Selective etching of PDMS: etching as a negative tone resist

TL;DR

This paper introduces a novel method for using cured PDMS as a negative tone resist by selective proton microbeam irradiation and etching, enabling high-aspect-ratio microstructures with solvent resistance.

Contribution

It demonstrates for the first time how to apply additive-free cured PDMS as a negative resist using focused proton irradiation and selective etching.

Findings

High aspect ratio PDMS microstructures were successfully fabricated.

Irradiated PDMS structures resist certain solvents and acids.

Unirradiated PDMS is fully etched away by sulfuric acid.

Abstract

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 um and 100 um 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*10^15 ion*cm-2 fluence did not show any signs of degradation even after 19 hours of etching. As a demonstration, 45 um and 100 um 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.