Selective etching of PDMS: Etching as a negative tone resist

Highlights: • 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. In this work authors present for the first time how to apply the additive-free, cured PDMS as a negative tone resist material, demonstrate the creation of PDMS microstructures and test the solvent resistivity of the created microstructures. The PDMS layers were 45 μm and 100 μm thick, the irradiations were done with a focused proton microbeam with various fluences. After irradiation, the samples were etched with sulfuric acid that removed the unirradiated PDMS completely but left those structures intact that received high enough fluences. The etching rate of the unirradiated PDMS was also determined. Those structures that received at least 7.5 × 10¹⁵ ion × cm⁻² fluence did not show any signs of degradation even after 19 h of etching. As a demonstration, 45 μm and 100 μm tall, high aspect ratio, good quality, undistorted microstructures were created with smooth and vertical sidewalls. The created microstructures were immersed into numerous solvents and some acids to test their compatibility. It was found that the unirradiated PDMS cannot, while the irradiated PDMS microstructures can resist to chloroform, n-hexane, toluene and sulfuric acid. Hydrogen fluoride etches both the unirradiated and the irradiated PDMS.