About holographic lithography for grating coupler fabrication in gallium nitride grown by MOVPE on sapphire substrate

Abstract

The use of the holographic lithography method for sub-nano pattering of photoresist layer deposited on bare sapphire substrate as well as on GaN grown by metaloorganic vapour phase epitaxy on Al 2 O 3 is reported. Positive photoresist Shipley SPR700 was first diluted with photoresist thinner and then spin-coated on prepared substrates to obtain layers of final thickness of 227nm. Thin photoresist layer was exposed in the holographic setup with wavelength of 355nm to produce the surface relief grating. After development SEM observations reveled well-defined valleys and ridges of diffraction grating in SPR700 deposited on gallium nitride layer whereas the whole structure on sapphire was strongly affected by the speckles created by reflection from the unpolished back surface of the sapphire substrate. Latter, we confirmed with transmission spectroscopy, that even small amount of light transmitted through the substrate, which is back reflected by the unpolished back-surface of sapphire, can strongly disturb nano-sized features in photoresist.