm thick aluminium layers, respectively. Curve f represents the growth stress of titanium on an amorphous alumina substrate film (10 nm), prepared by reactive evaporation of aluminium in oxygen w26x. From the large reduction of the initial compressive stress (compare curves a and b) built up during the deposition of the first titanium monolayer it is seen that already the deposition of approximately 0.4 nm aluminium on TiO2 significantly alters the chemical properties
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of the interface and reduces the interaction of the arriving titanium atoms with the substrate interphase. However, the titanium growth stress at higher thickness is again comparable to that on the bare TiO2 (curve a). This is interpreted to show that the high degree of order of the TiO2 interface is preserved and consequently epitaxial growth is still representative for the growth of the titanium film (tensile lattice mismatch at the interface) on this surface. The small increase in the thickness of the aluminium interlayer from 0.4 to 0.6 and 0.8 nm completely changes the general shape of the stress vs. thickness curve of titanium on this substrate. First, the initial compressive stress is further reduced and secondly, a tensile stress is built up only to a film thickness of approximately 10 nm, and changes in the incremental film stress (slope) indicate the development of compressive stress contributions. At an aluminium interlayer thickness of 2.3 nm even the initial growth stress is tensile and reaches a maximum at approximately 5 nm. Above this thickness the stress built up in the titanium film is compressive. After deposition a tensile stress change is measured. This stress vs. thickness curve is identical with that shown in curve f and a fingerprint for the growth of a polycrystalline titanium film on an amorphous alumina substrate. The transition from epitaxial film growth on the crystalline TiO2 to the growth of a polycrystalline titanium film on the amorphous alumina is also indicated by the increase in the tensile stress changes with time after the end of the titanium deposition, interpreted to indicate an increasing stress contribution due to recrystallization in the polycrystalline titanium film formed. When the aluminium interlayer is deposited at the higher substrate temperature of 300 8C this transition in the growth stress of the titanium film, discussed above, is extended to significantly higher aluminium interlayer thickness (more than 4 nm), which is interpreted to indicate that the thickness of the amorphous oxide formed is higher, which agrees well with the larger compressive stress measured during the deposition of the first few monolayers aluminium (Fig. 1). In summary, the trends seen in the stress vs. thickness curves of titanium, shown in Fig. 2, are interpreted to indicate a gradual transition from epitaxial growth (tensile interface strain) of the titanium film on the highly crystalline TiO2 substrate to the growth of a polycrystalline film on the amorphous alumina substrate formed in the oxidationyreduction reaction between TiO2 and aluminium at the interface. In view of these and earlier experimental results, we believe that the substrateyfilm interface and the film strain built up in this interphase region primarily determine the growth stress of thin films with high adatom mobility at higher film thickness. As discussed above, this stress model is at variance with models proposed recently by Chason et al. w11x, Friesen and Thompson w13x and Nix and Clemens w28x, in which
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Fig. 3. Internal stress of titanium films generated during and after evaporation at 120 8C substrate temperature onto TiO2 yAl bilayer substrates: Thickness of aluminium interlayer: 3.4 nm, curve a; 7 nm, curve b; 15 nm, curve c; 50 nm, curve d; 100 nm, curve e.
it is assumed that the compressive stress in the continuous film originates from processes occurring on the growing film surface. 3.2.2. Aluminium interlayer with 2.5–100 nm thickness Increasing the thickness of the aluminium interlayer to higher values once again gives rise to characteristic changes in the stress vs. thickness curve of a titanium film deposited onto this bilayer substrate. These changes are due to the fact that with increasing interlayer thickness the aluminium is only partially oxidized and the surface of the substrate onto which the titanium film is deposited eventually becomes metallic. The results of this series of experiments are summarized in Fig. 3. The dominating new feature is a tensile stress built up during deposition of the first titanium monolayer. This tensile stress contribution increases with the thickness of the aluminium interlayer. The magnitude of the tensile stress is strongly influenced by differences in the surface topography of the aluminium substrate film but limited to the first few titanium monolayers w29x. The observation that the interaction between the depositing titanium atoms and the aluminium interface is limited to the interface is also confirmed by a number of spectroscopic investigations w30–33x. We assume that the sudden change in slope after deposition of the first
few monolayers may be due to the formation and glide of dislocations, which reduces this giant interface strain. The stress vs. thickness curve above monolayer thickness in curve a and b is still comparable with that discussed above for the aluminium oxide substrate. We take this to indicate island growth on portions of remnant amorphous oxide and the formation of a polycrystalline film on these substrate areas. Consequently, the decreasing tensile stress in the thickness range up to approximately 5 nm, as well as the smaller compressive stress at even higher titanium thickness and finally the decreasing tensile stress change after deposition are the expected trends in terms of our stress model and indicate a decreasing amount of polycrystalline titanium growing on the respective substrate interface. On substrates with an aluminium interlayer thickness above approximately 15 nm (curve c) all the characteristic features of the growth stress of the polycrystalline film on an amorphous substrate have disappeared. Further increasing the thickness of the aluminium interlayer only affects the tensile stress built up during deposition of the first few titanium monolayers. The subsequent growth stress during the titanium deposition as well as the stress change with time after deposition is compressive. On the basis of preliminary experiments at higher substrate temperature, performed in this laboratory, we interpret this compressive stress to be due to interdiffusion of titanium into the aluminium substrate andyor alloy formation, which is slow at this substrate temperature and continues for some time after the titanium deposition has been completed. The film microstructure deduced from the stress vs. thickness curves on the basis of the proposed stress model is clearly confirmed by the plan view micrographs of the respective multilayer film system shown in Fig. 4. Fig. 4a shows the microstructure of the bare TiO2substrate film, which contains large crystalline domains. These domains are still visible after the deposition of a 30 nm thick titanium film on this substrate (Fig. 4b), confirming the formation of a highly ordered titanium film. As is seen in Fig. 4c, the highly crystalline character of the TiO2 substrate is eliminated after the deposition of a 2.3 nm thick aluminium interlayer, resulting in the growth of a polycrystalline titanium film. This change in microstructure of the respective film is also reflected in the electron diffraction patterns. 4. Conclusion In this work we studied the influence of substrate properties on the growth of titanium films by in situ internal stress measurements under UHV-conditions. When an increasing number of aluminium monolayers are deposited on top of a highly ordered TiO2-substrate, the latter is transformed to amorphous aluminium oxide in an oxidationyreduction reaction at the substrate inter-
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As soon as metallic aluminium is present on the substrate surface the dominating new feature in the growth stress is a large tensile stress, which is built up during the deposition of the first titanium monolayer, which increases with the thickness of the aluminium layer. References
Fig. 4. Electron micrographs of (a) TiO2 substrate (ds10 nm); (b) a titanium film (ds30 nm) on TiO2 substrate shown in (a); (c) a titanium film (ds30 nm) on a TiO2yAl bilayer substrate. Aluminium interlayer (ds2.3 nm) and titanium film were evaporated at 120 8C.
face, and finally gets covered by metallic aluminium. The variation in the substrate properties is reflected in significant changes in the growth stress of a titanium film deposited on the respective substrate. This result is interpreted to show that a transition from epitaxial film growth on the crystalline TiO2 to the growth of a polycrystalline titanium film on the amorphous alumina.
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