Anisotropic Coarsening of Two-Dimensional Surface Domains in Copolymer Thin Films

Effect of temperature on the morphology and kinetics of surface pattern formation in thin block copolymer films

Hole formation and growth on the top layer of thin symmetric diblock copolymer films, forming an ordered lamellar structure parallel to the solid substrate (silicon wafer) within these films, is investigated as a function of time (t), temperature (T), and film thickness (l), using a high-throughput experimental technique. The kinetics of this surface pattern formation process is interpreted in terms of a first-order reaction model with a time-dependent rate constant determined uniquely by the short-time diffusive growth kinetics characteristic of this type of ordering process. On the basis of this model, we conclude that the average hole size, lambda(h), approaches a steady-state value, lambda(h)(t-->infinity) identical with lambda(h,infinity)(T), after long annealing times. The observed change in lambda(h,infinity)(T) with temperature is consistent with a reduction of the surface elasticity (Helfrich elastic constant) of the outer block copolymer layer with increasing temperature. We also find that the time constant, tau(T), characterizing the rate at which lambda(h)(t) approaches lambda(h,infinity)(T), first decreases and then increases with increasing temperature. This temperature variation of tau(T) is attributed to two basic competing effects that influence the rate of ordering in block copolymer materials: the reduction in molecular mobility at low temperatures associated with glass formation and a slowing of the rate of ordering due to fluctuation effects associated with an approach to the block copolymer film disordering temperature (T(d)) from below.

Self-assembly of a diblock copolymer on a patterned surface with low-energy electron beam

A pattern was generated by 500 eV electron beam irradiation on benzaldimine monolayer through a grid and subsequent hydrolysis of nonirradiated regions. While we tried to assemble a block copolymer, polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP), on the pattern, we observed that the polarity difference between the two different regions was not right for discriminating the wetting behavior of two blocks of the polymer. Among various modifications of the retrieved amine, it was found that tribromoacetaldehyde was suitable for this end. Surprisingly, treatment of the aldehyde gave a surface preferring the polystyrene block to poly(4-vinylpyridine) block, while the irradiated section favored the latter block. As a result, island morphology was observed on the tribromoacetaldimine region and hole morphology on the irradiated region when the film thickness was 1.3Lo. Contact angle data were consistent with the observed symmetric wetting on the former region and the asymmetric one on the latter.

Stringlike structure formed in thin films of a lamella-forming diblock copolymer

Thin films of a lamella-forming polystyrene-block-poly(2-vinylpyridine) diblock copolymer, PS-P2VP, were annealed by a tetrahydrofuran (THF) vapor treatment. The thin films immediately developed a multilayered lamellar structure, similar to those known for conventional heat treatment. However, in the case of the vapor treatment, the steps moved over a significantly larger distance than the case of the heat treatment. This process strikingly developed stringlike objects (strings) that appeared near the steps. The strings were found to be cylindrical domains of poly(2-vinylpyridine) blocks (P2VP) partitioned from P2VP lamella during the step motion and sandwiched by the polystyrene phase that results in perforating the P2VP lamella. The string formation seemed to be associated with the motion of the step, which was considerably faster in the vapor treatment process than in the heat treatment process.

How surface topography relates to materials' properties

The topography of a surface is known to substantially affect the bulk properties of a material. Despite the often nanoscale nature of the surface undulations, the influence they have may be observed by macroscopic measurements. This review explores many of the areas in which the effect of topography is macroscopically relevant, as well as introducing some recent developments in topographic analysis and control.

Late-stage coarsening of an unstable structured liquid film

Thin films of poly(styrene-b-methyl methacrylate) diblock copolymers above the bulk order-disorder transition temperature, within a certain thickness range, are structurally unstable on SiO(x)/Si substrates. They dewet autophobically, forming droplets on a self-assembled brush. We investigated the late-stage evolution, coarsening, of the droplets on the brush. The average droplet cross-sectional area <S> increased with time, <S> proportional, variant t(gamma). This was accompanied by a decrease in the number of droplets per unit area with time, N(t) proportional, variant t(-gamma). We analyzed the droplet size distribution, F(S/<S>) vs S/<S>, and found that the shape of the curve was virtually identical at different stages throughout the process. This suggests that a structural self-similarity is associated with the process. A comparison of F(S/<S>) vs S/<S> data with distributions based on Ostwald ripening and coalescence cluster coarsening (dynamic and static) mechanisms strongly indicates that the dominant coarsening mechanism involved motion of droplets across the brush and subsequent coalescence, i.e., dynamic coalescence coarsening, not Ostwald ripening.

Viscous flow and coarsening of microdomains in diblock copolymer thin films

For thin block copolymer (BCP) films on a homogeneous substrate, a fast domain growth exponent has recently been observed in experiments. This growth exponent is larger than what one would expect from theoretical considerations of two-dimensional coarsening of small molecular liquid mixtures in the viscous regime. Thus it is not clear whether the growth kinetics is truly different for block copolymer films, or if the observed anomalous exponent is a result of a possible dimensional crossover from two to three dimensions. To address part of this question, we have carried out numerical simulations of ordering and domain growth in a two-dimensional system of BCP melts. The model calculations reported here explicitly include viscous, hydrodynamic flow and provide a scaling description of the growth of domains in a quenched BCP system. Our results indicate that the growth kinetics of BCP melts in two dimensions belong to the same dynamical universality class of small molecular liquid mixtures in the viscosity dominated regime.