A new publication from Opto-Electronic Sciences; DOI 10.29026/oes.2024.230035, discusses supercritical metalens for photolithography.
Lithography is an important part of micro-nano processing. In the past few decades, the continuous improvement in lithography resolution and the continuous reduction in node size have supported the rapid development of the semiconductor industry following Moore's Law and brought profound changes to the economy and the whole society. In conventional lithography systems, optical components such as refractive and reflective lenses are bulky, difficult to fabricate and assemble, and the size of the far-field focal spot is restricted by the diffraction limit. To achieve higher resolution, shorter wavelengths are adopted, enforcing demanding requirements on the materials and fabrications. In recent years, the development of metalenses has provided new possibility for the design of lithography lenses.
Metalens, wavelength-scale in thickness, is composed of thousands to millions of subwavelength-scale dielectric/metal resonance units arranged on the transverse plane. It can apply subwavelength-scale pixel level amplitude/phase modulations to the incident beam, far exceeding conventional optical lenses in the controlling of light. Supercritical metalens (SCL) can break the conventional diffraction limit by introducing violent phase modulation and achieve a focal spot smaller than the diffraction limit in the far field, thus is expected to bring innovations in imaging, lithography, and other fields. Using SCL to simplify the optical system in lithography and improve the lithography resolution is of great significances in both theory and application.
The authors of this article designed and fabricated SCL for direct laser writing (DLW) lithography, using the dielectric aluminum nitride material. Such SCLs have been verified to have a focal spot smaller than the diffraction limit in the far field and been experimentally demonstrated for the first time to achieve higher lithography resolution than Fresnel zone lens (FZL).
In the DLW system (Fig. 1a), the laser beam is shaped and adjusted before reaching the metalens (ML) which further focuses it upon the photoresist (PR). The photoresist moves with a high-precision piezo-stage of which moving trajectory determines the pattern to write and moving speed determines the exposure dose. The metalens is made of aluminum nitride pillars located on a sapphire substrate. The pillar height is 200 nm (Fig. 1b), and pillars of two different diameters are arranged in a certain pattern (Figs. 1c, d). These two types of pillars have different phase responses to the incident light, and through reasonably arranging them, the light can interfere constructively into a smaller bright spot at the center of the far-field focal plane, while suppressing the side lobes. The unit structure's response to light is calculated by full-wave simulation, the overall response of the metalens and the propagation process are calculated by a vectorial diffraction theory, and the SCLs are optimized by the particle swarm optimization method.
Setting the side lobe intensity to be no greater than 5% (10%) of the central bright spot, SCL05 (SCL10) is obtained through optimization. The central focal spots of both SCL05 and SCL10 are smaller than the diffraction limit (wavelength 405 nm, numerical aperture 0.447, diffraction limit 453 nm). In the measured results (Fig. 2), the size of the central bright spot is in good agreement with the simulation, and the side lobes are also well suppressed. In the DLW experiment, the classic grating patterns of different periods are adopted for testing. Compared with FZL, SCLs write the structures which can be resolved more clearly, and thus can achieve higher resolution (Fig. 3).
SCL can break the conventional diffraction limit in the far field, providing a new solution to improving the resolution of DLW lithography. With higher NA, shorter wavelength, and more judicious design, SCL-based DLW has the potential to push resolution to sub-100 nm and beyond, to be another powerful technology for micro-nano-fabrication. AlN-based metasurfaces at UV and deep-UV wavelengths may also find wider applications in imaging, biomedical and other fields.
Keywords: metalens / direct laser writing / supercritical lens / diffraction limit / ultraviolet metasurface
