Organizational Characteristics of a Block Copolymer Film

Organizational characteristics of a hexagonally close packed morphology block copolymer (BCP) film are studied and analyzed to assess order, to explore the connections between error rates and activation processes. Summary of Research: Self-organization has been suggested as a possible path to getting small dimensions where conventional optical lithography approaches break down. We have explored BCP approaches which allow different periodic structural forms and dimensions between 5-50 nm. Perpendicularly standing pore arrays of copolymer film are formed when the surface is energy neutral. This results in absence of preferential wetting from either block [1]. We graft the random copolymer to the substrate by annealing it in vacuum for 24 hours followed by rinsing with toluene to remove the excess random copolymers, forming a polymer brush on the surface. Polystyrene block co-methyl methacrylate (PS(68k)-b-PMMA(33.5k)) is then spun coated onto the surface and annealed in vacuum at 180°C for 10 hours. The raised temperature imparts mobility to the polymer. The polystyrene (PS) and poly(methyl methacrylate) (PMMA) blocks are covalently bonded to each other but are also immiscible in each other. In the presence of thermal energy, periodic array results from this repulsion. After the film has assembled, we crosslink the PS and degrade the PMMA by deep ultraviolet exposure. The PMMA block can then be removed by rinsing in acetic acid. We now have a PS mask with a periodic array of holes. The periodic arrays have been analyzed in this effort to determine their order parameters. Figure 1 shows an example with areaSEM ≈ 3238.7 × areapore. If for some reason there are regions on the substrate where the surface energy is not neutral, there will be preferential wetting by one of the blocks. First we wish to quantify how many PMMA cylinders are aligned parallel instead of perpendicular to the substrate due Figure 1: SEM image of block co-polymer film with Fourier transform of part of it (inset). to this preferential wetting, and also the spread in the center to center distance or domain spacing between all pores. In order to find the spread in the domain spacing, we use triangulation to first identify all the edges between pores. Figure 2 shows a histogram of all the edges. To quantify the defects in our sample, e.g., the places where we observe cylinders parallel instead of perpendicular pores we plot pores which are connected (Figure 3). Finally we calculate the translational and orientation correlation lengths. We observe from the Fourier transform of the micrograph (Figure 1 inset) that the intensity forms a ring denoting a more isotropic than crystalline ordering. Hence we use as our order parameter for translational correlation. We calculate the distance between all pairs of pores, histogram using a bin size of 4 nm and plot the normalized distribution