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Growth behavior and surface morphology of Ag rough thin films deposited on silicone oil surfaces
Thin Solid Films 342 (1999) 30±34
Growth behavior and surface morphology of Ag rough thin ®lms deposited on silicone oil surfaces Chun-Mu Feng a, b,*, Hong-Liang Ge a, Miao-Rong Tong b, Gao-Xiang Ye a, Zheng-Kuan Jiao a a
Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China Central Laboratory, Hangzhou University, Hangzhou 310028, People's Republic of China
b
Received 5 August 1997; accepted 24 July 1998
Abstract The growth behavior and surface morphology of a rough ®lm system, deposited on silicone oil drop surfaces by a r.f. magnetron sputtering method, have been studied. An anomalous ®lm growth relaxation is observed during the deposition process. The relaxation rate is extremely sensitive to the substrate temperature. The surface morphology at the micrometer length scale is very susceptible to the substrate temperature, the incident r.f. capacity, and the nature of the solid substrate on which the oil drop is dripped. A discussion on the physical origins of these phenomena is also presented. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Deposition process; Surface morphology; Sputtering; Scanning electron microscopy
1. Introduction There is a very sensitive and complex dependence of ®lm microstructure on growth conditions. Especially, the nature of substrate and the deposition method play an important role in the microstructure and physical properties of thin solid ®lms [1±4]. For this reason, many studies have recently been devoted to investigating conditions to create a particular morphology and understanding the dynamics of interface or surface evolution [5±10]. In general, rough thin ®lms are made by the ion bombardment method, non-equilibrium growth technique, and location dependent deposition rate (LDDR) method [11±13]. The bilateral rough ®lms have been fabricated by using rough substrates [3]. Recently, a new rough ®lm, deposited on the liquid substrates, has been prepared and exhibits a distinct surface structure at the micrometer length scale [14]. The experimental results indicate that, just as the solid substrates, the liquid substrates may also be very useful for both fundamental and practical purposes. In this paper, we present a study of the growth behavior and surface morphology of Ag rough thin ®lms on the silicone oil surfaces. In particular, we notice that there is an anomalous ®lm growth relaxation during the deposition process. The relaxation rate is strongly dependent on the substrate temperature. The surface morphology at the * Corresponding author. Tel.: 186-571-517-2211; fax: 186-571-204242.
micrometer length scale is very sensitive to the substrate temperature and the incident r.f. capacity, and it is also related to the nature of the solid substrates. We will show that these results are of paramount importance for us to understand both nucleation and growth of islands on the liquid substrates. 2. Experimental details The sample preparation method has been described in our previous work [14]. A small pure silicone oil drop(with diameter 3 mm) was dripped on a piece of glass or crystal Si(111) surface and then the free spreading of the oil drop formed the oil/glass of oil/Si(111) substrate (Fig. 1). The conductivity of the oil is less than 10 210 (V cm) 21 and its vapor pressure is less than 10 26 Pa, which are good enough for our purpose. The metallic atoms were deposited on both the oil drop surface and the other area of glass or Si(111). The substrate temperature T ranged from 17 to 1008C. The incident r.f. capacity could be adjusted accurately. The sputtering target was a metallic Ag (purity 99.99%) disk with diameter D 81 mm. The target±substrate distance was about 60 mm. The chamber was ®rst evacuated to 2 £ 1024 Pa, and then ®lled with 99.999% pure Ar gas. The ®lms were deposited under Ar gas pressure of 0.2 Pa. When the incident r.f. capacity was 15, 30 and 50 W, the typical deposition rate on the substrate was about 0.4, 0.7 and 1 nm/s in proper order. However, the growth rate of the
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(98)0115 1-1
C.M. Feng et al. / Thin Solid Films 342 (1999) 30±34
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®lms on the oil surface could not be measured quantitatively at this stage since it is no longer a constant during the deposition process due to the liquid surface effect (see Section 3.1). Therefore, the nominal thickness of the rough ®lm on the oil surface was characterized by the sheet resistance. Since d, the thickness of the ®lm on the glass or Si(111) surface was much bigger than that of the Ê ) (see Section 3), the ®lm on the oil surface (d . 1000 A resistance of the rough ®lm on the oil surface could be measured by the four-probe method in situ (Fig. 1b). In order to describe the growth behavior of thin ®lms on the liquid surface we introduced the relaxation time t associated with the approach to the disappearance of the liquid surface effect (see Sections 3.1 and 3.2). In our experiment, t was the deposition time needed to reach the critical value pc of the surface coverage fraction in Fig. 1b. The samples, deposited on the oil surface (Fig. 1a), were transferred from the chamber to the scanning electron microscope (SEM) through air to study their surface morphology. 3. Results and discussion 3.1. The anomalous ®lm-growth relaxation An interesting observation is that it takes an anomalous long deposition time t to arrive at the critical value pc of the
Fig. 2. The SEM photograph of two of Ag thin ®lms deposited on oil/glass substrate at C 50 W and T 300 K. The sheet resistance is (a) Rs 5 £ 105 V, (b) Rs 5 V. The deposition time is (a) t t 260 s, (b) t 290 s, where t is much longer than that ( , 15 s) of glass.
Fig. 1. Sketch of the substrate components and the measurement set-up of the sheet resistance. Both these substrates were simultaneously put into the chamber for the following needs: (a) SEM observation, (b) measuring the relaxation time and the sheet resistance (for characterizing the nominal ®lm thickness) in situ.
surface coverage fraction on the oil surface (Fig. 2a). That is, the growth rate is very small during that time. Then the growth rate increases quickly with the surface coverage fraction. Therefore, the continuous ®lm with a distinct morphology is formed shortly after the time t (Fig. 2b). It indicates that there is an anomalous ®lm growth relaxation during the deposition process. We propose that this behavior is mainly caused by the liquid surface effect since such a phenomenon has never happened in other ®lm systems deposited on solid substrates [1±10]. When the sputtered atoms strike the oil surface separately, some atoms with lower energy tend to stay on the liquid surface, and others with higher energy penetrate the liquid surface. However, the independent deposition atoms on the liquid surface can easily be evaporated again [14]. Therefore, the growth rate is small at the early stage of the deposition process. On the other hand, the local temperature of the liquid surface would increase due to the strong strike between the liquid surface and the deposition atoms. Higher temperature would obviously result in both evaporation of the liquid surface (including the second evaporation) and penetration of the deposition atoms. The penetration effect would increase the concentration of the metallic atoms in the liquid surface layer. When the concentration reaches a proper level, a great number of atomic clusters would be formed and gradually be exposed on the liquid surface due to the evaporation effect. Then the exposed clusters would form the percolation structure (Fig. 2a). So this indirect production pattern of the
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C.M. Feng et al. / Thin Solid Films 342 (1999) 30±34
clusters would greatly prolong the period of the nucleation on the liquid surface. However, if the surface coverage fraction increases. the scale of the clusters increases and the new clusters are constantly formed. They would result in the rapid increase of the growth rate. Finally, the liquid surface is fully covered with the metallic Ag atoms and the growth rate approaches the value of the deposition rate on the substrate. 3.2. The temperature dependence of the relaxation rate In Section 3.1, we see that there exists a relaxation mechanism for ®lm growth which arises because of the effect of the liquid surface on the deposition atoms, and t is a characteristic time associated with the approach to the disappearance of the liquid surface effect. For this reason, t should stand for the relaxation time for ®lm growth on the liquid substrate. In order to understand the relaxation mechanism well, we measured the temperature dependence of the relaxation rate. Fig. 3 shows that the relaxation rate R ( 1/t ) changes sensitively with the substrate temperature T. We propose that, when the substrate temperature is lower, the surface tension coef®cient of the silicone oil is relatively larger. Then quite higher energy is needed for the deposition atoms to penetrate the liquid surface. Therefore, most of the atoms would stay on the liquid surface and can easily be evaporated again. So the relaxation rate is small. However, as the substrate temperature goes up, the penetration effect would obviously increase due to the reduction of the surface tension in the early deposition stage. Then the density of the deposition atoms in the liquid surface layer would increase quickly and the clusters would form in a shorter time. There-
Fig. 4. The SEM photograph of two of Ag rough ®lms deposited on the oil/ Si(111) substrate at C 50 W. The substrate temperature and deposition time are (a) T 300 K, t 255 s, (b) T 330 K, t 120 s. The two samples have an approximate sheet resistance Rs 5 V.
fore, the relaxation rate increases sensitively. In Fig. 3, it is also found that, when both the capacity and the temperature are ®xed, the relaxation rate on the oil/Si(111) substrate is bigger than that of the oil glass. We suggest that this difference mainly results from the spreading effect of the oil drop on the solid surfaces. The nucleation of islands is related to the surface potential and the surface potential is dependent on the spreading status of the oil drop on the solid surfaces. Therefore, the relaxation rate has relation to the nature of the solid substrates. 3.3. The surface morphology
Fig. 3. The relaxation rate R ( 1/t ) as a function of the substrate temperature T at C 30 W, where t is an average of three repeat measurements. The sheet resistance ranges from 10 4 to 10 5 V. Empty circles: the oil drops are dripped on Si(111) surfaces. Solid circles: the oil drops are dripped on glass surfaces.
Figs. 2, 4 and 5 show the SEM photograph of the Ag samples, in which a characteristic structure at the micrometer length scales is observed. The anomalous morphology is related to the nucleation and growth of islands (or clusters) on the oil surface. Since the thermal expansive coef®cient of the oil is bigger than that of the metallic Ag ®lm, we propose that the expansion and contraction of both the Ag ®lm and the oil substrate should also make contributions to the formation of the morphology. Fig. 4 exhibits two representative SEM photographs of the Ag thin ®lms deposited on the oil/Si(111) substrate at 300 and 330 K. The surface morphology has a distinct branched structure and the orientation of these branches is preferential. This is pronounced at 330 K where the independent branches are preferentially arranged in the one direction of the oil surface. We propose that this preference
C.M. Feng et al. / Thin Solid Films 342 (1999) 30±34
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the ®lm and exhibits an obvious tendency of directed growth around islands. We suggest that this phenomenon mainly results from the anisotropy of the local surface tension. This anisotropy is dependent on the local spreading status of the oil and the spreading status is related to the local temperature of the oil surface. When the lower capacity is applied, the sputtered atoms possess a lower energy and most of the atoms tend to stay on the oil surface. The change of the local temperature would be smaller due to the serious second evaporation. Therefore, the anisotropy of the local surface tension is not clear and the ®lm grows isotropically around islands (Fig. 5a). At the higher capacity, however, the sputtered atoms have a larger energy and most of them penetrate the liquid surface. They would result in the greater increase of the local temperature. Then the anisotropy of the local surface tension will become obvious. Therefore, the ®lm shows an distinct branched structure around islands (Fig. 5b). 4. Conclusion
Fig. 5. The SEM photograph of two of Ag rough ®lms deposited on oil/ Si(111) substrate at T 300 K. The incident r.f. capacity and deposition time are (a) C 15 W, t 900 s, (b) C 50 W, t 245 s. The two samples have an approximate sheet resistance Rs 5 £ 10 V.
is mainly caused by the anisotropy of the surface tension of the oil drop on the Si(111) terrace. The ®lm growth is related to the surface potential and the surface potential is dependent on the tension force. Since the spreading rate of the oil drop on the Si(111) plane is different in some directions, the shape of the oil drop is not a perfect circle. This would result in the anisotropy of the liquid surface tension. Then the gradients of the surface potential are different in the two mutual perpendicular directions. Therefore, the growth rate of the ®lm on the liquid surface shows two different speeds in both these directions. When the temperature goes up, the anisotropy of the liquid surface would increase due to the enhancing of the spreading effect. Therefore, this preference will become more obvious. In Figs. 2b and 4a, one ®nds that, when the capacity and temperature is ®xed, the surface morphology is related to the nature of the solid surface below the oil drop. The ®lm deposited on the oil/glass substrate exhibits a distinct-disordered structure (Fig. 2b) since the spreading of the oil drop on glass is homogeneous in all directions. This result indicates again that the ®lm growth is dependent on the spreading status of the oil drop on the solid surfaces. Fig. 5 shows that the ®lm growth around islands is susceptible to the incident r.f. capacity. The ®lm deposited at C 15 W exhibits a snow¯ake-like structure. If C 50 W, however, a characteristic-branched structure appears on
In conclusion, we have studied the growth behavior and surface morphology of an Ag thin ®lm system, in which the liquid substrates are used. The anomalous ®lm-growth relaxation, which has relation to the interaction between the deposition atoms and the liquid substrate, is found in this system. The relaxation rate is strongly dependent on the substrate temperature mainly due to the evaporation effect and the penetration effect. The surface morphology is very susceptible to the substrate temperature, the incident r.f. capacity, and the nature of the solid substrate since the ®lm growth is dependent on the spreading status of the oil drop on the solid surface. We believe that the relaxation effect presented in this paper is very important for two reasons: (1) It could be universal in the growth of thin ®lms due to the presence of the interaction between the deposition atoms and the substrate. (2) It would be helpful for us to understand the dynamic process of ®lm growth deeply. However, further study on the relaxation mechanism is still needed. This work is presently in progress. Acknowledgements This work was supported by the Chinese Natural Science Foundation (grant no. 19504003) and the Zhejiang Provincial Natural Science Foundation of China (grant no. 195021). References [1] J. Krim, I. Heyvaert, C. Van Haesendonck, Y. Bruynseraede, Phys. Rev. Lett. 70 (1993) 57. [2] Y. Song, S.I. Lee, J.R. Gaines, Phys. Rev. B 46 (1992) 14.
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C.M. Feng et al. / Thin Solid Films 342 (1999) 30±34
[3] G.X. Ye, J.S. Wang, Y.Q. Xu, Q.R. Zhang, Solid State Commun. 88 (1993) 275. [4] Y. Yagil, G. Deutscher, D.J. Bergman, Phys. Rev. Lett. 69 (1992) 1423. [5] F. Wu, S.G. Jaloviar, D.E. Savage, M.G. Lagally, Phys. Rev. Lett. 71 (1993) 4190. [6] C. Quaeyhaegens, G. Knuyt, J.D. Haen, H.M. Stals, Thin Solid Films 258 (1995) 170. [7] M. Park, S.J. Krause, S.R. Wilson, Appl. Phys. Lett. 64 (1994) 721. [8] K.E. Johnson, D.D. Chimbliss, R.J. Wilson, S. Chiang, J. Vac. Technol. A 11 (1993) 1654.
[9] M. Hohage, M. Bott, M. Morgenstern, Z. Zhang, T. Michely, G. Comsa, Phys Rev. Lett. 76 (1996) 2366. [10] D.E. Jesson, K.M. Chen, S.J. Pennycook, T. Thundat, R.J. Warmack, Phys Rev. Lett. 77 (1996) 1330. [11] P. Meakin, Prog. Solid State Chem. 20 (1990) 135. [12] F. Family, Physica (Amsterdam) A 168 (1990) 561. [13] G.X. Ye, Y.Q. Xu, H.L. Ge, Z.K. Jiao, Q.R. Zhang, Phys. Lett. A 198 (1995) 251. [14] G.X. Ye, Q.R. Zhang, C.M. Feng, H.L. Ge, Z.K. Jiao, Phys. Rev. B 54 (1996) 14754.
Growth behavior and surface morphology of Ag rough thin ®lms deposited on silicone oil surfaces Chun-Mu Feng a, b,*, Hong-Liang Ge a, Miao-Rong Tong b, Gao-Xiang Ye a, Zheng-Kuan Jiao a a
Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China Central Laboratory, Hangzhou University, Hangzhou 310028, People's Republic of China
b
Received 5 August 1997; accepted 24 July 1998
Abstract The growth behavior and surface morphology of a rough ®lm system, deposited on silicone oil drop surfaces by a r.f. magnetron sputtering method, have been studied. An anomalous ®lm growth relaxation is observed during the deposition process. The relaxation rate is extremely sensitive to the substrate temperature. The surface morphology at the micrometer length scale is very susceptible to the substrate temperature, the incident r.f. capacity, and the nature of the solid substrate on which the oil drop is dripped. A discussion on the physical origins of these phenomena is also presented. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Deposition process; Surface morphology; Sputtering; Scanning electron microscopy
1. Introduction There is a very sensitive and complex dependence of ®lm microstructure on growth conditions. Especially, the nature of substrate and the deposition method play an important role in the microstructure and physical properties of thin solid ®lms [1±4]. For this reason, many studies have recently been devoted to investigating conditions to create a particular morphology and understanding the dynamics of interface or surface evolution [5±10]. In general, rough thin ®lms are made by the ion bombardment method, non-equilibrium growth technique, and location dependent deposition rate (LDDR) method [11±13]. The bilateral rough ®lms have been fabricated by using rough substrates [3]. Recently, a new rough ®lm, deposited on the liquid substrates, has been prepared and exhibits a distinct surface structure at the micrometer length scale [14]. The experimental results indicate that, just as the solid substrates, the liquid substrates may also be very useful for both fundamental and practical purposes. In this paper, we present a study of the growth behavior and surface morphology of Ag rough thin ®lms on the silicone oil surfaces. In particular, we notice that there is an anomalous ®lm growth relaxation during the deposition process. The relaxation rate is strongly dependent on the substrate temperature. The surface morphology at the * Corresponding author. Tel.: 186-571-517-2211; fax: 186-571-204242.
micrometer length scale is very sensitive to the substrate temperature and the incident r.f. capacity, and it is also related to the nature of the solid substrates. We will show that these results are of paramount importance for us to understand both nucleation and growth of islands on the liquid substrates. 2. Experimental details The sample preparation method has been described in our previous work [14]. A small pure silicone oil drop(with diameter 3 mm) was dripped on a piece of glass or crystal Si(111) surface and then the free spreading of the oil drop formed the oil/glass of oil/Si(111) substrate (Fig. 1). The conductivity of the oil is less than 10 210 (V cm) 21 and its vapor pressure is less than 10 26 Pa, which are good enough for our purpose. The metallic atoms were deposited on both the oil drop surface and the other area of glass or Si(111). The substrate temperature T ranged from 17 to 1008C. The incident r.f. capacity could be adjusted accurately. The sputtering target was a metallic Ag (purity 99.99%) disk with diameter D 81 mm. The target±substrate distance was about 60 mm. The chamber was ®rst evacuated to 2 £ 1024 Pa, and then ®lled with 99.999% pure Ar gas. The ®lms were deposited under Ar gas pressure of 0.2 Pa. When the incident r.f. capacity was 15, 30 and 50 W, the typical deposition rate on the substrate was about 0.4, 0.7 and 1 nm/s in proper order. However, the growth rate of the
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(98)0115 1-1
C.M. Feng et al. / Thin Solid Films 342 (1999) 30±34
31
®lms on the oil surface could not be measured quantitatively at this stage since it is no longer a constant during the deposition process due to the liquid surface effect (see Section 3.1). Therefore, the nominal thickness of the rough ®lm on the oil surface was characterized by the sheet resistance. Since d, the thickness of the ®lm on the glass or Si(111) surface was much bigger than that of the Ê ) (see Section 3), the ®lm on the oil surface (d . 1000 A resistance of the rough ®lm on the oil surface could be measured by the four-probe method in situ (Fig. 1b). In order to describe the growth behavior of thin ®lms on the liquid surface we introduced the relaxation time t associated with the approach to the disappearance of the liquid surface effect (see Sections 3.1 and 3.2). In our experiment, t was the deposition time needed to reach the critical value pc of the surface coverage fraction in Fig. 1b. The samples, deposited on the oil surface (Fig. 1a), were transferred from the chamber to the scanning electron microscope (SEM) through air to study their surface morphology. 3. Results and discussion 3.1. The anomalous ®lm-growth relaxation An interesting observation is that it takes an anomalous long deposition time t to arrive at the critical value pc of the
Fig. 2. The SEM photograph of two of Ag thin ®lms deposited on oil/glass substrate at C 50 W and T 300 K. The sheet resistance is (a) Rs 5 £ 105 V, (b) Rs 5 V. The deposition time is (a) t t 260 s, (b) t 290 s, where t is much longer than that ( , 15 s) of glass.
Fig. 1. Sketch of the substrate components and the measurement set-up of the sheet resistance. Both these substrates were simultaneously put into the chamber for the following needs: (a) SEM observation, (b) measuring the relaxation time and the sheet resistance (for characterizing the nominal ®lm thickness) in situ.
surface coverage fraction on the oil surface (Fig. 2a). That is, the growth rate is very small during that time. Then the growth rate increases quickly with the surface coverage fraction. Therefore, the continuous ®lm with a distinct morphology is formed shortly after the time t (Fig. 2b). It indicates that there is an anomalous ®lm growth relaxation during the deposition process. We propose that this behavior is mainly caused by the liquid surface effect since such a phenomenon has never happened in other ®lm systems deposited on solid substrates [1±10]. When the sputtered atoms strike the oil surface separately, some atoms with lower energy tend to stay on the liquid surface, and others with higher energy penetrate the liquid surface. However, the independent deposition atoms on the liquid surface can easily be evaporated again [14]. Therefore, the growth rate is small at the early stage of the deposition process. On the other hand, the local temperature of the liquid surface would increase due to the strong strike between the liquid surface and the deposition atoms. Higher temperature would obviously result in both evaporation of the liquid surface (including the second evaporation) and penetration of the deposition atoms. The penetration effect would increase the concentration of the metallic atoms in the liquid surface layer. When the concentration reaches a proper level, a great number of atomic clusters would be formed and gradually be exposed on the liquid surface due to the evaporation effect. Then the exposed clusters would form the percolation structure (Fig. 2a). So this indirect production pattern of the
32
C.M. Feng et al. / Thin Solid Films 342 (1999) 30±34
clusters would greatly prolong the period of the nucleation on the liquid surface. However, if the surface coverage fraction increases. the scale of the clusters increases and the new clusters are constantly formed. They would result in the rapid increase of the growth rate. Finally, the liquid surface is fully covered with the metallic Ag atoms and the growth rate approaches the value of the deposition rate on the substrate. 3.2. The temperature dependence of the relaxation rate In Section 3.1, we see that there exists a relaxation mechanism for ®lm growth which arises because of the effect of the liquid surface on the deposition atoms, and t is a characteristic time associated with the approach to the disappearance of the liquid surface effect. For this reason, t should stand for the relaxation time for ®lm growth on the liquid substrate. In order to understand the relaxation mechanism well, we measured the temperature dependence of the relaxation rate. Fig. 3 shows that the relaxation rate R ( 1/t ) changes sensitively with the substrate temperature T. We propose that, when the substrate temperature is lower, the surface tension coef®cient of the silicone oil is relatively larger. Then quite higher energy is needed for the deposition atoms to penetrate the liquid surface. Therefore, most of the atoms would stay on the liquid surface and can easily be evaporated again. So the relaxation rate is small. However, as the substrate temperature goes up, the penetration effect would obviously increase due to the reduction of the surface tension in the early deposition stage. Then the density of the deposition atoms in the liquid surface layer would increase quickly and the clusters would form in a shorter time. There-
Fig. 4. The SEM photograph of two of Ag rough ®lms deposited on the oil/ Si(111) substrate at C 50 W. The substrate temperature and deposition time are (a) T 300 K, t 255 s, (b) T 330 K, t 120 s. The two samples have an approximate sheet resistance Rs 5 V.
fore, the relaxation rate increases sensitively. In Fig. 3, it is also found that, when both the capacity and the temperature are ®xed, the relaxation rate on the oil/Si(111) substrate is bigger than that of the oil glass. We suggest that this difference mainly results from the spreading effect of the oil drop on the solid surfaces. The nucleation of islands is related to the surface potential and the surface potential is dependent on the spreading status of the oil drop on the solid surfaces. Therefore, the relaxation rate has relation to the nature of the solid substrates. 3.3. The surface morphology
Fig. 3. The relaxation rate R ( 1/t ) as a function of the substrate temperature T at C 30 W, where t is an average of three repeat measurements. The sheet resistance ranges from 10 4 to 10 5 V. Empty circles: the oil drops are dripped on Si(111) surfaces. Solid circles: the oil drops are dripped on glass surfaces.
Figs. 2, 4 and 5 show the SEM photograph of the Ag samples, in which a characteristic structure at the micrometer length scales is observed. The anomalous morphology is related to the nucleation and growth of islands (or clusters) on the oil surface. Since the thermal expansive coef®cient of the oil is bigger than that of the metallic Ag ®lm, we propose that the expansion and contraction of both the Ag ®lm and the oil substrate should also make contributions to the formation of the morphology. Fig. 4 exhibits two representative SEM photographs of the Ag thin ®lms deposited on the oil/Si(111) substrate at 300 and 330 K. The surface morphology has a distinct branched structure and the orientation of these branches is preferential. This is pronounced at 330 K where the independent branches are preferentially arranged in the one direction of the oil surface. We propose that this preference
C.M. Feng et al. / Thin Solid Films 342 (1999) 30±34
33
the ®lm and exhibits an obvious tendency of directed growth around islands. We suggest that this phenomenon mainly results from the anisotropy of the local surface tension. This anisotropy is dependent on the local spreading status of the oil and the spreading status is related to the local temperature of the oil surface. When the lower capacity is applied, the sputtered atoms possess a lower energy and most of the atoms tend to stay on the oil surface. The change of the local temperature would be smaller due to the serious second evaporation. Therefore, the anisotropy of the local surface tension is not clear and the ®lm grows isotropically around islands (Fig. 5a). At the higher capacity, however, the sputtered atoms have a larger energy and most of them penetrate the liquid surface. They would result in the greater increase of the local temperature. Then the anisotropy of the local surface tension will become obvious. Therefore, the ®lm shows an distinct branched structure around islands (Fig. 5b). 4. Conclusion
Fig. 5. The SEM photograph of two of Ag rough ®lms deposited on oil/ Si(111) substrate at T 300 K. The incident r.f. capacity and deposition time are (a) C 15 W, t 900 s, (b) C 50 W, t 245 s. The two samples have an approximate sheet resistance Rs 5 £ 10 V.
is mainly caused by the anisotropy of the surface tension of the oil drop on the Si(111) terrace. The ®lm growth is related to the surface potential and the surface potential is dependent on the tension force. Since the spreading rate of the oil drop on the Si(111) plane is different in some directions, the shape of the oil drop is not a perfect circle. This would result in the anisotropy of the liquid surface tension. Then the gradients of the surface potential are different in the two mutual perpendicular directions. Therefore, the growth rate of the ®lm on the liquid surface shows two different speeds in both these directions. When the temperature goes up, the anisotropy of the liquid surface would increase due to the enhancing of the spreading effect. Therefore, this preference will become more obvious. In Figs. 2b and 4a, one ®nds that, when the capacity and temperature is ®xed, the surface morphology is related to the nature of the solid surface below the oil drop. The ®lm deposited on the oil/glass substrate exhibits a distinct-disordered structure (Fig. 2b) since the spreading of the oil drop on glass is homogeneous in all directions. This result indicates again that the ®lm growth is dependent on the spreading status of the oil drop on the solid surfaces. Fig. 5 shows that the ®lm growth around islands is susceptible to the incident r.f. capacity. The ®lm deposited at C 15 W exhibits a snow¯ake-like structure. If C 50 W, however, a characteristic-branched structure appears on
In conclusion, we have studied the growth behavior and surface morphology of an Ag thin ®lm system, in which the liquid substrates are used. The anomalous ®lm-growth relaxation, which has relation to the interaction between the deposition atoms and the liquid substrate, is found in this system. The relaxation rate is strongly dependent on the substrate temperature mainly due to the evaporation effect and the penetration effect. The surface morphology is very susceptible to the substrate temperature, the incident r.f. capacity, and the nature of the solid substrate since the ®lm growth is dependent on the spreading status of the oil drop on the solid surface. We believe that the relaxation effect presented in this paper is very important for two reasons: (1) It could be universal in the growth of thin ®lms due to the presence of the interaction between the deposition atoms and the substrate. (2) It would be helpful for us to understand the dynamic process of ®lm growth deeply. However, further study on the relaxation mechanism is still needed. This work is presently in progress. Acknowledgements This work was supported by the Chinese Natural Science Foundation (grant no. 19504003) and the Zhejiang Provincial Natural Science Foundation of China (grant no. 195021). References [1] J. Krim, I. Heyvaert, C. Van Haesendonck, Y. Bruynseraede, Phys. Rev. Lett. 70 (1993) 57. [2] Y. Song, S.I. Lee, J.R. Gaines, Phys. Rev. B 46 (1992) 14.
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C.M. Feng et al. / Thin Solid Films 342 (1999) 30±34
[3] G.X. Ye, J.S. Wang, Y.Q. Xu, Q.R. Zhang, Solid State Commun. 88 (1993) 275. [4] Y. Yagil, G. Deutscher, D.J. Bergman, Phys. Rev. Lett. 69 (1992) 1423. [5] F. Wu, S.G. Jaloviar, D.E. Savage, M.G. Lagally, Phys. Rev. Lett. 71 (1993) 4190. [6] C. Quaeyhaegens, G. Knuyt, J.D. Haen, H.M. Stals, Thin Solid Films 258 (1995) 170. [7] M. Park, S.J. Krause, S.R. Wilson, Appl. Phys. Lett. 64 (1994) 721. [8] K.E. Johnson, D.D. Chimbliss, R.J. Wilson, S. Chiang, J. Vac. Technol. A 11 (1993) 1654.
[9] M. Hohage, M. Bott, M. Morgenstern, Z. Zhang, T. Michely, G. Comsa, Phys Rev. Lett. 76 (1996) 2366. [10] D.E. Jesson, K.M. Chen, S.J. Pennycook, T. Thundat, R.J. Warmack, Phys Rev. Lett. 77 (1996) 1330. [11] P. Meakin, Prog. Solid State Chem. 20 (1990) 135. [12] F. Family, Physica (Amsterdam) A 168 (1990) 561. [13] G.X. Ye, Y.Q. Xu, H.L. Ge, Z.K. Jiao, Q.R. Zhang, Phys. Lett. A 198 (1995) 251. [14] G.X. Ye, Q.R. Zhang, C.M. Feng, H.L. Ge, Z.K. Jiao, Phys. Rev. B 54 (1996) 14754.