Fine tuning of lithographic masks through thin films of PS-b -PMMA with different molar mass by rapid thermal processing

The self-assembly of asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer based nanoporous thin films over a broad range of molar mass (M_n) between 39 kg·mol^-1 and 205 kg·mol^-1 is obtained by means of a simple thermal treatment. In the case of standard thermal treatments, the self-assembly process of block copolymers is hindered at small M_n by thermodynamic limitations and by a large kinetic barrier at high M_n. We demonstrate that a fine tuning of the annealing parameters, performed by a Rapid Thermal Processing (RTP) machine, permits us to overcome those limitations. Cylindrical features are obtained by varying M_n and properly changing the corresponding annealing temperature, while keeping constant the annealing time (900 s), the film thickness (∼30 nm), and the PS fraction (∼0.7). The morphology, the characteristic dimensions (i.e., the pore diameter d and the pore-to-pore distance L₀), and the order parameter (i.e., the lattice correlation length) of the samples are analyzed by scanning electron microscopy and grazing-incidence small-angle X-ray scattering, obtaining values of d ranging between 12 and 30 nm and L₀ ranging between 24 and 73 nm. The dependence of L₀ as a 0.67 power law of the number of segments places these systems inside the strong segregation limit regime. The experimental results evidence the capability to tailor the self-assembly processes of block copolymers over a wide range of molecular weights by a simple thermal process, fully compatible with the stringent constraints of lithographic applications and industrial manufacturing.