edfas.org 27 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 3 where the waveguide is broken up into a metamaterial over a certain length. This way, the optical index of the waveguide is increased gradually, and reflections are minimized.[10] The metamaterial consists of Si blocks, which gradually increase in size, then form a contiguous structure and finally end up as the waveguide. Figure 7 shows 3D STEM imaging (STEM electron to- mography) of a spot size converter at the transition point between single Si blocks and a contiguous structure. Advanced imaging techniques like these can help the development team find design improvements of the optical device and can eliminate reflections and mitigate insertion loss. CONCLUSION Silicon photonics is growing steadily and is now in volume production on 300 mm wafers. It promises very high bandwidth information transfer at relatively low power consumption. Photonic failure analysis has plenty of new challenges to overcome. PICs need to be optimized for both electrical and optical performance, which requires new processes and thus introduces new failure modes. Most importantly, optical FA needs to develop new techniques, especially in the area of fault isolation. Finding the location where the optical power is lost or where scattering diminishes the optical performance is of crucial importance. With the growing presence of photonic circuits, failure analysis in the electro-optical domain promises to become exciting and a highly rewarding field to work in. ACKNOWLEDGMENTS The authors would like to thank William Taylor for fruitful and insightful discussions. REFERENCES 1. V. Gupta: “GlobalFoundries Silicon Photonics,” Consortium for On-Board Optics (COBO), September 1, 2020. 2. https://www.yolegroup.com/product/report/silicon-photonics-2022, accessed April 4, 2023. 3. M. Ragowski, et al.: “45nm CMOS - Silicon Photonics Monolithic Technology (45CLO) for Next-generation, Low Power and High Speed Optical Interconnects,” T3H.3, 2020 Optical Fiber Communications Conference (OFC), 6-10 March 2020. 4. Y. Bian, et al.: “Hybrid III-V Laser Integration on a Monolithic Silicon Photonic Platform,” M5A.2, 2021 Optical Fiber Communications Conference (OFC), 6-10 June 2020. (continued on page 30)
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