Patterning dewetting in thin polymer films by spatially directed photocrosslinking

Abstract

In this report we examine the dewetting of spin-cast poly (styrene) films in a confined geometry. We designed a platform for laterally confining PS by photo-patterning crosslinks in spin-coated thin films. Heating the patterned film above the glass transition temperature of PS results in localized dewetting patterns in regions that were not crosslinked, while the crosslinked pattern serves as a rigid barrier that confines the retraction of the uncrosslinked polymer in micron-sized domains. The barriers also provide a favorable surface that the liquid PS wets onto, forming a rim at the boundary of crosslinked and uncrosslinked polymer. The resulting patterns are shown to be dependent on the irradiation and annealing time, the dimensions of the uncrosslinked region and the thickness of the film.

Introduction

Self-organization processes offer exciting opportunities in generating micro- and nano-scale architectures and materials. The resulting interfacial structures generated by such processes can imbue a material with unique physical properties. For example, the undulating pattern of the inner membrane of the mitochondria results in a very large surface area, increasing the amount of possible locations for the presentation of ATP synthesizing proteins [1]. The surfaces of many organisms contain micro- and nano-structured motifs that provide important functions such as the self-cleaning abilities of the lotus leaf [2], [3] and wings of the cicada [4], water-harvesting by the Namib desert beetle [5] and the hydrophobic “water-walking” legs of the water strider [6], [7]. A common goal in materials science is the utilization of self-organizing processes to create three-dimensional structures and patterns on surfaces to facilitate the fabrication of devices. Pertinent applications include anti-fog and anti-reflection coatings [8], water-harvesting surfaces [9], decontaminating surfaces [10], surface roughness-enhanced adsorption [11], surface-enhanced spectroscopic signals [12], micro- and nano-containers [13], [14], [15], [16] and the study of fluid flow in confined spaces [17], [18].

Many reports have described methods that allow for polymer films and other materials to organize into characteristic structures [19], [20], [21], [22], [23]. Surface tension can lead to interesting morphological structures in polymer films. An important surface-tension driven process is the dewetting of thin polymer films where a macromolecular layer retracts from a surface that does not favor spreading [24], [25], [26]. A classic example is the dewetting of poly styrene (PS) on silicon wafers. Smooth thin (typically less than 100 nm) PS films can be prepared on Si substrates by spin-coating, however upon heating the films above T_g, the polymer dewets, forming droplets with finite contact angles. Dewetting proceeds by the formation of isolated holes, followed by their growth through the retraction of the hole perimeter into the liquid film. A rim of liquid is created ahead of the retracting front, which eventually contacts other rims forming ribbons. The ribbons eventually break into drops splayed in a cellular structure. Additionally, a bicontinuous pattern has been observed for very thin films [27]. The origin of the holes at the earliest stages of the dewetting process has been attributed to two mechanisms: spinodal decomposition and nucleation onto defect sites [28]. In the spinodal mechanism, thermal undulations (with wavelength λ) increase the interfacial area, which raises the free energy (in proportion to the surface tension, γ). The undulations also change the local film thickness h, and hence locally change the free energy per unit area of the van der Waals interactions in the film:

$$F(h)=-A/12\pi h^2\text{,}$$

where A is the Hamaker constant of the polystyrene/SiO₂ system. When the spinodal parameter d²F/dh² < 0 (i.e. A > 0), the van der Waals interactions cause the film to rupture for wavenumbers (2πλ) smaller than a critical value as given by:

$$k_c^2=-1/\lambda d^{2}F/{\mathit{dh}}^2\text{.}$$

The second mechanism of hole formation derives from heterogeneities (particles, air bubbles, etc.) in the film, which nucleate holes as the polymer dewets around the defect.

In some cases it is possible to control the spatial layout of surface structures that form as a result of dewetting or other instabilities by confining surface organization to a defined geometry [29], [30], [31], [32], [33], [34], [35], [36], [37]. In this report we examine pattern formation when PS dewets in a confined geometry defined by spatially directed photocrosslinking. Dewetting of the thin film is confined to masked regions of a photolithographically patterned film (i.e., negative patterning). The masked regions remain uncrosslinked, allowing the polymer to flow when annealed at elevated temperature. We have previously shown that a bifunctional benzophenone derivative, bis-BP, can photo-crosslink PS films and that the resulting films resist dewetting when heated above the glass transition temperature, T_g, or exposed to solvent vapor [38]. The photochemical nature of the method renders the technique amenable to patterning by irradiation through a photomask as depicted in Fig. 1. A spin-cast thin polymer film on a substrate that favors dewetting is patterned with regions of stable branched/crosslinked polymer and unstable linear polymer. Morphological changes occur under thermal annealing to form dewetting patterns that reflect the initial mask pattern used to induce spatially directed crosslinking.