Recent advancements in artificial intelligence (AI) and large language models have increased demand for data centers. However, the strain on data centers has prompted the need for efficient communication links to handle the massive amounts of data required to train AI models. Fiber optics, a technology used for long-distance communication for decades, is now being adopted for short-distance intra-data center communication due to its superior performance compared to classical electrical links.

Researchers have successfully implemented fiber optics for communication between chips inside the same package in a breakthrough development. This requires converting the data stream from electrical to optical signals, achieved through optical transceivers. The most widely used technology for fabricating these optical transceivers is silicon photonics.

Silicon photonics involves integrating active photonic devices, such as modulators and photodetectors, with electronic drivers for powering the devices and reading incoming data. To achieve a tight integration with low parasitic capacitance, researchers have employed 3D stacking technology by stacking the electronic chip (EIC) on top of the photonic chip (PIC).

A recent study published in the Journal of Optical Microsystems investigated the thermal impact of this 3D integration. The design of the photonic chip includes an array of ring modulators known for their temperature sensitivity. In demanding environments like data centers, these ring modulators require active thermal stabilization, implemented using integrated heaters. However, minimizing the power required for thermal stabilization is crucial to improve energy efficiency.

The research team from KU Leuven and Imec in Belgium conducted experiments to measure the heater efficiency of the ring modulators before and after the flip-chip bonding of the EIC on the PIC. They found a significant relative loss of -43.3% in efficiency after bonding, indicating a substantial impact. 3D finite element simulations revealed that this loss was attributed to heat spreading in the EIC, which should be avoided. Ideally, all heat generated in the integrated heater should be contained close to the photonic device. Additionally, the thermal crosstalk between the photonic devices increased by up to +44.4% after bonding the EIC, making individual thermal control more challenging.

To address these thermal challenges, the researchers conducted a thermal simulation study. They explored various design variables and found that increasing the spacing between microbumps and the photonic device while decreasing the interconnect linewidth can minimize the thermal penalty of 3D integration and improve heater efficiency.

In conclusion, recent AI and fiber optics advancements have revolutionized data center communication. The adoption of fiber optics for short-distance intra-data center communication, along with the integration of silicon photonics, has improved performance and efficiency. However, the thermal impact of 3D integration between electronic and photonic chips poses challenges. The research conducted by the KU Leuven and Imec team highlights the importance of quantifying and mitigating the thermal effects of 3D integration to optimize the performance of integrated heaters in photonic devices.

References

• Coenen, D., Kim, M., Oprins, H., Ban, Y., Velenis, D., Van Campenhout, J., & De Wolf, I. (2023). Thermal modeling of hybrid three-dimensional integrated, ring-based silicon photonic–electronic transceivers. Journal of Optical Microsystems, 4(01). https://doi.org/10.1117/1.JOM.4.1.011004
• SPIE. (2023, December 7). Research investigates thermal impact of 3D stacking photonic and electronic chips. Phys.Org; SPIE. https://phys.org/news/2023-12-thermal-impact-3d-stacking-photonic.html