Thin film lithium niobate on silicon enables large-scale optical interconnects for machine learning

The growing demand for artificial intelligence and cloud computing is driving the need for faster, more energy-efficient data transfer, and researchers are now addressing this challenge with innovative approaches to optical interconnects. Lingfeng Wu, Zhonhao Zhou, and Weilong Ma of Chongqing United Microelectronics Center Co., together with their colleagues, successfully integrated thin-film lithium niobate with active silicon photonics, demonstrating a major advance in the field. This achievement overcomes previous limitations in combining these materials and enables the creation of a single chip that incorporates high-performance modulators, photodetectors, and passive components. As a result, the integrated optical links deliver bandwidths in excess of 60 GHz, supporting high-speed data transmission, establishing a scalable platform for future energy-efficient, high-capacity systems, and paving the way for significant improvements in data center and computing infrastructure.

Integration of thin film lithium niobate silicon photonics

Scientists have developed a new method to integrate thin-film lithium niobate (TFLN) and active silicon photonics to address critical needs for high-bandwidth, low-power optical interconnects. Unlike previous approaches, the research team developed a unique process that completes all silicon CMOS steps before introducing the TFLN material. This involves bonding the TFLN die to a silicon wafer using trench-based technology to create a platform for cross-integrating modulators, photodetectors, and passive optics. The fabrication process begins with the creation of a standard silicon waveguide, followed by germanium epitaxy and silicon doping, all protected by a silicon dioxide layer.

The researchers then deposited an etch stop layer over the modulation region before continuing with silicon nitride deposition, heater fabrication, and metallization. Critical steps include drilling trenches in the silicon for bonding, then removing the titanium nitride layer and precisely bonding the TFLN die. Subsequent processing removes the remaining material, allowing for the definition of the TFLN waveguide and the fabrication of the modulation sections, including the SU8 overcladding, titanium termination resistor, and gold electrodes. Finally, pad openings for the heater and germanium photodetector are created to complete the integrated photonic circuit.

Efficient optical coupling between the materials is paramount, and the team designed a vertical adiabatic coupler (VAC) to connect the silicon and TFLN waveguides. The silicon waveguide tapers from 450nm to 180nm over 200μm, whereas the TFLN waveguide maintains a width of 1.5μm in the VAC region and expands to 2.5μm in the modulation section. This design achieves a coupling efficiency of over 97%, which corresponds to only 0 losses.

It exhibits resistance to bonding variations of 11 dB per coupler, up to ±300nm lateral offset and ±20nm thickness variation. Similar tapered couplers connect silicon and silicon nitride waveguides and achieve coupling losses as low as 0.06 dB. Regarding photodetection, a horizontal PIN structure with a germanium layer grown directly on silicon ensures high optical absorption over a device length of 55 μm. Finally, the team implemented an inverted taper to couple light from an on-chip silicon waveguide into a single-mode optical fiber, achieving a coupling loss of 1.

6 dB for TE polarization. The resulting integrated modulator utilizes an unbalanced Mach-Zehnder interferometer structure fabricated from silicon and TFLN components. This device exhibits a half-wave voltage of 4.4 V with a 4 mm length modulator, which corresponds to a voltage length product of 2.8 V cm, demonstrating the potential for energy-efficient high-capacity optical communication systems.

TFLN silicon integration for optical interconnects

Scientists have successfully integrated thin-film lithium niobate (TFLN) with fully functional silicon photonics, achieving a breakthrough in integrated photonics and establishing a new platform for high-speed optical interconnects. This work demonstrates for the first time back-end-of-line integration of TFLNs onto active silicon via a trench-based die-to-wafer bonding process, enabling joint integration of high-performance modulators, photodetectors, and passive components on a single chip. In this process, TFLN is introduced after all CMOS-compatible silicon manufacturing steps are completed, ensuring compatibility with existing manufacturing technologies. The integrated platform incorporates essential optical transceiver components, including silicon and silicon nitride passive components, fiber couplers, thermo-optic phase shifters, germanium photodetectors, and multilayer metallization.

The researchers designed a vertical adiabatic coupler (VAC) in the trench to enable near-lossless mode transition between silicon and TFLN waveguides, achieving a coupling efficiency of over 97% with only 0.11 dB of loss. Simulations confirm that the process tolerates lateral offsets of up to ±300 nm and adhesive layer thickness variations of ±20 nm, improving manufacturing robustness. Measurements demonstrate that the fabricated on-chip optical link achieves an electrical-to-electrical bandwidth of over 60 GHz and supports data transmission rates of 128 GBaud on-off keying (OOK) and 100 GBaud 4-level pulse amplitude modulation (PAM4).

The combination of silicon and silicon nitride achieves a low interlayer optical loss of 0.06 dB per coupler, and the silicon waveguide and integrated germanium photodetector ensure high optical absorption. This integrated platform establishes a path to energy-efficient, high-capacity systems and has potential applications in data center interconnects, microwave photonics, and high-density on-chip data links for future wafer-scale computing.

Achieves 128 Gbaud with TFLN and silicon integration

This study demonstrates a new platform for integrating thin-film lithium niobate (TFLN) and active silicon photonics, achieving a significant advance in heterogeneous optical interconnect technology. Scientists were able to successfully bond TFLNs to silicon after completing standard silicon processing, overcoming previous limitations that limited their integration into passive components. The resulting chip incorporates a high-speed germanium photodetector and a TFLN modulator and demonstrates an electrical-to-electronic bandwidth of over 60 GHz, supporting data transmission rates of 128 Gbaud with on-off keying and 100 Gbaud with pulse amplitude modulation 4 signaling. This integration leverages the low loss and high modulation efficiency of TFLNs with the established CMOS compatibility and integration density of silicon photonics to combine the advantages of both materials.

This platform can also incorporate silicon nitride, allowing efficient optical coupling between TFLN, silicon, and silicon nitride layers on the same chip. The researchers acknowledge that design optimization has the potential to further improve modulator and detector performance, match the bandwidth of state-of-the-art germanium photodetectors, and reduce the size of TFLN modulators. This work establishes a scalable foundation for energy-efficient, high-capacity optical systems with potential applications in complex functions such as coherent transceivers, microwave photonics, and photonic computing.