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Programmable Nanopatterns by Controlled Debonding of Soft Elastic Films Nandini Bhandaru,† Ashutosh Sharma,*,‡ and Rabibrata Mukherjee*,† †
Instability and Soft Patterning Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India ‡ Department of Chemical Engineering and Nanoscience Center, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India ABSTRACT: We report a facile patterning technique capable of creating nanostructures with different feature heights (hS), periodicities (λS), aspect ratios (AR), and duty ratios (DR), using a single grating stamp with fixed feature height hP and periodicity λP. The proposed method relies on controlling the extent of debonding and morphology of the contact instability features, when a rigid patterned stamp is gradually debonded from a soft elastic film to which it was in initial conformal contact. Depending on whether the instability wavelength (λF scales with the film thickness hF as λF ≈ 3hF) and the periodicity of the stamp feature (λP) are commensurate or not, it is possible to obtain features along each stamp protrusion when λF ≈ λP or patterns that span several stripes of the stamp when λF > λP. In both cases, the patterns fabricated during debonding are taller than the original stamp features (hS > hP). We show that hS can be modulated by controlling the extent of debonding as well as the shear modulus of the film (μ). Additionally, when λF > λP, progressive debonding leads to the gradual peeling of replicated features, which, in turn, allows possible tuning of the duty ratio (DR) of the patterns. Finally we show that by the simultaneous modulation of AR, DR, and hS, it becomes possible to create surfaces with controlled wettability. KEYWORDS: soft lithography, nanopatterning, debonding, contact instability, elastic film
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flat stamp by applying an external electric field14 or a thermal gradient15 or even in the absence of any field due to the action of attractive electrostatic forces between the mask and a thermally softened polymer film in lithographically induced self-assembly (LISA).16 However, in all of these approaches, spacers with definite heights are necessary to maintain uniform separation between the stamp and film because the pattern morphology largely depends on the ratio between the film thickness and air gap. TheChange Figure 2B (Replace “θE” with “θ*”) Change Figure 3B, C, D (Replace “θE” with “θ*”) delicate experimental protocol renders these approaches rather difficult for bulk nanopatterning applications despite the richness of science associated with them. Of particular interest is surfaces with tall nanostructures (a few hundreds of nanometers to a few microns), which have wide application in hydrophobic and self-cleaning surfaces. The fabrication of such surfaces by any soft lithographic technique would require a stamp with tall features. However, such a stamp
INTRODUCTION The development of various soft-patterning techniques including soft lithography and nanoimprint lithography in the mid-1990s opened up new vistas in the nanopatterning of soft surfaces,1−3 which find application in the fabrication of organic electronic circuits,4 nanobiotechnology and tissue engineering scaffolds,5,6 hydrophobic and self-cleaning surfaces, lab-on-chip devices, microfluidic mixers, etc.7,8 Despite the ease of implementation and low cost, most of these embossing- or imprinting-based techniques are limited by the availability of an appropriate master or stamp because most of the methods are capable of generating a perfect negative replica of the original stamp with no possible option for tuning the pattern morphology or dimension. As a result, individual lithographically fabricated masters are necessary to create every new pattern. In order to reduce the dependence of soft nanofabrication on the primary master, efforts are underway to develop novel techniques that are capable of generating patterns that are not a mere negative replica of the original master, based on innovative approaches such as shrinking of a patterned hydrogel,9 swelling and deswelling of a stamp during patterning,10 stress relaxation in a viscoelastic film,11,12 a surface-initiated polymerization reaction in the presence of a stamp, etc.13 Ordered patterns have also been obtained with a © XXXX American Chemical Society
Special Issue: Focus on India Received: July 24, 2016 Accepted: November 1, 2016
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DOI: 10.1021/acsami.6b09127 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. (A) Variation of the shear modulus of the elastic films (μ) with the cross-linker concentration (CL). (B) Variation of the pattern height (hS) as a function of μ for different hF. Insets B1 and B2: hS for films with μ = 0.08 and 0.43 MPa, respectively. In both the cases, hF ≈ 4.0 μm. (C) Variation of the normalized snap-off distance (dp/de) with nondimensional stiffness ratio (ΔGhF/μde2).
have a dominant wavelength (λF) that scales as 3 times the film thickness (λF ≈ 3hF).19−24 During debonding, a strong hysteresis is observed as the patterns persist much beyond dC because of the existence of several intermediate stages corresponding to local minima in the energy landscape, arguably because of adhesion and pinning of the film along of the contours of the stamp features, and frictional resistance.26−29 When the patterns are made permanent just ahead of detachment, it becomes possible to create structures with hS > hP. We show that the extent of stretching of the patterns vis-a-vis the final feature height of the structures can be tuned by varying hF and μ of the film. The concept is first demonstrated using a flat stamp, and, subsequently, a grating patterned stamp is used to obtain ordered debonded features, with additional control on DR and AR of the patterns in certain cases. We finally show that it becomes possible to create surfaces with controlled wettability by modulation of hS, DR, and AR. The proposed method does not need the application of any external field, and pattern formation takes place directly in the solid state, thereby eliminating the need for thermal annealing of the film to soften the polymer, which simplifies the hardware requirement of the system.
is expensive and difficult to fabricate by standard photolithography because the thin photoresist layer needs to be replaced with a hard etch mask such as a metal or oxide before subsequent deep etching can be performed for creating deep features.17 Of course, by using a topographically patterned stamp and by varying the separation distance between the stamp and film, it is indeed possible to create patterns with different heights following either electrohydrodynamic instability14 or LISA.18 As was already pointed out, these approaches are difficult to successfully implement for patterning over large areas. Thus, no patterning technique exists to date that can produce taller features starting from a shallow stamp except techniques relying on EHD or LISA, which have limited practical applicability for bulk nanofabrication. In this article, we report a novel patterning technique based on the partial debonding of a thin elastic film from a patterned stamp, which allows the creation of patterns that are taller than the original stamp features (hS > hP), thereby offering an easy approach for fabricating tall structures. The technique offers additional flexibility because it also becomes possible to tune the periodicity and duty ratio of the structures by suitably adjusting the film thickness with respect to the periodicity of the stamp (λP) and the shear modulus of the film (μ). The technique relies on the spontaneous instability observed on the surface of a thin soft elastic film when it is brought into the proximity of a rigid substrate and the bonding−debonding hysteresis associated with it.19−26 The surface of a thin elastic film becomes unstable with the appearance of labyrinth-like isotropic structures when it approaches a rigid contactor, and the gap (d) between the two is hP) in both cases, additional tuning of λS and DR of the patterns becomes possible when λP and λF are noncommensurate, offering excellent morphology control. Figure 2A captures various aspects of debonding of a flat film from a patterned stamp, when λP and λF are commensurate (λP ≈ λF ≈ 3hF). Figure 2B shows the morphology of the patterns obtained (just ahead of complete detachment) on the surface of a film with hF ≈ 1 μm by debonding it from a grating patterned stamp with λP = 3.0 μm, line width lP = 1.5 μm, DR = lP/λP = 0.5, and hp = 120 nm. Figure 2C highlights the strength of the present technique, and it can be seen that hS > hP for all films with μ up to ≈2.85 MPa. The plot also shows that hS progressively reduces with an increase in μ due to a gradual increase in the stiffness of the film. While the trend is identical with that observed in Figure 1B with a flat stamp, hS is observed to be even higher when a topographically patterned stamp is used for a film with identical hF and μ. The additional stretching is attributed to the debonding mechanism when a patterned stamp is used and is schematically shown in Figure 2A. Initially, when the stamp is in conformal contact with the film, the local thickness of the film below the stamp protrusions (h1) is lower than that below the stamp grooves (h2). During debonding, the areas of the film below the stamp protrusions snap off first with the appearance of cavities, as shown schematically in frame A2 of Figure 2A. This is expected based on eq 1, which predicts lower dp over zones of the film where the local thickness is less.26 At this stage, parts of the films within the stamp grooves remain pinned and undergo further vertical stretching with progressive debonding. In comparison to the debonding from a flat stamp, parts of the film pinned within the stamp grooves experience additional frictional resistance at the side walls of the stamp features. This additional friction delays the detachment of the film from the patterned stamp, resulting in features with taller hS than that obtained with a flat stamp when debonding from a film having identical hF and μ. An example of this is shown in Figure 2B, where patterns with hS = 413.2 ± 5.3 nm are obtained from a stamp with hP = 120 nm. The aspect ratio of the patterns (AR = hS/0.5λP) has enhanced significantly to ≈0.28 compared to that of the stamp, which has AR = 0.10. Although we could not fully quantify, we have mentioned that debonding was performed very slowly because rapid withdrawal of the stamp results in shallow structures, and it clearly indicates that the frictional resistance is dominated by dynamic friction because it is wellknown that the coefficient of dynamic friction reduces with increased normal force.35 When pulled rapidly, the frictional resistance reduces and the structures stretch less before snapoff. Thus, a combination of enhanced adhesion due to increased interfacial contact area between the stamp and film as well as strong dynamic frictional resistance experienced by the patterns
(up to 6 months), scanning them at periodic intervals, and comparing with earlier scans. No appreciable deviation in the feature height of the samples was observed, even after 6 months. The wettability of the structured surfaces was investigated using a contact-angle goniometer (290 G1, Ramehart, USA).
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RESULTS AND DISCUSSION In the contact instability of thin elastic films, it has been shown by several groups that the instability wavelength is independent of the exact μ of the film.19−25 However, theoretically it has been predicted that the maximum debonding distance (dp) depends on μ and is given by the relationship26,27 d p/de ≈ (ΔGhF /μde 2) p
(1)
where ΔG is the work of adhesion between the stamp and film (≈50 mN/m, calculated for a silica contactor and a Sylgard-184 film) and de is the equilibrium separation distance between the film and contactor at complete contact (=0.158 nm). dp is considered to be identical with hS, which is obtained from the experiments. The nondimensional term (ΔG/de2)/(μ/hF) represents the ratio of the stiffness of the interaction potential to the elastic stiffness of the film and is referred to in the literature as the compliance of the film.26,27 The exponent p represents a nonlinear dependence between dp and the compliance of the film, and a positive value of it (as predicted in simulations) indicates faster debonding for stiffer films. The relationship in eq 1 suggests that by tuning μ it is possible to increase the height of the features, which has been recently shown based on simulations.29 However, actual tuning of dp by varying μ has never been experimentally demonstrated before. In order to establish this, we initially performed debonding experiments with films of different μ and hF (varying between 1.0 and 7.0 μm) using a flat contactor. The corresponding values of hS are plotted in Figure 1B, which shows that, for every hF, hS attains a maximum at μ ≈ 0.08 MPa. Beyond this point, hS reduces with a gradual increase in μ, while λP remains nearly constant. This can be seen from the atomic force microscopy (AFM) images shown in insets B1 and B2 of Figure 1B. Further, by performing some debonding experiments under an optical microscope, we observe that, in films with lower μ (≤1.50 MPa), the instability structures remain firmly pinned to the stamp during the entire debonding process until a sudden snap-off, leading to catastrophic adhesive failure with the complete disappearance of the patterns. Thermodynamically this occurs when the system remains trapped in its original deep energy minimum without any intermediate relaxation during debonding. This, in turn, leads to the accumulation of a large amount of elastic stresses near the pattern edges during pull-off, which, in turn, leads to the sudden snap-off.26,27 In contrast, in films with higher μ (≥1.50 MPa), the fractional area of contact (α) is seen to decrease gradually during debonding because the patterns peel from the sides. This leads to reduced pinning, which, in turn, favors easy detachment and eventually results in shallower structures. By using the value of hF as dp from Figure 1B and plotting dp/de as a function of ΔGhF/μde2 in Figure 1C, we obtain the range of p and its variation with μ. The value of p decreases from ≈0.77 to 0.60 as μ increases from 0.08 to 2.85 MPa (inset of Figure 1C). The range of p is consistent with the theoretical predictions made earlier by Sarkar et al.26,27 Interestingly, for μ < 0.06 MPa (CL ≤ 2.5%), the films tend to become too viscous, and as a result, hS drops drastically to rather low values due to flowinduced flattening. D
DOI: 10.1021/acsami.6b09127 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. (A) Schematic of the debonding sequence of a patterned contactor from a film in the initial conformal contact, when λF > λP (noncommensurate). (B−D) Progressive change in DR and lS of the patterns due to enhanced detachment of the stamp. (B) DR = 0.85, hS = 233.9 ± 6.3 nm, lS = 22.95 μm. (C) DR = 0.66, hS = 288.9 ± 4.8 nm, lS = 17.83 μm. (D) DR = 0.11, hS = 896.3 ± 7.2 nm, and lS = 3.02 μm. In all of these cases, hF = 9.0 μm, λP = 3.0 μm, hP = 120 nm, and λS = 27.0 μm. Insets show the water contact angle (θ*) on the respective surfaces.
was debonded from this stamp. For a bilayer with this specific configuration, λ F = 0.82h F , which made λ F and λ P commensurate30 and resulted in patterns with hS = 95.1 ± 3.2 nm (RP = 2.37), from a stamp having hP = 29 nm (Figure 2E). This observation highlights the strength of the proposed technique in fabricating high AR structures with lateral resolution down to 100 nm. Next, we look into the debonding of a patterned stamp when λP and λF are noncommensurate. It was already mentioned that in such a situation although the instability patterns get aligned under the patterned stamp, they follow their natural instability wavelength λF rather than λP imposed by the stamp.32 We have utilized the mismatch between λP and λF to create patterns with different DR by controlling the extent of debonding. The schematic in Figure 3A shows the debonding sequence, which shows that the instability pattern spans several stamp patterns, the extent of which depends on the relative magnitude of λP and λF. The effect of noncommensuration between λP and λF can be seen in Figure 3B, which shows the morphology of the patterns when a stamp with λP = 3.0 μm is debonded from a film with hF ≈ 9 μm. The cavities appear at every λS ≈ 27 μm; that is, each instability pattern spans under eight stamp protrusions, faithfully obeying the relationship λF = 3hF. The corresponding hS and DR of the structures are 233.9 ± 6.3 nm and 0.85, respectively. Further debonding results in an increase in hS and a reduction of DR as a result of the gradual widening of the cavities due to peeling of the structures from the sides. However, the peel front does not propagate continuously, and the inward retraction of the peel front gets arrested at each stamp protrusion.38 A higher normal force is required to overcome every pinning event, which, in turn, leads to stretching of the patterns. This allows the simultaneous tuning of both DR and hS with progressive debonding, as can be
during stamp pull-off along the side walls of the stamp features results in taller structures. In order to highlight the role of frictional resistance imposed by the side walls of the stamp grooves during debonding, stamps with identical λP but with different hP were debonded from films with identical hF (≈1.0 μm) and μ (≈1.2 MPa). Figure 2D summarizes the results of these experiments, which reveal that not only hS but also RP, the ratio between hS and hP (RP = hS/hP), decreases with a reduction in hP. This is a very fascinating observation because it highlights the dependence of the frictional coefficient on the surface area, which is unique to the submicron scale and nanoscale.36 The result clearly highlights that the frictional coefficient increases with increasing groove depth (hP) and thus engenders greater stretching of the patterns before detachment. For hP < 10 nm, the patterns became isotropic, which implies that the contactor effectively behaves like a flat one and fails to laterally confine the instability structures. A major limitation of elastic contact instability particularly in the context of nanofabrication is the rapid enhancement of the scaling factor between λF and hF to values much higher than 3 in films with hF < 1 μm due to the strong influence of surface tension,25 which significantly hinders the utility of this form of instability in creating truly nanoscale features. We could circumvent this problem and obtain patterns with lateral resolution down to ≈100 nm, using an elastic bilayer, where the scaling prefactor can be tuned to values much lower than 3, by adjusting the thickness ratio of the two layers.30,37 For this purpose, we used a silicon stamp with λP = 200 nm, lP = 95.6 nm, and hP = 29 nm, fabricated by FIB writing (Auriga Compact, Zeiss; probe current = 5 Pa; area dwell time = 0.05 ms). An elastic bilayer comprising a softer film on top of a stiffer film, with a thickness ratio of 1:6 and total hF ≈ 250 nm, E
DOI: 10.1021/acsami.6b09127 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces seen in Figure 3C,D, where structures with hS = 288.9 ± 4.8 and 896.3 ± 7.2 nm with corresponding DR ≈ 0.66 and 0.11 are obtained during progressive debonding. Thus, by the careful choice of hF and λP, it becomes possible to obtain a myriad of ordered structures with different λS, hP, and DR by controlling the extent of debonding, f rom a single stamp. We have so far discussed the possible fabrication of patterns with different combinations of hS, λS, and DR from a single stamp, based on controlled debonding from films with different initial thicknesses (hF). It is well-known that surfaces with regular nano- or mesoscale structures exhibit either Cassie or Wenzel wetting regimes, depending on the roughness ratio of the surface (RF).39 The minimum roughness ratio (RC) required for achieving the Cassie state of wetting is given by RC = −
1 cos θE
not, it is possible to obtain patterns at the length scale of each stamp pattern (λP = λS, lP = lS) or patterns that are wider than the original stamp patterns (λP > λS and lP > lS), respectively, including patterns that are taller than the original stamp pattern (hS > hP). In the latter case, it becomes possible to tune DR in addition to hS by varying the extent of debonding. The two situations also lead to a transformation in the debonding mechanisms from catastrophic adhesive failure to the peeling mode.26 The work is also the first experimental study that reports the debonding of a rigid stamp (flat/pattered) from a soft elastic film in the parallel-plate geometry because all earlier studies involved either peeling of a flexible cover plate or a spherical punch following the probe tack geometry.38 The parallel-plate configuration allows large-area patterning (cm2) and is thus ideally suited for bulk nanoapplications such as selfcleaning coatings, microfluidics, adhesion, photonics, plasmonics, etc. As an example, we have also shown the possible modulation of wetting of the surfaces.
(2)
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For a Sylgard-184 flat surface, the equilibrium contact angle θE = 104.8°. The corresponding value of RC = 3.91 for achieving the Cassie state. We note that, despite creating surfaces with tall structures, RF varies between 1.02 and 1.28 (values mentioned in the respective figures), all of which are lower than RC, and therefore it is expected that the wetting regimes will be Wenzel in all cases. The apparent contact angle in the Wenzel state, θ* (cos θ* = RF cos θE), and experimentally obtained values for the different structures are as follows. Figure 2B predicted θ* = 109.4° and the experimentally obtained θ* = 113.3°; Figure 3B predicted θ* = 105.0° and experimental θ* = 122.4°; Figure 3C predicted θ* = 105.2° and experimental θ* = 115.2°; Figure 3D predicted θ* = 105.8° and experimental θ* = 107.5°. The results show that, by the proposed method, it becomes possible to tune the wettability of the surfaces based on the simultaneous modulation of hS, λS, and DR. Interestingly, in most cases the experimentally obtained values of θ* are close to the predicted values of θ* except for Figure 3B,C, where the experimentally obtained values of θ* are significantly higher than the predicted values. At this point, it is difficult to comment about the precise reason that is responsible for this deviation. For Figure 3B, it might be possible that the channels are narrow, and, consequently, the entrapped air fails to escape when the water drop comes in contact with the surface. As a result, the wetting regime might be in a mixed state, which is considered responsible for the sticky hydrophobicity of rose petals.40 Additionally, the AFM images in Figure 3B,C clearly show the existence of a secondary roughness on the replicated stripes spanning several stamp features, which might also be responsible for enhancing θ* to values higher than that predicted from the Wenzel equation. Nonetheless, the observations show the ability to achieve a higher degree of superhydrophobicity even on a surface with relatively low RF probably because of complex wetting states close to the surface, which requires detailed investigations in future.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
Useful discussions with Prof. G. Harikrishnan of the Indian Institute of Technology Kharagpur and the contribution of Mr. B. Prabhakar, in designing the experimental setup are gratefully acknowledged. R.M. acknowledges support of the Department of Science & Technology, New Delhi, India, for funding the research under its SERI program by a research grant [DST/ TM/SERI/DSS/361(G)]. N.B. acknowledges funding of her fellowship from Centre of Excellence in Microfluidics, funded by SRIC, Indian Institute of Technology Kharagpur.
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NOMENCLATURE λF dominant wavelength of the contact instability, μm λP periodicity of the patterns on the stamp, μm λS periodicity of the structures obtained on the film upon controlled detachment, μm μ shear modulus of the film, MPa θE equilibrium water contact angle, deg θ* apparent contact angle in the Wenzel state, deg AR aspect ratio of the patterns CL concentration of the cross-linker in a Sylgard-184 solution, % (w/w) d separation distance between the thin elastic film and rigid contactor, nm dC critical inter surface separation distance, below which the van der Waals forces become actives and deforms the film surface, nm de equilibrium separation distance between the film and contactor at complete contact, nm dp maximum debonding distance between the thin elastic film and rigid contactor, nm DR duty ratio of the patterns ΔG work of adhesion between the stamp and film, mN/m G′ storage modulus of the film, MPa G″ loss modulus of the film, MPa
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CONCLUSION As a final summary, we have presented a facile nanopatterning technique based on physical confinement of the elastic instability patterns during debonding using a simple grating patterned stamp. We have shown that the method is capable of producing patterns that are taller than the stamp features. Further, depending on whether λP and λF are commensurate or F
DOI: 10.1021/acsami.6b09127 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces h1 h2 hF hP hS lP lS p RC RF RP
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local thickness of the film in conformal contact with the stamp below the stamp protrusions, μm local thickness of the film in conformal contact with the stamp below the stamp grooves, μm elastic film thickness, μm feature height of the patterns on the stamp, nm feature height of the structures obtained on the elastic film, nm line width of the patterns on the stamp, nm line width of the obtained structures on the elastic film surface, nm slope of the curve showing the dependence of dp on ΔG, hF, and μ minimum roughness ratio required for achieving the Cassie state of wetting roughness ratio, the ratio of the true area of the patterned surface to the apparent area ratio between hS and hP
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DOI: 10.1021/acsami.6b09127 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
