State-of-the-art integrated circuits are currently already involving multi-layered architectures that
rapidly extend towards three-dimensional (3D) semiconductor structures. To overcome out-of-plane
alignment issues, we present an original method to employ conventional deep-UV lithography on
pre-structured silicon substrates to pattern monolithic 3D nanostructures with N layers with only
M = 1 alignment step (instead of M = (N −1) steps as in layer-by-layer assembly). The exemplary
nanostructures studied here consist of arrays of pores that intersect at oblique angles. To this end,
we project elliptical apertures on the sidewalls of V-trenches at controllable depths relative to the
wafer’s top surface. We create V-trenches in Si by hot HF 50% and KOH 25% etching with θ = 54.7◦
sloping sidewalls. We successfully spin-coat photoresist on the sloping sidewalls of the V-trenches
with a thickness between 50 and 800 nm. The photoresist is remarkably homogeneous, better than
±4 nm along the 2.6 μm depth of the V-trench sidewall. Using a single deep-UV lithography
exposure at λ = 193 nm (with a 25 mJ/cm2 dose and a nominal focus at 500 nm depth below the
Si wafer top-surface) we successfully expose apertures at various depths in the photoresist on the
θ = 54.7◦ sloping sidewall surfaces of the V-trenches. The apertures have diameters between d = 180
nm and 500 nm, to as deep as Δz∗ = 1.8 μm below the wafer surface. We develop an analytical
model to successfully describe lithographic exposure on an inclined surface, which allows to sculpt
3D nanostructures on inclined surfaces. In future, aperture arrays on inclined surfaces may serve
as etch masks to create monolithic and controllable 3D nanostructures deep inside silicon which
will eliminate alignment steps, thereby increasing the yield and throughput and reducing costs and
usage of chemical and material resources.

D. A. Grishina, E. Yüce, C. A. M. Harteveld, and W. L. Vos

3D photonic crystals, Anderson localization, etch mask, nanofabrication, silicon, silicon nanophotonics, UV lithography