A Silicon-Embedded Transformer for High-Efficiency, High-Isolation, and Low-Frequency On-Chip Power Transfer

In this brief, a backside silicon-embedded transformer (BSET) with an improved isolation structure is proposed and demonstrated. The interleaved transformer coils are embedded inside a silicon substrate from the backside and connected to the front-side through vias. The isolation between the coils is achieved by the oxide layer between the Cu coil and the Si substrate, as well as the BCB (BenzoCycloButene) layer covering the backside of the transformer. The 2-mm² BSET fabricated shows a best reported monolithic transformer efficiency of over 80% at a low frequency of 20 MHz. A 380 V isolation capability is achieved and shows the potential for various applications, such as USB (Universal Serial Bus) isolation. Only three masks are required for the fabrication. This technology is very suitable for on-chip isolated power transfer applications.