Polymer Engineering Faculty Research
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.