pubs.acs.org/NanoLett
Spontaneous Formation of Periodic Nanostructures by Localized Dynamic Wrinkling Se Hyun Ahn,† and L. Jay Guo*,‡ †
Department of Mechanical Engineering and ‡ Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109 ABSTRACT Micro/nanoscale periodic structures are widely used in display, optics and bio industries as key functional elements. We present a novel nanopatterning method, localized dynamic wrinkling (LDW), which creates micro/nanoscale metal gratings continuously by simply sliding a flat edge of a hard material over a thin metal surface coated on a polymer layer. The LDW process presented in this paper is an etching- and template-free nanopatterning technology based on nanoscale wrinkling phenomenon. This simple process enables spontaneous formation of large-area metal gratings with controllable periodicity from micrometer down to 120 nm. KEYWORDS Wrinkling, periodic structures, nanopatterns, template-free patterning
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anograting structures can be fabricated by techniques such as electron beam lithography, laser interference lithography, nanoimprint lithography (NIL), and nanoinscribing.1-4 Even though direct-write electron beam lithography can produce gratings of arbitrary period and has excellent resolution down to sub-10 nm, the extremely low process throughput renders the technique impractical for many applications such as flat panel displays that require large patterned area and high throughput. Another nanofabrication technology, the low-cost and highthroughput NIL and even the recently developed high-speed roll to roll nanoimprinting,5,6 requires original stamps containing prefabricated nanoscale structures, while the fabrication of large area stamp can be challenging and expensive.7 The nanoinscribing process introduced recently also requires a prepatterned grating mold.4 Parallel to the development of lithographic-based nanopatterning methods, a whole different class of patterning approach based on the spontaneous formation of ordered surface wrinkle patterns has been investigated in the past decade.8-14 Wrinkling or buckling of membranes exposed to compressive stress is a commonly observed physical phenomenon, where due to mechanical instability the membrane takes specific forms of deformation to release the strain. However, the nature of the nonlinear behavior in wrinkling not only makes it difficult to theoretically analyze but also difficult to experimentally control the process, thereby limiting its practical applications.15 For better repeatability, most studies on wrinkling phenomenon use a thin and stiff film on a thick and prestretched soft
elastomeric substrate such as poly(dimethylsiloxane) (PDMS) to generate strains and subsequently form spontaneous wrinkles upon relaxing the stained polymer.8,9,15,16 Uniaxially strained membrane forms well-ordered grating patterns that can be utilized in a variety of applications such as stamps for microcontact printing, cell alignment, and surface metrology.17-19 However, experimentally it is very difficult to apply purely uniaxial strain to a large-size membrane. As a result, creating well-oriented wrinkles in practically large area remains a challenge. Moreover, patterns formed on a soft elastomeric substrate, such as PDMS have only limited utility, and require additional steps to transfer the pattern on to a desired substrate for specific applications. Here we report a novel technique, termed localized dynamic wrinkling (LDW), which enables continuous formation of micro/nanoscale gratings in a thin metallic film coated on common polymer substrates, such as polyethylene terephthalate (PET) and polycarbonate (PC). LDW shares the basic principle as the buckling (wrinkling) phenomenon of a thin and stiff film on a compliant substrate under uniaxial stress, but it uses the moving edge of a cleaved Si wafer to exert stress to the metal film coated on the polymer substrate along the uniquely defined movement direction, and sequentially generates localized wrinkles in the metal film in a dynamic and controllable fashion. More interestingly, the period and geometry of the patterned gratings can be determined by material parameters such as the thickness of the metal layer, the type of metal and backing polymer substrate. Furthermore, the nature of the sequential formation of localized wrinkle along the Si edge moving direction makes LDW easily applicable to a high-speed and continuous roll-to-roll process. In this work, we demonstrated the
* To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (734) 647-7718. Fax: (734) 763-9324. Received for review: 08/1/2010 Published on Web: 09/07/2010 © 2010 American Chemical Society
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FIGURE 1. (a) Schematic of pattern formation by the LDW mechanism. Finite element models showing (b) localized wrinkle formation by engaging a sharp edge of solid material on the film surface and (c) the distributed bulk wrinkling when the supporting bulk polymer is uniformly contracted. (d) Sequential steps of the LDW process to create discrete metal gratings.
fabrication of gold nanogratings with line width from micrometer scale down to 120 nm on plastic substrates. In the LDW process illustrated in Figure 1a, the moving sharp edge of a cleaved Si wafer locally creates a wrinkle in a metal-coated polymer followed by several wrinkles with rapidly decaying amplitude, and the continuous motion of the sharp edge produces periodic metal grating patterns in a sequential manner. This is confirmed by finite element simulation shown in Figure 1b, showing that the wrinkle is localized just ahead of the sharp edge of the hard material pressing on the film. As a comparison, Figure 1c shows the simulated result of wrinkle formation distributed across the whole surface when the same film is subject to a uniform stress. The difference between the localized wrinkling and the bulk wrinkling can be clearly seen. In our experiment, gold film of various thicknesses (10-100 nm) was deposited on two types of plastic substrates, polyethylene terephthalate (125 µm thick) and polycarbonate (125 µm thick). A well-cleaved Si wafer is clamped by a static holder that is tilted with adjustable angle to the substrate. To ensure conformal contact between the Si edge to the metal-coated polymer substrate, the substrate is placed on a 5-DOF stage and cushioned by a rubber film. The sharp Si edge slides over the metal surface when the polymer substrate is pulled with controlled speed. The contact force between the cleaved Si edge and the substrate is controlled by a z-direction mi© 2010 American Chemical Society
crostage and monitored by a flexible force sensor (Tekscan. Inc.). In the LDW process, both the gold film and the underlying polymer layer plastically deform when subject to a sufficiently strong normal and shear stress by the moving Si edge. Figure 1d illustrates how the LDW process creates discrete metal gratings. When a flat Si edge slides on the metal-coated substrate with proper pressure, localized wrinkle starts to develop under the shear (friction) and normal force. As the Si wafer moves further, the wrinkle fully grows and defines its period (λ) and amplitude (A). Then, the Si edge slips over the wrinkled bump and finalizes the pattern geometry, depressing the amplitude (A′) of the wrinkled film while maintaining a constant period (λ). The process repeats itself and sequentially creates another wrinkle under the same mechanism, and finally periodic grating patterns are generated. In this process if the friction force is higher than the fracture strength of the metal film, discrete metal gratings are created by fracturing the metal film between the wrinkles. Higher friction force gives larger spacing (S) between the metal lines but interestingly, the period of the pattern remains nearly constant. LDW provides a way to continuously generate large-area nanometal gratings with few defects. Defects mainly in the form of missing or nonstraight lines are mostly from the imperfection of the cleaved Si wafer having terraces, which results in nonuniformity in the direction and the magnitude 4229
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FIGURE 2. (a) A SEM image of 970 nm period gold grating pattern on a PET film fabricated by the LDW process. Inset (a-1) is a magnified image of (a). (b) An atomic force microscope image of 780 nm period pattern fabricated by LDW (30 nm thick gold on a PET film).
FIGURE 3. (a) Dependence of the pattern period in LDW on the gold thickness, temperature, and the types of polymer. (b-d) Corresponding SEM images of LDW grating pattern on different gold thicknesses (100, 50, and 10 nm) on a PET substrate at 100 °C yielding different periods (2.3 µm, 1.2 µm, and 150 nm, respectively).
of the local contact force where the Si edge engages the metal film, but this limitation can be overcome by preparing better cleaved Si. Figure 2a (and inset, a-1) shows an 800 nm line width, 970 nm period grating pattern in a 50 nm thick gold on a PET substrate by the LDW process. In the fabrication of this pattern, a well-cleaved 1 cm wide Si wafer was clamped by a tilted (40°) holder and scanned over the gold surface from bottom to top direction while keeping a contact force of 7.25 N normal to the surface. The resulting patterned area is about 1 cm by 5 cm. As seen from the scanning elctron microscopy (SEM) (and inset), well-oriented discrete gold gratings are formed over a large area. The period is very uniform with an overall deviation of less than © 2010 American Chemical Society
5% across the whole patterned surface. An atomic force microscope image of 780 nm period pattern fabricated by LDW (30 nm thick gold on a PET) shows 250 nm peak-tovalley difference (Figure 2b). A prominent characteristic in the LDW process is that the grating period is determined by a few material parameters and therefore can be controlled. The results are summarized in Figure 3a. Each data point in this plot represents the average value from three separate experiments. In this plot, substrates of different Young’s modulus was used, either provided by using different materials (PET vs PC), or by the same material (PET) but at different temperatures (room temperature vs 100 °C) and showed the dependence of 4230
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pattern period on the substrate modulus. For the heating process, the cleaved Si wafer was heated by a conductive heating element with a temperature controller, similar to that described in a previous work.4 Experimental results clearly show that the harder substrate (room temperature PET, E ) 3.0 GPa) produces smaller periodic pattern, while the softer one (PC at 160 °C, E ) 50 MPa) results in larger periodic pattern. Another interesting finding is that the period follows a linear dependence on the metal thickness. Figure 3b-d shows the corresponding SEM images of grating pattern by LDW process for different gold thicknesses (100, 50, and 10 nm) on a PET substrate at 100 °C, producing different periods (2.3 µm, 1.2 µm, and 150 nm, respectively). Since the line pattern in a very thin gold layer (10 nm, Figure 3d) is mechanically weak, it tends to be distorted by the physical contact during sweeping. We expect that a more precise control of minimum required force to form a well-ordered pattern can prevent the distortion of gratings in sub 100 nm line width. Interestingly, the linear trend of pattern period in terms of the metal thickness and the dependence on the modulus of the polymer layer has a excellent agreement with the previous theoretical study on bulk wrinkling20
( )
λeq ) 2πh
E¯ 3E¯s
1/3
(1)
where h is the thickness of stiff layer, and E¯ and E¯s correspond to the plane strain modulus of the stiff layer and the compliant substrate, respectively. This strong correlation further supports that the basic mechanism of the LDW process is due to the wrinkling phenomenon in a stressed bilayer system. In addition to dimensional control, the processing speed is also a very important factor in nanofabrication. We found that LDW can provide continuous formation of nanoscale metal grating pattern at high speed. In our experiment we used speeds up to 50 mm/sec and could obtain good pattern formation. To investigate the effect of sliding speed on the pattern geometry and period, a PET substrate covered by 100 nm thick gold was moved with respect to a fixed Si wafer edge by microlinear motor that provides speed range from 24 to 266 µm/sec and by a AC motor that provides much higher speeds, for example, 5-50 mm/sec. Our experiments using PET substrate covered by 100 nm thick gold show constant period (1.56 µm with a standard deviation of 0.03 µm) in the wrinkle pattern regardless of the sliding speed of the Si edge within that range. This effect can be explained because the speed of elastic wave (wrinkling) is extremely high on the order of 109 µm/sec,21 therefore the localized wrinkle by the LDW process can reach the fully developed equilibrium state in a very short engaging time as the sharp Si edge scans over the wrinkle. This is a very © 2010 American Chemical Society
FIGURE 4. Room-temperature LDW patterning (50 nm gold-coated PET) with increased applied force in normal component: (a) 3.25, (b) 4.25, and (c) 5.25 N.
promising and useful result because one can easily obtain repeatable results of ordered grating pattern without having to precisely control the scanning speed. Moreover, it is possible to speed up the LDW process even further to produce the same quality nanopatterns, which is highly desirable for practical manufacturing processes. Next, we studied the influence of the applied force on the wrinkle pattern formation. As shown in Figure 4, experiments performed on a 50 nm thick gold on PET substrate exhibits different behaviors under different force condition. The exerted normal force was monitored by a flexible PZT sensor. When a small force is applied (3.25 N, Figure 4a), 4231
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FIGURE 5. (a) Schematic of pattern transferring step after LDW using UV curable epoxysilicone as an adhesive layer. (b) Initial LDW gold pattern on a fluoro-silane surface-treated PET. (c) Transferred pattern on a glass substrate showing inverse profile of initial pattern (b).
only very shallow wrinkles with less well-defined grating patterns are observed. With increased force (4.25 N, Figure 4b), the wrinkles become deeper, still with not well-oriented pattern in the gold film. When the applied force reaches a certain value (5.25 N, Figure 4c), all wrinkles are well developed and uniformly oriented perpendicular to Si edge moving direction. Saturation occurs for applied forces higher than 5.25 N without showing significant difference in either the pattern period or the pattern straightness. Discrete metal lines are formed when the friction force is greater than the fracture strength of the metal film (Figure 3b). This can be explained by the physics of surface buckling under twodimensional (2D) strain, ε011 and ε022, where ε011 and ε022 denote the principle strain in each orthogonal direction, respectively.20,22 A small |ε022/ε011| value results in more anisotropic 0 0 /ε11 | value produces and straight wrinkles, while a larger |ε22 more isotropic, complex wrinkle patterns. In addition, the amplitude of the wrinkle pattern increases with an applied force. In LDW process, a small applied force not only 0 generates a strain in the Si edge movement direction (ε11 ) 0 but also a comparable amount of lateral strain (ε22) in the transverse direction, which results in more complex 2D patterns with small amplitude. On the other hand, higher applied force in LDW generates a large strain in the Si edge 0 movement direction (ε11 ) resulting in large wrinkle amplitude, while at the same time effectively suppresses the film 0 deformation in the transverse direction (ε22 ) by strongly pressing the substrate. Such a directional strain distribution produces more anisotropic and straight periodic patterns shown in Figure 4c. Such a self-constraint effect in LDW is very significant for practical application, because it eliminates the need of any special uniaxial strain device to create well oriented wrinkle patterns. © 2010 American Chemical Society
To create high quality and periodic wrinkling patterns by LDW process, the substrate should have certain degree of compliance. For further practical applications requiring specific type of substrate such as Si, the metal grating fabricated by LDW can be transferred to a second substrate. Since in general, gold has low adhesion to most of the plastic surfaces, one can take this advantage to transfer gold pattern to other substrates after LDW. For even better results in the gold pattern transfer, one can use antisticking surfacetreated plastic substrate in LDW. We demonstrated that the metal grating pattern produced by LDW process could be transferred to a glass substrate by using a UV curable epoxysilicone23 as an adhesive layer (Figure 5a). Figure 5b is the SEM image of the initial gold pattern on a PET substrate with surface-treated by (1H,1H,2H,2H-perfluorodecyl trichlorosilane), and Figure 5c is the transferred pattern on a glass substrate showing an inverse profile of that in Figure 5b and with exactly the same period. We would like to suggest one of the potential applications of the LDW-formed metal nanogratings. Nanometal gratings have been developed as transparent electrodes to substitute the widely used conductive oxide ITO in display and solar cell applications.24,25 However, the metal transferring technique used in refs 25-27 requires a prefabricated PDMS template, and moreover, it requires successive metal deposition and cleaning processes to repeat the transfer process. In contrast, LDW can directly generate nanometal gratings on plastic substrates without using any prepatterned template. The metal lines generated by the LDW process generally have small spacings and hence do not provide enough transparency as transparent electrodes. However, this drawback can be overcome by uniaxially stretching the plastic substrate with LDW-formed metal lines. As illustrated in 4232
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FIGURE 6. (a) Uniaxial stretching with heat (T > Tg) to enlarge openings between the metal wires. (b) An SEM of LDW pattern in 50 nm gold-coated PET. (c) A 20% strained LDW pattern. (d) A 30% strained LDW pattern. (e) Measured transmittance of each listed sample.
Ω for a 5 mm by 5 mm square), which is favorable to the transparent electrode application. Since our stain-generating device used for stretching does not provide purely uniaxial strain but also creates a biaxial component, the stretched gratings are buckled due to a lateral compressive strain. We are certain that higher quality transparent electrodes based on such metal wires can be produced if improved experimental setups are used. In summary, in contrast to most micro/nanofabrication processes such as photolithography or NIL that rely on “replication” scheme using prepatterned masks or stamps, LDW can create periodic nanopatterns by just a flat edge of any hard material. We demonstrated the formation of gold nanograting patterns having line width down to 120 nm using the LDW method. The pattern geometry and the period in LDW can be controlled by using different metal thicknesses, types of polymers, and temperature conditions. Since the patterning mechanism in LDW is new and different from well-understood surface bulk wrinkling, the theoretical model for this process has not been estab-
Figure 6a, the metal pattern by LDW having small openings is stretched orthogonal to the grating direction with heat (raising the temperature higher than glass transition temperature of the polymer substrate) to obtain larger openings. Figure 6b is the initial grating pattern fabricated by LDW on a 50 nm gold-coated PET substrate with openings less than 20 nm. Twenty percent stretching at 200 °C results in wider opening (Figure 6c) and 30% stretching produces even larger openings (Figure 6d). For comparison, a continuous gold layer with the same thickness on a PET substrate has been stretched. Compared to the LDW-formed gratings, the bare gold layer with 20% stretching results in nonuniformly fractured metal film in much larger dimension (ca. 10 µm). Figure 6e is the corresponding transmittance of each sample listed. A 50 nm bare gold film on a PET substrate shows about 20% maximum transmittance at 510 nm wavelength. LDW patterned film has slightly enhanced transmittance (ca. 27%) due to tiny openings. However, a 30% strained LDW pattern provides much enhanced transmittance (ca. 50%) still showing good electric conductivity (a resistance of 26.8 © 2010 American Chemical Society
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