Building reliable 3D photonic integrated circuits and cavities at the wafer scale

TL;DR

This paper introduces a scalable fabrication method for reliable 3D photonic integrated circuits with low loss and high uniformity, enabling advanced optical devices and systems.

Contribution

It presents a novel etch-back assisted chemical mechanical polishing process and a $ppa$-engineered taper design to improve wafer-scale 3D PIC fabrication.

Findings

Achieved wafer-scale uniform spacer layer using E-CMP.

Demonstrated a $ppa$-engineered taper with 75% higher reliability.

Realized 3D PICs with low propagation loss and low 3D transition loss.

Abstract

Three-dimensional (3D) photonic integrated circuits (PIC) are emerging as an indispensable scheme for high density and multifunctional photonic systems. However, the wafer-scale scaling of PICs towards a 3D configuration is constrained by two key factors: (i) the trade-off between inter-layer taper efficiency and footprint, and (ii) wafer-scale uniformity of inter-layer transition loss. In this work, we introduce etch-back assisted chemical mechanical polishing (E-CMP) to achieve high wafer-scale uniformity of the spacer layer. Moreover, we break the efficiency-footprint trade-off by demonstrating a novel $\kappa$-engineered taper, achieving a reliability metric that is 75\% higher than the traditional linearly tapered structure. Building on these design and fabrication developments, we enable reliable 3D PICs with typical loss of 0.077 and 0.068 dB/cm on two silicon nitride (SiN) waveguide layers and typical 3D transition loss as low as 6 mdB. Furthermore, the low 3D transition loss enables the first class of 3D high-Q optical cavities occupying two distinct device layers, providing new design space for high-Q optical cavities. The scalable fabrication process and design methodology provide routes for wafer-scale reliable 3D PICs that are promising in a series of applications ranging from photonic interconnects and computing networks to high-density photonic sensors and nonlinear photonics.