Selective etching of polylactic acid in poly(styrene)-block-poly(d,l) lactide diblock copolymer for nanoscale patterning

Self-assembled thin films of a lamellar forming polystyrene-block-poly(d,l) lactide (PS-b-PLA) block copolymer (BCP) contain a "reactive" block that can be readily removed to provide a template for substrate pattern formation. Various methods of PLA removal were studied here with a view to develop the system as an on-chip etch mask for substrate patterning. Solvo-microwave annealing was used to induce microphase separation in PS-b-PLA BCP with a periodicity of 34 nm (L_o) on silicon and silicon on insulator (SOI) substrates. Wet etches based on alkaline and enzymatic solutions were studied in depth. Fourier transform-infrared (FT-IR) analysis showed that basic hydrolysis using sodium hydroxide (NaOH) or ammonium hydroxide (NH ₄OH) solutions resulted in greater PLA removal in comparison to an enzymatic approach using Proteinase K in a Tris-HCl buffer solution. However, in the enzymatic approach, the characteristic self-assembled fingerprint patterns were retained with less damage. Comparison to a dry etch procedure using a reactive ion etch (RIE) technique was made. A detailed study of the etch rate of PS and PLA homopolymer and PS-b-PLA shows depending on DC bias, the etch selectivity of PLA and PS (dPLAdPS ) can be almost doubled from 1.7 at DC bias 145 V to 3 at DC bias 270 V.