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Deterministic soliton microcombs in Cu-free photonic integrated circuits

Nature volume 646, pages 843–849 (2025)Cite this article

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Abstract

Chip-scale optical frequency combs based on microresonators (microcombs) have provided access to optical combs with GHz-to-THz repetition rates, broad bandwidth, compact form factors and compatibility with wafer-scale manufacturing1. Si3N4 photonic integrated circuits emerged as a leading platform and have been used in nearly all system-level demonstrations so far, ranging from optical communications2, parallel lidar3, optical frequency synthesis4, low-noise microwave generation5 to parallel convolutional processing6. Yet, transitioning to real-world deployment outside laboratories has been compounded by the difficulty of deterministic soliton microcomb generation, primarily due to strong thermal instabilities. Although a variety of techniques have been developed to initiate soliton generation, including pulsed pumping, fast scanning and auxiliary-laser pumping7,8,9,10,11, these techniques do not eliminate thermal effects and often compromise microcomb performance, either by adding additional complexity or by reducing the accessible soliton existence range. Here we overcome thermal effects and demonstrate deterministic soliton generation in Si3N4 photonic integrated circuits. We trace thermal effects to unexpected copper impurities within the waveguides, which originate from residual contaminants in CMOS-grade Si wafers and are gettered into Si3N4 during fabrication. By developing copper removal techniques, we substantially reduce copper concentration and thereby mitigate thermal effects. We demonstrate successful dissipative Kerr soliton generation with arbitrary laser scanning profiles and slow laser scanning. Our techniques can be readily applied to front-end-of-line processing of Si3N4 devices in foundries, removing a key obstacle to the deployment of soliton microcomb technology.

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Fig. 1: Thermal absorption-induced instabilities in DKS generation in Si3N4 microresonators.
Fig. 2: Copper-induced thermal absorption loss in Si3N4 microresonators: origins and impacts on DKS generation.
Fig. 3: Substrate preparation techniques for copper-impurity-free PICs.
Fig. 4: Deterministic DKS generation in Si3N4 microresonators fabricated with copper-impurity-free silicon substrates using the photonic Damascene process.

Data availability

The experimental datasets and scripts used to produce the plots in this paper are available at Zenodo50 (https://doi.org/10.5281/zenodo.15773976).

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Acknowledgements

We thank Ali Torkaman for contributions to soliton generation experiments. The Si3N4 samples were fabricated in the EPFL Center of MicroNanoTechnology. The SIMS measurements were performed by X. Liu at Eurofins EAG Laboratories. This work was supported by the Horizon Europe EIC transition programme under grant no. 101136978 (CombTools) and funded by the Swiss State Secretariat for Education, Research and Innovation. This work was supported by the Swiss National Science Foundation under grant agreement no. 216493 (HEROIC). This work was further sponsored by the Army Research Office and was accomplished under Award Number W911NF-25-2-0145. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Office or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.

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Author notes

  1. Rui Ning Wang

    Present address: Luxtelligence SA, Lausanne, Switzerland

Authors and Affiliations

  1. Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland

    Xinru Ji, Xurong Li, Zheru Qiu, Rui Ning Wang, Marta Divall, Andrey Gelash, Grigory Lihachev & Tobias J. Kippenberg

  2. Institute of Electrical and Micro Engineering, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland

    Xinru Ji, Xurong Li, Zheru Qiu, Rui Ning Wang, Marta Divall, Andrey Gelash, Grigory Lihachev & Tobias J. Kippenberg

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Contributions

T.J.K. conceived the idea and the concept. X.J. designed the Si3N4 samples. A.G. performed the simulation. X.J., X.L., R.N.W. and Z.Q. fabricated the Si3N4 samples. X.J. designed and performed the Cu-gettering experiment with assistance from Z.Q.; M.D. coordinated the SIMS measurements with Eurofins EAG Laboratories. X.J. and X.L. characterized and analysed the Si3N4 samples. X.L. performed the soliton generation experiment with assistance from G.L.; X.J. and X.L. wrote the paper with input from all co-authors. T.J.K. supervised the project.

Corresponding author

Correspondence to Tobias J. Kippenberg.

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Competing interests

T.J.K. is a cofounder and shareholder of LiGenTec SA, a start-up company offering Si3N4 PICs as a foundry service.

Peer review

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Nature thanks Martino Bernard, Mher Ghulinyan and David Moss for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 SIMS depth profiles of 14 metal impurities in a fully SiO2-cladded Si3N4 waveguide.

The x-axis represents sputtering depth, the y-axis denotes metal species, and the z-axis indicates impurity concentration. Background colors indicate the layer structure. Sample ID: D100_01_F1_C7.

Extended Data Fig. 2 Copper contamination impact on thermal absorption and soliton generation in Si3N4 microresonators.

a, Schematic of the Cu contamination experiment, where a Cu-coated Si wafer is placed beneath the Si3N4 chip during a 1000 °C, 4 hour thermal process in N2. Device ID: D163_01_F2_C3. b, Histogram of intrinsic loss rate κ0/2π before and after Cu diffusion. c, Kerr-normalized thermal response measurements before and after deliberate copper diffusion, at the same resonance near 1550 nm in TE polarization. d, Measured soliton step lengths in TE polarization, showing shorter steps in microresonators after Cu diffusion. e, SIMS depth profiles revealing Cu incorporation into Si3N4 waveguides after thermal treatment. SIMS results are calibrated to the Si3N4 filling ratio in the mixed Si3N4/SiO2 waveguide layer.

Extended Data Fig. 3 Pathway of Cu diffusion into Si3N4 waveguides during device fabrication.

a, Schematic of the photonic Damascene process for low-loss Si3N4 waveguide fabrication, including preform etching, reflow, LPCVD Si3N4 and SiO2 deposition, planarization, and annealing. b-e, Depth-resolved Cu concentration profiles: b, after preform etching and reflow; c, post Si3N4 deposition and planarization; d, after Si3N4 annealing (1200 °C, 11 hours), showing Cu incorporation into the Si3N4 waveguide; e, post SiO2 cladding deposition and annealing, revealing Cu localization exclusively within the Si3N4 waveguide core.

Extended Data Fig. 4 SIMS characterization of Cu concentration in Si wafers coated with LPCVD and PECVD Si3N4 films.

The deposited Si3N4 films, measuring  ~200 nm in thickness for both LPCVD and PECVD, were annealed at 800 °C for varying durations. SIMS analysis reveals higher detectable Cu concentrations in PECVD Si3N4 films compared to LPCVD films, indicating enhanced gettering efficiency in PECVD films due to a higher density of Cu trapping sites. In contrast, LPCVD Si3N4 films exhibit a constant Cu level regardless of annealing time.

Extended Data Fig. 5 Optical loss, thermal absorption, and SIMS characterization of gettered versus non-gettered Si3N4 microresonators.

a, Resonance intrinsic linewidth (κ0/2π) measurements from 1310-1630 nm (TE polarization), comparing gettered (blue) and non-gettered (red, labeled ‘normal’) devices. Sample IDs: gettered (D163_03_F1_C7_1_04), non-gettered (D163_11_F1_C7_2_04). b, Histogram of κ0/2π for devices in a. c, Kerr-normalized thermal response measurements at resonances near 1520, 1530, 1550, and 1560 nm for both devices. Colored regions denote thermal-dominated regimes at low modulation frequencies. d, Soliton step formation during pump laser scanning. e, Comparative SIMS depth profiles of Cu contamination in gettered versus non-gettered devices.

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Ji, X., Li, X., Qiu, Z. et al. Deterministic soliton microcombs in Cu-free photonic integrated circuits. Nature 646, 843–849 (2025). https://doi.org/10.1038/s41586-025-09598-4

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