Appl Phys A (2011) 103: 139–147 DOI 10.1007/s00339-010-6157-3
Marangoni effect induced micro/nano-patterning on Sb2 Te3 phase change thin film by laser pulse Aihuan Dun · Jingsong Wei · Fuxi Gan
Received: 1 October 2010 / Accepted: 2 December 2010 / Published online: 18 December 2010 © Springer-Verlag 2010
Abstract Thermocapillary and chemicapillary effects are known to coexist in a material molten pool when irradiated by a pulse laser. According to the effects, we fabricate various patterns with different shapes on a Sb2 Te3 phase change thin film by precisely adjusting the pulse energy. In this process, the laser power is fixed at 5.0 mW, and the pulse width is adjusted from 100 ns to 5 ns. The shape of the patterns gradually changes from a dimple-bowl shape, a dome shape, a “Sombrero” shape to a deep-bowl shape following an increase in the pulse energy, which corresponds to the crystallization-threshold, bump-threshold, rupture-threshold, and ablation-threshold of the material. The different patterns are the results of the competition between the thermocapillary and chemicapillary effects in the molten pool, which determine the nature of the flow and lead to the different patterns in different laser parameters.
1 Introduction Focused laser beam-assisted patterning and surface modification processes are widely researched and used in a variety of industrial applications [1–8]. Micro/nano-patterning of surfaces of various materials, such as metals, polymer, silicon, and semiconductor, are often in thin film form with a thickness ranging from several nanometers to micrometers [9]. In this process, the interaction between laser and A. Dun · J. Wei () · F. Gan Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China e-mail:
[email protected] A. Dun Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
materials usually involves melting and resolidification, even vaporization and ablation when the laser pulse energy is high enough to exceed the relevant threshold of the materials. Surface patterning features are sensitively dependent on laser pulse energy, substrate material, and the material preparation procedure [3, 10]. Many different bump patterns, such as bowl-shape, dome-shape, sombrero-shape, jetshape, and so on, can be obtained by precisely controlling the laser parameters and selecting some special materials. This transient formation process of patterns takes only several tens to hundred nanoseconds and the size of the patterns often ranges from a nanometer to a sub-micrometer, which makes it difficult to achieve the whole process of the bump growth [3]. One predominant model explaining the formation mechanism is the Marangoni effect [1–4, 11], which is the phenomenon of fluid flow caused by surface-tension gradients. The Marangoni effect was introduced by Marangoni [12] and has attracted great interest in industry and science. It consists of the thermocapillary effect and chemicapillary effect. The thermocapillary effect results from the thermal potential of a temperature gradient caused by a laser beam with Gaussian intensity distribution, and the chemicapillary effect is generated by the chemical potential caused by a compositional gradient from the temperature-activated vaporization and diffusion of solute specials [1]. In general, the two effects coexist during the whole process, and the competition between them determines the nature of the material flow and leads to the different patterns for different laser energy parameters. Sb2 Te3 phase change materials have been extensively used as optical and nonvolatile electrical data storage media due to the large difference of optical reflectivity and electrical resistivity between crystalline and amorphous states [13–22]. Sb2 Te3 phase change materials are very sensitive
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to temperature [14, 17], thus the Marangoni effect can be observed more than other materials when induced by pulse lasers and different patterns can be formed accordingly. In this work, we report the different laser-induced patterning on the Sb2 Te3 phase change thin films according to the Marangoni effect. Various patterns with different shapes are obtained, and the size of patterns is measured to be submicrometers or nanometers. The mechanism of the pattern formation is analyzed based on the surface tension driving the material flow in the molten pool, which depends not only on the surface temperature, but also on the surfactant concentration. This patterning technology can be useful in scientific and industrial fields.
2 Experimental details The amorphous Sb2 Te3 thin films were directly deposited on the glass substrate by the radio frequency (RF) magnetroncontrol sputtering method at room temperature. The background pressure was approximately 1.5 × 10−4 Pa and the sputtering pressure was about 0.8 Pa of Ar environment. The sputtering power was 50 W and the thin film thickness was 50 nm. The micro/nano-patterning was directly formed on this thin film using direct laser writing system, where a laser beam with of wavelength of 650 nm was used and focused through an objective lens with a numerical aperture of 0.90. In laser writing process, the laser power was firstly fixed at 5.0 mW, and the pulse width was adjusted from 100 ns to 5 ns. In addition, in order to observe the typical pattern shapes, we also used higher laser powers between 6.5 mW to 8 mW to irradiate the film. The pattern shapes were observed by an atomic force microscope (AFM, Veeco, Multimode V).
3 Results and discussions 3.1 Principle of Marangoni effect [1–4, 11, 12] According to the Marangoni effect, when the material surface is heated to melting temperature by the laser energy, the surface tension of the molten flow depends not only on the surface temperature gradient, but also on the surfactant concentration gradient. The surface temperature gradient corresponds to the thermocapillary effect due to the different thermal potential, and the surfactant concentration gradient corresponds to the chemicapillary effect due to the different chemical potentials in the molten pool. The thermocapillary effect usually pushes the molten materials toward the cold peripheral region with higher surface tension, whereas the chemicapillary effect often pushes the liquid material toward the regions with higher surface energy and lower surfactant
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concentration. The thermocapillary and chemicapillary effects often coexist during the whole process, and the competition between them determines the nature of the molten flow and leads to the different patterns in different laser energy conditions. When a focused Gaussian laser beam is used as the energy source, many different surface patterns, such as bowlshape, dome-shape, and “Sombrero”-shape, can be formed due to the competition between the thermocapillary and chemicapillary effects. If the laser energy is slightly higher than the melting threshold, the temperature gradient along the radial direction generates, the material partially melts. Moreover, the thermocapillary effect is dominant and drives the material from the hot center to the cold periphery, and it then forms the patterns with a central depression and a surrounding rim, which looks like a dimple-bowl. As the laser energy increases to another threshold, the chemicapillary effect is strengthened and some materials in molten pool are removed by the vaporization of adsorbed oxygen and water, as well as the phase change materials and the native oxide layer. A surfactant concentration gradient is established in the molten pool and carries the material toward the center. The bottom of the “bowl” then becomes flat, and a round, smooth central dome starts growing out. For even higher laser powers, the height of the dome gradually increases due to the continuous increase of the chemicapillary effect, finally reaching a maximum value that is larger than the height of the rim, thus the pattern shape looks like a “Sombrero.” The chemicapillary effect also reaches its peak value and is completely dominant. If the laser power becomes higher, the center part of the molten zone attains a higher temperature and the thermocapillary effect becomes dominant again. This effect pushes the material outward, causing the central dome to broaden and develops into a deeper depression or dimple in its middle part. By further increasing the laser power to a much higher level, the “Sombrero” shape gradually disappears, an irregular crater becomes observable, and a deep-bowl shaped pattern is produced. The whole process makes a Marangoni effect cycle, as shown in Fig. 1. In order to further verify the Marangoni effect cycle experimentally, Fig. 1 also give some typical experimental results, where the dimple-bowl shape, central dome growing out, Sombrero-shape, depression in the middle of dome, and deep-bowl shape are marked as ‘A’, ‘B’, ‘C’, ‘D’, and ‘E’, respectively. The corresponding profiles and laser parameters applied of different patterning shapes (circled by dashed rectangle) are also presented in Fig. 1. The experimental parameters for ‘A’, ‘B’, ‘C’, ‘D’, and ‘E’ are P (laser power) = 8 mW, τ (laser pulse width) = 5 ns; P = 5 mW, τ = 25 ns; P = 5 mW, τ = 30 ns; P = 5 mW, τ = 33 ns; P = 5 mW, τ = 42 ns, respectively. The detailed experimental results and analysis will be given in the following sections.
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Fig. 1 Schematics of Marangoni effect cycle and the corresponding typical experimental results. For the experimental results, AFM three-dimensional photos and the corresponding cross-section profiles (marked by dashed rectangle) of different patterning shapes (black scale plate in the pictures represents 1 µm) are given. ‘A’,‘B’,‘C’,‘D’, and ‘E’ marked in the pictures correspond to the five typical shapes, and the corresponding laser parameters are P (laser power) = 8 mW, τ (laser pulse width) = 5 ns; P = 5 mW, τ = 25 ns; P = 5 mW, τ = 30 ns; P = 5 mW, τ = 33 ns; P = 5 mW, τ = 42 ns, respectively. In this schematic, we assume the film thickness is very low (50 nm) and the thin film is completely melted by the laser pulse
In this Marangoni effect cycle schematics, we assume the film thickness is very low (50 nm) and the thin film is completely melted by a laser pulse. From the AFM photos, we can see that the experimental results agree well with the principle. ‘A’, ‘C’, and ‘E’ pictures in Fig. 1 represent the
three patterns with typical shapes, which will be discussed in the next section. ‘B’ and ‘D’ pictures in Fig. 1 are two critical states for this process. ‘B’ picture in Fig. 1 shows the central bump starts growing out, and ‘D’ picture in Fig. 1 shows the central bump begins rupture, which corresponds
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Fig. 2 AFM three-dimensional photos of the three typical pattern units with unique shape, and the inset in the photo is the corresponding cross-section profile. (a) Dimple-bowl shape. The laser power is 8 mW and the laser pulse width is 5 ns. (b) “Sombrero”-shape. The laser power is about 6.5 mW and laser pulse width is 30 ns. (c) Deep-bowl-shape. The laser power is about 6.5 mW and laser pulse width is 40 ns
to two threshold effects of this material. The two critical states also verify the fact that the whole process mainly involves fluid motion under this laser irradiation condition. The slight rupture shown in the profile of ‘D’ picture in Fig. 1 also means that the liquid begins to move to the cold periphery from the hot center.
3.2 Different patterns fabricated on Sb2 Te3 phase change materials Fig. 2 shows the three typical pattern units with unique shape. For clear observation, we used high laser powers. Fig. 2(a) is the AFM photo of the typical dimple-bowl shape
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Fig. 3 AFM three-dimensional photos of regular and uniform patterns with different shapes fabricated on Sb2 Te3 phase change material film by laser power of 5.0 mW and different laser pulse widths. (Two tracks for one pulse width.) The insets on the right side of the photos marked by ‘A’, ‘B’, ‘C’ are the profiles corresponding to the areas marked by rectangles ‘A’, ‘B’, ‘C’ on the left. (a) Patterns with laser pulse width from 100 ns to 60 ns. (b) Patterns with laser pulse width from 50 ns to 10 ns. (c) Patterns with laser pulse width from 95 ns to 55 ns. (d) Patterns with laser pulse width from 45 ns to 5 ns. (e) Patterns with laser pulse width from 36 ns to 32 ns. (f) Patterns with laser pulse width from 31 ns to 26 ns
and the inset is the corresponding cross-section profile. The laser power is 8 mW and the laser pulse width is 5 ns. As shown in the photo, the depth of the dimple-bowl is only about 6 nm below the surface. It is noted that the surrounding rim in the dimple-bowl shape patterns according to Marangoni effect is not clear to observe in our experiment and only a dimple crater occurs with different color between the laser irradiated area and the as-deposited film. We think that the Sb2 Te3 phase change materials are very sensitive to temperature and the response time for phase change is in several tens of nanoseconds. In such a short time scale and low laser energy, the material does not have enough time to move toward the peripheral region; thus the material just changes from amorphous to crystalline state due to the low crystallization temperature of about 125°C [23]. As this change occurs, the material will experience a volume shrinkage caused by the transformation because the density
of the crystalline material is slightly higher than the amorphous state [24]. But we still can see the slight rim from the cross-section profile, which also confirms the fact that the material experiences fluid motion under laser irradiation. Fig. 2(b) shows an AFM photo of the typical pattern unit with “Sombrero”-shape, and the inset in the photo is the corresponding cross-section profile. The laser power is about 6.5 mW and laser pulse width is about 30 ns. We can see that the deep ring surrounding the bump center is about 25 nm in depth, and the bump center is about 50 nm in height, which makes it similar to a “Sombrero”-shape. Fig. 2(c) shows an AFM photo of the typical pattern unit with deep-bowl-shape, and the inset in the photo is the corresponding cross-section profile. The laser power and laser pulse width are about 6.5 mW and 40 ns, respectively. We can see that the bump ring surrounding the hole is about 50 nm in height and the center hole is about 50 nm in depth,
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Fig. 3 (Continued)
which is approximately the thickness of the film. The profile of the central hole area also shows that the bottom of the hole is smooth, which indicates that the film is completely melted by the laser pulse and also makes it like a deep-bowl. In order to investigate the regularity of the patterns induced by laser pulse, the regular and uniform patterns with different shapes are fabricated on Sb2 Te3 phase change material film. The AFM three-dimensional photos are shown in Fig. 3. In this process, the laser power was fixed at 5.0 mW, and the corresponding pulse widths were adjusted from 100 ns to 5 ns to generate the patterns as is shown in Fig. 4. The patterns with deep-bowl-shape, “Sombrero”shape, dome-shape, and dimple-bowl-shape are sequentially obtained as the laser pulse width decreases. The insets on the right side of the photos marked by ‘A’, ‘B’, ‘C’ are the crosssection profiles corresponding to the areas marked by rectangles ‘A’, ‘B’, and ‘C’ on the left. In Fig. 3(a)–(d), the diam-
eters of the patterns decrease by decreasing the pulse width. The maximum diameter of the patterns is about 1 µm, and the minimum size is only about 200 nm, which can hardly be observed in the photos. According to the results above, we can realize the micro/nano-fabrication by precisely controlling the laser parameters. A jump between the patterns of deep-bowl-shape and “Sombrero”-shape can be seen, and the depression or dimple in the middle of the dome is not observed in Fig. 3(a)–(d). We refine the scale of the pulse width between 25 ns and 35 ns, and the results are shown in Fig. 3(e)–(f). One can find that the gradually changing process as the laser pulse width can be seen. Both the diameter and the height increase by increasing the laser pulse width according to the AFM analysis. Figure 4(a)–(f) show the AFM two-dimensional photos of patterns corresponding to Fig. 3(a)–(f), respectively.
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Fig. 3 (Continued)
Their corresponding pulse widths are also listed on the right side of the images. As shown in the pictures, three regimes emerge as the pulse energy increases according to the experimental results. The first regime finishes at about 20 ns. In this time scale, the thermocapillary effect is dominant and drives the material from the hot center to the cold periphery, then forms the patterns with a dimple-bowl shape. The material structure changes from amorphous to crystalline. We do not find the initial pulse energy for this regime, which corresponds to the first threshold, known as the crystallizationthreshold. The second regime starts at about 20 ns and ends at about 34 ns, including the process where the rounded and smooth central dome grows out to the dome gradually increasing to its maximum value. Meanwhile, the process of the chemicapillary effect is gradually strengthened to its peak value, which continuously carries the material in
the molten pool toward the center part and finally forms the pattern with a “Sombrero” shape. The initial and final pulse energies correspond to two thresholds, known as the bump-threshold and rupture-threshold, respectively. From the picture, we see that the “Sombrero” shape pattern begins to depress or dimple in its middle at the pulse width of 34 ns, which is the sign of pattern rupture. When the pulse width exceeds 34 ns, it enters the third regime, in which the thermocapillary effect becomes dominant again. The central dome is gradually broadened following the increase of pulse energy, and the bowl shape patterns become deeper and deeper. If the pulse energy is further increased, the final result of this regime may be ablated to form a hole in the middle, which corresponds to the ablation-threshold of the material. A further explanation of these threshold effects will be presented in future research.
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Fig. 4 AFM two-dimensional photos of patterns with different shapes fabricated on Sb2 Te3 phase change material film by laser power of 5.0 mW and different laser pulse widths. (Two tracks for one pulse width.) (a) Patterns with laser pulse width from 100 ns to 60 ns. (b) Patterns with laser pulse width from 50 ns to 10 ns. (c) Patterns with laser pulse width from 95 ns to 55 ns. (d) Patterns with laser pulse width from 45 ns to 5 ns. (e) Patterns with laser pulse width from 36 ns to 32 ns. (f) Patterns with laser pulse width from 31 ns to 26 ns
Obtaining different patterns on the Sb2 Te3 phase change materials with very low pulse laser energy is very reproducible and efficient. We can fabricate large-area periodic varieties of patterns by precisely controlling the laser parameters and film conditions to meet different requests. We hope that the Marangoni effect induced micro/nanopatterning is useful for scientific and industrial applications in the future.
process, determine the nature of the flow, and lead to the different patterns in different laser parameters. Acknowledgements This work is partially supported by the Natural Science Foundation of China (Grant Nos. 50772120, 60977004 and 11054001), Shanghai Rising Star Tracking Program (10QH1402700), and the Basic Research Program of China (Grant No. 2007CB935400).
References 4 Conclusion We fabricated various patterns with different shapes on the Sb2 Te3 phase change thin films according to the laser pulse induced Marangoni effect. As the pulse energy increases, the shape of the patterns gradually change from dimple-bowl shape, dome shape, “Sombrero” shape, to deep-bowl shape, which correspond to the different thresholds of the material under pulse laser irradiation, such as crystallizationthreshold, bump-threshold, rupture-threshold and ablationthreshold. The different pattern shapes are the results of the competition between the thermocapillary effect and the chemicapillary effect in the molten pool, which are caused by the temperature gradient and compositional gradient, respectively. The two effects often coexist during the whole
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