Overcoming historical barriers
Silicon, the cornerstone of modern electronics, photovoltaics, and photonics, has traditionally been limited to surface-level nanofabrication due to challenges with existing lithographic techniques. Available methods either fail to penetrate the wafer surface without causing alterations or are limited by the micron-scale resolution of laser lithography in silicon. In the spirit of Richard Feynman’s famous dictum, “There’s plenty of room at the bottom,” this advancement is in line with the vision of exploring and manipulating matter at the nanoscale. The innovative technique developed by Bilkent’s team overcomes current limitations, enabling the controlled fabrication of nanostructures buried deep within silicon wafers with unprecedented control.
The team addressed the dual challenges of complex optical effects within the wafer and the inherent diffraction limit of laser light. They overcame these issues by using a special type of laser pulse, created by an approach called spatial modulation of light. The non-diffracting nature of the beam overcomes the optical scattering effects that have hitherto hampered precise energy deposition, inducing extremely small and localized voids within the wafer. This process is followed by an emergent seeding effect, where pre-formed underlying nanovoids establish a strong field enhancement around their immediate vicinity. This new fabrication regime marks an order of magnitude improvement over the state of the art, enabling feature sizes down to 100 nm.
“Our approach is to localize the laser pulse energy in a semiconductor material in an extremely small volume, so that we can exploit emerging field amplification effects analogous to those in plasmonics. This leads to subwavelength and multidimensional control directly inside the material,” explains Professor Tokel. “We can now fabricate embedded nanophotonic elements in silicon, such as nanogratings with high diffraction efficiency and even spectral control.”
The researchers used spatially modulated laser pulses, technically corresponding to a Bessel function. The non-diffracting nature of this special laser beam, created using advanced holographic projection techniques, allows for precise localization of the energy. This leads to temperature and pressure values high enough to modify the material at a small volume. Remarkably, the resulting field enhancement, once established, is maintained through a “seeding” mechanism. Simply put, the creation of earlier nanostructures helps to fabricate later nanostructures. The use of laser polarization provides additional control over the alignment and symmetry of the nanostructures, allowing for the creation of various nanolattices with high precision.
“By exploiting the anisotropic feedback mechanism present in the laser-material interaction system, we achieved polarization-controlled nanolithography in silicon,” said Dr. Asgari Sabet, first author of the study. “This capability allows us to guide the alignment and symmetry of nanostructures at the nanoscale.”
The research team demonstrated large-area volumetric nanostructuring with features beyond the diffraction limit, validating the feasibility of buried nanophotonic elements. These advances have important implications for the development of nanoscale systems with unique architectures. “We believe that the emerging design freedom in the most important technological material will find exciting applications in electronics and photonics,” Tokel said. “Beyond-diffraction-limit features and multidimensional control imply future advances, such as metasurfaces, metamaterials, photonic crystals, many information processing applications, and even 3D integrated electronic-photonic systems.”
“Our results introduce a new manufacturing paradigm for silicon,” concludes Professor Tokel. “The ability to fabricate at the nanoscale directly inside silicon opens a new regime, towards deeper integration and advanced photonics. We can now start asking whether full three-dimensional nanofabrication in silicon is possible. Our study is the first step in this direction.”
About the researchers
The research team consists of Rana Asgari Sabet, Aqiq Ishraq, Alperen Saltik, Mehmet Bütün and Onur Tokel, all affiliated with the Department of Physics and the National Research Center for Nanotechnology at Bilkent University. Their expertise covers various fields, including optics, materials science and nanotechnology.
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