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Current nanostructure fabrication by etching is usually limited to planarstructures as they are defined by a planar mask. The realisation ofthree-dimensional (3D) nanostructures by etching requires technologies beyondplanar masks. We present a method to fabricate a 3D mask that allows to etchthree-dimensional monolithic nanostructures by using only CMOS-compatibleprocesses. The mask is written in a hard-mask layer that is deposited on twoadjacent inclined surfaces of a Si wafer. By projecting in single step twodifferent 2D patterns within one 3D mask on the two inclined surfaces, themutual alignment between the patterns is ensured. Thereby after the maskpattern is defined, the etching of deep pores in two oblique directions yieldsa three-dimensional structure in Si. As a proof of concept we demonstrate 3Dmask fabrication for three-dimensional diamond-like photonic band gap crystalsin silicon. The fabricated crystals reveal a broad stop gap in opticalreflectivity measurements. We propose how 3D nanostructures with five differentBravais 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 ofcavities. In general, the 2D patterns for the different surfaces can becompletely independent and still be in perfect mutual alignment. Indeed, weobserve an alignment accuracy of better than 3.0 nm between the 2D maskpatterns on the inclined surfaces, which permits one to etch well-definedmonolithic 3D nanostructures.

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