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Stress Engineered Polymeric Nanostructures by Self-Organized Splitting of Microstructures Danish Faruqui and Ashutosh Sharma* Department of Chemical Engineering, Indian Institute of Technology, Kanpur, UP 208016, India
We propose a generic, simple, and parallel nanofabrication technique to obtain three-dimensional multiscale nanostructures based on stress-induced cracking of thin film polymeric microstructures at their sharp edges. The technique was applied to obtain an array of nanochannels on large areas (∼squared centimeters) with channel widths and heights of ∼100 nm. The technique offers flexibility in nanofabrication by providing very simple controls on the feature size, aspect ratio, and the complexity of the nanostructures thus obtained. Patterning of substrates on the nanoscale is of technological importance in the fabrication of opto-electronic devices, display devices, antireflective coatings, micro-/nano-eletro-mechanical systems, chemical or biological sensors, DNA enrichment and other biological applications, lab-on-a-chip diagnostic devices, nanofluidics, super hydrophobic surfaces, etc. Numerous soft-lithography techniques such as the nanoimprint lithography,1 microcontact printing, and replica molding have aimed at reducing the pattern dimensions to sub-micrometers without the use of serial and slow processing techniques such as the ion-beams and e-beams that are often not suitable for large areas and mass production. Fabrication of three-dimensional (3D) nanochannels has received increasing attention due to their potential application in nanobiochips, nanofluidics, and lab-on-a-chip type of diagnostic devices. Also, single-molecule analysis has received tremendous attention in recent years, and a critical issue in such analysis is the ability to spatially constrain molecules of interest within a nanoscale confinement zone. Spatial localization helps in reducing Brownian motions and provides a fixed location for the molecule. Different methodologies to achieve this goal have been reported including the use of suspended microdroplets,2 trapping molecules within nanoscale porous gels,3 and confining molecules within sub-micrometer silica capillaries.4–7 However, imbedded capillary nanochannels are not readily integrated with the larger microfluidics network.8 More recently, lab-on-a-chip technology using nanochannels fabricated on planar substrates has been investigated and employed as singlemolecule analysis platforms that provide system-level functionality such as analyte separation, reagent and analyte delivery, and integrated sensing.9–12 The reported techniques9–19 for the fabrication of nanoscale 3D structures either require the use of a mould/mask or a “writing” tool or the use of complex serial physical or chemical operations to reduce the size order of initially transferred pattern or structure. The techniques employed thus far are mostly topdown methods of fabrication. We propose here a new fabrication technique based on self-organized stress-induced cracking of microstructures to produce multiscale micro- or nanostructures. The recent developments in the top-down fabrication of nanochannels include reversal imprint lithography (RIL) and its variants which are multistep processes. Nakajima et al.16 reported fabrication of multilayered nanochannels employing RIL using a single kind of polymer throughout to ensure homogeneity of the nanostructures thus formed. The method * To whom correspondence should be addressed. E-mail: ashutos@ iitk.ac.in. Tel.: + 91-512-2597026. Fax: + 91-512-2590104.
requires a precise thermal differential between the stages holding mould and the substrate. The reported feature sizes of bi- and trilayered nanochannels fabricated by RIL are around 400 and 160 nm. Another recent development by Kehagias et al.17 to fabricate fine nanochannels employed a serial process combining the advantages of both reverse nanoimprint lithography and contact ultraviolet lithography (RUVNIL). However, the method is appropriate only for photosensitive polymers. The reported feature size of embedded nanochannels is 400 nm in thickness and 350 nm in height. The minimum width of the nanochannels in this method is limited by light diffraction and light reflection on the substrate during the UV exposure. In addition, interplay of different process parameters including application of high pressure (∼40 bar), temperature, and UV exposure is critical to obtain defect free structures. Tas et al.18 demonstrated another top-down method for the fabrication of nanochannels much lower in feature size (∼400 nm) than the photolithographic resolution employed (1 µm). It was made possible due to a combination of conventional photolithography and subsequent thin film deposition and sacrificial etching (done using reactive ion etching (RIE)). Several groups19–24 have also reported direct creation of nanometer scale depressions in biological substrates using active enzymes delivered via scanning probe microscopes. Such serial techniques include protease nanolihography, dip-pen lithography, and nanofountain pen lithography.19–24 These techniques require optimization of many operational parameters and operate at small scan velocities of 1-10 µm/s thus limiting the throughput and its applicability to generate multiple nanochannels over a large area (∼squared centimeters). The smallest cantilever nanopipettes available are of the order of ∼100 nm which sets the upper limit over the feature size of nanochannels possible through these techniques. In this paper, we present a novel generic fabrication technique based on self-organization to form large-area arrays of nanostructures by a simple and cost effective method requiring no specialized equipment beyond photolithography with micrometer resolution or a micromask/mould. The proposed technique for nanofabrication with polymers is based on the following logic. It is known that the thin films of spin coated high molecular weight glassy polymers develop significant differential residual stresses across the film thickness because of the rapid freezing of the nonequilibrium entangled chain configurations during the solvent evaporation.25,26 Upon thermal annealing of such films near or above their Tg, the
10.1021/ie7017838 CCC: $40.75 2008 American Chemical Society Published on Web 05/14/2008
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relaxation of residual stresses occurs very slowly and it is an important factor in the spontaneous breakup and dewetting of initially smooth thin polymer films.25,26 Further processing of such films to fabricate microstructures on their surfaces by imprint lithography, micromolding, capillary lithography, etc. can create further stresses, and more importantly, produce a lateral distribution of residual stresses.27–29 Micropatterning of a spin-coated polymer film can thus produce focusing of stresses more selectively at sharp corners, which become potential sites for the initiation of cohesive failure by crack-formation and propagation, which relaxes the residual stresses. The fine cracks thus produced add another hierarchy of a smaller scale structure to the parent microstructure. This generic technique is employed here to produce a parallel array of nanochannels (∼100 nm) over a large area (∼squared centimeters) starting with a parent microstructure composed of triangular ridges on the scale of micrometers. The technique is not restricted to certain class of polymers (e.g., radiation sensitive polymers) and has no optical limitations on the feature size obtained. Nanochannels are fabricated in the same polymer that is finally used, thus ensuring uniformity of properties (refractive index, surface energy, etc.) in the structure formed. The nanochannels thus formed are surrounded by thick walls thereby enhancing their mechanical stability. Materials and Methods of Fabrication The polymer used in this study was poly(methyl methacrylate) (PMMA) [weight-average molecular weight (Mw) 189 094, weight-average molecular weight/number-average molecular weight (Mw/Mn) 5.09]. In a typical experiment, silicon wafers (1.5 cm × 1.5 cm) with a native oxide layer were cleaned using a standard cleaning procedure. After the cleaning, the wafers were immediately used as substrates for the spin coating. Experiments with cleaned quartz used as the substrate produced identical results. Thin films of PMMA were created by spin coating the polymer in analytical-grade acetic acid solvent. Subsequently, PMMA films were dried in vacuum at room temperature and then annealed under vacuum to remove any residual solvent. Next, the polymer film thus obtained was annealed at 40-50 °C above the glass transition temperature of the polymer with a simple microstriped mould of periodicity 1500 nm left in conformal contact with the polymer film to utilize the capillary force lithography as a tool for initial pattern transfer. During this stage, the polymer melts and flows into the grooves of the master in contact. The lateral feature size of initial pattern transferred was equal to that of master stamp used. However, the cross-section of the pattern replicated on the polymer film was found to be not perfectly rectangular because the molten polymer did not completely fill the grooves of the master. In fact, after the removal of the master and on further extended annealing, micro-stripes formed on the PMMA film surface showed perfectly triangular cross-sections, thus leading to the buildup of localized stresses at the tip of the triangles. Due to further annealing, breakage was observed to start on the pointed tips of the triangular stripes to relieve the stresses. The cleavage of the microstructure propagated with annealing time to reach the silicon substrate. The initial triangular stripes were thus split into two parts, each of which forms a side wall to the nanochannel that appears in the middle because of the stress-induced splitting of the stripe. The propagation of cleavage could be easily calibrated by changing the annealing time and temperature and thus, the depth of the nanochannels could be controlled. Under the chosen conditions, 100 nm wide and 120
nm deep fine nanochannels arranged in parallel could be formed. Since the micromaster stamp used is metallic and flexible, this technique can be easily extended to polymers with a high glass transition temperature and/or fabrication of nanochannels on curved surfaces. Further, the technique presented is generic and the feature size of 100 nm reported here is not the minimum possible but actually depends on the periodicity of the master stamp and thickness of polymer film used. Use of finer striped moulds and very thin polymer films can further bring down the feature size of three dimensional nanostructures formed. Results and Discussion The final nanochannels that formed starting from the cracking of the parent microstructure follow a topographical evolution process because of a stress-induced self-organization of structures. This structural evolution ending in the nanochannels can be summarized by three characteristic stages. These stages are discussed below. The results shown are for a 50 nm spin coated PMMA film on a flat quartz substrate and a microstriped thin metal foil of 1500 nm periodicity used as the master for the initial micropatterning of the PMMA film by the capillary flow lithography. The metal foil master had alternating microstripes/ grooves of rectangular cross sections with ∼100 nm height. Stage I: Formation of Triangular Cross-Section Surface Microstructures. After replicating the pattern of micromould onto the PMMA film, the sample was annealed in vacuum for 30 h at 160 °C, which is above the glass transition temperature of the polymer. After annealing, the samples were cooled down to room temperature and the atomic force microscopy (AFM) scans (Figure 1a and b) of PMMA surface at this stage reveal well-aligned, sharp triangular stripes of width 1.5 µm and of 100 nm height. These triangular stripes are remarkably identical to one another in height and width and are obtained without defect on a large area. It is interesting to note that although the metallic master foil used to transfer the pattern on PMMA surface had rectangular microgrooves, the conditions employed for this stage of topographical evolution result in a perfectly triangular cross section of the raised stripes on the PMMA surface. The reason triangular stripes are imprinted by using a master with rectangular cross-sectional grooves is because the thickness of polymer film was very small (∼50 nm), as compared to the 130 nm high stripes/grooves of the master stamp. Thus, the amount of polymer in the thin film is not enough to fill the entire rectangular groove of the master stamp while imprinting. Under these conditions, the filling of grooves is incomplete and the polymer flows in to form the higher aspect ratio triangular stripes, rather than lower aspect ratio rectangular stripes, on the polymer surface. The initial structure formed by the capillary lithography increasingly transformed to perfectly triangular ridges shown in Figure 1 by annealing of the structure. However, it may be noted that the triangular structures shown can also be produced more directly, for example, by imprint lithography by employing a suitable stamp with triangular protrusions/grooves. The next step of the technique is thus not dependent on the microfabrication method used for the creation of the parent microstructure with sharp corners. Stage II: Genesis of Nanochannels by Crack Initiation in Triangular Microstructures. Further annealing of triangular stripes obtained in stage I at 160 °C for 10 h initiated the stressinduced cracking of the triangular stripes starting at their sharp tips. The AFM scan (Figure 2) of PMMA surface taken at the room temperature reveals the onset of breakage. As can be seen,
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Figure 2. Stage II: onset of breakup of the triangular stripes starting from their tips. The PMMA surface obtained in stage I was annealed for another 10 h in vacuum at 160 °C (total annealing time 40 h after pattern transfer from master). Scan area: 10 µm × 10 µm.
Figure 1. Stage I: PMMA surface structure formed by pattern transfer from a rectangular metal foil grating of 1.5 µm pitch and subsequent 30 h of annealing in vacuum at 160 °C. (a) AFM cross-sectional analysis and (b) 3D view (scan area: 10 µm × 10 µm) clearly reveal the triangular cross section of the PMMA surface pattern.
the breakage starts to propagate from the tip of the triangular stripes where the stress concentration is highest due to the geometry of the structure. After realization of stage II, it was desired to control the breakage process thus initiated to get nanochannel structures with much higher aspect ratios. At this stage, the aspect ratio of triangular stripes is 100/1800 (length/ width) as shown in Figure 1a and Figure 2. As shown below, the aspect ratio of the nanochannels formed by the vertical crackpropagation increases to ∼1.2. The nanochannel formation occurs by fully developing the rudimentary nanochannels already present in the form of cracks at the tip of the triangular stripes. Stage III: Nanochannel Formation by Controlled Crack Propagation. Further annealing of samples in stage II above for 10 h at the same temperature (∼160 °C) was found enough for the breakage that initiated from the tip of triangular stripes to reach their bases. This vertical crack-propagation split the initial polymeric microtriangle into two parts that form the
side walls of the nanochannel that appears in the middle. The AFM scans (Figure 3a-c) of the split PMMA surface reveal a very fine periodic network of high aspect ratio nanochannels thus formed. It is especially interesting to note that the crack propagation occurs cleanly and vertically without any significant lateral meandering of the crack as it grows downwards. Thus, the elevated levels of stress at the center line of the polymeric triangles combined with a slow crack propagation to release the residual stresses produce a very uniform fidelity of the nanochannel network. The nanochannels obtained in this example are of ∼100 nm width and of almost equal height. The other striking feature of the nanochannels obtained employing the presented technique is that the nanochannels are imbedded in relatively thick microsized walls of the polymer formed by the bifurcation of the initial triangular stripes. Thus, the nanochannels thus formed should have superior mechanical properties for a wide range of practical applications that require minimal flow induced deformation of the structure. Further, it is noteworthy that the nanochannels can be formed directly in the polymer chosen for the final application, which ensures homogeneity of properties over the entire structure and their potential for nanofluidic, photonic, and bio applications. Also, the nanochannels formed are well separated from one another at the micrometer level, which may be another key aspect in their applications. It is also interesting to note that the nanochannels thus formed are surface structures that are initially not covered or embedded, unlike those formed by RUVNIL/RIL processes. Depending on the application, these can either be covered by a sheet of polymer/glass etc. or can be used as a large area surface structure/mask/mould/ master for further processing. The large area AFM scans (Figure 3c and d) further show that the technique is capable of forming an array of nanochannels on large areas. The height and width of nanochannels formed by this technique can be controlled by varying the thickness of polymeric film and the periodicity of master used. Also, by using a complex master instead of a simple rectangular grating used in the above example, it appears possible to fabricate more complex three dimensional nanostructures using the same generic technique of stress-induced self-organized crack propagation. In the experiments presented here, the aspect ratio of the nanochannels is almost 20 times higher than that of the parent microstructure generated on the polymer layer using the striped master stamp. Thus, unlike the nanoimprint processing that
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Figure 3. Stage III: complete splitting of PMMA triangular stripes at their center and the formation of stress-induced nanochannels. The PMMA surface obtained in stage II was further annealed for 10 h in vacuum at 160 °C. The periodically arranged nanochannels are ∼100 nm with almost similar height and width and flanked by thick microwalls. (a) Cross-sectional view of the nanochannels. (b-d) 3D views covering different scan areas: 5 µm × 5 µm, 10 µm × 10 µm, 15 µm × 15 µm, respectively.
requires e-beam or ion beam lithographies, one can use photolithography for the generation of the initial microstamp in our technique. Summary We have demonstrated a novel method for the fabrication of large area arrays of nanochannels in a polymer based on selforganized breakage of a microstructure with triangular crosssection ridges. The nanochannels are formed by a vertical splitting of the triangular ridges induced by relaxation of the pre-existing stresses during annealing. The main advantage of the technique is its simplicity and versatility as opposed to other top-down methods of fabricating three dimensional nanostructures of similar feature sizes. Neither the initial pattern transfer process, which employs capillary force lithography as a tool, nor the subsequent vacuum annealing require any differential thermal control across the sample. Also, there is no requirement of application of high pressure in the entire process or UV curing, thus removing any optical limitations on the feature size. The technique is not limited to the use of radiation-sensitive materials. An additional benefit of the presented technique is that the channels thus formed have relatively very thick walls of the same polymer which enhances the mechanical stability of channels and also separate them from one another by large distances which are among the key requirements in their commercial applications. As used in the example shown here, if the initial master stamp is a thin flexible foil, the technique can be easily applied to curved surfaces as the flexibility of the master can ensure its conformal contact to facilitate good initial pattern transfer. The technique is suitable for fabrication of nanostructures over large areas (∼squared
centimeters) limited only by the microstamp used for the generation of the parent microstructure. The underlying physics of the process indicates that the feature size and complexity of 3D nanostructures can be controlled by varying the polymeric layer thickness, periodicity, and complexity of the master stamp used for the fabrication of the parent microstructure and the annealing time/temperature. The technique first requires fabrication of an initial microstructure consisting of triangular ridges with sharp corners. This can be obtained either directly by nanoimprint lithography or by partially filling the rectangular groves of a master by the capillary flow lithography followed by a controlled annealing of the microstructure. In the latter technique which is used here, a proper selection of the initial film thickness and the dimensions of the microstructure on the master stamp are both important in the fabrication of the initial microstructure with triangular ridges. Clearly, the volume of the grooves on the master stamp should be less than the volume of the polymer available in the thin film. The use of a relatively thick film or the use of a master with smaller grooves would completely fill the grooves of the master thus producing an initial polymer microstructure with rectangular ridges that is not suitable for the production of nanochannels by a symmetrical splitting of the microstructure. Further, the height of the nanochannels is limited by the depth of the grooves on the master and the thickness of the surrounding nanochannel walls is governed by the width of the grooves (wall thickness ∼ groove width/2). Finally, although the residual stress in the patterned polymer increases in the higher molecular weight polymer, its relaxation may also becomes slower.25–29 Thus, there may be many optimal conditions on a combination of processing parameters such as the molecular weight and the
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annealing time and temperature. Work is currently underway in our group to probe the optimal conditions and the limits of the method. The drastic reduction in feature size obtained using this simple technique results due to self-organization of the initial features formed and hence does not require any series of additional complex operation for size reduction. The presented process is a robust and promising technology to fabricate novel/in-demand highly functional three dimensional nanostructures in a well reproducible and highly cost effective manner as it includes no complex or serial processes and has the capability of producing such structures on very large areas without any additional operation or operation time. Acknowledgment This work was supported by the DST Unit on Nanosciences at IIT Kanpur. A.S. thanks the Unilever Corporate Research for the invitation to the 1st Unilever International Conference on Multi-scale Structures and Dynamics of Complex Systems, Beijing, and acknowledges fruitful discussions with Vijay M. Naik who has inspired our work in the area of self-assembled multiscale structures over the years. Literature Cited (1) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Nanoimprint Lithography. J. Vac. Sci. Technol. B. 1996, 14184, 4129. (2) Barnes, M. D.; Ng, K. C.; Whittenm, W. B.; Ramsey, J. M. Detection of Single Rhodamine 6G Molecules in Levitated Microdroplets. Anal. Chem. 1993, 65, 2360. (3) Dickson, R. M.; Norris, D. J.; Tzeng, Y.-L.; Moerner, W. E. ThreeDimensional Imaging of Single Molecules Solvated in Pores of Poly(acrylamide) Gels. Science 1996, 274, 966. (4) Lyon, W.; Nie, S. Confinement and Detection of Single Molecules in Submicrometer Channels. Anal. Chem. 1997, 69, 3400. (5) Lee, Y. H.; Maus, R. G.; Smith, B. W.; Winefordner, J. D. LaserInduced Fluorescence Detection of a Single Molecule in a Capillary. Anal. Chem. 1994, 66, 4142. (6) Guenard, R. D.; King, L. A.; Smith, B. W.; Winefordner, J. D. TwoChannel Sequential Single-Molecule Measurement. Anal. Chem. 1997, 69, 2426. (7) Zander, C.; Drexhage; et al. Single-Molecule Counting and Identification in a Microcapillary. Chem. Phys. Lett. 1998, 286, 457. (8) Sivanesan, P.; Li, Y.; Okamoto, K.; Polymer Nanochannel Fabrication and Analysis of Single Protein Molecules Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems, Miami, FL, Feb 2005; pp 762-765. (9) Ponniah, S.; Kenji, O.; et al. Polymer Nanochannels Fabricated by Thermomechanical Deformation for Single-Molecule Analysis. Anal. Chem. 2005, 77, 2252–2258. (10) Haab, B.; Mathies, R.; et al. Single-Molecule Detection of DNA Separations in Microfabricated Capillary Electrophoresis Chips Employing Focused Molecular Streams. Anal. Chem. 1999, 71, 5137.
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ReceiVed for reView December 29, 2007 ReVised manuscript receiVed March 9, 2008 Accepted March 11, 2008 IE7017838
