(302f) 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.