Two-Stage Buried Power Rail for Tight Cell Spacing

The patent extends the buried power rail formation process to the backside of the substrate, utilizing the third dimension (depth/backside access) to create enlarged BPR structures. Through backside etching and isotropic silicon etching, the BPR is expanded vertically and laterally from the backside, allowing larger critical dimensions without increasing frontside cell-to-cell spacing. This dimensional transition resolves the contradiction by maintaining high integration density while achieving low resistance through three-dimensional BPR structuring.

The buried power rail is divided into two distinct stages: a first stage formed through the substrate thickness with standard dimensions, and a second stage formed from the backside with enlarged critical dimensions. The first BPR section connects to frontside power delivery networks, while the second BPR section provides low-resistance connection to backside power delivery networks. This segmentation allows each stage to be optimized for its specific function, resolving the contradiction between small frontside footprint and large BPR cross-section.

The patent transitions from two-dimensional BPR expansion (lateral only) to three-dimensional expansion by accessing the backside of the substrate. The second BPR stage is formed with larger critical dimensions through backside etching and isotropic silicon etching, which expand the BPR volume without increasing the frontside lateral footprint. This allows reduced resistance through larger BPR cross-section while maintaining tight cell-to-cell spacing on the frontside.

Instead of forming the enlarged BPR section from the frontside (which would require increased cell-to-cell space), the patent inverts the approach by forming the second BPR stage from the backside of the substrate. The backside access allows lateral and vertical expansion of the BPR without interfering with frontside cell layout, effectively inverting the conventional BPR formation sequence and direction to resolve the space-resistance contradiction.

The BPR formation process is segmented into two distinct stages: first BPR formation through the substrate thickness using conventional frontside processes, and second BPR formation from the backside using backside access etching and isotropic silicon etching. Each stage serves a specific function - the first stage provides connection to frontside power networks, while the second stage provides low-resistance connection to backside power networks. This segmentation enables resistance reduction that cannot be achieved with a single stage, while maintaining reasonable process complexity through modular fabrication steps.

The two-stage approach enables three-dimensional BPR structuring that combines frontside and backside formation processes. The first stage creates the initial BPR structure through the substrate, and the second stage adds volume and reduces resistance from the backside. This multi-dimensional approach achieves superior resistance characteristics compared to single-stage processes, while the modular nature of the two stages keeps process complexity manageable.