Researchers have developed an ultralow-loss photonic integrated circuit (PIC) platform utilizing germano-silicate. This new platform, known for its high performance in optical fibers, is now manufactured using a fully CMOS-foundry-compatible process.
Background
Operating at shorter wavelengths (400–1,100 nm) in photonic applications typically faces increased waveguide losses due to surface Rayleigh scattering and rising absorption. Materials like silica and germano-silicate are effective for short-wavelength operation in optical fibers due to low absorption. However, their integration into planar photonic circuits has been challenging due to fabrication complexities or reliance on suspended geometries.
Fabrication Process
The fabrication process for these germano-silicate PICs is designed to be fully CMOS-compatible. It involves depositing a 4-μm-thick germano-silica layer (25 mol% GeO2) onto a 15-μm-thick thermal oxide layer on a silicon wafer using plasma-enhanced chemical vapor deposition (PECVD) at approximately 270 °C. This low-temperature deposition ensures an anneal-free thermal budget for the entire process.
Ridge waveguides are patterned using ruthenium and silica hard masks, deep-ultraviolet (DUV) lithography, and inductively coupled plasma (ICP) etching. A furnace annealing step is typically applied to reflow waveguide sidewalls, smoothing roughness and reducing scattering losses, while the underlying thermal oxide remains unaffected. An optional upper cladding layer, either phosphorus-doped silica or ICP-PECVD silica, can be added post-annealing to ensure acoustic confinement or device protection. DUV-stepper lithography is used for precise patterning, critical for dispersion engineering.
Key Results and Discussion
The germano-silicate PICs exhibit record-low waveguide propagation losses from violet to telecom bands. Resonator Q factors exceed 180 million across this spectrum, peaking at 463 million at 1,064 nm, which translates to a waveguide loss of 0.08 dB m−1. This loss figure is comparable to early low-loss optical fibers. The platform achieved a loss of 0.49 dB m−1 at 458 nm, representing a 13-dB improvement over previous records in the visible and short-NIR ranges.
Beyond loss performance, the platform demonstrates material and structural advantages:
- Dispersion Engineering: DUV-stepper-defined waveguides enabled soliton microcomb generation.
- Acoustic Mode Confinement: Verified through stimulated Brillouin scattering (SBS) gain spectrum characterization. Integrated germano-silicate resonators produced a high-coherence Brillouin laser with a lasing frequency shift of 9.68 GHz.
- Large Mode Area (LMA): Enhances hybrid-integrated low-noise lasers. The LMA (28.06 μm2 for Ge-silica) significantly suppresses thermal refractive noise (TRN). Self-injection locking (SIL) of semiconductor diode lasers with these microresonators reduced frequency noise by 46 dB, achieving Hz-level fundamental linewidths. SIL of commercial Fabry–Pérot diode lasers resulted in fundamental linewidths of 15 Hz at 632 nm, 12 Hz at 512 nm, and 90 Hz at 444 nm.
Conclusion
The germano-silicate ultralow-loss platform represents a significant advance in integrated photonics, achieving over 10 dB improvement in quality factor in the violet wavelength range and for anneal-free processing. It features engineered dispersion, acoustic mode confinement, and thermal stability. The platform achieves ultralow losses without post-processing thermal annealing in the telecom band, offering a 10-fold reduction in anneal-free waveguide loss over prior records, facilitating heterogeneous integration. This technology aims to enable fiber-like loss on a chip for applications such as optical clocks and quantum sensors.