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

TL;DR

This paper introduces a scalable method to fabricate high-performance nanophotonic metasurfaces using deep-UV lithography, enabling practical applications in biosensing and photonic systems.

Contribution

A wafer-scale fabrication technique for quasi-BIC metasurfaces using DUVL, overcoming resolution limits and enabling CMOS-compatible sensing.

Findings

qBIC resonances with Q-factors of 150 were achieved using DUVL-fabricated silicon nitride hole arrays.

Spatial uniformity was maintained despite nanoscale variations due to the nonlocal nature of qBIC resonances.

Refractive index sensing achieved 129 nm/RIU sensitivity using CMOS camera-based interrogation.

Abstract

High quality-factor
(Q) dielectric metasurfaces operating
in the
visible to near-infrared range require sub-200 nm features, limiting
fabrication to expensive, low-throughput electron beam lithography.
Here, we demonstrate wafer-scale metasurfaces fabricated using deep
ultraviolet lithography (DUVL), a workhorse technology in the semiconductor
industry. Using a radius and depth perturbation technique in a hole
array patterned into a silicon nitride slab, we achieve quasi-bound
states in the continuum (qBIC) resonances with measured Q-factors
of 150. We introduce DUV exposure dose as a Q-factor engineering parameter
and demonstrate how hole depth control circumvents DUVL resolution
limits. Despite stochastic nanoscale variations, the fabricated metasurfaces
exhibit spatial uniformity, a consequence of the nonlocal nature of
the qBIC resonances. Refractive index sensing demonstrates 129 nm/RIU
sensitivity while maintaining CMOS camera-based resonance shift interrogation.
This work bridges scalable semiconductor manufacturing with high-performance
nanophotonics, establishing a practical pathway for commercializing
metasurface-based biosensors, on-chip spectrometers, and integrated
photonic systems.

Figures

Full text

Optical metasurfaces, nanoengineered thin films with subwavelength structures, have opened new possibilities to precisely control the phase,? direction, ?,? polarization,? and dispersion? properties of light. Beyond far-field wavefront modification capabilities, optical metasurfaces can be designed to confine incident light into subwavelength mode volumes, creating high quality-factor (Q) photonic cavities with long-lived resonances. Such resonant metasurfaces have become essential platforms for applications requiring enhanced light-matter interactions, including lasing,? nonlinear frequency conversion,? quantum photonics,? and molecular biosensing. ?−? ?

There has been remarkable progress in high-Q metasurfaces utilizing diverse material combinations, including metals,? semiconductors, ?,? dielectrics,? and 2D materials.? Moreover, an expansive toolbox of geometric design strategies has been accumulated, achieving various physical resonance mechanisms. Among these approaches, metasurfaces fabricated from high-refractive-index, low-loss dielectrics and leveraging bound states in the continuum (BICs) have proven particularly powerful. ?,? While true BIC modes are nonradiative and cannot couple to free space, their radiative counterpartsquasi-BICs (qBICs)enable tunable radiative losses.? QBICs can be created by introducing structural asymmetries into the constituent building blocks of the metasurfaces, providing far-field accessibility to otherwise dark modes,? positioning them as promising candidates for next-generation photonic technologies.

In particular, qBIC metasurfaces hold strong promise as label free biosensors that address key gaps in medical diagnostics and real time patient monitoring. ?,?,? To translate these devices into user-friendly, real-world tools, two bottlenecks must be overcome: (1) fabrication must be scalable for affordability, and (2) the optical readout must be compatible with cost-effective hardware such as CMOS cameras and LED illumination for imaging-based interrogation. Generating qBIC resonances in the CMOS-detectable spectral range (λ < 900 nm) requires ordered dielectric nanostructures with critical dimensions below 200 nm, a regime inaccessible to conventional ultraviolet (UV) lithography. Consequently, researchers have predominantly relied on costly and low-throughput electron beam lithography (EBL) to fabricate high-Q metasurfaces.

Alternative approaches have emerged to address the scalable manufacturing bottleneck: nano imprint lithography (NIL)? and self-assembly processes, including glass dewetting? and nanosphere lithography ?,? offer improved throughput and reduced costs. However, these methods currently lack the large-area pattern fidelity and geometric design flexibility required for complex architectures and industry-compatible continuous operation necessary for commercial viability. Therefore, developing qBIC metasurfaces with CMOS-detectable resonances using scalable manufacturing is an unmet need. ?−? ?

Deep UV lithography (DUVL), which has been the workhorse for the semiconductor industry, presents a compelling solution to metasurface manufacturing challenges. ?,? As a mature, high-throughput technology operating at wafer scale down to 200 nm resolution, DUVL offers scalability, reliability, and geometric versatility, well-suited for practical metasurface production.

In contrast to serial EBL, which can generate a nanopatterned active device area of a few (∼2–5) mm^2^ per multihour exposure at typical low beam currents, DUVL enables full-wafer patterning in ∼1–2 min, delivering orders-of-magnitude higher throughput in device fabrication. While previous reports have demonstrated DUVL-fabricated metalenses,? beam steering devices,? and mid-IR resonators,? the critical dimension of those structures remain within the DUVL capabilities. Whereas the wafer-scale fabrication of qBIC metasurfaces operating in the visible to near-IR range demands a stringent control of feature sizes at or below 200 nm, a challenging resolution at the practical limits of DUVL.

Here, we demonstrate qBIC metasurfaces operating in the visible to NIR range with sub-200 nm features, fabricated on 4-in. wafers containing multiple chips by pushing DUVL to its resolution limits (Figurea, b). Our approach employs a C-4 symmetry-broken “double-hole” design? patterned in silicon nitride (Si_3_N_4_) thin film, where alternating holes are reduced in radius by an asymmetry parameter Δr in orthogonal lateral directions (Figurec). Beyond conventional radius modulation, we introduce hole depth as a complementary Q-factor tuning mechanism, achieved simply by adjusting lithographic exposure dose (Figured). Shallow, partially etched holes enhance radiative coupling to free space, reducing the Q-factor of the qBIC mode (Figuree). This depth-tuning strategy circumvents DUVL resolution constraints that would otherwise limit further radius reduction. Remarkably, the qBIC mode exhibits robust spectral and spatial performance even with stochastic depth variations across the chip. This is a consequence of the qBIC mode’s nonlocal character, where collective unit cell interactions govern the ensemble optical response. This dual-parameter control (hole radius and depth) substantially expands the accessible Q-factor design space, offering enhanced flexibility for practical deployment of DUVL-fabricated dielectric metasurfaces in real-world applications.

First, we designed qBIC resonances based on the Brillouin zone folding (BZF) approach in a Si_3_N_4_ thin film (160 nm) deposited by low-pressure chemical vapor deposition (LPCVD) on 4-in. silica wafers to achieve high-quality, low-loss (n > 1.99, k ∼ 0) dielectric material. Our metasurface design is based on periodic nanohole arrays etched into the Si_3_N_4_ film. In this reference single-hole design, a square lattice of circular holes with radius r_1_ = 90 nm and period P = 510 nm, supports only guided-mode resonances (Figurea, top). Reducing the radius of every other hole by Δr creates a double-hole design with enlarged unit cell area (P ≈ 721 nm) (Figurea, bottom). This symmetry breaking introduces BZF from X to X′ and couples otherwise dark modes to free space, as shown in the band diagram (Figurec).

Simulated transmission spectra at normal incidence (Γ-point) show the emergence of sharp qBIC resonances in both TE and TM polarizations with increasing Δr (Figureb). Measurements confirm the appearance of high-Q resonances under similar conditions (Figured). Extracted Q-factors from simulation and experiment follow the expected inverse-square dependence, Q ≈ a/Δr ^2^ + b, consistent with radiative leakage induced by symmetry breaking (Figuree). The observed discrepancy between simulated and experimental results (Figureb and ?d) likely arise from geometric deviations between the idealized simulated structure and the fabricated device. Such variations modify the effective refractive index and shift the relative spectral positions of the qBIC TE and TM modes. For example, in Figure S1, we present three designs where variations in hole geometry bring TE and TM modes into close spectral proximity or even invert their spectral ordering, resulting in partial or complete spectral overlap of the two resonances.

Near-field simulations further reveal distinct modal confinement (Figuref). The TM-polarized qBIC concentrates the field inside the air holes (|E/E_0_| ≈ 38.1), while the TE-polarized qBIC localizes near the Si_3_N_4_ surface (|E/E_0_| ≈ 21.3). These results establish periodic radius perturbations as a robust strategy to realize tunable high-Q resonances in all-dielectric metasurfaces.

We used a 248 nm (KrF) DUV stepper to fabricate the double-hole metasurfaces. This process requires coating the Si_3_N_4_ layer with a 60 nm bottom antireflection layer (BARC), followed by a 230 nm thick DUV chemically amplified photoresist. During DUV exposure, we tested different exposure doses from low (40 mJ/cm^2^) to high (61 mJ/cm^2^). The dose, defined as the amount of energy delivered per unit area, directly affects how well the photoresist is patterned and developed. Such “dose matrices” are a standard calibration process in nanofabrication for identifying the conditions that lead to optimal feature fidelity, sidewall profile, and etch characteristics. Subsequently, DUV resist is developed, and the BARC layer is dry-etched in an O_2_ inductively coupled plasma (ICP) to clear the holes. Next, exposed Si_3_N_4_ in the holes was ICP etched in situ with a CF_4_/O_2_ and the remaining DUV resist, and BARC was stripped off.

During characterization, we observed that the exposure dose primarily influenced the small hole (r_2_ in Figure), including its depth, diameter, and random occurrence per area at lower exposure doses (Figurea). Scanning electron microscopy (SEM) (Figureb) and atomic force microscopy (AFM) (Figurec, and Figure S2) analyses showed that at lower doses, the small holes were often missing, or they appeared shallower than the total Si_3_N_4_ film thickness, and their occurrence was random. We explain this dose-dependent small hole depth variation and stochastic occurrence with optical proximity effects and uncontrollable interference effects resulting from the use of a coherent KrF laser source in the DUVL system (ASML PAS 5500/300). At lower doses, the small holes with radius r_2_ = 90 nm receive a lower than critical dose for 100% photoresist exposure, thus after development, the resist is only partially removed from the holes. The residual resist in the small holes prevents the BARC from being fully etched in O_2_ ICP. Consequently, this leads to varying hole depths with low-dosed small holes exhibiting shallower Si_3_N_4_ etching. Moreover, due to small spatial dose fluctuations caused by laser interference, some of the small holes do not get exposed at all, leading to the stochastic nature of small hole disappearance across the metasurface area.

As the dose increased toward 61 mJ/cm^2^, the central holes became well-defined, etched through the full 160 nm Si_3_N_4_ layer, and consistently matched the depth of the outer holes, forming a complete and uniform double-hole design. Quantitative AFM measurements of the central hole depth as a function of the exposure dose (Figureg) confirmed this trend, with shallow (6.9 nm on average, n = 100) depths observed at 49 mJ/cm^2^ and depths approaching 160 nm at 61 mJ/cm^2^. This dose-dependent hole formation follows the typical contrast curve of a photoresist, where small changes in exposure energy can translate into large variations in developed resist depth, especially near the linear region of the curve.

To investigate how dose-controlled geometric variations affect the optical response, we measured transmission spectra of the fabricated double-hole metasurfaces using a tunable light source and hyperspectral imaging using a scientific CMOS camera in the 400–1000 nm spectral range. At 49 mJ/cm^2^ exposure, the far-field spectra exhibited a pronounced transmission dip at ∼ 760 nm, corresponding to the qBIC TE mode (Figured). Counterintuitively, the resonance dip weakened with increasing dose and vanished entirely at 61 mJ/cm^2^, contrary to expectations from a fully formed double-hole metasurface with a fixed Δr designed to support a radiative qBIC mode.

To investigate this unexpected experimental finding, we performed numerical simulations of the double-hole unit cell (Δr = 15 nm), varying the small hole depth according to experimental AFM measurements (Figureg). The simulations revealed that fully etched small holes, as achieved at 61 mJ/cm^2^, produce significantly sharper qBIC resonances (Q-factor ∼ 2637) (Figurese and ?f). In contrast, shallow small holes broaden the resonance and enhance radiative coupling. Although the simulations assume perfect periodicity and do not capture the stochastic hole formation observed experimentally at lower doses, the robust signal from 49 mJ/cm^2^ demonstrates resilience to structural disorder. We further confirmed the robustness of the qBIC mode by showing that chips taken from various locations across the same wafer, exposed at 49 mJ/cm^2^ dose, exhibited consistent resonance (Figure S6).

We attribute the absence of detectable resonance at 61 mJ/cm^2^ to spectral undersampling, where the qBIC mode becomes too narrow to be resolved by our optical system with a spectral resolution of 2.5 nm. At intermediate doses, such as 53 mJ/cm^2^ and 55 mJ/cm^2^ the resonance is partially detectable but incompletely resolved, limiting the accuracy of the measured Q-factors. These results establish that, beyond the conventional radius perturbation approach, exposure dose-controlled hole depth and occupancy provide an alternative pathway for Q-factor engineering in DUVL-manufactured metasurface platforms.

Having identified robust resonance performance at low exposure doses where small holes are shallow and stochastically formed, we investigated whether this structural nonuniformity generates any spatial variations in resonance properties. Performing pixel-by-pixel spectral analysis, we extracted the resonance wavelength (λ_ res ) and Q-factor distributions by Fano fitting. At 49 mJ/cm^2^ (Figurea,g), 53 mJ/cm^2^ (Figureb,h), and 55 mJ/cm^2^ (Figurec,i), both λ res _ and Q-factor distributions follow Gaussian distributions with tight standard deviations, indicating high spatial uniformity across the measured area (200 μm^2^). The corresponding spatial maps in Figured–f for λ_ res _ and Figurej–l for Q-factor visually confirm this consistency. The minor horizontal fringes are likely artifacts from fabrication. To corroborate the experimental findings, we performed numerical simulations on a metasurface comprising a 10 × 10 array of double-hole unit cells with randomized central-hole depth variations, mimicking low-exposure-dose fabrication conditions (Figure S3). The simulation results confirmed the spatial uniformity of the resonance. We excluded the 61 mJ/cm^2^ dose from this analysis because, as discussed above, the ultrahigh Q-factors exceed our spectral measurements’ resolution limit.

This degree of spatial uniformity is striking, given the depth variations of the small hole at these doses (Figureg). We attribute this robustness to the nonlocal character of guided-mode resonances from which the qBIC mode is derived. ?−? ? ? The qBIC modes emerge from coherent coupling across multiple unit cells, such that the collective optical response averages over local structural imperfections. The insensitivity to nanoscale fabrication imperfections represents a critical advantage for scalable manufacturing. The nonlocality in metasurfaces inherently relaxes lithographic tolerance requirements, enabling high-performance qBIC metasurfaces fabricated using industrial semiconductor manufacturing tools.

Finally, to validate the refractive index sensing functionality of our platform, we performed proof-of-concept measurements by exposing the metasurface to aqueous glycerol solutions at 0–50% concentration. For each sample, we acquired hyperspectral data cubes and extracted resonance wavelengths, averaging a sensor area of 65 μm × 65 μm to ensure robust statistics. The qBIC resonance exhibits a linear redshift with increasing glycerol concentration (Figurea), following λ_ res _= 0.18 nm/(% glycerol) × C + 788.36 nm, where C is the glycerol concentration (in %, v/v). This corresponds to a bulk sensitivity of 129 nm/refractive index unit (RIU), confirming the metasurface’s sensitivity to refractive index changes.?

Beyond conventional spectral tracking, we explored single-wavelength intensity readout, which is a simpler detection scheme advantageous for multiplexed or high-throughput applications. By fixing the probe wavelength at λ = 789 nm (near the resonance inflection point) and monitoring transmittance, we observed intensity changes at different concentrations. Pixel-level intensity histograms (Figureb) and corresponding spatial maps (Figurec) show a clear trend of increasing intensity with higher glycerol content, indicating spectral red-shift through the detection window (Figured). The intensity variations in Figureb,c primarily originate from the intrinsic spatial heterogeneity of the sharp metasurface resonance in its dry state. This is supported by our pixel-level analysis presented in intensity histogram and sensor image at the resonance wavelength (Figure S4) and the extracted per-pixel spectra shown in Figure S5.

To quantitatively assess the refractive index discrimination capability of this intensity-based sensing approach, we generated receiver operating characteristic (ROC) curves from the pixel-level intensity distributions (Figuree). The ROC analysis demonstrates excellent classification performance, with area-under-curve (AUC) values closer to 1.00 for concentrations above 20% and 0.65 for 10%. This performance indicates that even modest refractive index changes (0.014 RIU per 10% glycerol concentrations) are reliably distinguishable. Because the ROC analysis uses large-area pixel-level intensity distributions referenced to the water control, intrinsic intensity heterogeneity is inherently accounted for, an approach we verified in our prior work.? Overall, these results demonstrate the wafer-scale all-dielectric metasurface’s potential for label-free optical sensing using both spectral shift and single-wavelength intensity readout from hyperspectral imaging data.

In summary, we demonstrated wafer-scale fabrication of high-Q dielectric metasurfaces with sub-200 nm features using DUVL, a critical milestone for translating nanophotonic devices from laboratory prototypes to manufacturable technologies. By employing the Brillouin zone folding strategy and DUVL exposure dose-controlled hole depth tuning, we introduced a scalable approach for Q-factor engineering with measured Q-factors ∼150. Moreover, the nonlocal qBIC resonances demonstrated strong tolerance to stochastic structural imperfections across the metasurface, enabling CMOS compatible optical interrogation. Proof-of-concept refractive index sensing validates the platform’s potential for practical biosensing applications, demonstrating a sensitivity of ∼129 nm/RIU and high-fidelity imaging-based sample discrimination via single-wavelength interrogation. Critically, the ability to tailor Q-factors to match detection system bandwidths enables optimized signal-to-noise ratios for specific sensing modalities, from high-resolution spectroscopy to rapid CMOS camera-based imaging.

This work addresses a longstanding barrier to metasurface commercialization by uniting semiconductor-compatible manufacturing with design flexibility, previously achievable only through low-throughput techniques. Specifically, translating metasurfaces for biosensing requires large numbers of sensor chips for bioassay development and preclinical studies, an impractical task with EBL-fabricated devices.? To overcome this, we prioritized wafer-scale fabrication, spatial uniformity, and CMOS camera–based optical interrogation over solely maximizing Q-factor or sensitivity. If the current RI sensitivity does not meet clinically relevant ranges for specific biomarkers, metasurface geometry and materials can be further optimized. Importantly, the experimentally measured Q-factor should be considered a lower bound rather than a fundamental limitation, as electromagnetic simulations indicate the potential for significantly higher values. Overall, the scalability, reproducibility, and cost-effectiveness of our approach position DUVL-fabricated qBIC metasurfaces as practical platforms for next-generation biosensing arrays, on-chip spectroscopy, and integrated photonic systems requiring cavity-enhanced light–matter interactions within a CMOS-detectable wavelength regime.

Supplementary Material