High-yield wafer-scale fabrication of ultralow-loss, dispersion-engineered silicon nitride photonic circuits

Abstract: Low-loss photonic integrated circuits (PIC) and microresonators have enabled novel applications ranging from narrow-linewidth lasers, microwave photonics, to chip-scale optical frequency combs and quantum frequency conversion. To translate these results into a widespread technology, attaining ultralow optical losses with established foundry manufacturing is critical. Recent advances in fabrication of integrated Si3N4 photonics have shown that ultralow-loss, dispersion-engineered microresonators can be attained at die-level throughput. For emerging nonlinear applications such as integrated travelling-wave parametric amplifiers and mode-locked lasers, PICs of length scales of up to a meter are required, placing stringent demands on yield and performance that have not been met with current fabrication techniques. Here we overcome these challenges and demonstrate a fabrication technology which meets all these requirements on wafer-level yield, performance and length scale. Photonic microresonators with a mean Q factor exceeding 30 million, corresponding to a linear propagation loss of 1.0 dB/m, are obtained over full 4-inch wafers, as determined from a statistical analysis of tens of thousands of optical resonances and cavity ringdown with 19 ns photon storage time. The process operates over large areas with high yield, enabling 1-meter-long spiral waveguides with 2.4 dB/m loss in dies of only 5x5 mm size. Using a modulation response measurement self-calibrated via the Kerr nonlinearity, we reveal that, strikingly, the intrinsic absorption-limited Q factor of our Si3N4 microresonators exceeds a billion. Transferring the present Si3N4 photonics technology to standard commercial foundries, and merging it with silicon photonics using heterogeneous integration technology, will significantly expand the scope of today's integrated photonics and seed new applications.

• The paper presents a novel fabrication method that yields ultralow-loss Si3N4 waveguides on 4-inch wafers with Q-factors exceeding 30 million.
• Utilizing the photonic Damascene process with DUV lithography, the method enables production of meter-long PICs with losses as low as 1.0 dB/m.
• The research indicates that managing scattering losses via Kerr nonlinear calibration may push intrinsic Q-factors beyond 10^9, enhancing device performance.

High-Yield Wafer-Scale Fabrication of Ultralow-Loss, Dispersion-Engineered Silicon Nitride Photonic Circuits

The paper presents a significant advancement in the manufacturing of silicon nitride (Si$_3$N$_4$) photonic integrated circuits (PICs), with a focus on achieving ultralow optical losses at a wafer-scale level. This development marks a substantial enhancement in the ability to produce large-scale, low-loss PICs, paving the way for advanced applications in various fields, including nonlinear photonics, laser optics, and quantum technologies.

Summary of Main Findings

This study demonstrates a successful fabrication technology for producing ultralow-loss, tight-confinement, dispersion-engineered Si$_3$N$_4$ waveguides. Noteworthy achievements include:

• Photonic Microresonators: The authors achieve a mean quality (Q) factor exceeding $30 \times 10^6$ over a full 4-inch wafer, equating to a linear propagation loss of just 1.0 dB/m. This is verified through a comprehensive statistical analysis of tens of thousands of optical resonances.
• Wafer-scale Yield and Performance: Utilizing the photonic Damascene process with deep ultraviolet (DUV) stepper lithography, the process attains high yield and performance across 4-inch wafers, allowing for the production of waveguides up to 1-meter long with minimal loss (2.4 dB/m in 5x5 mm$^2$ dies).
• Intrinsic Absorption Limit: By employing a Kerr nonlinear calibration method, the research reveals that the intrinsic absorption-limited Q factor of these Si$_3$N$_4$ microresonators exceeds $10^9$, where scattering, rather than absorption, primarily influences current optical losses.

Implications and Future Directions

The improvements in ultralow-loss Si$_3$N$_4$ photonics present strategic implications for both academic research and industrial applications:

• Enhanced Nonlinear Photonics: The reduced optical losses and high-Q factors support the development of sophisticated nonlinear photonic devices. Applications in dissipative Kerr soliton microcombs and integrated frequency combs are particularly promising, with potential extensions to telecommunications and optical computing systems.
• Commercial Fabrication Compatibility: The integration of the photonic Damascene process in standard CMOS foundries aligns with trends towards large-volume commercial deployment. This synergy can facilitate the broader adoption of Si$_3$N$_4$ PICs in commercial applications, driving cost reductions and accelerating development cycles.
• Advanced System-Level Devices: The capacity to fabricate densely packed, meter-long PICs unlocks new possibilities for creating devices such as traveling-wave parametric amplifiers and mode-locked lasers, particularly when combined with rare-earth doping techniques or heterogeneous material integrations.

Conclusion

By successfully fabricating high-Q Si$_3$N$_4$ waveguides with ultralow losses on a wafer-scale using standard CMOS-compatible techniques, the research provides a solid foundation for developing next-generation PIC technologies. Future research could focus on further refining fabrication processes to reduce scattering losses and improving integration techniques with existing silicon photonic technologies to enhance the performance and functionality of optical systems. This work represents a crucial stride towards realizing ultralow-loss photonic circuits in mainstream applications, offering substantial opportunities for innovation in the years to come.