Functional Polymer Micro- and Nanostructures by Nanoimprint

Xing Cheng, Hyunsoo Park and Dehu Cui

Texas A&M University, College Station, TX, USA, chengx@ece.tamu.edu

ABSTRACT Thermal nanoimprint is a physical molding technique

for thermoplastic polymer patterning. In this paper, we explore the application of nanoimprint for nondestructive patterning of functional polymers. We found ubiquitous polymer chain alignment in polymer micro- and nanostructures patterned by nanoimprint. This discovery indicates nanoimprint can not only pattern functional polymers but also be used as a tool to finetune the properties of functional polymers. We also develop a novel scheme to remove residual layer in nanoimprint without dry etching. Isolated functional polymer structures are obtained by this technique. Finally, we demonstrate that this modified nanoimprint scheme can also be used to fabricate polymer nanowires. Coupled with nanoimprint’s high-resolution, high-throughput and low-cost characteristics, the techniques developed in this work are expected to have significant impact on patterning functional polymer micro- and nanostructures for novel device applications in flexible electronics, optoelectronics, sensors and bioengineering.

Keywords: nanoimprint, functional, polymer, fabrication

1 INTRODUCTION Nanoimprint is a high-resolution, high-throughput and

low-cost patterning technique for soft materials. It has attracted enormous attention since its inception in 1996 [1, 2]. Despite its short development time, nanoimprint has emerged as a strong candidate for next-generation lithography for microelectronic fabrication at 22 nm node and beyond. Although current main stream nanoimprint research focuses on UV-based nanoimprint for microelectronic fabrication [3], there are growing interests in direct-patterning of soft materials which remain as active components in final device by nanoimprint, particularly functional polymers.

Functional polymers, such as polymer semiconductors, nonlinear optical polymers and piezoelectric polymers, are gaining increasing interests nowadays because of their potential in building novel active devices such as flexible electronics and sensors. In this paper, we present our work in functional polymer patterning by nanoimprint. Not only nanoimprint can successfully pattern those functional polymers into micro- and nanostructures in a nondestructive fashion, but also it improves the properties of those functional polymers through flow-induced polymer chain orientation during nanoimprint.

2 FUNCTIONAL POLYMER

PATTERNING BY NANOIMPRINT To fabricate functional polymer micro- and

nanostructures by nanoimprint, nanoimprint molds are first fabricated by conventional microelectronic fabrication techniques. Molds with structures of several to tens of μm can be fabricated in silicon dioxide or fused silica by photolithography and reactive-ion etching. Molds with submicron or nanostructures can be fabricated by electron-beam lithography and reactive-ion etching. The molds are coated with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) to reduce polymer-mold adhesion for easy mold releasing after nanoimprint. This surfactant coating also prevents the formation of nanoimprint defects.

Since nanoimprint is a physical molding process, polymers can be forced to flow to fill up trench areas on mold surface at a temperature above their glass transition point (Tg). After cooling and mold releasing, a pattern complementary to mold structure is created in polymer film. For mold pattern replication with high fidelity, it is desirable to perform nanoimprint at conditions where polymer melt viscosity is low. This can be achieved by using low molecular weight polymers and performing nanoimprint at a temperature much higher than polymer Tg. However, higher temperature increases the chances of breaking chemical bonds in functional polymers such as conjugated polymers. In most cases, optimal nanoimprint temperature is around 50 to 70 degrees above polymer Tg.

Figure 1 shows scanning electron microscope (SEM) images of representative microstructures nanoimprinted in functional polymers such as poly (3-decylthiophene) (P3DT) and poly(vinylidene fluoride) (PVDF). Similar to other commonly used polymers such as poly(methyl methacrylate) (PMMA), micron and submicron scale patterns are well formed in those functional polymers. However, unlike amorphous PMMA, semicrystalline polymers such as P3DT and PVDF may have increased tendency to crystallization under favorable thermal history that is imposed during the nanoimprint process. In confined micro- and nanoscale dimensions, it has been found that PVDF undergoes unique crystallization process that improves its piezoelectric property [4]. Patterning functional polymers into micro- and nanostructures by nanoimprint provides a platform for studying polymer crystallization at micro- and nanoscale.

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3 CHAIN ORIENTATION IN NANOIMPRINTED POLYMER MICRO-

AND NANOSTRUCTURES The physical properties of functional polymers are

usually affected by their chain configuration. Significant amount of effort has been devoted to achieve chain ordering in functional polymers to improve existent or impart new properties [5-9].

Recently, we observed ubiquitous chain ordering in polymer micro- and nanostructures patterned by thermal nanoimprint. The origin of this chain orientation is flow-induced chain alignment. Polymer materials are well known to exhibit chain orientation during melt flow [10] as illustrated in Figure 2. During the formation of the polymer microstructures in nanoimprint, polymer chains will naturally align due to melt flow. Anisotropy of material properties, such as refractive index and optical absorption, emerges in polymer micro- and nanostructures after cooling [11].

Optical birefringence due to chain alignment in nanoimprinted microstructures can be observed with polarizing optical microscope. Figure 3(a) shows an optical microscope image of nanoimprinted PVDF lines. In crossed-polarizers (Figure 3(b)), the lines appear as bright because the polarization direction of the incident light is rotated by the birefringent PVDF microstructure. The amorphous background appears dark because this region is optically isotropic. In this case the polymer chains are aligned perpendicular to PVDF lines. This can be verified by rotating the sample by 45o to align the PVDF lines with the polarization directions of the polarizer and the analyzer. In this situation, PVDF microstructures will not rotate the polarization direction of the incident light, which is then blocked by the analyzer and the structures appear dark (Figure 3(c)).

In our experiments, we observe that chain alignment in

polymer structures after nanoimprint is ubiquitous. Since the polymer melt flow pattern during nanoimprint depends on the size, shape and depth of the mold patterns, the

Figure 2. A schematic of polymer chain orientation during nanoimprint.

(a)

(b)

(b)

(c)

Figure 3. (a) Optical microscope image; (b) Same structure viewed in polarized microscope; (c) Sample rotated by 45o with respect to (b).

(a)

Figure 1. (a) PVDF dots patterned by nanoimprint (scale bar is 10 μm); (b) P3DT disks patterned by nanoimprint (scale bar is 40 μm).

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direction and degree of chain alignment in functional polymers can be easily adjusted by designing a specific mold structures. The nanoimprint processing parameters, such as nanoimprint temperature and the cooling rate after nanoimprint, also affect the degree of chain alignment in final structure. Because polymer properties heavily depend on chain conformation, this opens up an easy yet powerful route toward manipulating polymer properties by nanoimprint.

4 NONDESTRUCTIVE RESIDUAL LAYER REMOVAL FOR FUNCTIONAL

POLYMER STRUCTURES In conventional nanoimprint lithography, there is

always a residual layer left after nanoimprint. The residual layer is typically removed by oxygen dry etching. However, dry etching is not compatible with functional polymers because it will break chemical bonds and damage the functional polymer. For many applications, such as integrated circuits based on conjugated polymer thin-film transistors, isolated structures are needed to provide good device isolation. In this case, new method to nondestructively remove residual layers must be developed.

Recently we develop a technique to remove polymer residual layer after nanoimprint without dry etching [12]. The basic idea is to transfer the polymer film onto mold surface to expose the residual layer (Figure 4(a)). The mold with the polymer film is then soaked in solvent to remove the exposed residual layer (Figure 4(b)). After complete residual layer removal, the polymer microstructure is transferred from the mold to a substrate by transfer bonding [13], which leads to isolated functional polymer microstructures without residual layer. Figure 5 shows fluorescent microscope images of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) patterned by nanoimprint. Conventional nanoimprint leaves

a residual layer between grating lines (Figure 5(a)), while our new method yields isolated MEH-PPV lines of 5 μm. The dark region between MEH-PPV lines indicates complete removal of the residual layer.

The new method we developed resolves a major limitation in nanoimprint technique. This presents opportunities to pattern isolated functional polymer micro- and nanostructures for novel device applications. Although other patterning techniques, such as ink-jet printing, can also achieve isolated functional polymer microstructures, they are limited in pattern resolution [14, 15]. Coupled with nanoimprint’s sub-10 nm resolution, we believe this technique can be easily extended into patterning functional polymer structures with sub-100 nm dimensions. In addition to patterning structures for practical applications, the technique also allows us to pattern isolated polymer nanostructures to study fundamental structure-property relations in functional polymers at nanoscale.

5 FUNCTIONAL POLYMER NANOWIRES BY NANOIMPRINT

Nondestructive removal of residual layer in nanoimprint

also presents a route toward polymer nanowire fabrication. As shown in Figure 4 (b), individual polymer nanowires can be achieved in mold trenches after residual layer removal. If those nanowires are detached from mold surface, we can obtain polymer nanowires suspended in solution. Detaching nanowire from mold can be achieved by ultrasonic agitation or lift-off in solvent after transfer-bonding nanowires onto a PMMA surface.

(a)

(b)

Figure 4. Schematics of residual layer removal without dry etching. (a) Transfer polymer film onto mold surface after nanoimprint; (b) Soak mold in solvent to remove residual layer.

(b)

Figure 5. Fluorescence microscope images of MEH-PPV with (a) and (b) without residual layers. The line width is 5 μm.

(a)

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Figure 6 shows an SEM image of poly(3-hexylthio-phene) (P3HT) nanowires obtained by lift-off. Individual P3HT nanowires are achieved. In this work, we demonstrate only 350nm size nanowires. However, this technique is not limited to this size. Since the polymer nanowire width is determined by the mold structures, sub-100 nm polymer nanowires can be easily achieved if molds with sub-100 nm structures are fabricated. Future work will extend current results to achieve sub-100 nm polymer nanowires. Potential applications of such polymer nanowires include low-cost polymer solar cell devices, polymer sensors with large surface areas, and polymer scaffolds for tissue engineering.

6 CONCLUSION In this work, we have demonstrated that thermal

nanoimprint is an ideal technique for functional polymer patterning. Micro- and nanostructures can be easily obtained in various functional polymers. Moreover, new residual layer removal technique we developed enables nondestructive patterning of functional polymers for integrated device and system applications. The residual layer removal technique can also be used to create polymer nanowires. The techniques developed here are widely applicable to a variety of functional polymers. It is expected to accelerate the inroad of functional polymers in flexible electronics, optoelectronics, sensors and biomedical engineering applications to benefit from their superior material properties. Finally, chain ordering in polymer micro- and nanostructures after nanoimprint presents a new way to finetune polymer properties. Compared with other functional polymer patterning techniques such as ink-jet printing, nanoimprint holds advantages in pattern resolution and the ability to manipulate chain conformation. In addition to engineering applications, nanoimprint can also be used to create polymer nanostructures with predefined chain alignment for fundamental study of polymer properties at nanoscale and the structure-property relations in functional polymers.

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Figure 6. An SEM image of 350 nm P3HT nanowire.

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