3D nanostructures by stacking pre-patterned fluid-supported single-crystal Si membranes

Abstract

The fabrication of complex three-dimensional (3D) structures at sub-100 nm resolution presents a difficult challenge. 3D photonic crystals that contain waveguides, resonant cavities, filters or other devices, and require deep-sub-100 nm dimensional control, are a particular example of this challenge. Multilayer 3D structures can be formed by stacking and bonding thin membranes that have been patterned in advance. This approach enables the full panoply of 2D planar-fabrication techniques to be employed. Membranes containing patterns that are not perfectly regular will exhibit in-plane distortion unless their intrinsic stress is zero. To minimize the effects of intrinsic stress we float individual membranes on the surface of a liquid. Thin single-crystal Si membranes on an oxide substrate are first patterned and then removed by etching the oxide in hydrofluoric acid. The freed Si membranes readily float on the liquid surface, aided by the hydrophobic nature of H-terminated Si. The authors describe methods for cleaning, patterning, manipulating, bonding and stacking such freely floating membranes.