Diblock copolymers self-assemble into periodic arrays of microdomains with feature sizes of typically 10-50 nm, and have been used as self-assembled ?resists? to define periodic patterns useful in making a wide range of devices such as silicon capacitors and transistors, photonic crystals, and patterned magnetic media. However, the lamellar, cylindrical or spherical microdomains in diblock copolymers generally form grating patterns, or close-packed structures with hexagonal symmetry. This restricts their device applications, making it desirable to create self-assembled patterns with a wider range of geometries and applications. To overcome this limitation, this collaborative project between the Massachusetts Institute of Technology (MIT) and the University of Bristol in the United Kingdom will develop triblock terpolymers which form thin films with a diverse range of geometries. The work includes design of triblock terpolymers to form films with specific geometries such as widely or closely-spaced lines, lines with specific edge modulations, junctions, or bends, or arrays of cylinders or spheres in non-close-packed arrangements; synthesis of polymers with appropriate block chemistry, interactions and volume fractions; understanding processing effects including substrate treatment and annealing processes; modeling the self-assembly; and generation of magnetic nanoparticles within one block to form functional nanostructures directly. Central to this work is an investigation of templating of triblock terpolymers using substrate chemistry and topography, so that the self-assembly can be guided to nanoscale precision.
Students will be trained and mentored in an international setting, including the synthetic chemistry of triblock terpolymers, block copolymer lithography and templating, and the fabrication of functional nanostructures. The investigators will collaborate on developing teaching materials including lectures and demonstrations on self-assembly, and on dissemination of this work to a wide audience through school and community college outreach and use of resources such as the MIT OpenCourseWare initiative.
Block copolymers are a class of polymers in which each molecule is made up of two or more regions which are chemically different. In a triblock terpolymer, there are three different regions of the molecule which can be connected end-to-end to make a linear chain, or connected at a junction point to make a 3-armed molecule. If the component blocks of the block polymer are made of immiscible materials, then the block polymer can self-assemble to form regions of each of the different blocks. The region size and geometry are governed by the composiiton and chain length of the blocks. The self assembled structures are highly regular and as a result have many uses. For example, consider a polymer where one block forms parallel cylinders inside the other block. By etching out the cylindrical block, a porous membrane can be made useful as a filter. In this project, thin films of triblock terpolymers were made and their geometry was explored. First, the materials were designed and then synthesized using chemical methods developed at University of Bristol, UK, the international collaborator in this proposal. Then the materials were coated as thin films onto substrates, annealed, and etched to reveal their underlying structure. Finally, they were used as etch masks to pattern other materials, a process used in microelectronics to create devices. The unique discoveries in the project included novel synthetic methods for making triblock terpolymers containing silicon and iron in one of the blocks that could open the way to making a much wider array of new materials. The work showed that triblock terpolymers form a great variety of geometries, ranging from patterns that look like checkerboards to tiling patterns and structures with 90 degree bends. These patterns are not typically formed by self assembly, yet they are important in making microelectronic devices where square or rectangular grids and meshes are often required. Examples of patterned structures, such as arrays of square pits or pillars and meshes, were made, and some of these were used to template other structures such as magnetic and ferroelectric oxide films (see Figures). This pattern transfer process was considerably benefitted by the presence of the silicon and iron-containing block. This work has therefore shown how triblock terpolymers can provide key capabilities for microelectronic device fabrication. In addition, the faculty and students involved in this international collaboration visited each others’ laboratories and learned each others techniques, providing new training opportunities and exposure to a wider field of science. The researchers also participated in outreach activities including summer training for high school teachers, presentation of an event at the Cambridge Science Festival, and on-line learning.
