Overcoming interconnect obstacles with co-packaged optics (CPO)

Over the last few years, there has been growing interest across the global semiconductor packaging industry with a new approach. Co-packaged optics (CPO) involves integrating optical fibers, used for data transmission, directly onto the same package or photonic IC die as semiconductor chips.

Traditionally, semiconductor packaging has used copper interconnects, but these can consume large amounts of power and lead to signal weakening at high frequencies when the distance is further than a couple of meters.

With CPO, the optical components are integrated directly into a package, and the long copper trace between the switch and the optical module is replaced with short, high-integrity connections. Optical signaling uses far less power at high data rates than electrical signaling. As CPO reduces the distance between optical components and the semiconductor dice, this lowers latency, improves high-speed signal integrity, and accelerates data transfer.

All of which are fundamental for the next generation of AI devices for high performance computing (HPC) inside the data center systems. Nevertheless, there are obstacles that need to be overcome with CPO and when designing photonic packages, especially for integrated photonic circuits or photonic chips. This is why advances in photonic package design are coming to the forefront.

Overcoming CPO obstacles

When co-packaging photonics with electronics, there can be signal integrity issues. Electrical crosstalk must be reduced to improve signal quality. Using short interconnects and low-parasitic layouts are the most appropriate tactics when used alongside co-design tools for optical optimization. Signal integrity can be ensured without requiring complex routing or more space, as optical interconnects can support multi-terabit-per-second data rates over long distances with only minor signal loss.

Mounting a large photonic IC die onto a laminate or organic substrate can be problematic. Due to the coefficient of thermal expansion (CTE) mismatch between the substrate and the photonic IC die, non-negligible die warpage may occur. This warpage can significantly degrade optical signal performance in the photonic IC waveguides during data transmission, leading to substantial reductions in optical signal power and quality.

In addition, excessive warpage may introduce mechanical stress in the photonic IC die, altering its material properties and further impacting optical performance. While using a ceramic substrate could mitigate these issues, it’s more costly and is not widely adopted today.

Dealing with temperature variations can be a concern with photonic devices, but efficient thermal management and thorough thermal design can help to improve performance and reliability. Integrating photonics with electronics may require thermoelectric coolers (TECs) and heat sinks along with smart thermal simulations throughout the design process.

Sub-micron alignment is also a complex technical task. Optical misalignment can lead to significant insertion losses, as well as disrupting device performance. Leveraging passive alignment techniques with etched features or alignment markers may mean lower levels of accuracy, but this is the lowest cost. Active alignment, using real-time optical feedback, results in better performance and efficiency, though it’s far more complex and costly.

Addressing challenges when testing optical components involves using built-in test waveguides, automated optical probing systems, and standardized test procedures during and after packaging. Integrating optical and electrical components into a single package not only makes the manufacturing process more complicated, the associated risks and costs are also greater due to the different assembly phases. It’s possible to cut through the complexity and improve yields by using standardized processes for CPO assembly.

The future of CPO and photonic package design

As a result of the growing interest in CPO and photonic packaging, there have been advances in photonic package design. CPO enables faster data transmission and improved power-efficiency when compared to the conventional copper-based interconnects approach. It has many advantages, including high-speed communication and lower power consumption, but there are also concerns related to signal integrity, thermal management, optical alignment, and costs.

Advances in photonic package design can overcome these obstacles and help electronic design engineers create new architectures that would not be viable with traditional semiconductor packaging. As the semiconductor industry continues to rapidly evolve, with more complex devices requiring high-performance, compact and power-efficient chips, CPO with advanced photonic package design will become increasingly important.

Dr Larry Zu is CEO of Sarcina Technology.

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