The Thermal Impact of 3D Integration on Silicon Photonics Explored
Researchers investigate the efficiency loss of ring modulators in 3D photonic-electronic integration
As AI models and large language models like ChatGPT continue to advance, the strain on data centers becomes more apparent. These models require massive amounts of data to train, necessitating efficient communication links between processing units and memory. While fiber optics has long been the solution for long-distance communication, the industry is now adopting it for short-distance intra-data center communication as well. This shift is driven by the superior performance of fiber optics compared to classical electrical links. With recent technological developments, the industry is now exploring the possibility of using optical interconnects for communication between chips within the same package.
To achieve this, data streams must be converted from the electrical to the optical domain using optical transceivers. Silicon photonics has emerged as the leading technology for fabricating these transceivers. However, the active photonic devices within the chip, such as modulators and photodetectors, still require connections with electronic drivers for power and data transmission. To address this, researchers have turned to 3D stacking technology, which allows for the integration of electronic and photonic chips with low parasitic capacitance.
Investigating the Thermal Impact
In a recent study published in the Journal of Optical Microsystems, a research team from KU Leuven and imec in Belgium delved into the thermal impact of 3D integration on silicon photonics. The team focused on the design of photonic chips, specifically an array of ring modulators known for their temperature sensitivity. These modulators require active thermal stabilization to operate effectively in demanding environments like data centers. Integrated heaters were employed to achieve this thermal stabilization, with a focus on minimizing power consumption for energy efficiency.
Measuring Efficiency Loss
To gauge the impact of 3D integration on heater efficiency, the researchers conducted experiments before and after flip-chip bonding of the electronic chip (EIC) on the photonic chip (PIC). The results revealed a significant relative loss of -43.3% in efficiency after bonding. Further analysis through 3D finite element simulations attributed this loss to heat spreading in the EIC. Ideally, all heat generated in the integrated heater should be contained close to the photonic device. The increased heat spreading observed after bonding the EIC led to a rise of up to +44.4% in thermal crosstalk between the photonic devices, complicating individual thermal control.
Minimizing Thermal Impact
Understanding the thermal impact of 3D photonic-electronic integration is crucial, as is finding ways to prevent efficiency loss in heaters. To address this, the researchers conducted a thermal simulation study, exploring various design variables to increase heater efficiency. The study revealed that by increasing the spacing between µbumps and the photonic device and decreasing the interconnect linewidth, the thermal penalty of 3D integration can be minimized.
Conclusion:
The integration of electronic and photonic chips through 3D stacking technology offers numerous advantages in terms of performance and compactness. However, this study highlights the importance of considering the thermal impact of such integration. The research conducted by the team from KU Leuven and imec sheds light on the efficiency loss experienced by ring modulators in 3D photonic-electronic integration and provides insights into mitigating this issue. By optimizing design variables, it is possible to minimize the thermal penalty and enhance the efficiency of heaters in silicon photonic-electronic transceivers. As the demand for AI and large language models continues to grow, addressing the thermal challenges associated with 3D integration will be crucial to ensure the continued advancement and efficiency of data centers.