Local laser annealing for amorphous/polycrystalline silicon hybrid photonics on CMOS

Deposited photonics represents a promising avenue for monolithic back-end integration on CMOS, yet encounters challenges in simultaneously enhancing waveguide loss and modulation dynamics. In this paper, a novel amorphous/polycrystalline hybrid scheme for deposited silicon photonics on CMOS was proposed, which utilizes mask-assisted local laser annealing to crystallize the active region of low-loss amorphous silicon (α-Si) PICs only into high-mobility polycrystalline silicon (poly-Si). The feasibility of key techniques such as laser annealing of α-Si thin films, laser activation of doping ions, and mask-assisted local laser annealing of photonic devices is validated. A comparative study between excimer laser annealing and solid-state laser annealing of α-Si is conducted, examining the impacts of pre-dehydrogenation, doping, etching depth, laser pulse energy density, and pulse number. During mask-assisted laser annealing the necessity of a buffer layer between the mask and the α-Si to prevent metal contamination is highlighted. The mask-assisted local laser annealing technique effectively mitigates the optical loss increase by ∼140 dB/cm typically associated with laser crystallization in a α-Si racetrack resonator and reduces the coupling loss in grating couplers by ∼8 dB/pair. Mask-assisted laser annealing not only facilitates high-yield wafer-level active deposited photonics but also allows for leveraging the strengths of both α-Si and poly-Si within a single photonic integrated circuit. This work provides technological insights and valuable guidance for the development of high-performance deposited silicon photonics.