Sidewall Patterning in 3D Micro/Nanosystems: A Review.

Current micro/nanosystems mainly rely on a planar fabrication framework, where structures are built layer-by-layer on flat surfaces. This conventional approach leaves vertical sidewalls underutilized, posing geometric limits in packaging density, three-dimensional (3D) interconnects, and multi-surface functionalization. To overcome these constraints, sidewall patterning has emerged as a promising strategy, enabling 3D integrated circuits, templates for directed nanostructure synthesis, and microfluidic drag reduction. Nevertheless, traditional photolithography and non-photolithographic techniques face challenges when applied to vertical or curved 3D surfaces. Unidirectional radiation and restricted focal depths prevent high-fidelity pattern transfer, even when using soft lithography, scanning probes, or nanoimprinting. To address these geometric and mechanical barriers, our group has developed several approaches for patterning the sidewalls of microsystems, which are the primary focus of this review. Building upon our approaches, this review further surveys related sidewall-patterning strategies, including micro-transfer printing, multi-stimuli-responsive mechanics, block copolymer self-assembly, two-photon polymerization, and laser-induced forward transfer. Collectively, these techniques expand the capabilities of sidewall engineering and provide valuable insights into next-generation 3D micro- and nanomanufacturing.