Low-Loss AlN Waveguides

• Low-loss AlN waveguides are photonic guiding structures made from aluminum nitride, offering wide transparency (≈200 nm to 13.6 μm) and minimal intrinsic absorption.
• They leverage precise fabrication methods—such as sputtering, ALD passivation, and rapid thermal annealing—to control sidewall roughness and achieve record propagation losses (e.g., 0.137–0.154 dB/cm at telecom wavelengths).
• These waveguides are integral to high-performance integrated circuits, enabling high Q-factor resonators, efficient fiber–chip interfaces, and applications in quantum, nonlinear, and telecommunication photonics.

Low-loss aluminum nitride (AlN) waveguides are photonic guiding structures fabricated from AlN, optimized to achieve minimal propagation loss across a broad spectral bandwidth—spanning ultraviolet (UV), visible, and telecommunication wavelengths. Leveraging AlN’s wide bandgap ($\approx$ 6.2 eV), high refractive index, broad transparency (from $\approx$200 nm to 13.6 μm), and inherent nonlinear/piezoelectric properties, these waveguides serve as fundamental building blocks for integrated photonic circuits demanding high Q-factor resonators, low insertion loss, efficient fiber-chip interfaces, and compatibility with advanced nonlinear and quantum optical applications.

1. Material Properties and Loss Mechanisms

AlN is a wide-bandgap semiconductor characterized by low intrinsic absorption, making it exceptionally suitable for photonics in the UV, visible, and infrared regimes (Pernice et al., 2012, Liu et al., 2018). The transparency window ($\approx$200 nm – 13.6 μm) is enabled by its direct bandgap and high crystal quality. Propagation loss in AlN waveguides is determined primarily by:

• Material absorption: Nominal absorption coefficient $<10^{-11}$ at telecom wavelengths, verified by ellipsometry (Singh et al., 19 Aug 2024).
• Scattering losses: Dominated by sidewall roughness, which enters as Rayleigh scattering ($\propto 1/\lambda^4$), particularly impactful at shorter wavelengths (Videnov et al., 27 Aug 2025, West et al., 2018). Typical RMS roughness values fall in the 1–3 nm range but are strongly process-dependent.
• Interface and grain boundary losses: Relevant for sputtered polycrystalline AlN; mitigated through process control and post-deposition treatments (Singh et al., 19 Aug 2024).
• Waveguide geometry and etch profiles: Steep sidewall angles ($\approx$68°–80°), achieved via optimized ICP-RIE with Cl$_2$/BCl$_3$/Ar, minimize scattering losses (Pernice et al., 2012, West et al., 2018).

Mitigation of loss mechanisms requires meticulous control of film growth, pattern transfer, and post-fabrication surface engineering. ALD passivation and rapid thermal annealing (RTA) present substantial improvements by passivating etched defects and reducing high-frequency roughness (Videnov et al., 27 Aug 2025).

2. Fabrication Processes and Innovations

AlN waveguide technology spans several deposition/growth methods, each offering distinct control over material properties and loss characteristics:

Technique	Typical Substrate	Loss Range (at λ)
Sputtering	Si/SiO₂, sapphire	0.137–0.8 dB/cm (1.3–1.55 μm) (Singh et al., 19 Aug 2024, Pernice et al., 2012)
MOCVD	Sapphire	0.14–0.2 dB/cm (1.55 μm)
MOVPE-AlGaN/AlN	Sapphire	2.3–2.5 dB/cm (785 nm) (Gündogdu et al., 2023)
ALD (for cladding/passivation)	Sapphire	2 dB/cm (852 nm) (Videnov et al., 27 Aug 2025)

Key process elements include:

• High-resolution EBL with shape-based proximity effect correction, ODUS (over-dose under-size): Reduces mask-induced roughness and enables tight pattern fidelity (Videnov et al., 27 Aug 2025).
• RIE/ICP etching with endpoint detection and double-step hard mask: Controls sidewall angle, minimizes roughening, and sustains pattern transfer accuracy (Singh et al., 19 Aug 2024, West et al., 2018).
• ALD passivation (Al₂O₃): Ultra-thin conformal layers ($\sim$1 nm) used for chemical defect passivation at the sidewall-cladding interface, showing loss reductions of several dB/cm (Videnov et al., 27 Aug 2025).
• Rapid thermal annealing (RTA): 400 °C for 60 s soak period, further reduces scattering loss post-cladding (Videnov et al., 27 Aug 2025).

Process integration with standard CMOS (Si substrates, SiO₂ buried oxide, low-temperature deposition) is routine for sputtered AlN, facilitating large-scale photonic circuit fabrication (Singh et al., 19 Aug 2024).

3. Optical Characterization and Performance Metrics

Characterization of low-loss AlN waveguides relies on direct transmission, cutback measurements, and resonator Q-factor analysis.

• Propagation loss ($\alpha$): Extracted via linear fit: $T(L) = T_0 - \alpha L$, where $T(L)$ is transmission vs length (in dB) (Singh et al., 19 Aug 2024).
• Q-factor ($Q$): $Q = \lambda/\Delta\lambda$, with $\Delta\lambda$ as FWHM of the resonance. High-Q values ($>10^5$–$4.4\times 10^5$) indicate minimal loss, especially in ring and nanobeam resonators (Pernice et al., 2012, Pernice et al., 2012).
• Loss-to-Q conversion: Commonly via

$$\alpha = \frac{10 \log_{10}(e) \lambda}{2\pi n_g Q_{\text{int}}}$$

(with $n_g$ group index, $Q_{\text{int}}$ intrinsic Q) (Pernice et al., 2012, West et al., 2018).

• Sidewall roughness: Direct impact on Rayleigh scattering and loss scaling, confirmed via AFM and cross-sectional SEM (Videnov et al., 27 Aug 2025, Singh et al., 19 Aug 2024).

Leading results:

• Sputtered AlN on Si/SiO₂: 0.137 ± 0.005 dB/cm at 1310 nm, 0.154 ± 0.008 dB/cm at 1550 nm (best reported for C-band, first report for O-band) (Singh et al., 19 Aug 2024).
• Single-crystal AlN on sapphire (post-ALD and RTA): 2.0 ± 0.3 dB/cm at 852 nm (Videnov et al., 27 Aug 2025).
• Polycrystalline sputtered AlN: 0.8 dB/cm at 1550 nm (Pernice et al., 2012).
• AlGaN/AlN heterostructures: 2.3–2.5 dB/cm at 785 nm (Gündogdu et al., 2023).
• UV AlN microrings: Q = 2.1 × 10⁵ and $\sim$8 dB/cm loss at 390 nm (Liu et al., 2018).
• ALD alumina waveguides: $<$3 dB/cm loss at 371 nm, Q = 470,000 at 405 nm (West et al., 2018).

4. Integrated Functionality: Couplers, Modulators, and Bandgap Structures

Low-loss AlN waveguides serve as the backbone for high-performance integrated photonic devices:

• Adiabatic fiber–chip interfaces: Stepwise tapering in 600 nm-thick AlN, insertion loss –0.97 dB (1550 nm TM), –2.6 dB (780 nm TM) (Zhao et al., 2019). Coupling efficiencies reach >80%.
• Photonic crystal cavities: Q = 146,000, tunable extinction ratio $>$15 dB by controlling waveguide–cavity coupling gaps (Pernice et al., 2012).
• Micro-ring resonators: Q = 440,000, extinction ratios $>$30 dB in critical coupling regime at telecom (Pernice et al., 2012). At visible wavelengths (770 nm), Q > 30,000.
• Broadband directional couplers: Three-waveguide geometry enables spectrally flat 50:50 or arbitrary splitting, extinction ratio $>$35 dB over a 60 nm bandwidth (Stegmaier et al., 2013).
• Electro-optic modulation: Resonance tuning via Pockels effect (dominant $r_{33}$ tensor component), modulation up to 4.5 Gb/s, energy per bit $<$10 fJ (Xiong et al., 2014).
• On-chip mode converters and waveplates: Variable cross-section, twisted beam rotators and mode converters achieve propagation losses of 0.14 dB/cm and coupling loss down to 0.19 dB (Sun et al., 2021).

The precise control of coupling, extinction ratio, mode transformation, and integrated filtering functions is made possible by the underlying low propagation loss and tailored waveguide geometries.

5. Process Development and Loss Mitigation Strategies

The prevailing limitation for AlN, especially sub-micron, single-mode waveguides, is Rayleigh scattering from sidewall roughness and chemical defects. Process innovations:

• EBL Patterning: 4 nm shot pitch, shape-based PEC, ODUS strategy; enables $\sim$7$$dB/cm loss reduction (<a href="/papers/2508.20245" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">Videnov et al., 27 Aug 2025</a>).</li> <li><strong>ICP-RIE etch optimization:</strong> Maintains sidewall verticality and reduces roughness (<a href="/papers/1205.1665" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">Pernice et al., 2012</a>, <a href="/papers/1808.00429" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">West et al., 2018</a>).</li> <li><strong>ALD passivation:</strong> Ultra-thin Al₂O₃ ($$\sim$1 nm) coats chemically reactive sites, reducing loss by conformally smoothing interfaces and passivating dangling bonds (Videnov et al., 27 Aug 2025).
• RTA: 400 °C anneal, soak 60 s, further loss reduction by healing sidewall defects (Videnov et al., 27 Aug 2025).
• Sputter process parameters: High purity target, pure N₂ atmosphere, controlled temperature (700 °C), base pressure $<$10⁻⁷ mbar, yielding low impurity films ($\sim$2.1 nm RMS roughness) (Singh et al., 19 Aug 2024).
• ALD alumina for UV and blue: Sidewall roughness mitigation via careful mask and etch optimization, with absorption loss shown $<$1% of total (West et al., 2018).

A plausible implication is that further improvement requires post-etch smoothing and defect passivation, especially for quantum/UV applications.

6. Application Domains and Integrated Circuit Implications

Low-loss AlN waveguides underpin key application areas:

• Quantum photonics: Enables low-noise transmission for entangled photon pair and squeezed light generation, supports quantum information processing with atom–photon interfaces at 852 nm (Zhao et al., 2019, Videnov et al., 27 Aug 2025).
• Nonlinear optics: High $\chi^{(2)}$and$\chi^{(3)}$$susceptibilities exploited for second harmonic generation, frequency conversion, parametric oscillation, phase matching via mode engineering (<a href="/papers/2312.03128" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">Gündogdu et al., 2023</a>, <a href="/papers/1807.10716" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">Liu et al., 2018</a>).</li> <li><strong>Telecommunication photonics:</strong> Sputtered AlN achieves record losses in O- and C-bands ($$\sim$0.14–0.16 dB/cm), compatible with standard silicon substrates, scalable to large wafers (Singh et al., 19 Aug 2024).
• Visible/UV integrated optics: Facilitates on-chip sensors, spectroscopy, and quantum circuits, with Q > 10⁵ and loss down to 2 dB/cm at 852 nm (for atomic transitions) (Videnov et al., 27 Aug 2025, Liu et al., 2018).
• Integrated waveplates and rotators: 3D beam manipulation in low-loss glass waveguides guides future hybrid PIC design (Sun et al., 2021).

A plausible implication is that sidewall engineering and passivation (e.g., ALD) are essential for scaling AlN PICs into the visible/UV quantum regime, and that sputtered AlN platforms now rival MOCVD performance at lower thermal budgets for CMOS compatibility.

7. Comparative Performance and Future Outlook

Recent literature establishes self-consistent benchmarks for propagation loss reduction:

System	Propagation Loss (dB/cm)	Key Process Elements	Application Domain
Sputter AlN (Si/SiO₂)	0.137 (1310 nm), 0.154 (1550 nm) (Singh et al., 19 Aug 2024)	DC magnetron, optimized RIE, BARC litho	Telecom PIC
ALN/AlGaN Heterostructure	2.3–2.5 (785 nm) (Gündogdu et al., 2023)	MOVPE, HTA, buffer optimization	Nonlinear quantum photonics
Single-crystal AlN (sapphire, ALD+RTA)	2.0 (852 nm) (Videnov et al., 27 Aug 2025)	Shape-PEC EBL, ALD passiv., RTA	Atomic quantum platforms
ALD alumina (blue/UV)	1.3–3.0 (371–405 nm) (West et al., 2018)	Full-etch, SiO₂ hard mask, optimized ICP	Biochemical sensing, quantum optics

Data consistently show that sidewall roughness mitigation, chemical passivation, and post-processing (ALD+RTA) yield substantial reductions in propagation loss, approaching the theoretical limit imposed by bulk material.

A plausible implication is that scaling to larger wafers and integrating passive and active elements—modulators, couplers, nonlinear cavities—is now practical within a CMOS-compatible, low-thermal-budget foundry environment. Future research avenues may target further reductions in UV/visible loss and integration of complex functionalities for scalable quantum photonic circuits.

In summary, low-loss AlN waveguides now achieve record propagation losses ($\sim$0.14–0.2 dB/cm at telecom, $<$2 dB/cm at atomic/visible, $<$3 dB/cm at shortwave UV/blue), combining process innovations in lithography, etching, passivation, and annealing. These advances have enabled robust, scalable, and multifunctional photonic integrated circuits spanning telecommunication, nonlinear, and quantum optical domains.