A path to fully monolithic silicon RF front-ends with 3D sequential integration

CEA-Leti and STMicroelectronics presented their results at IEDM 2025, highlighting key enablers for a new high-performance, versatile RF Si platform that co-integrates active and passive devices for RF and Optical FEM

Their paper outlines the 3D sequential integration of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs), RF SOI switches, and high-quality passives on a single wafer, creating a path to highly integrated, low-parasitic, and cost-efficient systems for next-generation wireless and wireline communications.

The paper, titled “Unlocking high-performance Si RF platforms with SiGe HBT and RFSOI switch technologies”, entails how those high-performance components can be built on the same silicon wafer instead of being packaged or stacked as separate chips. This can be done through 3D sequential integration, which involves layering device types such as SiGe HBT and SOI CMOS switches, without overheating the lower tiers during processing.

Thibaud Fache, lead author of the paper, CEA-Leti, said, “Our team has shown that high-performance SiGe HBT has been conceived to be compatible with the top-tier fabrication steps, and that trap-rich isolation can be localized and thermally robust, and low-temperature (600°C) SOI switches can match state-of-the-art figures of merits. These results pave the way to an all-silicon RF front-end module that is efficient and cost-effective.”

The team demonstrated that a localised trap-rich layer can achieve FR isolation and linearity comparable to those of commercial trap-rich substrates, withstand thermal cycles to 600 °C, and preserve the performance of the underlying SiGe HBT layer.

Extending silicon’s reach of sequential integration into the RF domain enables a future of more energy-efficient and accessible connectivity. Completely integrated RF and optical front ends could simplify manufacturing, facilitating the cost-effective deployment of dense wireless networks necessary for smart cities, autonomous systems, and AI data centres.

Thomas Bordignon, co-author, STMircoelectronics, said, “This joint result demonstrates a credible path from advanced research to manufacturable solutions. By combining CEA-Leti’s sequential integration know-how with ST’s RF technology expertise, we’re enabling the co-integration of state-of-the-art SiGe HBT, switches, and passive devices, that are key elements of the FEM.”

The device’s process flow does not rely on new equipment or costly steps, making it industrially viable due to the relatively relaxed thermal budget constraints of the bottom tier. The team demonstrated high-quality, low-loss RF switches fabricated at 600 °C, retaining the performance of the SiGe HBT below and confirming the feasibility of a fully integrated silicon RF platform.