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Technical Insight

Magazine Feature
This article was originally featured in the edition:
Issue 1 2023

High-performance monolithically integrated edge couplers

News

Monolithic silicon photonics (SiPh) technology enables the large-scale fabrication of state-of-the art photonics and CMOS devices on the same silicon-on-insulator (SOI) substrate with ultra-low parasitics, allowing to build faster, and more energy efficient transceivers than the conventional hybrid integrated solutions. The realization of scalable, cost-effective and power-efficient optical inputs/outputs (I/Os) on the monolithic SiPh platform remains a significant challenge. In this article, GlobalFoundries researchers report on recent advances in their monolithically integrated high-performance edge coupling solutions, including V-groove-based fiber attach and cavity-based laser attach leveraging hybrid flip-chip bonding.

BY YUSHENG BIAN, TAKAKO HIROKAWA, KOUSHIK RAMACHANDRAN, KAREN NUMMY, KEN GIEWONT AND TED LETAVIC, GLOBALFOUNDRIES

The ever-increasing demand in global data communication is driving the emergence of low-cost, disruptive interconnect solutions that can simultaneously meet the energy consumption, bandwidth and throughput requirements. SiPh based optical interconnection technology has been widely recognized as a key enabler to overcome the power and bandwidth capacity bottlenecks encountered by its electrical counterparts [1].

The well-established complementary metal oxide semiconductor (CMOS) manufacturing infrastructure, along with many intrinsic advantages of Si material such as high refractive index and optical transparency, has rendered SiPh a viable solution towards high-performance, cost-effective, and mass-manufacturable photonic integrated circuits (PICs).

Moreover, the ability to implement wavelength division multiplexing and advanced modulation schemes, in conjunction with high performance electronic components and CMOS-compatible materials onto the SiPh platform has further enabled unprecedented bandwidth scalability and naturally led to complex photonic systems suitable for a variety of applications such as datacom, telecom, automotive LiDAR, artificial intelligence, quantum computing and bio-chemical sensing.

Monolithic SiPh platform
Among the existing SiPh solutions that have been demonstrated so far, monolithic SiPh technology has been identified as one of the most promising candidates to simultaneously meet the scalability, power consumption and cost efficiency demands for the next generation optical interconnection schemes [2]. By enabling the large-scale fabrication of state-of-the art photonics and CMOS devices on the same SOI substrate, monolithic SiPh technology holds great potential for ultra-low parasitic integration [3], and allows to build faster, and more energy efficient transceivers than the conventional hybrid integrated solutions leveraging 2.5D or 3D packaging [4].

Figure 1. Monolithic SiPh platform simultaneously integrating advanced photonic devices and CMOS components on the same SOI substrate.

Fig. 1 shows the cross-section of a representative monolithic CMOS SiPh platform. By leveraging dual Si thicknesses and dual contact modules, this platform enables seamless integration of the best-in-class active and passive photonic devices with high-speed CMOS transistors. The front end of line (FEOL), middle of line (MOL) and back end of line (BEOL) of the stack-up are co-optimized for both photonics and electronics, while satisfying stringent integration requirements and maintaining excellent fabrication fidelity for all the pre-existing devices with varying feature sizes. The RF-friendly BEOL stack is highly attractive for building complex logic circuits, high-Q inductors and low-loss transmission lines, while simultaneously serving as high-quality contact interfaces for advanced active photonic devices.

The incorporation of silicon nitride (SiN) material has further enriched the capability of the monolithic SiPh platform [5]. Benefiting from the optimized stack-up and integration flow, low-loss and robust SiN passive photonic components can be realized, with extended capabilities in nonlinear loss mitigation, high power handling and thermal stabilization. Moreover, the combination of Si and SiN waveguides (WGs) provides additional degrees of design freedom to enable a variety of advanced devices featuring expanded functionalities such as low loss waveguiding, efficient polarization management, compact interlayer transition, high performance edge and vertical coupling with optical fibers, to name a few [6].

Monolithically integrated fiber attach solution
A high-performance optical I/O interface that allows for the efficient coupling with a standard single mode fiber (SMF) or fiber array has been recognized as a key enabler to improve the scalability and cost efficiency of SiPh interconnects. To realize robust and highly efficient fiber to PIC coupling on the monolithic platform, advanced crystallographic etch module and state-of-the-art 300mm manufacturing processes are adopted to form high quality V-grooves with high structural yield that allow high accuracy fiber attach with passive alignment.

To enable efficient coupling between the SMF and edge coupler (EC) while preventing mode leakage into the substrate, the BEOL cladding above the spot size converter (SSC) is replaced with a thick, homogeneous dielectric layer, and the Si substrate underneath is partially removed and filled with index matching liquid during the measurement (See Fig.1 and Figs.2 (a)-(b)).