Self-Organized Nano-Lens Arrays by Intensified Dewetting of Polymer Thin-Film

Dynamics of instabilities and structure formation in thin polymer films have been widely studied because of their increasing applications in the areas of coatings (thermal and electrical insulation, optical, protective, biological etc.), plastic electronics, organic light emitting diodes (OLEDs), membranes, lubricants and adhesives. In many such applications instabilities are undesired and have to be suppressed for the product to remain functional. However, the study of instabilities and their kinetics in ultrathin films (<100 nm) can provide some important insights into the inter-surface potentials that cannot be directly measured otherwise. It also can be used to generate useful meso- and nano-scale patterns over large areas (~cm²). Therefore, the spontaneous dewetting of ultrathin polymer films on a non-wetting substrate has been a subject of great interest owing to its scientific and technological importance.

Dewetting on a flat homogeneous surface starts with the nucleation of randomly placed holes at a certain mean separation (λ), which grow and coalesce with time and result in randomly placed droplets of the polymer. Average diameter as well as the mean separation of dewetted structures are a function of initial thickness (h) and the interfacial tension of the polymer film. However, randomness of the dewetted structure limits the usefulness of this method in some applications as a potential soft patterning technique. Various strategies have been explored to impose a long range-order in the dewetted structures. One strategy for the alignment of the dewetted structure is to combine it with other top-down lithographic approaches such as controlled dewetting on topographically or chemically patterned substrates. Dewetting of ultrathin polymers films on physico-chemically patterned substrates has also been extensively studied both theoretically and experimentally. However, despite its promising scientific and technological advantages, the feature-size generated by the self-organized dewetting has two major limitations on the pattern resolution and its aspect ratio. The first limitation arises from the weak van der Waals destabilizing force and high surface tension (γ), both of which impose a severe limit on minimum feature size, which is related to the wavelength, (λ) of the long-wave instability in spinodal dewetting:
λ = [ -8π²γ/(∂φ/∂h)]^1/2

Where h is the film thickness and f is the destabilizing intermolecular potential (~ h^-3 for van der Waals interaction). In a model system of polystyrene (PS) thin-film on silicon substrate, this limits the spot size to >1µm even in case of films as thin as 10 nm. Other limitation is the very small contact angle (< 10°) and thus the aspect ratio of dewetted structures in air is rather small.

We have recently developed a novel method which overcomes the limitations on the feature size and aspect ratio to a great extent. The PS thin (< 100 nm) films were destabilized by immersing into an optimal mixture of solvent (methyl-ethyl ketone and acetone) and water at room temperature. Selective diffusion of solvent molecules into the polymer matrix brings down its glass transition temperature (T_g) below room temperature and thus causes the instability and dewetting of the film. However, water being the majority phase in the bounding media inhibits the dissolution of PS. Room temperature dewetting in the liquid media provides the advantage of cleaner environment, greater control over the shape of structure and better defect control as compared to thermal annealing or dewetting in air. Further it prevents the sensitive materials from harsh conditions (UV, laser, electron beam, heating high vacuum etc.) of patterning. Interestingly, dewetting under the water-solvent mix reduces the droplet diameter by more than one order of magnitude compared to dewetting in air. We have also investigated the underlying mechanisms of intensification of surface instability under water-solvent mix including the length scale of instability before fragmentation of the film into droplets and compared the conditions for the formation of ordered patterns on topographically structured substrates in air and under the water-organic mix. Toward these ends, we first systematically investigate the length scale of instability, droplet diameters and inter-droplet spacing as a function of film thickness in order to clearly differentiate these from the well-known aspects of spontaneous dewetting in air. Finally, we explore the conditions for the formation of ordered patterns on topographically structured 1-D and 2-D substrates where dewetting occurs in highly confined but structured spaces. We demonstrate the use this technique for fabrication of ordered arrays of polymeric nano-lenses of tunable curvature by controlled dewetting under the water-organic mix.

We have achieved more than an order of magnitude reduction in the feature size of the dewetted structures and thus fabricate structures as small as ~60 nm. The mean separation of these structures is also reduced close to ~200 nm. Further, the mechanisms of miniaturization of the interfacial instability was explored and it was found that the thickness dependence of the characteristic length scales of dewetted structures is weaker (λ~h^1.51) as compared to the instability caused by van der Waals attraction (λ~h²). Moreover, a far greater control over the shape and aspect ratio of the dewetted structures is demonstrated as the contact angles can be tuned in the range of 40-140°. The ability to tune the curvature of the dewetted structures can be exploited in the fabrication of polymeric nano-lenses. Further, the controlled dewetting under the water-solvent mix on topographically patterned substrates was also employed for fabrication of sub-micron ordered structures. It was also shown that under the same conditions, dewetting in air produces no dewetting, incomplete dewetting or is incapable of producing ordered structures owing to a large length scale of instability in air. A further miniaturization of length scales because of the 2-D confinement is also demonstrated.

Figure 1. PS droplets of different contact angles and size visualized in the scanning electron microscope in transverse view.

Figure 2. 2-D array of polymeric nano-lenses obtained from the dewetting of PS thin film on the topographically patterned substrate. (Scale bar: 2 mm)

Field-induced self-organized patterning in ultra-thin polymer films produces structures that are limited to few microns to tens of microns in size because of the high energy penalty for the surface deformations on small scales and because of weak destabilizing van der Waals forces. We have resolved this long standing problem of miniaturization of length scales in thin film self-organization to sub-100 nm scale by reducing the interfacial tension and intensifying the field using a water-solvent mixture for dewetting. This proves to be a simple, powerful, flexible and inexpensive technique for the room temperature fabrication of sub-micron polymeric structures such as nano-lenses and their ordered arrays.

In conclusion, dewetting under water-solvent mixture takes the physical self-assembly of polymer thin-films to its limits by producing sub-micron structures of various degrees of ordering and tunable shapes. Among other things, such patterns have the potential to be used as tunable polymeric micro-lenses and lens-arrays, modifiers for the optical properties, and as delivery and positioning tools for a variety of functional materials that can be mixed with the polymer.

References

1.      Ankur Verma and Ashutosh Sharma, “Enhanced Self-organized Dewetting of Ultrathin Polymer Films under Water-organic Solutions: Fabrication of Submicrometer Spherical Lens Arrays”, Advanced Materials 2010, 22, 5306-5309.

2.      Ankur Verma and Ashutosh Sharma, “Submicrometer Pattern Fabrication by Intensification of Instability in Ultrathin Polymer Films under a Water-Solvent Mix”, Macromolecules 2011, communicated.