Lossless, Non-Volatile Post-Fabrication Trimming of PICs via On-Chip High-Temperature Annealing of Undercut Waveguides

TL;DR

This paper introduces a novel, CMOS-compatible high-temperature annealing method for permanently trimming photonic integrated circuits, effectively compensating manufacturing deviations with high precision and stability, without additional processing steps.

Contribution

The authors present the first demonstration of a nonvolatile, high-temperature thermal treatment for post-fabrication trimming of PICs that is simple, energy-efficient, and compatible with existing CMOS processes.

Findings

Achieved a permanent refractive index reduction of 0.0173 in silicon waveguides.

Demonstrated high-resolution tuning of up to 5.25 bits across broadband spectrum.

Maintained stability of the trimmed state for over 218 days.

Abstract

Limited by equipment precision, manufacturing deviations in waveguide width, etch depth, and layer thickness inevitably occur in photonic integrated circuits (PICs). These variations cause initial phase errors, compromising the reliability of phase-sensitive devices such as Mach-Zehnder Interferometers (MZI) and microring resonators. To overcome this, we report a nonvolatile, near-lossless post-trimming method utilizing sufficient high-temperature thermal treatment for undercut waveguides, reported here for the first time to the best of our knowledge. This CMOS-compatible approach requires no additional processes or equipment, enables simple electrical heating for trimming, and retains long-term stability after high-temperature removal, ensuring high energy efficiency. Transmission electron microscopy indicates that high-temperature thermal treatment induces irreversible lattice expansion in silicon waveguides, leading to a reduction in the real refractive index and enabling compensation for process errors. Experimental results using MZIs confirm a permanent refractive index reduction of 0.0173 and high-resolution tuning up to 5.25 bits, effective across a broadband spectrum and stable for over 218 days after final trimming. Furthermore, 15 MZIs on a single wafer are precisely calibrated to BAR, CROSS, or orthogonal states, demonstrating the method universality. This practical and scalable technique enables reliable post-fabrication trimming for next-generation low-cost, energy-efficient PIC applications such as optical switches and optical computing.