Silicon nitride (Si3N4) photonic integrated circuits have garnered significant attention in recent years due to their CMOS-compatible fabrication processes and unique advantages across a wide range of applications. 1) Si3N4 waveguides exhibit ultra-low propagation loss, making them ideal for building high-quality factor microcavities and long optical path interferometric structures, thereby enabling the integration of various fiber-optic applications. 2) Si3N4 waveguides offer excellent transparency over a broad wavelength range from 400 nm to 2350 nm, making them suitable for photonic devices operating from the visible to the near-infrared spectrum, such as those used in biosensing, quantum communication, and spectroscopic analysis. 3) Si3N4 waveguides exhibit extremely low two-photon absorption (TPA), allowing them to perform well under high-power conditions and enabling efficient nonlinear optical processes such as four-wave mixing, optical parametric oscillation, and supercontinuum generation. 4) The thermo-optic coefficient of Si3N4 is significantly lower than that of silicon, contributing to improved thermal stability of devices under varying temperature conditions. This makes Si3N4 well-suited for high-precision interferometric measurements and systems with stringent thermal control requirements.
However, Si3N4 can only be used to construct passive devices and cannot support essential photonic system functionalities such as light generation, modulation, amplification, and detection. Therefore, the heterogeneous integration of active materials is particularly important for unlocking the full potential of the Si3N4 photonic platform. Among these, III-V materials are capable of enabling a wide range of high-performance active devices. Integrating III-V materials with Si3N4 can greatly expand the application scope of the platform. Nonetheless, significant challenges exist in integrating Si3N4 waveguides with III-V devices due to the large refractive index contrast, which prevents efficient evanescent coupling, as well as limitations in conventional wafer bonding techniques, where Si-based and III-V processes are tightly coupled, restricting platform scalability.
To address these challenges, a research team from the Interuniversity Microelectronics Centre (imec) and Ghent University, in collaboration with Thales (France) and III-V Lab, has developed a heterogeneous integration scheme based on micro-transfer printing technology, enabling the realization of a narrow-linewidth, tunable on-chip light source based on a Si3N4 laser cavity. Relevant research results were recently published in Photonics Research, Volume 12, Issue 11, 2024. [Biwei Pan, Jerome Bourderionnet, Vincent Billault, Guenole Dande, Marcus Dahlem, Jeong Hwan Song, Sarvagya Dwivedi, Diego Carbajal Altamirano, Cian Cummins, Sandeep Seema Saseendran, Philippe Helin, Joan Ramirez, Delphine Néel, Emadreza Soltanian, Jing Zhang, Gunther Roelkens, "III-V-on-Si3N4 widely tunable narrow-linewidth laser based on micro-transfer printing," Photonics Res. 12, 2508 (2024)]
The team proposed using plasma-enhanced chemical vapor deposition (PECVD) to directly grow an amorphous silicon refractive index transition layer, achieving low-loss evanescent coupling between the Si3N4 waveguide and the III-V gain waveguide. This approach is CMOS-compatible and avoids wafer bonding of crystalline silicon-based transition layers, greatly simplifying the fabrication process. In addition, by optimizing the process flow, the team successfully demonstrated the micro-transfer printing integration of pre-fabricated III-V devices. In this scheme, both the III-V and Si3N4 fabrication processes are carried out on their native substrates. After integration, only redistribution of the III-V electrodes is required. This allows for complete decoupling of the silicon-based and III-V processes, enabling each to be manufactured and quality-controlled independently in separate facilities. As a result, integration yield is significantly improved, paving the way for large-scale industrial deployment.
The fabrication process of the on-chip laser is illustrated in Figure 2 (a)–(p). First, on a III-V wafer with a release layer, the required structures for the III-V gain region—including ridge waveguides, planarization layers, electrodes, and window openings—are fabricated, along with tether structures and substrate release processes necessary for micro-transfer printing. Next, on a silicon substrate, the material deposition, structural definition, and planarization of the Si3N4 waveguide and the amorphous silicon refractive index transition layer waveguide are completed. Finally, heterogeneous integration of the III-V thin-film devices onto the Si3N4 wafer is carried out via micro-transfer printing. After integration, no further processing of either the silicon-based or III-V materials is needed—only the final redistribution of the III-V electrodes is required to complete device fabrication. Utilizing a high-Q Si3N4 optical cavity and a novel cascaded microring resonator design, the laser achieves 6.3 mW output power in the Si3N4 waveguide, a wavelength tuning range of 54 nm across the C+L band, and a linewidth narrower than 25 kHz.
Professor Gunther Roelkens stated: "This study presents a micro-transfer printing-based heterogenous integration approach that fully decouples the III-V and silicon-based fabrication processes, allowing quality control of the Si3N4 photonic link and the active III-V devices to be independently conducted at their respective foundries. This paves the way for high-yield, large-scale integration and diverse applications of silicon nitride photonics."
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Figure 1 Microscope picture of III-V-on-Si3N4 laser
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Figure 2 Schematic fabrication process flow of III-V-on-Si3N4 laser
