Leti, ST Enable Monolithic RF Modules

At IEDM, CEA-Leti and STMicroelectronics described a path towards monolithic RF front-end modules

By Mark LaPedus

At the IEEE International Electron Devices Meeting (IEDM) in San Franciso, CEA-Leti and STMicroelectronics presented a paper that described a path towards the development of a monolithic silicon RF front-end module (FEM) using 3D sequential integration.

Basically, an FEM integrates various RF chips and components in a module. An FEM handles the RF functions in wireless and wireline systems. For example, a smartphone incorporates an FEM as well as a processor, memory, CMOS image sensors and many other chips.

3D sequential integration consists of a process, where you stack devices on top of each other in a sequential manner. CEA-Leti is a French R&D organization, while STMicroelectronics is a Geneva, Switzerland-based chip vendor.

The paper from Leti and STMicroelectronics detailed the 3D sequential integration of silicon-germanium (SiGe) heterojunction bipolar transistors (HBT), RF SOI switches and passives on a single wafer.

The paper, entitled “Unlocking High-Performance Si RF Platforms with SiGe HBT and RFSOI Switch Technologies,” described how those high-performance components can be built directly on the same silicon wafer rather than packaged or stacked as separate chips. This can be achieved through 3D sequential integration without overheating the lower tiers during processing.

“This work presents the key enablers for a new high-performance and versatile RF silicon platform cointegrating the best-in-class active and passive devices used in an RF FEM,” said CEA-Leti’s Thibaud Fache, the lead author of the paper.

“It relies on the 3D sequential integration of a standalone SiGe HBT and a local trap rich (bottom tier) with the CMOS and RF switch (top tier) and the back-end-of-the line (for routing and passive devices),” Fache said in the paper.

“Finally, the state-of-the-art performance is demonstrated for the LT (600°C) PDSOI switch exhibiting RON × COFF= 96fs and RFV max=2.7V. These results validate the feasibility of the envisioned platform, which is the integration pinnacle of an RF Si monolithic front-end module,” Fache 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,” Fache added. “These results pave the way to an all-silicon RF front-end module that is efficient and cost-effective.”

“This joint result demonstrates a credible path from advanced research to manufacturable solutions,” said ST’s Thomas Bordignon, co-author of the paper. “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.”