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

TL;DR

This paper introduces a CMOS-compatible method to create a single-step 3D etch mask enabling the fabrication of complex monolithic 3D nanostructures in silicon, demonstrated through photonic crystals and various lattice structures.

Contribution

The authors develop a novel 3D mask fabrication technique that allows for precise, aligned etching of complex 3D nanostructures in silicon using only CMOS-compatible processes.

Findings

Achieved better than 3.0 nm alignment accuracy between mask patterns.

Demonstrated fabrication of 3D photonic band gap crystals with broad stop gaps.

Proposed methods to realize five different Bravais lattice structures.

Abstract

Current nanostructure fabrication by etching is usually limited to planar structures as they are defined by a planar mask. The realisation of three-dimensional (3D) nanostructures by etching requires technologies beyond planar masks. We present a method to fabricate a 3D mask that allows to etch three-dimensional monolithic nanostructures by 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 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 realised, 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 for the different surfaces can be completely independent 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.