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

Abstract

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.

Cartesian light: Unconventional propagation of light in a three-dimensional superlattice of coupled cavities within a three-dimensional photonic band gap

“…For a normalized pore radius r/a = 0.24-as considered here-a maximum relative bandwidth ω pbg /ω c = 25.3% occurs for = 12.1 typical of silicon [39,40], with ω pbg the frequency width of the band gap, and ω c the band gap's center frequency. 3D inverse woodpile crystal nanostructures have been fabricated from a number of different high-refractive index backbones [41][42][43][44][45][46]. In nanophotonic experiments, the potential of silicon inverse woodpiles was demonstrated by the observation of a broad 3D photonic band gap for many angles [47], as well as a strong spontaneous emission inhibition of embedded quantum dots [48].…”

Three-dimensional photonic band gap cavity with finite support: Enhanced energy density and optical absorption

“…Since this structure is rela-tively straightforward to define, it has been realized by various nanofabrication techniques and high-index backbones [43][44][45][46]. Moreover, 3D inverse woodpile photonic crystals made from silicon have been fabricated by our group using CMOS-compatible nanofabrication methods that were developed in collaboration with high-tech industry [47][48][49]. To functionalize inverse woodpile crystals, our group has proposed a design to create a resonant cavity in the 3D inverse woodpile crystal structure, whereby the photons are tightly confined in the proximal region of two orthogonal defect pores that have a radius smaller than all other pores in the bulk of the crystal, as illustrated in Fig.…”

Nanomanufacturing—Perspective and applications

“…By projecting two different 2D patterns within one 3D mask on the two inclined surfaces in a single step, a mutual alignment between the patterns is ensured (Fig. 18) [82].…”

“…18. Single-step etching mask fabrication on two perpendicular surfaces with a single 2D mask projected on a 3D surface [82]. Fig.…”