3D-Integrated SRAM Components for High-Performance Microprocessors

3D integration is an emergent technology that has the potential to greatly increase device density while simultaneously providing faster on-chip communication. 3D fabrication involves stacking two or more die connected with a very high density and low-latency interface. The die-to-die vias that comprise this interface can be treated as regular on-chip metal due to their small size (on the order of 1 mum) and high speed (sub-FO4 die-to-die communication delay). The increased device density and the ability to place and route in the third dimension provide new opportunities for microarchitecture design. In this paper, we focus on the 3D-integrated designs of SRAM structures. We show that the dense die-to-die vias enable 3D-integrated SRAM components that are partitioned at the level of individual wordlines or bitlines. This results in a wire length reduction within SRAM arrays, and a reduction in the area footprint, which reduces the wires required for global routing. The wire length reduction provides simultaneous latency and energy reduction benefits, e.g., 47 percent latency reduction and 18 percent energy reduction for a 4 MB 4-die stacked 3D SRAM array. A 3D implementation of a 128-entry multiported SRAM array achieves a 36 percent latency improvement with a simultaneous energy reduction of 55 percent. As planar designs adapt high-performance techniques such as hierarchical wordlines to improve performance, 3D integration provides even larger benefits, making it a desirable technology for high-performance designs. For the 4 MB SRAM array, the 3D-integrated designs provide additional latency reduction benefit over the planar designs when hierarchical wordlines are implemented in both planar and 3D designs.