OFC 2025: TSMC's Silicon Nitride (SiN) Progress on Its Silicon Photonics Platform

🧩 Architecture and integration concept

• TSMC introduced a compact universal photonic engine (CUPE) that supports short-reach, high-speed electro-optical interconnect (EIC to PIC).

• Integrated components include:

  • Low-loss silicon grating couplers

  • Silicon nitride H-couplers (expanding I/O options)

🔧 SiN process and optimization strategy

• Uses a low-temperature PECVD process to deposit silicon nitride, avoiding thermal damage to the high-performance silicon active devices built in the front-end process.

• Module-level optimization of the PECVD process improves thickness and refractive-index uniformity:

  • 3σ thickness variation: 2.3%

  • 3σ refractive-index variation: 0.2%

• After optimization, MZI channel drift is reduced, which benefits filter applications (e.g., MZI-based mux/demux).

📉 SiN waveguide and component loss performance

Component type	Loss before optimization (dB/cm)	Loss after optimization (dB/cm)
Single-mode waveguide	~0.4	0.16
Multimode waveguide	~0.4	0.12
Baseline: LPCVD	Higher even without optimization	-

📐 Special component design and performance

• Bends:

  • Conventional bend loss is high because of the low index contrast.

  • An advanced design reduces bend radius and scattering loss: 0.005 dB / 30 µm radius

• Other passive components:

  • Taper: < 0.005 dB

  • Crossing: ~0.05 dB, crosstalk < -50 dB

  • 1x2 MMI: ~0.1 dB, bandwidth > 18 nm

🔄 SiN-to-Si waveguide transition

• Main challenge: a large TM-mode index mismatch, causing high loss and reflection.

• Solution: locally thinning the silicon (from 270 nm) to improve TE/TM mode index matching.

  • Result: insertion loss ~0.05 dB (TE / TM)

↔️ Edge coupler design

• Uses a small tip (< 200 nm), a self-aligned structure and substrate removal.

• Performance:

  • TE/TM insertion loss < 1 dB

  • PDL < 0.2 dB

  • 3σ variation: 0.3–0.5 dB

  • Bandwidth > 100 nm

  • Supports FAUs with up to 8 channels

🧰 PDK status

• A SiN component library has been established, including:

  • Single-mode / multimode waveguides

  • Bends, tapers, crossings

  • 1x2 MMI, edge coupler, SiN-Si transition

🔚 Summary

• Low-loss, highly uniform SiN components have been successfully integrated into TSMC's silicon photonics platform.

• Without affecting the performance of silicon active devices.

• Promising for high-speed datacom, especially for flexible and diverse optical I/O needs.

Slides from the presentation follow below