Recess Photomask Contact Lithography and the fabrication of coupled silicon photonic and plasmonic waveguide switches

A novel lithographic method is presented, based on the use of a mask aligner in the contact mode with a modified photomask, the so-called recess photomask; its goal is the printing of submicron-sized patterns into deep cavities of a chip, which is not manageable with conventional contact lithography. The photomask (initially a standard photomask) is etched in designated sections in order to obtain a surface nearly conformal to the topography of the wafer to be patterned and allowing to maintain the ultimate resolution of UV mask aligners in the contact mode, down to similar to 0.5 mu m critical dimension at 250 nm wavelength in spite of the difficult topography of the chip. The technique is successfully applied to the integration of polymer and gold plasmonic structures in silicon photonic SOI chips with 1.9 mu m deep cavities intended to accommodate the plasmonic waveguide switching technology. In this specific application, coupling of silicon photonic waveguides with small-footprint plasmonic waveguides can assure ultrafast and low-power electrically-driven thermo-optical switching. The method is generic as it can be applied to any pre-existing resist process and has no theoretical recess depth limit. It is potentially widely applicable to emerging technologies in fields including optoelectronic and integrated optical communications devices, but also potentially MEMS and MOEMS. (C) 2015 Elsevier B.V. All rights reserved.