Selective etching of PDMS: Etching as a negative tone resist

•Etching pure, additive-free and cured PDMS as a negative tone resist is presented.•Selective etching was done by concentrated sulfuric acid at 35 °C.•Etching rate of the cured, unirradiated PDMS was determined.•Various high aspect ratio structures were created in 45 μm and 100 μm thick PDMS.•Solvent and acid resistivity of the created structures were tested.