Using spatially-modulated laser beams and anisotropic seeding, researchers at Bilkent University demonstrated a way to control nanofabrication deep inside silicon bulk material, rather than just on the surface of a silicon chip. This approach surpasses the feature-size limitations of current lithographic techniques, enabling the fabrication of silicon structures with feature sizes ranging from 80 nm to 120 nm. The method could advance the development of 3D nanophotonics, micro- and nanofluidics, and integrated photonic systems for the NIR to MIR regime. Silicon, a foundational material in modern electronics, photovoltaics, and photonics, has traditionally been limited to surface-level nanofabrication. Existing lithographic techniques either cannot penetrate the silicon wafer surface without causing alterations, or are limited by the micron-scale resolution of laser lithography within silicon. The researchers sought to fabricate structures inside silicon that were smaller than 1 μm, using a method that offered subwavelength and multidimensional control. The method they developed for creating controlled nanostructures within bulk silicon material is based on 3D nonlinear laser nanolithography and near- and far-field seeding effects. “Our approach is based on localizing the energy of the laser pulse within a semiconductor material to an extremely small volume, such that one can exploit emergent field enhancement effects analogous to those in plasmonics,” said professor Onur Tokel. “This leads to subwavelength and multidimensional control directly inside the material.” The team used spatially modulated laser pulses corresponding to how a Bessel beam functions. The nondiffracting nature of this type of laser beam, which the researchers created using holographic projection techniques, suppresses the effects of optical scattering and enables precise energy localization. This leads to a high temperature and a pressure value strong enough to modify the silicon material at a small volume. The realization of energy concentration at the nanoscale allows nanovoids to be created within the irradiated volumes. The resulting field enhancement sustains itself through a seeding-type mechanism. After small, localized voids inside the wafer are created, a seeding effect causes preformed, subsurface nanovoids to establish strong field enhancement around their immediate neighborhood. This effect is analogous to the hot spots observed in plasmonics, but is achieved deep inside the wafer. Simply put, the initial nanostructures that are created support the fabrication of subsequent nanostructures. Laser polarization provides additional control over the alignment and symmetry of the nanostructures, enabling diverse nanoarrays to be formed with a high degree of precision. The anisotropic feedback from the preformed subsurface structures enables the researchers to establish control of the nanofabrication capability inside the silicon. “By leveraging the anisotropic feedback mechanism found in the laser-material interaction system, we achieved polarization-controlled nanolithography in silicon,” said researcher Asgari Sabet. “This capability allows us to guide the alignment and symmetry of the nanostructures at the nanoscale.” The team demonstrated large-area volumetric nanostructuring with feature sizes beyond the diffraction limit and multidimensional confinement. The buried nanostructures developed by the team had feature sizes down to 100 ± 20 nm, which is an order-of-magnitude improvement over the state of the art. “We can now fabricate nanophotonic elements buried in silicon, such as nanogratings, with high diffraction efficiency and even spectral control,” Tokel said. The new approach could accelerate the development of nanoscale systems with exceptional architectures. “We believe the emerging design freedom, in arguably the most important technological material, will find exciting applications in electronics and photonics,” Tokel said. “The beyond-diffraction-limit features and multidimensional control imply future advances, such as metasurfaces, metamaterials, photonic crystals, numerous information processing applications, and even 3D integrated electronic-photonic systems.” The nanograting capability demonstrated by the team is a step toward these advances, and according to the researchers, constitutes the first multilayer silicon photonics. “Our findings introduce a new fabrication paradigm for silicon,” Tokel said. “The ability to fabricate at the nanoscale directly inside silicon opens up a new regime, toward further integration and advanced photonics. We can now start asking whether complete three-dimensional nanofabrication in silicon is possible. Our study is the first step in that direction.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-024-49303-z).
