A new fabrication method for 3D Si-based photonic crystal structures

We have designed and developed a new, simplified 3-dimensional (3D) Photonic Crystal (PhC) fabrication technique that can be used to fabricate a nanoscale 3D structure from the 2D surface of a Si (or SOI) wafer with a single lithography and self-aligned etching sequence. This technique produces deep trenches with controlled width variation along the vertical direction. Using an alternating sequence of Bosch etches, a combined cryogenic etching and/or chemical etching process, allows the Bosch etched layers to maintain the width defined by the mask, while the cryogenic/chemical etched layer creates a lateral undercut that decreases the width beneath the surface. The result is a 3D lattice structure with a stack of vertical square grids. This paper reports the experimental procedures and results of fabrication of a 3D lattice structure that forms an array of hollow-core waveguides. We also compare several different etch recipes for the attempt to produce a uniform structure with smooth walls. These techniques will reduce overall fabrication cost, increase yield and are compatible with CMOS processing. Using this method, one can fabricate a variety of Si/SOI based 3D PhC structures including hollow-core, high contrast grating, waveguide arrays.