A Holistic Evaluation of Buried Power Rails and Back-Side Power for Sub-5 nm Technology Nodes

Buried power rail (BPR) and back-side power delivery grid have been proposed as solutions to scaling challenges that arise beyond the 5-nm technology node, mainly to lower IR drop and further shrink area. This article demonstrates a holistic evaluation of this technology and its variants at the microprocessor level. This is carried out by taking an Arm Cortex-A53 design through the standard-VLSI physical design implementation flow on Imec’s iN6 node, equivalent to the industry 3-nm technology node, which features the buried power technology. The power, performance, area, on-chip IR drop, and off-chip voltage droop metrics are benchmarked, and implications on power gating are explored. An extensive Design-Technology-Co-Optimization (DTCO) study of the back-side power grid is presented to enhance the decoupling capacitance by sweeping associated technology parameters showcasing further optimization opportunities in manufacturing. The conclusions of this work highlight that the front-side (FS) power delivery network (PDN) with buried rails achieves a 25% lower on-chip IR drop and 17% lower off-chip voltage droop (power supply noise) resulting in 21% lower guard band voltage. On the other hand, the back-side power grid with BPRs achieves 85% lower on-chip IR drop and 30% off-chip voltage droop resulting in 60% lower guard band voltage. In addition, the impact of BPRs, and back-side power grids on power gated designs are evaluated.