Thin-film Lithium Niobate on Silicon Enables Large-Scale Optical Interconnects for Machine Learning

The increasing demands of artificial intelligence and cloud computing are driving a need for faster, more energy-efficient data transfer, and researchers are now addressing this challenge with innovative approaches to optical interconnects. Lingfeng Wu, Zhonghao Zhou from Chongqing United Microelectronics Center Co., and Weilong Ma, alongside colleagues, demonstrate a significant advance in this field by successfully integrating thin-film lithium niobate with active silicon photonics. This achievement overcomes previous limitations in combining these materials, enabling the creation of a single chip that incorporates high-performance modulators, photodetectors, and passive components. The resulting integrated optical links achieve greater than 60GHz bandwidth and support high-speed data transmission, establishing a scalable platform for future energy-efficient, high-capacity systems and paving the way for substantial improvements in data centre and computing infrastructure. Thin-Film Lithium Niobate Silicon Photonics Integration Scientists have pioneered a new method for integrating thin-film lithium niobate (TFLN) with active silicon photonics, addressing a critical need for high-bandwidth, low-power optical interconnects. The research team developed a unique process, completing all silicon CMOS steps before introducing the TFLN material, a departure from previous approaches. This involves bonding TFLN dies to a silicon wafer using a trench-based technique, creating a platform for co-integrating modulators, photodetectors, and passive optical components. The fabrication process begins with standard silicon waveguide creation, followed by germanium epitaxy and silicon doping, all protected by a silicon dioxide layer. Researchers then deposited an etching stop layer at the modulation region before continuing with silicon nitride deposition, heater fabrication, and metallization. A crucial step involves opening trenches in the silicon for bonding, followed by removal of a titanium nitride layer and precise bonding of the TFLN dies. Subsequent processing removes the remaining material, allowing for definition of the TFLN waveguides and fabrication of the modulation section, including an SU8 over-cladding, titanium termination resistors, and gold electrodes. Finally, pad openings are created for heaters and germanium photodetectors, completing the integrated photonic circuit. Efficient optical coupling between materials is paramount, and the team engineered vertical adiabatic couplers (VACs) to connect silicon and TFLN waveguides. Silicon waveguides taper from 450nm to 180nm over 200μm, while the TFLN waveguide maintains a width of 1. 5μm in the VAC region, expanding to 2. 5μm in the modulation section. This design achieves a coupling efficiency exceeding 97%, corresponding to a loss of only 0. 11 dB per coupler, and demonstrates tolerance to bonding variations of up to ±300nm lateral offset and ±20nm thickness variation. Similar tapered couplers connect silicon and silicon nitride waveguides, achieving a coupling loss as low as 0. 06 dB. For photodetection, a horizontal PIN structure with a germanium layer directly grown on silicon ensures high optical absorption over a 55μm device length. Finally, the team implemented inverse tapers to couple light from the on-chip silicon waveguides to single-mode optical fibers, achieving a coupling loss of 1. 6 dB for TE polarization. The resulting integrated modulator utilizes an unbalanced Mach-Zehnder interferometer structure, fabricated with silicon and TFLN components. The device exhibits a half-wave voltage of 4. 4V over a 4mm-long modulator, corresponding to a voltage-length product of 2. 8V·cm, demonstrating the potential for energy-efficient, high-capacity optical communication systems.,. TFLN Silicon Integration For Optical Interconnects Scientists have achieved a breakthrough in integrated photonics by successfully integrating thin-film lithium niobate (TFLN) with fully functional silicon photonics, establishing a new platform for high-speed optical interconnects. This work demonstrates the first back-end-of-line integration of TFLN onto active silicon via a trench-based die-to-wafer bonding process, allowing for the co-integration of high-performance modulators, photodetectors, and passive components on a single chip. The process introduces TFLN after completing all CMOS-compatible silicon fabrication steps, ensuring compatibility with existing manufacturing techniques. The integrated platform incorporates essential building blocks for optical transceivers, including silicon and silicon nitride passive components, fiber couplers, thermo-optic phase shifters, germanium photodetectors, and multilayer metallization. Researchers engineered vertical adiabatic couplers (VACs) within the trenches to enable nearly lossless mode transitions between silicon and TFLN waveguides, achieving a coupling efficiency exceeding 97% with a loss of only 0. 11 dB. Simulations confirm the process tolerates lateral offsets of up to ±300nm and variations in adhesive layer thickness of ±20nm, enhancing manufacturing robustness. Measurements demonstrate the fabricated on-chip optical links achieve greater than 60GHz electrical-to-electrical bandwidth, supporting data transmission rates of 128-GBaud on-off keying (OOK) and 100-GBaud 4-level pulse amplitude modulation (PAM4). Silicon-to-silicon nitride coupling achieves interlayer optical loss as low as 0. 06 dB per coupler, while germanium photodetectors, integrated with a silicon waveguide, ensure high optical absorption. This integrated platform establishes a pathway for energy-efficient, high-capacity systems, with potential applications in data center interconnects, microwave photonics, and dense on-chip data links for future wafer-scale computing.,. TFLN and Silicon Integration Achieves 128 Gbaud This research demonstrates a new platform for integrating thin-film lithium niobate (TFLN) with active silicon photonics, achieving a significant advance in heterogeneous optical interconnect technology. Scientists successfully bonded TFLN to silicon after completing standard silicon processing, overcoming previous limitations that restricted integration to passive components. The resulting chip incorporates high-speed germanium photodetectors and TFLN modulators, demonstrating greater than 60GHz electrical-to-electrical bandwidth and supporting data transmission rates of 128 Gbaud with On-Off Keying and 100 Gbaud with Pulse Amplitude Modulation 4 signaling. This integration combines the benefits of both materials, leveraging the low loss and high modulation efficiency of TFLN with the established CMOS compatibility and integration density of silicon photonics. The platform also allows for the inclusion of silicon nitride, enabling efficient optical coupling between TFLN, silicon, and silicon nitride layers on the same chip. Researchers acknowledge that the performance of the modulators and detectors can be further improved through design optimizations, potentially matching the bandwidth of state-of-the-art germanium photodetectors and reducing the size of the TFLN modulators. This work establishes a scalable foundation for energy-efficient, high-capacity optical systems, with potential applications extending to complex functionalities such as coherent transceivers, microwave photonics, and photonic computing. 👉 More information 🗞 Heterogeneous back-end-of-line integration of thin-film lithium niobate on active silicon photonics for single-chip optical transceivers 🧠 ArXiv: https://arxiv.org/abs/2512.07196 Tags: