Layout Considerations of Logic Designs Using an N-layer 3D Nanofabric Process Flow

In the past few years, novel fabrication schemes such as parallel and monolithic 3D integration have been proposed to keep sustaining the need for more powerful integrated circuits. By stacking several devices, wafers, or dies, the footprint, delay, and power can be decreased when compared to traditional 2D implementations. While parallel 3D does not enable very fine-grained vertical connections, monolithic 3D currently only offers a limited number of transistor tiers due to the high cost of the additional masks and processing steps, limiting the benefits of using the third dimension. In this paper, we introduce an innovative planar circuit netlist and layout approach, which enables a new 3D integration flow called 3D Nanofabric. The flow, consisting of N identical vertical tiers, is aimed at single instruction multiple data processor Arithmetic Logic Units (ALUs). By using a single metal routing layer for each vertical tier, the process flow is significantly simplified since multiple vertical layers can potentially be patterned at once, similar to the 3D NAND flash process. In our study, we thoroughly investigate the layout constraints arising from the Nanofabric flow and the unique metal layer rule and propose several ways to overcome them. We then show that by stacking 32 layers to build a 32-bit ALU, the footprint is reduced by 8.7x when compared to a conventional 7nm FinFET implementation.