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An experimental study of the influence of film edges and imperfections on buckling morphologies of quenched iron films
Thin Solid Films 519 (2011) 7936–7939
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
An experimental study of the influence of film edges and imperfections on buckling morphologies of quenched iron films Sen-Jiang Yu a,⁎, Miao-Gen Chen a, Yong-Ju Zhang b, Hong Zhou a, Ping-Zhan Si a a b
Department of Physics, China Jiliang University, Hangzhou 310018, PR China Department of Physics, Taizhou University, Linhai 317000, PR China
a r t i c l e
i n f o
Article history: Received 15 January 2011 Received in revised form 5 July 2011 Accepted 11 July 2011 Available online 21 July 2011 Keywords: Buckling Edge effect Stress Quenching Sputtering
a b s t r a c t Iron films, quenched by silicone oil during deposition, have been prepared on glass substrates by a DCmagnetron sputtering method. The role of film edges and imperfections on the morphologies of buckle driven delaminations in the films has been investigated. The buckling patterns are found to initiate usually from areas such as the film edges, spreading fronts of silicone oil and other imperfections in the films. When the buckling patterns grow near the fronts of silicone oil, they tend to form disordered telephone cord buckle networks. When the patterns initiate from the film edges, they generally have first a straight-sided shape perpendicular to the edges, and then propagate a few micrometers apart in bifurcation or telephone cord structure. The growth of the buckling patterns can be well controlled by introducing some imperfections into the glass substrates before deposition. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
The presence of residual stress was frequently observed in films and coatings fabricated by vapor deposition and sputtering process. It has been reported that thin layers containing large residual stresses are susceptible to delamination and/or buckling, resulting in interesting topographical patterns such as straight-sided [1], circular [2] and telephone cord structures [3]. The basic mechanism governing the propagation of the buckles, as well as their morphological tendencies, has been largely worked out and validated by experiment [1–8]. However, the initiation and propagation of the buckles from film imperfections receive little attention. Recently Moon et al. studied growth and morphology of buckle delamination on pre-patterned regions of low adhesion at the interface between the film and substrate [9]. More recently Abdallah et al. focused on the effect of etch defects and edge defects on buckle initiation and propagation in patterned ITO layers on a polymer substrate [10]. In this work, the influence of film edges and imperfections on buckling morphologies in silicone oil quenched iron films has been investigated systematically. It has been found that the film imperfections, including free edges, artificial edges, spreading fronts of silicone oil, introduced impurities before and during deposition etc., play an important role in determining and controlling the morphological tendencies, initiation and growth behaviors of the buckling patterns.
The iron films were prepared by using room temperature DCmagnetron sputtering method under 0.8 Pa Ar gas (purity 99.999%). An iron (purity 99.9%) disk with diameter D = 60 mm was used as the sputtering target. The DC sputtering power P was 85 W, corresponding to the deposition rate f ≈ 18 nm/min. The deposition time t ranged from 10 s to 12 min, which was controlled precisely by a computer. After deposition, the film thickness was calibrated by a profilometer (α-step 200 profilometer, TENCOR). The surface morphologies of the samples were examined with an optical microscope (Leica DMLM), equipped with a charge-coupled device (CCD) camera (Leica DC 300). In our experiment, the quenched Fe films can be fabricated by two following techniques. First, a drop of silicone oil (DOW CORNING 705 Diffusion Pump Fluid with a vapor pressure below 10-8 Pa at room temperature) with a diameter 2–4 mm was dripped on a flat glass surface before deposition. Second, the glass substrate was composed of two parts: about 25 × 16 mm2 frosted surface and 25× 9 mm2 flat surface. Some silicone oil was painted onto the frosted glass surface before sputtering. During deposition, the temperature of the silicone oil would increase slightly due to the bombardment of the depositing atoms and heating of the target source, and therefore the silicone oil layer and oil drop can spread steadily onto the flat glass surface [11,12]. The increased temperature of the film sample detected by a thermocouple, which contacted with the back face of the sample, was about 10 K, and it changed with the sputtering power and deposition time. As a result, the Fe films deposited on the glass substrates are quenched by the silicone oil gradually during deposition. The details of the formation of the quenched Fe films can be found in our previous work [13].
⁎ Corresponding author. Tel.: + 86 571 86835756; fax: + 86 571 86914404. E-mail addresses: [email protected], [email protected] (S.-J. Yu). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.07.024
S.-J. Yu et al. / Thin Solid Films 519 (2011) 7936–7939
3. Results and discussion The typical surface morphologies of the quenched Fe films prepared by two techniques described above are shown in Fig. 1, where (A) and (B) represent the cases of oil drop and oil layer, respectively. Generally, the spreading front of the silicone oil drop is a perfect circle, and therefore the quenched Fe film in this case exhibits a ring shape, as shown in Fig. 1(A). It is clear that the ideal front of the silicone oil layer should be a straight line. However, the actual front of silicone oil layer in most cases is an irregular curve due to the inhomogeneity of the boundary between the frosted and flat glass surfaces, as shown in Fig. 1(B). When the Fe films are covered with the silicone oil during deposition, the kinetic energy of the Fe atoms (or clusters) will be dissipated and the oil molecules will block the thermal diffusion of the Fe atoms (or clusters). The quenching process generates a large number of defects in the films, and thus resulting in the development of a high compressive stress in the Fe films. The compressive stress is relieved by buckling and delamination of the film from its substrate. The typical overviews of the buckling morphologies are shown in Fig. 1(A, B). In Fig. 1(C), the buckling patterns are shown at a higher resolution, so that more details of the patterns become observable. It
Fig. 1. Typical buckling morphologies near the spreading fronts of silicone oil. (A) The case of silicone oil drop. (B) The case of silicone oil layer. (C) The higher magnification buckling patterns of (B).
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has been proved that buckling patterns exhibiting telephone cord mode usually permit an optimal residual compressive stress release [6]. Our experimental observations indicate that the buckling patterns usually initiate from the fronts of the silicone oil and then propagate randomly to form irregular buckling networks. In some cases, the buckle patterns can propagate through the fronts of silicone oil and spread into the unquenched Fe films up to a certain distance, as shown in Fig. 1. The propagation distance is of the order of tens micrometers. It should be noted that we did not observe any buckling patterns in the unquenched Fe films that are far away from the fronts of silicone oil. The experiment also shows that the buckle width, maximum height and delamination length increase with the film thickness linearly [13]. Larger delamination length and width correspond to a larger delamination region, which permits larger residual compressive release. Many previous studies showed that the buckling patterns always start from the imperfections of thin films because the local internal stress near imperfection is quite large and the adhesion energy between film and substrate in the imperfection region is comparatively small [14,15]. Our previous work has demonstrated that the mechanical properties, including microstructure, internal stress, adhesion energy etc., of quenched and unquenched Fe films are quite different [16]. Therefore, the film regions near the fronts of silicone oil should contain a large number of imperfections. Because the internal stress is compressive in quenched Fe film while it is tensile in unquenched film [16], the buckling patterns generally initiate in the quenched region near the fronts of silicone oil. The buckling patterns can propagate in the unquenched region for a certain distance, indicating that the mechanical properties of the unquenched film near the fronts of silicone oil should be different from those of the unquenched film far away from the oil fronts. We propose that the silicone oil molecules may diffuse into the interface between the film and the substrate under the atmosphere pressure, reducing the adhesion and initiating the buckles. The propagation distance of the buckles in unquenched film should be related to the diffusion region of the silicone oil. Further studies on the microstructures and mechanical properties of the Fe films in different areas should be interesting. Given a long enough deposition time, the silicone oil would spread through the edges of the glass substrates, and thus the fronts of the silicone oil and the film free edges are overlapped. In this case, the buckling patterns tend to nucleate at the film edges for the edges are imperfections of the film. Fig. 2 shows the typical buckling morphologies near the film edges. The buckling patterns exhibit a straight-sided shape first and then propagate a few micrometers into bifurcation structures on the top, as shown in Fig. 2(A). The initial straight-sided buckles are almost perpendicular to the film edges, and the average spacing between the neighboring straight buckles is in the range of 10–30 μm. Further propagation of the bifurcation structures results in the formation of telephone cord structures, as shown in Fig. 2(B). The impurities such as large particles from the target are sometimes observed in the films. The typical buckling morphologies near the impurities are shown in Fig. 2(C), in which the buckling patterns nucleate first at the impurities and then radiate outwards. Similarly, the buckles first have straight-sided shapes and then transform into bifurcation or telephone cord structures gradually. According to the previous studies [1–10,13–16], the buckle delaminations generally localize and propagate across the film in telephone cord buckles or straight-sided blisters. It is well known that the telephone cord buckles are generated under equi-biaxial stresses, while the straight-sided blisters form under uniaxial stresses. Because the residual compressive stress in a homogeneous film is equi-biaxial, the telephone cord structures are commonly observed in various film/ substrate specimens while the straight-sided wrinkles are very rare in the experiment. In an equi-biaxial state of stress, there is neither a preferred orientation for the buckles, nor a reason for the buckles to
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S.-J. Yu et al. / Thin Solid Films 519 (2011) 7936–7939
formation of bifurcation or telephone cord structures. The length l characterizing the transition from uniaxial stress to equi-biaxial stress is in the range of 30–60 μm in our experiment. If the substrates are homogeneous and isotropic, the deposited films are generally in the equi-biaxial stress state and tend to form disordered buckling networks, as observed in many experiments. If the substrates are anisotropic in nature [19] or are patterned prior to film deposition by lithographic and other techniques [9,10], the delamination phenomena can be well controlled and finally form ordered buckling patterns. In the experiment, we employed a very simple method to pattern the glass substrates. The flat glass surface was wiped by using a piece of ethanol dipped absorbent gauze along a certain direction before deposition. A larger number of impurities would remain on the substrate surface after evaporation of the ethanol. After deposition, the buckle driven delaminations start to nucleate at the impurities because the local compressive stress near impurity is quite large and the adhesion between film and substrate at impurity region is comparatively small [14,15]. Fig. 3 shows the typical buckling morphologies of Fe films on the patterned glass substrates. It is interesting that the buckling patterns are aligned parallel because the impurities that remained on the substrates are generally ordered along the wiping trace of ethanol. In the case of two or more ethanol wiping directions in the Fe film, the buckling patterns propagate along these directions simultaneously. However, the optimal direction for buckle growth is determined by the competition of the impurities in different wiping directions. The typical morphology is shown in Fig. 3(B), in which four different buckling regions could be seen obviously. In most cases, the buckling patterns look like parallel dotted lines with a slight delamination of the film at each dot, which actually looks like a circular blister. The smaller blisters are frequently observed to possess perfect circular configuration. Once the size is beyond a critical value, however, the circular blister exhibits wavy configuration at its edge [2]. The neighboring circular blisters in the same line tend to connect one another and finally the telephone cord buckles appear.
Fig. 2. Typical buckling morphologies near the film free edges. (A) Transition of a straight-sided shape to bifurcation structure. (B) Transition of a straight-sided shape to telephone cord structure. (C) Radiate shape near the dotted film impurities.
form ordered patterns. Therefore the buckling patterns tend to form irregular telephone cord buckle networks, as shown in Fig. 1. When film edges are present, the stress in the film will no longer be uniform or equi-biaxial. There is a strong orientation to the stress in the vicinity of the film edges. Bowden et al. [17,18] discussed the stress state near the film edges or steps and the two stress components perpendicular and parallel to the film edges can be respectively expressed as −x = l σx = −σ0 1−e
ð1Þ
−x = l σy = −σ0 1−νm e
ð2Þ
where σ0 is the value of equi-biaxial stress far away from the film edge (minus represents the stress is compressive), σx is the stress in the x-direction (perpendicular to the film edge as shown in Fig. 2(A)) and σy is the stress in the y-direction (parallel to the film edge), and x is measured starting from the film edge. For iron, the Poisson's ratio νm is 0.293. It is clear that if x = 0, i.e., at the film edge, σx = 0 and σy ≈ 0.7σ0. That is to say, only the parallel stress component exists at the film and it is relieved by formation of straight-sided buckles perpendicular to the film edges. With increasing the distance x, σx increases quickly while σy increases slowly, and finally appears the equi-biaxial stress state (σx = σy = σ0), which is then relieved by
Fig. 3. Typical buckling morphologies in the Fe films on patterned glass substrates wiped by ethanol prior to deposition. (A) There is only one wiping direction of ethanol and (B) there are two wiping directions of ethanol in the local film region.
S.-J. Yu et al. / Thin Solid Films 519 (2011) 7936–7939
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patterns tend to grow in regions outside of the pit only, as shown in Fig. 4(C). In this case, the buckling patterns have first a straight-sided shape perpendicular to the pit edges and then propagate a few micrometers apart in a bifurcation structure, which are quite similar to the buckling morphologies near the film free edges as seen Fig. 2. 4. Conclusions In summary, quenched Fe films containing high compressive stresses have been prepared by using the mobile and expansive properties of silicone oil. The influence of film edges and imperfections on buckling morphologies of these films is investigated in detail. The buckling patterns initiate from the imperfections of the films such as the film free edges, pit edges, spreading fronts of silicone oil and introduced impurities etc. The buckling patterns tend to form disordered telephone cord buckle networks in areas near the fronts of silicone oil. In areas near the film free edges or pit edges, the buckling patterns tend to form parallel straight-sided blisters at the edges, and then change into bifurcation or telephone cord structures gradually. If the glass substrates are patterned before deposition, the morphologies and growth behaviors of the buckling patterns can be well controlled. Acknowledgments
Fig. 4. Typical buckling morphologies near the pit edges created by a rough sand paper. (A) Overview of buckling morphologies in a pit. The black arrow denotes propagation of individual telephone cord buckle from the pit edge. (B) Parallel circular or straight blisters along the pit edges shown at a higher magnification. (C) Propagation of straight-sided buckles outside of the pit when the pit is quite deep.
In order to further understand the influence of edges and imperfections on the buckling morphologies of quenched Fe films, we employed another simple method to pattern the glass substrate in the experiment. Before deposition, the cleaned flat glass surface was rubbed by using a rough sand paper. After deposition, buckling patterns initiate near the rubbed pits and nicks as imperfections of the Fe film. Fig. 4(A) shows the overview of buckling morphologies near a pit created by the rough sand paper. It is clear that the buckling patterns generally grow around the pit edge. We could also observe that individual telephone cord buckle starts from the pit edge and then propagates into the homogeneous film regions, as seen as the black arrow in Fig. 4(A). If the pit is comparatively shallow and the films in and out of the pit are continuous, the buckling patterns tend to form circular or parallel straight blisters along the pit edges, as shown in Fig. 4(A, B). Moreover the blisters can propagate across the edge lines and grow in and out of the pits simultaneously. With increasing the distance from the pit edge, the parallel circular or straight blisters disappear and then wavy or telephone cord buckles form gradually. If the pit is quite deep and the film in the pit is much inhomogeneous, the buckling
We thank Ping-Gen Cai and Zhi-Wei Jiao for useful discussions and technical assistance. This work was supported by the National Natural Science Foundation of China (Grant Nos. 10874159, 11074227) and by the Zhejiang Provincial Natural Science Foundation (Grant Nos. Y7080042, Y6090542, R6110362). References [1] C. Coupeau, Thin Solid Films 406 (2002) 190. [2] J.W. Hutchinson, M.D. Thouless, E.G. Liniger, Acta Metall. Mater. 40 (1992) 295. [3] A.A. Abdallah, D. Kozodaev, P.C.P. Bouten, J.M.J. den Toonder, U.S. Schubert, G. de With, Thin Solid Films 503 (2006) 167. [4] G. Gille, B. Rau, Thin Solid Films 120 (1984) 109. [5] J.W. Hunchinson, Z. Suo, Adv. Appl. Mech. 29 (1992) 64. [6] B. Audoly, Phys. Rev. Lett. 83 (1999) 4124. [7] M.W. Moon, H.M. Jensen, J.W. Hutchinson, K.H. Oh, A.G. Evans, J. Mech. Phys. Solids 50 (2002) 2355. [8] O. van der Sluis, A.A. Abdallah, P.C.P. Bouten, P.H.M. Timmermans, J.M.J. den Toonder, G. de With, Engin. Fract. Mech. 78 (2011) 877. [9] M.W. Moon, K.R. Lee, K.H. Oh, J.W. Hutchinson, Acta Mater. 52 (2004) 3151. [10] A.A. Abdallah, P.C.P. Bouten, J.M.J. den Toonder, G. de With, Surf. Coat. Technol. 205 (2011) 3103. [11] C.M. Feng, H.L. Ge, M.R. Tang, G.X. Ye, Z.K. Jiao, Thin Solid Films 342 (1999) 30. [12] X.M. Tao, G.X. Ye, Q.L. Ye, J.S. Jin, Y.F. Lao, Z.K. Jiao, Phys. Rev. B 66 (2002) 115406. [13] S.J. Yu, Y.J. Zhang, M.G. Chen, Thin Solid Films 518 (2009) 222. [14] J.W. Hutchinson, M.Y. He, A.G. Evans, J. Mech. Phys. Solids 48 (2000) 709. [15] M.W. Moon, J.W. Chung, K.R. Lee, K.H. Oh, R. Wang, A.G. Evans, Acta Mater. 50 (2002) 1219. [16] S.J. Yu, Y.J. Zhang, H. Zhou, P.G. Cai, M.G. Chen, Appl. Surf. Sci. 256 (2009) 909. [17] N. Bowden, S. Brittain, A.G. Evans, J.W. Hutchinson, G.M. Whitesides, Nature 393 (1998) 146. [18] W.T.S. Huck, N. Bowden, P. Onck, T. Pardoen, J.W. Hutchinson, G.M. Whitesides, Langmuir 16 (2000) 3497. [19] F. Zhao, B. Wang, X. Cui, N. Pan, H. Wang, J.G. Hou, Thin Solid Films 489 (2005) 221.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
An experimental study of the influence of film edges and imperfections on buckling morphologies of quenched iron films Sen-Jiang Yu a,⁎, Miao-Gen Chen a, Yong-Ju Zhang b, Hong Zhou a, Ping-Zhan Si a a b
Department of Physics, China Jiliang University, Hangzhou 310018, PR China Department of Physics, Taizhou University, Linhai 317000, PR China
a r t i c l e
i n f o
Article history: Received 15 January 2011 Received in revised form 5 July 2011 Accepted 11 July 2011 Available online 21 July 2011 Keywords: Buckling Edge effect Stress Quenching Sputtering
a b s t r a c t Iron films, quenched by silicone oil during deposition, have been prepared on glass substrates by a DCmagnetron sputtering method. The role of film edges and imperfections on the morphologies of buckle driven delaminations in the films has been investigated. The buckling patterns are found to initiate usually from areas such as the film edges, spreading fronts of silicone oil and other imperfections in the films. When the buckling patterns grow near the fronts of silicone oil, they tend to form disordered telephone cord buckle networks. When the patterns initiate from the film edges, they generally have first a straight-sided shape perpendicular to the edges, and then propagate a few micrometers apart in bifurcation or telephone cord structure. The growth of the buckling patterns can be well controlled by introducing some imperfections into the glass substrates before deposition. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
The presence of residual stress was frequently observed in films and coatings fabricated by vapor deposition and sputtering process. It has been reported that thin layers containing large residual stresses are susceptible to delamination and/or buckling, resulting in interesting topographical patterns such as straight-sided [1], circular [2] and telephone cord structures [3]. The basic mechanism governing the propagation of the buckles, as well as their morphological tendencies, has been largely worked out and validated by experiment [1–8]. However, the initiation and propagation of the buckles from film imperfections receive little attention. Recently Moon et al. studied growth and morphology of buckle delamination on pre-patterned regions of low adhesion at the interface between the film and substrate [9]. More recently Abdallah et al. focused on the effect of etch defects and edge defects on buckle initiation and propagation in patterned ITO layers on a polymer substrate [10]. In this work, the influence of film edges and imperfections on buckling morphologies in silicone oil quenched iron films has been investigated systematically. It has been found that the film imperfections, including free edges, artificial edges, spreading fronts of silicone oil, introduced impurities before and during deposition etc., play an important role in determining and controlling the morphological tendencies, initiation and growth behaviors of the buckling patterns.
The iron films were prepared by using room temperature DCmagnetron sputtering method under 0.8 Pa Ar gas (purity 99.999%). An iron (purity 99.9%) disk with diameter D = 60 mm was used as the sputtering target. The DC sputtering power P was 85 W, corresponding to the deposition rate f ≈ 18 nm/min. The deposition time t ranged from 10 s to 12 min, which was controlled precisely by a computer. After deposition, the film thickness was calibrated by a profilometer (α-step 200 profilometer, TENCOR). The surface morphologies of the samples were examined with an optical microscope (Leica DMLM), equipped with a charge-coupled device (CCD) camera (Leica DC 300). In our experiment, the quenched Fe films can be fabricated by two following techniques. First, a drop of silicone oil (DOW CORNING 705 Diffusion Pump Fluid with a vapor pressure below 10-8 Pa at room temperature) with a diameter 2–4 mm was dripped on a flat glass surface before deposition. Second, the glass substrate was composed of two parts: about 25 × 16 mm2 frosted surface and 25× 9 mm2 flat surface. Some silicone oil was painted onto the frosted glass surface before sputtering. During deposition, the temperature of the silicone oil would increase slightly due to the bombardment of the depositing atoms and heating of the target source, and therefore the silicone oil layer and oil drop can spread steadily onto the flat glass surface [11,12]. The increased temperature of the film sample detected by a thermocouple, which contacted with the back face of the sample, was about 10 K, and it changed with the sputtering power and deposition time. As a result, the Fe films deposited on the glass substrates are quenched by the silicone oil gradually during deposition. The details of the formation of the quenched Fe films can be found in our previous work [13].
⁎ Corresponding author. Tel.: + 86 571 86835756; fax: + 86 571 86914404. E-mail addresses: [email protected], [email protected] (S.-J. Yu). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.07.024
S.-J. Yu et al. / Thin Solid Films 519 (2011) 7936–7939
3. Results and discussion The typical surface morphologies of the quenched Fe films prepared by two techniques described above are shown in Fig. 1, where (A) and (B) represent the cases of oil drop and oil layer, respectively. Generally, the spreading front of the silicone oil drop is a perfect circle, and therefore the quenched Fe film in this case exhibits a ring shape, as shown in Fig. 1(A). It is clear that the ideal front of the silicone oil layer should be a straight line. However, the actual front of silicone oil layer in most cases is an irregular curve due to the inhomogeneity of the boundary between the frosted and flat glass surfaces, as shown in Fig. 1(B). When the Fe films are covered with the silicone oil during deposition, the kinetic energy of the Fe atoms (or clusters) will be dissipated and the oil molecules will block the thermal diffusion of the Fe atoms (or clusters). The quenching process generates a large number of defects in the films, and thus resulting in the development of a high compressive stress in the Fe films. The compressive stress is relieved by buckling and delamination of the film from its substrate. The typical overviews of the buckling morphologies are shown in Fig. 1(A, B). In Fig. 1(C), the buckling patterns are shown at a higher resolution, so that more details of the patterns become observable. It
Fig. 1. Typical buckling morphologies near the spreading fronts of silicone oil. (A) The case of silicone oil drop. (B) The case of silicone oil layer. (C) The higher magnification buckling patterns of (B).
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has been proved that buckling patterns exhibiting telephone cord mode usually permit an optimal residual compressive stress release [6]. Our experimental observations indicate that the buckling patterns usually initiate from the fronts of the silicone oil and then propagate randomly to form irregular buckling networks. In some cases, the buckle patterns can propagate through the fronts of silicone oil and spread into the unquenched Fe films up to a certain distance, as shown in Fig. 1. The propagation distance is of the order of tens micrometers. It should be noted that we did not observe any buckling patterns in the unquenched Fe films that are far away from the fronts of silicone oil. The experiment also shows that the buckle width, maximum height and delamination length increase with the film thickness linearly [13]. Larger delamination length and width correspond to a larger delamination region, which permits larger residual compressive release. Many previous studies showed that the buckling patterns always start from the imperfections of thin films because the local internal stress near imperfection is quite large and the adhesion energy between film and substrate in the imperfection region is comparatively small [14,15]. Our previous work has demonstrated that the mechanical properties, including microstructure, internal stress, adhesion energy etc., of quenched and unquenched Fe films are quite different [16]. Therefore, the film regions near the fronts of silicone oil should contain a large number of imperfections. Because the internal stress is compressive in quenched Fe film while it is tensile in unquenched film [16], the buckling patterns generally initiate in the quenched region near the fronts of silicone oil. The buckling patterns can propagate in the unquenched region for a certain distance, indicating that the mechanical properties of the unquenched film near the fronts of silicone oil should be different from those of the unquenched film far away from the oil fronts. We propose that the silicone oil molecules may diffuse into the interface between the film and the substrate under the atmosphere pressure, reducing the adhesion and initiating the buckles. The propagation distance of the buckles in unquenched film should be related to the diffusion region of the silicone oil. Further studies on the microstructures and mechanical properties of the Fe films in different areas should be interesting. Given a long enough deposition time, the silicone oil would spread through the edges of the glass substrates, and thus the fronts of the silicone oil and the film free edges are overlapped. In this case, the buckling patterns tend to nucleate at the film edges for the edges are imperfections of the film. Fig. 2 shows the typical buckling morphologies near the film edges. The buckling patterns exhibit a straight-sided shape first and then propagate a few micrometers into bifurcation structures on the top, as shown in Fig. 2(A). The initial straight-sided buckles are almost perpendicular to the film edges, and the average spacing between the neighboring straight buckles is in the range of 10–30 μm. Further propagation of the bifurcation structures results in the formation of telephone cord structures, as shown in Fig. 2(B). The impurities such as large particles from the target are sometimes observed in the films. The typical buckling morphologies near the impurities are shown in Fig. 2(C), in which the buckling patterns nucleate first at the impurities and then radiate outwards. Similarly, the buckles first have straight-sided shapes and then transform into bifurcation or telephone cord structures gradually. According to the previous studies [1–10,13–16], the buckle delaminations generally localize and propagate across the film in telephone cord buckles or straight-sided blisters. It is well known that the telephone cord buckles are generated under equi-biaxial stresses, while the straight-sided blisters form under uniaxial stresses. Because the residual compressive stress in a homogeneous film is equi-biaxial, the telephone cord structures are commonly observed in various film/ substrate specimens while the straight-sided wrinkles are very rare in the experiment. In an equi-biaxial state of stress, there is neither a preferred orientation for the buckles, nor a reason for the buckles to
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S.-J. Yu et al. / Thin Solid Films 519 (2011) 7936–7939
formation of bifurcation or telephone cord structures. The length l characterizing the transition from uniaxial stress to equi-biaxial stress is in the range of 30–60 μm in our experiment. If the substrates are homogeneous and isotropic, the deposited films are generally in the equi-biaxial stress state and tend to form disordered buckling networks, as observed in many experiments. If the substrates are anisotropic in nature [19] or are patterned prior to film deposition by lithographic and other techniques [9,10], the delamination phenomena can be well controlled and finally form ordered buckling patterns. In the experiment, we employed a very simple method to pattern the glass substrates. The flat glass surface was wiped by using a piece of ethanol dipped absorbent gauze along a certain direction before deposition. A larger number of impurities would remain on the substrate surface after evaporation of the ethanol. After deposition, the buckle driven delaminations start to nucleate at the impurities because the local compressive stress near impurity is quite large and the adhesion between film and substrate at impurity region is comparatively small [14,15]. Fig. 3 shows the typical buckling morphologies of Fe films on the patterned glass substrates. It is interesting that the buckling patterns are aligned parallel because the impurities that remained on the substrates are generally ordered along the wiping trace of ethanol. In the case of two or more ethanol wiping directions in the Fe film, the buckling patterns propagate along these directions simultaneously. However, the optimal direction for buckle growth is determined by the competition of the impurities in different wiping directions. The typical morphology is shown in Fig. 3(B), in which four different buckling regions could be seen obviously. In most cases, the buckling patterns look like parallel dotted lines with a slight delamination of the film at each dot, which actually looks like a circular blister. The smaller blisters are frequently observed to possess perfect circular configuration. Once the size is beyond a critical value, however, the circular blister exhibits wavy configuration at its edge [2]. The neighboring circular blisters in the same line tend to connect one another and finally the telephone cord buckles appear.
Fig. 2. Typical buckling morphologies near the film free edges. (A) Transition of a straight-sided shape to bifurcation structure. (B) Transition of a straight-sided shape to telephone cord structure. (C) Radiate shape near the dotted film impurities.
form ordered patterns. Therefore the buckling patterns tend to form irregular telephone cord buckle networks, as shown in Fig. 1. When film edges are present, the stress in the film will no longer be uniform or equi-biaxial. There is a strong orientation to the stress in the vicinity of the film edges. Bowden et al. [17,18] discussed the stress state near the film edges or steps and the two stress components perpendicular and parallel to the film edges can be respectively expressed as −x = l σx = −σ0 1−e
ð1Þ
−x = l σy = −σ0 1−νm e
ð2Þ
where σ0 is the value of equi-biaxial stress far away from the film edge (minus represents the stress is compressive), σx is the stress in the x-direction (perpendicular to the film edge as shown in Fig. 2(A)) and σy is the stress in the y-direction (parallel to the film edge), and x is measured starting from the film edge. For iron, the Poisson's ratio νm is 0.293. It is clear that if x = 0, i.e., at the film edge, σx = 0 and σy ≈ 0.7σ0. That is to say, only the parallel stress component exists at the film and it is relieved by formation of straight-sided buckles perpendicular to the film edges. With increasing the distance x, σx increases quickly while σy increases slowly, and finally appears the equi-biaxial stress state (σx = σy = σ0), which is then relieved by
Fig. 3. Typical buckling morphologies in the Fe films on patterned glass substrates wiped by ethanol prior to deposition. (A) There is only one wiping direction of ethanol and (B) there are two wiping directions of ethanol in the local film region.
S.-J. Yu et al. / Thin Solid Films 519 (2011) 7936–7939
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patterns tend to grow in regions outside of the pit only, as shown in Fig. 4(C). In this case, the buckling patterns have first a straight-sided shape perpendicular to the pit edges and then propagate a few micrometers apart in a bifurcation structure, which are quite similar to the buckling morphologies near the film free edges as seen Fig. 2. 4. Conclusions In summary, quenched Fe films containing high compressive stresses have been prepared by using the mobile and expansive properties of silicone oil. The influence of film edges and imperfections on buckling morphologies of these films is investigated in detail. The buckling patterns initiate from the imperfections of the films such as the film free edges, pit edges, spreading fronts of silicone oil and introduced impurities etc. The buckling patterns tend to form disordered telephone cord buckle networks in areas near the fronts of silicone oil. In areas near the film free edges or pit edges, the buckling patterns tend to form parallel straight-sided blisters at the edges, and then change into bifurcation or telephone cord structures gradually. If the glass substrates are patterned before deposition, the morphologies and growth behaviors of the buckling patterns can be well controlled. Acknowledgments
Fig. 4. Typical buckling morphologies near the pit edges created by a rough sand paper. (A) Overview of buckling morphologies in a pit. The black arrow denotes propagation of individual telephone cord buckle from the pit edge. (B) Parallel circular or straight blisters along the pit edges shown at a higher magnification. (C) Propagation of straight-sided buckles outside of the pit when the pit is quite deep.
In order to further understand the influence of edges and imperfections on the buckling morphologies of quenched Fe films, we employed another simple method to pattern the glass substrate in the experiment. Before deposition, the cleaned flat glass surface was rubbed by using a rough sand paper. After deposition, buckling patterns initiate near the rubbed pits and nicks as imperfections of the Fe film. Fig. 4(A) shows the overview of buckling morphologies near a pit created by the rough sand paper. It is clear that the buckling patterns generally grow around the pit edge. We could also observe that individual telephone cord buckle starts from the pit edge and then propagates into the homogeneous film regions, as seen as the black arrow in Fig. 4(A). If the pit is comparatively shallow and the films in and out of the pit are continuous, the buckling patterns tend to form circular or parallel straight blisters along the pit edges, as shown in Fig. 4(A, B). Moreover the blisters can propagate across the edge lines and grow in and out of the pits simultaneously. With increasing the distance from the pit edge, the parallel circular or straight blisters disappear and then wavy or telephone cord buckles form gradually. If the pit is quite deep and the film in the pit is much inhomogeneous, the buckling
We thank Ping-Gen Cai and Zhi-Wei Jiao for useful discussions and technical assistance. This work was supported by the National Natural Science Foundation of China (Grant Nos. 10874159, 11074227) and by the Zhejiang Provincial Natural Science Foundation (Grant Nos. Y7080042, Y6090542, R6110362). References [1] C. Coupeau, Thin Solid Films 406 (2002) 190. [2] J.W. Hutchinson, M.D. Thouless, E.G. Liniger, Acta Metall. Mater. 40 (1992) 295. [3] A.A. Abdallah, D. Kozodaev, P.C.P. Bouten, J.M.J. den Toonder, U.S. Schubert, G. de With, Thin Solid Films 503 (2006) 167. [4] G. Gille, B. Rau, Thin Solid Films 120 (1984) 109. [5] J.W. Hunchinson, Z. Suo, Adv. Appl. Mech. 29 (1992) 64. [6] B. Audoly, Phys. Rev. Lett. 83 (1999) 4124. [7] M.W. Moon, H.M. Jensen, J.W. Hutchinson, K.H. Oh, A.G. Evans, J. Mech. Phys. Solids 50 (2002) 2355. [8] O. van der Sluis, A.A. Abdallah, P.C.P. Bouten, P.H.M. Timmermans, J.M.J. den Toonder, G. de With, Engin. Fract. Mech. 78 (2011) 877. [9] M.W. Moon, K.R. Lee, K.H. Oh, J.W. Hutchinson, Acta Mater. 52 (2004) 3151. [10] A.A. Abdallah, P.C.P. Bouten, J.M.J. den Toonder, G. de With, Surf. Coat. Technol. 205 (2011) 3103. [11] C.M. Feng, H.L. Ge, M.R. Tang, G.X. Ye, Z.K. Jiao, Thin Solid Films 342 (1999) 30. [12] X.M. Tao, G.X. Ye, Q.L. Ye, J.S. Jin, Y.F. Lao, Z.K. Jiao, Phys. Rev. B 66 (2002) 115406. [13] S.J. Yu, Y.J. Zhang, M.G. Chen, Thin Solid Films 518 (2009) 222. [14] J.W. Hutchinson, M.Y. He, A.G. Evans, J. Mech. Phys. Solids 48 (2000) 709. [15] M.W. Moon, J.W. Chung, K.R. Lee, K.H. Oh, R. Wang, A.G. Evans, Acta Mater. 50 (2002) 1219. [16] S.J. Yu, Y.J. Zhang, H. Zhou, P.G. Cai, M.G. Chen, Appl. Surf. Sci. 256 (2009) 909. [17] N. Bowden, S. Brittain, A.G. Evans, J.W. Hutchinson, G.M. Whitesides, Nature 393 (1998) 146. [18] W.T.S. Huck, N. Bowden, P. Onck, T. Pardoen, J.W. Hutchinson, G.M. Whitesides, Langmuir 16 (2000) 3497. [19] F. Zhao, B. Wang, X. Cui, N. Pan, H. Wang, J.G. Hou, Thin Solid Films 489 (2005) 221.