Ordered to Isotropic Morphology
Transition in Pattern-Directed
Dewetting of Polymer Thin Films on Substrates with Different Feature
Heights
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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 (<b>H</b><sub><b>S</b></sub>) on the dewetting pathway and final pattern morphology by studying
the dewetting of polystyrene (PS) thin films on grating substrates
with identical periodicity (<b>λ</b><sub><b>P</b></sub> = 1.5 μm), but <b>H</b><sub><b>S</b></sub> 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 (<b>H</b><sub><b>S</b></sub> ≈ 120 nm); (2) aligned undulating ribbons (<b>H</b><sub><b>S</b></sub> ≈ 70–100 nm); (3) multilength
scale structures with coexisting large droplets uncorrelated to the
substrate and smaller droplets/ribbons aligned along the stripes (<b>H</b><sub><b>S</b></sub> ≈ 40–60 nm); and
(4) large droplets completely uncorrelated to the substrate (<b>H</b><sub><b>S</b></sub> < 25 nm). The distinct morphologies
across the categories are attributed to two major factors: (a) whether
the as-cast film is continuous (<b>H</b><sub><b>S</b></sub> <b>≤</b> 80 nm) or discontinuous (<b>H</b><sub><b>S</b></sub> <b>≥</b> 100 nm) and (b) in case
of a continuous film, whether the film ruptures along each substrate
stripe (<b>H</b><sub><b>S</b></sub> <b>≥</b> 70 nm) or with nucleation of random holes that are not correlated
to the substrate features (<b>H</b><sub><b>S</b></sub> <b>≤</b> 60 nm). While the ranges of <b>H</b><sub><b>S</b></sub> values indicated in the parentheses are
valid for PS films with an equivalent thickness (<b>h</b><sub><b>E</b></sub>) ≈ 50.3 nm on a flat substrate, a change
in <b>h</b><sub><b>E</b></sub> merely alters the cut-off
values of <b>H</b><sub><b>S</b></sub>, as the final dewetted
morphologies and transition across categories remain generically unaltered.
We finally show that the structures obtained by dewetting on different <b>H</b><sub><b>S</b></sub> 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