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Recently-demonstrated high-quality three-dimensional (3D) subsurface laserprocessing inside crystalline silicon (c-Si) wafers opens a door to a widerange of novel applications in multidisciplinary research areas. Using thistechnique, a novel maskless micro-pillars with precise control on the surfacereflection and coverage are successfully fabricated by etching the laserprocessed region of c-Si wafer. To achieve this, a particular selective wetchemical etching is developed to follow subsurface laser processing of c-Si toreveal the desired 3D structures with smooth surfaces. Here, we report thedevelopment of a novel chromium-free chemical etching recipe based on coppernitrate, which yields substantially smooth surfaces at high etch rate andselectivity on the both laser-processed Si surface and subsurface, i.e.,without significant etching of the unmodified Si. Our results show that theetch rate and surface morphology are interrelated and strongly influenced bythe composition of the adopted etching solution. After an extensivecompositional study performed at room temperature, we identify an etchant witha selectivity of over 1600 times for laser-modified Si with respect tounmodified Si. We also support our findings using density functional theorycalculations of HF and Cu adsorption energies, indicating significant diversityon the c-Si and laser-modified surfaces.

Development of a Selective Wet-Chemical Etchant for 3D Structuring of Silicon via Nonlinear Laser Lithography