Geometry-Independent Nanometric Planarization of 3D Nanostructures for Strain-Controlled Integration of 2D Materials

Emerging 3D nanoelectronic and nanophotonic architectures require dielectric integration with nanometric thickness control to enable vertical stacking and the deposition of strain-sensitive functional layers. High planarity combined with precise control of the supporting dielectric thickness is essential to deposit thin materials without inducing strain, deformation, or performance degradation. Conventional planarization approaches based on thick SiO2 deposition and chemical-mechanical polishing (CMP) offer limited thickness precision at the nanometric scale. Although Spin-On Glass (SOG) combined with etch-back processes improves thickness control, it has not yet enabled geometry-independent nanoscale planarity across dense 3D nanostructure networks. Here, we demonstrate a geometry-independent planarization strategy that embeds nanostructures of various dimensions and densities within a dielectric matrix while achieving nanometric control of both layer thickness and surface flatness. The approach relies on successive HSQ spin-on glass depositions that progressively smooth surface nonuniformities, combined with calibrated thickness adjustment through reactive ion etching assisted by laser interferometry for controlled etch-stop positioning. Multicycle HSQ deposition reduces surface height variations from approximately 40 nm to within ±2 nm across diverse nanostructure geometries. Deterministic positioning of the SiO2 surface relative to embedded silicon nanostructures is achieved with an accuracy of ±3-5 nm. The interface precision is validated using monolayer MoSe2 as a strain-sensitive optical probe due to its strain-tunable optical properties. Photoluminescence measurements demonstrate controlled near-field coupling for separations below approximately 15 nm while achieving minimal residual strain (<0.02%), in contrast to nonplanarized configurations exhibiting higher strain levels (∼0.06-0.08%). This planarization provides a platform for the strain-controlled integration of 2D materials and advanced thin films onto embedded 3D nanostructures.