Submicrometer Pattern Fabrication by Intensification of Instability in Ultrathin Polymer Films under a Water–Solvent Mix

Abstract

Dewetting of ultrathin (< 100 nm) polymer films, by heating above the glass transition, produces droplets of sizes of the order of microns and mean separations between droplets of the order of tens of microns. These relatively large length scales are because of the weak destabilizing van der Waals forces and the high surface energy penalty required for deformations on small scales. We show a simple, one-step versatile method to fabricate sub-micron (>~100 nm) droplets and their ordered arrays by room temperature dewetting of ultrathin polystyrene (PS) films by minimizing these limitations. This is achieved by controlled room temperature dewetting under an optimal mixture of water, acetone and methyl-ethyl ketone (MEK). Diffusion of organic solvents in the film greatly reduces its glass transition temperature and the interfacial tension, but enhances the destabilizing field by introduction of electrostatic force. The latter is reflected in a change in the exponent, n of the instability length scale, λ ~hn , where h is the film thickness and n = 1.510.06 in the case of water-solvent mix, as opposed to its value of 2.190.07 for dewetting in air. The net outcome is more than one order of magnitude reduction in the droplet size as well as their mean separation and also a much faster dynamics of dewetting. We also demonstrate the use of this technique for controlled dewetting on topographically patterned substrates with submicrometer features where dewetting in air is either arrested, incomplete or unable to produce ordered patterns.

Deformation and dewetting of thin liquid films induced by moving gas jets

“…For low film thicknesses, we measured power laws for the dryspot density very similar to those found by others for polystyrene melts on silicon wafers [18][19][20]27]. For our material system of polar liquids on polycarbonate, the dry-spot density coincides with the most unstable wavelength predicted by linear stability analysis for a Van der Waals type disjoining pressure with a value of the Hamaker coefficient in agreement with the retarded version of the Tabor-Winterton approximation.…”

“…7d illustrates a measurement close to that transition point. The power law N H $ h À4 min for low values of h min is close to the experimental observations on thin films of polystyrene on silicon wafers [18][19][20]22,27] and is consistent with our earlier analysis of 3EG on PC (see Fig. 6d for Re D > 100 in Ref.…”

“…Above a critical translation speed, however, a thin liquid film is left on the wafer, that spontaneously breaks up, dewets and leads to undesirable residual droplets on the substrate. Similar film rupture and dewetting phenomena have been described for thin layers of polymer melts [18][19][20][21][22][23][24][25][26][27], metals [28][29][30][31][32] and oil films submersed in water [33][34][35].…”

“…This may not be entirely consistent with our assumption of a uniform film thickness h = h min in the region of interest jxj < l 3 /2, upon which Eqs. (27)- (32) are based.…”

Synergism of Dewetting and Self-Wrinkling To Create Two-Dimensional Ordered Arrays of Functional Microspheres

“…Thus, in the past decades, enormous experimental and theoretical endeavors have been made to improve the ordering degree of the dewetting-induced features. 9−12,14−28 Previously, three preferred strategies have been introduced for directing dewetting: chemically 14,15 or topographically patterned substrates [9][10][11][20][21][22][23][24][25]28 or externally structured templates. 16 −19 In the above directed dewetting, lithography-based techniques such as photolithography or e-beam etching have been directly or indirectly involved in the fabrication of the required patterned substrates and structured templates.…”