Diana Grishina

Method for making a single-step etch mask for 3D monolithic nanostructures

Current nanostructure fabrication by etching is usually limited to planar structures as they are defined by a planar mask. The realization of three-dimensional (3D) nanostructures by etching requires technologies beyond planar masks. We present a method for fabricating a 3D mask that allows one to etch three-dimensional monolithic nanostructures using only CMOS-compatible processes. The mask is written in a hard-mask layer that is deposited on two adjacent inclined surfaces of a Si wafer. By projecting in a single step two different 2D patterns within one 3D mask on the two inclined surfaces, the mutual alignment between the patterns is ensured. Thereby after the mask pattern is defined, the etching of deep pores in two oblique directions yields a three-dimensional structure in Si. As a proof of concept we demonstrate 3D mask fabrication for three-dimensional diamond-like photonic band gap crystals in silicon. The fabricated crystals reveal a broad stop gap in optical reflectivity measurements. We propose how 3D nanostructures with five different Bravais lattices can be realized, namely cubic, tetragonal, orthorhombic, monoclinic and hexagonal, and demonstrate a mask for a 3D hexagonal crystal. We also demonstrate the mask for a diamond-structure crystal with a 3D array of cavities. In general, the 2D patterns on the different surfaces can be completely independently structured and still be in perfect mutual alignment. Indeed, we observe an alignment accuracy of better than 3.0 nm between the 2D mask patterns on the inclined surfaces, which permits one to etch well-defined monolithic 3D nanostructures.

The inside view of 3D photonic nanostructures

In nanofabrication, it is a major challenge to characterize the structure of a real sample, in particular of three-dimensional (3D) ones that control non-linear optics or quantum dot emission [1]. The challenge notably pertains to nanophotonic media whose properties are determined by their complex structure with feature sizes d comparable to or even less than the wavelength of light (d<2). These nanomaterials are from the outset opaque, thus optical microscopy has insufficient penetration depth, apart from limited resolution. While scanning electron microscopy (SEM) offers fantastic nm spatial resolution, it has a small penetration depth hence only the sample surface is viewed (see Fig. 1), but not the bulk. X-ray techniques are promising tools for Nanophotonics, in view of excellent penetration depth, non-destructive character, and nm spatial resolution. Therefore, we study a 3D Si photonic band gap crystal with a cubic diamond-like structure by X-ray tomography [2].