Ordered to Isotropic Morphology Transition in Pattern-Directed Dewetting of Polymer Thin Films on Substrates with Different Feature Heights

Abstract

Controlled dewetting of a thin polymer film on a topographically patterned substrate is an interesting approach for aligning isotropic dewetted structures. In this article, we investigate the influence of substrate feature height (H(S)) on the dewetting pathway and final pattern morphology by studying the dewetting of polystyrene (PS) thin films on grating substrates with identical periodicity (λ(P) = 1.5 μm), but H(S) varying between 10 nm and 120 nm. We identify four distinct categories of final dewetted morphology, with different extent of ordering: (1) array of aligned droplets (H(S) ≈ 120 nm); (2) aligned undulating ribbons (H(S) ≈ 70-100 nm); (3) multilength scale structures with coexisting large droplets uncorrelated to the substrate and smaller droplets/ribbons aligned along the stripes (H(S) ≈ 40-60 nm); and (4) large droplets completely uncorrelated to the substrate (H(S) < 25 nm). The distinct morphologies across the categories are attributed to two major factors: (a) whether the as-cast film is continuous (H(S)≤ 80 nm) or discontinuous (H(S)≥ 100 nm) and (b) in case of a continuous film, whether the film ruptures along each substrate stripe (H(S)≥ 70 nm) or with nucleation of random holes that are not correlated to the substrate features (H(S)≤ 60 nm). While the ranges of H(S) values indicated in the parentheses are valid for PS films with an equivalent thickness (h(E)) ≈ 50.3 nm on a flat substrate, a change in h(E) merely alters the cut-off values of H(S), as the final dewetted morphologies and transition across categories remain generically unaltered. We finally show that the structures obtained by dewetting on different H(S) substrates exhibits different levels of hydrophobicity because of combined spatial variation of chemical and topographic contrast along the surface. Thus, the work reported in this article can find potential application in fabricating surfaces with controlled wettability.

Transition from Spin Dewetting to continuous film in spin coating of Liquid Crystal 5CB

“…Disjoining pressure arising out of interfacial van der Waals’ interaction within the film leads to the growth of thermally excited surface capillary waves with time, leading to spontaneous rupture and dewetting of ultra-thin films, particularly on non wettable surfaces 6 . While such instability is undesirable from the standpoint of coatings 11 , pattern formation associated with it is gaining popularity as a viable non lithographic alternative for creating meso and nano structures, particularly on a topographically or chemically patterned substrate, which results in perfect ordering of the features 5 , 12 – 17 .…”

“…While initial experiments on spontaneous instability of ultra-thin films involved simple homo-polymers such as PS, PMMA 5 – 17 , dewetting of complex materials such as liquid crystal (LC) has also received significant attention. Dewetting of a LC thin films is intriguing due to the coupling of the structural effects arising from the anisotropic and oriented nature of the molecules along with the destabilizing van der Waal’s forces active in films thinner than ≈100 nm 18 – 34 .…”

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.…”

Nanostructure Formation by controlled dewetting on patterned substrates: A combined theoretical, modeling and experimental study

“…For example, in the production of nanodot arrays with templated substrates, regular arrays of nanodots are only obtained with an optimal thickness of deposited film. A very thin or thick film usually forms random distributed nanodots even with periodic templates262728. A systematic theoretical analysis that establishes the comprehensive relationships between the system parameters and the resultant nanostructures is crucial in order to fully exploit the potential of the SSD technique for nanofabrication.…”