With the large-scale deployment of AI large models and the continued advancement of high-performance computing, traditional copper interconnects are facing severe "communication bottlenecks" and "power walls." The essence of co-packaged optics (CPO) is to bring the switch ASIC, silicon photonic chips, lasers, and fiber arrays back onto a single packaging substrate. The transmission distance of electrical signals is drastically shortened from the "centimeter scale" to the "millimeter scale," significantly reducing electrical interconnect length and cutting power consumption by 30%–50%, making it a critical path to breaking through these bottlenecks.
Clearly, when optical engines are no longer "plugged" into the switch front panel but instead "cohabitate" with compute chips, the system becomes far more complex, and material selection becomes the decisive factor in whether CPO can reach mass production. In this round of material reconfiguration, silicon nitride (Si₃N₄) stands out as a special player-it plays two distinctly different yet critical roles in CPO: one, as a silicon nitride ceramic substrate, serving as the physical carrier and heat-dissipation base for CPO devices; the other, as a silicon nitride optical waveguide, serving as the optical signal transmission channel inside the photonic chip.
Silicon Nitride Ceramic Substrate: The Preferred Carrier for CPO Packaging
As mentioned above, in the high-density integrated packaging of CPO, optical engines and switch chips are tightly integrated within an extremely small space. System integration is higher, optoelectronic interconnect paths are shorter, and power consumption is lower-all good. However, with so many high-power devices packed together and operating simultaneously, a large amount of heat accumulates. Literature data show that for electronic devices, every 10°C rise in temperature typically reduces effective device lifetime by 30%–50%. If this heat cannot be dissipated in time, it will cause junction temperatures to rise rapidly, leading to performance degradation or even failure.
The packaging substrate is the primary heat-dissipating component. Among traditional materials, organic substrates (such as FR-4) have low thermal conductivity and poor CTE matching, failing to meet CPO's high-power dissipation requirements. Ceramic substrates, especially silicon nitride ceramic substrates, have become the key substrate material for CPO packaging due to their high strength, high thermal conductivity, and CTE matching.
Silicon nitride wins by being "well-rounded."
First, its thermal conductivity is remarkably good. In the early stages of research, the room-temperature thermal conductivity of Si₃N₄ was 20–70 W/(m·K), far lower than that of AlN and SiC, so its thermal performance did not attract much attention. In 1995, this perception changed. Scientist Haggerty, through classical solid-state transport theory, calculated that the low thermal conductivity of Si₃N₄ is mainly due to lattice defects and impurities, and predicted that its theoretical maximum could reach 320 W/(m·K). Thanks to joint efforts in research and industry, the thermal conductivity of silicon nitride ceramics has now surpassed 177 W/(m·K). Compared with traditional resin substrates, which have thermal conductivity below 1 W/(m·K), this is a game-changer.
Second, silicon nitride has a low coefficient of thermal expansion (CTE) of only about 3.2×10⁻⁶/°C, roughly one-third that of Al₂O₃. CPO packages contain multiple materials-silicon chips, III-V compound semiconductor chips (such as InP lasers)-each with different CTEs. When local heat surges, CTE matching directly determines solder joint lifetime under thermal cycling. Silicon nitride's CTE is very close to that of silicon-based materials, so using it as the substrate can greatly alleviate the strain mismatch between different materials during thermal cycles.
Third, silicon nitride's mechanical properties are outstanding. Its elastic modulus is 320 GPa, and its flexural strength is 920 MPa. Such superior mechanical performance means the substrate can be made thinner-silicon nitride can be reduced to 0.32 mm or even 0.25 mm, whereas aluminum nitride, due to brittleness, must be kept at 0.62 mm. The thinner substrate closes the thermal conductivity gap: although AlN has higher thermal conductivity than Si₃N₄, the overall thermal resistance of ultra-thin silicon nitride is essentially on par with that of thicker AlN. Moreover, silicon nitride has greater overall toughness, reducing edge chipping and breakage during industrial batch production, thus improving manufacturing yield.
In terms of cost, silicon nitride is considerably more expensive than alumina but only about 60% the price of aluminum nitride. Combined with TFC (thin-film ceramic) integrated modules formed by one-step co-sintering, overall production costs can be significantly reduced.
Therefore, industry insiders point out that in the entire optical communications industry chain and the CPO integrated packaging niche, silicon nitride is the core substrate material that best aligns with future technology trends, with overall suitability far exceeding that of aluminum nitride.
Silicon Nitride Optical Waveguide-The "Optical Signal Superhighway" of CPO
Unlike its macroscopic role as a packaging substrate, silicon nitride's other core role in CPO is as an optical waveguide material inside the photonic chip, responsible for optical signal transmission, routing, splitting, combining, and other key functions.
This application represents the most essential optical value of silicon nitride in CPO. It is primarily used to fabricate on-chip optical waveguides, microring resonators, optical frequency combs, wavelength-division multiplexers, polarization beam splitters, grating couplers, and all other passive optical devices, forming the complete optical network inside the CPO optical engine for signal distribution, transmission, filtering, and multiplexing/demultiplexing. It is often hybrid-integrated with InP active chips and has become the standard technology roadmap for NVIDIA's next-generation 3.2T CPO optical modules.
Why silicon nitride instead of silicon? Silicon nitride has a refractive index of about 1.98, providing optical field confinement between that of Si waveguides (≈3.4) and silica cladding (≈1.44), making it one of the most promising materials for designing edge couplers with high index contrast and small cross-sections. More importantly, in CPO high-power scenarios, silicon waveguides suffer from two-photon absorption and can burn out, whereas silicon nitride has a wide bandgap and does not suffer from this issue, making it an ideal choice for transmitting high-power optical signals.
In addition, silicon nitride optical waveguide process compatibility opens up heterogeneous integration opportunities. Silicon nitride thin-film deposition processes (LPCVD/PECVD/magnetron sputtering) are mature and CMOS-compatible. Using low-temperature PECVD deposition at process temperatures below 400°C, it does not damage front-end CMOS chips or III-V active chips, allowing monolithic integration directly on wafers, highly compatible with TSMC's COUPE silicon-photonics CPO process platform.
In summary, silicon nitride optical waveguides in CPO play a dual role as the on-chip optical transmission "superhighway" and as the "bridge" for fiber-to-chip optical coupling, making them the core functional material for building high-performance photonic integrated circuits.
Current Status of Technology and Industrialization
Silicon Nitride Substrates:
CETC (China Electronics Technology Group Corporation) has explicitly stated that its CPO ceramic substrates are in the final R&D sprint phase and are expected to reach mass production within three years. Fude Technology's TFC thin-film ceramic substrate products, with their high surface flatness and CTE matching with chips, are positioned as "one of the ideal carrier platforms for CPO packaging." Sinocera Materials disclosed in its investor relations records that its subsidiary, Sinocera Saichuang, has technical reserves for ceramic substrates for optical modules.
Silicon Nitride Optical Waveguides:
Overseas, Solindes Photonics has launched a silicon nitride micro-comb light source adapted for CPO, with a single device capable of outputting 28 wavelength channels and a optical conversion efficiency of 60%. At ISSCC 2026, TSMC unveiled its silicon-photonics CPO platform, adopting silicon nitride waveguides as the standard passive optical waveguide solution. Domestic photonic foundries have already completed PDK development for passive silicon nitride photonics and are gradually introducing samples for 800G and 1.6T CPO verification.