Review of glass substrates for co-packaged optics: fabrication and performance of integrated electro-optical structures

The development of artificial intelligence and high-performance computing has driven increasing demand for higher aggregate data throughput, single-channel transmission rates, and lower power consumption in data centers. Co-packaged optics (CPO) technology presents a promising solution to these challenges. Glass substrates, owing to their excellent electrical and optical properties, are a highly promising candidate for CPO platforms. This review focuses on glass substrates for large-scale integrated CPO applications. First, the materials of glass substrates are compared: alkali glass for high-performance ion-exchange (IOX) optical waveguides and alkali-free glass for reliable electrical packaging. Next, electrical interconnection processes—through-glass-via (TGV) and redistribution layer (RDL) fabrication—are detailed. TGV+RDL structures achieve bandwidths up to 110 GHz, with RDL line densities exceeding 500 lines/mm at 2 μm line/space. Standard 40 μm diameter TGVs provide densities of 100 to 2500 vias/mm2 at 20 to 130 μm pitch. Furthermore, we examine and compare several manufacturing processes for glass optical waveguides, which are critical for optical coupling in CPO. For optical coupling, various waveguide fabrication methods are analyzed, with IOX being the dominant technique, yielding propagation losses as low as 0.034 dB/cm. Finally, the applications, opportunities, and challenges for glass-based CPO are summarized.