Wafer-Scale All-Dielectric quasi-BIC Metasurfaces: Bridging High-throughput Deep-UV Lithography with Nanophotonic Applications

This paper introduces a method for fabricating high-Q dielectric metasurfaces using deep ultraviolet lithography (DUVL), making it possible to scale up the production of metasurfaces with sub-200 nm features. These metasurfaces leverage quasi-bound states in the continuum (qBIC) resonances, which have been achieved with Q-factors of up to 150, suitable for a wide range of applications in nanophotonics and biosensing.

Key Findings:

1. DUVL Fabrication: The authors push DUVL to its resolution limits by using a C-4 symmetry-broken "double-hole" design in a silicon nitride (Si₃N₄) thin film. This enables the creation of qBIC resonances with high Q-factors in the visible to near-infrared (NIR) range, which was previously difficult due to the small feature sizes required.

2. Q-Factor Engineering: The paper introduces exposure dose as a new parameter for tuning the Q-factor of these metasurfaces. By controlling the depth of the holes in the metasurfaces, the authors are able to circumvent DUVL’s resolution limits, achieving a broad range of Q-factors. This depth tuning technique is crucial in mitigating limitations caused by traditional lithographic techniques.

3. Fabrication Process: The fabrication process includes patterning a Si₃N₄ slab using a 248 nm KrF DUV stepper and varying the exposure dose to achieve depth variations in the hole structures. This process results in well-defined metasurfaces when exposed to higher doses, while lower doses lead to stochastic hole formation.

4. Optical and Resonance Performance: Transmission spectra and scanning electron microscopy (SEM) show the emergence of high-Q qBIC modes at specific wavelengths, with the Q-factors increasing as the small hole depths increase. Simulations further confirm that fully etched small holes (achieved at higher doses) produce sharper resonances and higher Q-factors.

5. Spatial Uniformity: Despite stochastic variations in hole depth and placement, the qBIC metasurfaces exhibit excellent spatial uniformity in resonance wavelength and Q-factor across the entire wafer. This is attributed to the nonlocal nature of the qBIC resonance, where collective unit cell interactions dominate, allowing the metasurfaces to perform consistently even with nanoscale imperfections.

6. Sensing Application: A proof-of-concept demonstration of refractive index sensing using the metasurfaces shows a sensitivity of 129 nm/RIU. By monitoring resonance shifts and intensity changes with a CMOS camera-based readout, the metasurfaces show high sensitivity, making them suitable for label-free optical sensing.

7. Commercial Viability: The use of DUVL provides a scalable and cost-effective pathway for fabricating these metasurfaces, which can be seamlessly integrated into semiconductor manufacturing processes. The ability to fabricate high-performance qBIC metasurfaces at wafer scale could revolutionize fields like biosensing, on-chip spectroscopy, and integrated photonics.

Conclusion:

This work bridges the gap between high-performance metasurface design and scalable, cost-effective manufacturing, leveraging DUVL to produce wafer-scale dielectric metasurfaces with tunable Q-factors. The successful demonstration of these metasurfaces for refractive index sensing paves the way for a broad range of applications in biosensing, spectroscopy, and integrated photonics.