Deterministic soliton microcombs in Cu-free photonic integrated circuits

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 pumping^7-11, 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.