Nanocellular formation of supercritical CO$_2$ in block copolymer thin films

The production of large-area structured surfaces with a featured size of nanometer scale still remains a challenge by conventional photolithography. In particular, block copolymer thin films have been considered as the ideal lithography templates. Here we present our novel supercritical carbon dioxide (scCO$_2$) process to fabricate thin films with a single layer of empty cells of a diameter of ca. 30 nm in block copolymer thin films. By absorbing CO$_2$ in CO$_2$-philic block domains of a block copolymer followed by depressurization, empty cells are introduced in CO$_2$-philic domains. This process was successful even in a block copolymer thin films with a thickness less than 100 nm. The typical nanocellular structures introduced by our novel scCO$_2$ process (10MPa) have an average spacing of 34 nm and a density of 9 $\times$ 10$^{10}$ cm$^{-2}$. The size and the spacing of such nanocells can be adjusted by changing saturation pressure of scCO$_2$. The obtained structures are significantly different from those expected from the volume ratio of domains swollen by CO$_2$. Spherical cells in block copolymer thin films are found even when the porosity is more than 30\%.