Polythiophene-based charge dissipation layer for electron beam lithography of zinc oxide and gallium nitride

Abstract

The ability of thin polythiophene layers to dissipate accumulated charge in the electron beam lithography (EBL) of wide bandgap semiconductors, such as zinc oxide and gallium nitride, is demonstrated. A quick and inexpensive processing method is demonstrated for EBL exposure of dense and high-resolution patterns in a hydrogen silsesquioxane (HSQ) negative-tone resist deposited on bulk ZnO samples and with GaN/AlN on sapphire substrates. For the former, experimental results are given for three different cases: where no charge dissipation layer was used as well as cases where 40-nm-thick Al and 100-nm-thick conductive polymer layers were used on the top of the HSQ resist. For the latter material, EBL exposure was investigated for pure HSQ and for HSQ with a thin conductive polymer layer on top. Based on the scanning electron microscope observations of the resulting photonic crystal (PhC) pattern, conventional Al and the proposed polymer approach were compared. Good agreement between these results is reported, while the new method considerably simplifies sample processing. Spin-coatable conducting polymer may be easily removed due to its solubility in water, which makes it a perfect solution for the processing of amphoteric oxide samples, i.e., zinc oxide. Gallium nitride processing also benefits from polymer dissipation layer usage due to extended exposure range and the avoidance of dense pattern overexposure in HSQ.