Block Copolymer Patterning for Creating Porous Silicon Thin Films with Tunable Refractive Indices.

We investigated the use of block copolymer (BCP) self-assembly for tuning the optical properties of silicon. We fabricated porous silicon by etching a hexagonally ordered pore pattern onto the surface of silicon wafers using poly(styrene-2-vinylpyridine) to prepare the etch mask. Contrary to typical BCP lithography, we did not need to use a range of different polymers to vary the pore size. We used the dry etching time as a way to increase the pore diameter and thus the porosity. The optical properties of the fabricated porous thin films were characterized by two effective medium approximations. Both the volume-averaging theory and the 2D Maxwell-Garnett theory gave similar effective refractive index values, although the latter was more accurate in predicting the film porosity. The refractive indices of the produced thin films could be varied by controlling the porosity. A maximum decrease of 30% in the refractive index was observed at 34% porosity compared to bulk silicon. We also demonstrated over a 60% decrease in the reflectance of silicon at 500 nm wavelength. The presented BCP method can be used to tailor semiconductor and dielectric layers for photonic applications without the size limitations of conventional lithography or the unpredictability of other pore-forming fabrication methods.