An experimental program is proposed aimed at discovering both new directed assembly methods and their limitations for achieving good translational and orientational order in 2D arrays of block copolymer domains, methods that could ultimately lead to nanopatterns that could be transferred to underlying substrates. Directed self-assembly techniques are proposed for investigation whereby the edges of regions defined by optical and electron beam lithography serve to register and template the order. Emphasis will be placed on understanding the ordering and disordering processes in melts of both single layer and multilayer films of spherical domain block copolymers and their blends with homopolymers in such regions. State-of-the art scanning force microscopy and grazing incidence small angle X-ray diffraction will be used in a complementary fashion to precisely define the order in these layers. Directed self-assembly is also of interest for cylindrical domain block copolymer films with cylinders parallel to the film surface. Here controlling the equilibrium concentration of dislocations and the thermal unbinding of their component disclinations using nearby channel edges is the primary challenge. These experiments will be supplemented with field theoretic numerical simulations of 2D block copolymer ordering in collaboration with Glenn Fredrickson at UCSB.
NON-TECHNICAL SUMMARY:
Patterning of regular features on the scale of 10 nanometers can enable applications such as ultrahigh density magnetic storage, ultraregular nanoporous filtration membranes and quantum dot arrays. Such patterning is well beyond what is possible directly using light (optical lithography) but this project aims to explore the possibilities of using optical lithography to create larger scale features that can direct the assembly of highly perfect arrays of much smaller block copolymer domains to create the desired patterns. Understanding the causes of imperfections in such arrays and the overall limits of the method are major goals. Undergraduates will be involved in the research both during the academic year and the summer. The research group will continue to host visiting graduate students from foreign countries for periods up to one year to promote international awareness. Major emphasis will be placed on developing graduate student communication and presentation skills, especially in the context of national and even international meetings. Interactions will be developed with nanotechnology groups at various industrial concerns interested in utilizing the results of this research.
Complementary experimental imaging and diffraction methods were developed to characterize the structure of AB diblock copolymer thin films as a function of the temperature and environment of annealing. When directed by substrate topological features these block copolymer films form exceptionally small scale patterns that can be transferred into an underlying substrate such as silicon, providing access to length scales that cannot be reached currently even by exceptionally costly optical lithography and thus these directed assembly methods are of intense interest to the microelectronics and magnetic storage industries. This project however has used the experimental methods above to demonstrate that a fundamental lower limit to the block copolymer domain spacing exists below which a large density of defects in the block copolymer pattern is generated, thus destroying the regularity of the pattern. This result agrees with an analytical theory of defect formation and implies an exceptional sensitivity to the lengths of the A and B blocks. Self-consistent field theory simulations were used to predict the minimum feature spacing for diblock copolymers of arbitrary composition thus providing guidance to researchers in industry who try to create small scale patterns by directed block copolymer assembly. These methods have been extended to films with multiple layers of block copolymer domains resulting in an improved understanding of the role that surfaces and interfaces play in determining the structure of such films. Several PhD graduate students participated in this research and have subsequently taken up research and development positions in US industry and national laboratories. Their experience here was enhanced by interaction with a number of visiting graduate students from foreign countries hosted by the research group and by travel to national synchrotron X-ray facilities to do experiments. Major emphasis was placed on developing their communication and presentation skills. All PhD students presented their work multiple times at the national March Meeting of the American Physical Society as well as at national meetings of the Materials Research Society and the American Institute of Chemical Engineers. Based on the research results reported above, interactions developed with directed assembled groups at various industrial concerns who were interested in utilizing the results of this research.
