The Last Level Cache (LLC) is the processor’s critical bridge between on-chip and off-chip memory levels - optimized for high density, high bandwidth, and low operation energy. To date, high-density (HD) SRAM has been the conventional device of choice; however, with the slowing of transistor scaling, as reflected in the industry’s almost identical HD SRAM cell size from 5 nm to 3 nm, alternative solutions such as 3D stacking with advanced packaging (i.e., hybrid bonding) are pursued (as demonstrated in AMD’s V-cache). Escalating data demands necessitate ultra-large on-chip caches to decrease costly off-chip memory movement, pushing the exploration of device technology towards monolithic 3D (M3D) integration, where transistors can be stacked in the back-end-of-line (BEOL) at the interconnect level. M3D integration requires fabrication techniques compatible with a low thermal budget (seconds) when used in a gain-cell configuration. This paper examines device, circuit, and system-level tradeoffs made when optimizing BEOL-compatible AOS-based 2-transistor gain cells (2T-GC) for LLC. A cache early-exploration tool, NS-Cache, is developed to model caches in advanced 7 & 3 nm nodes and is integrated with the Gem5 simulator to systematically benchmark the impact of the newfound density/performance when compared to HD-SRAM, MRAM, and 1T1C eDRAM alternatives for LLC.

Digital compute-in-memory (DCIM) has emerged as a promising solution to address scalability and accuracy challenges in analog compute-in-memory (ACIM) for next-generation AI hardware acceleration. In this work, we present a comprehensive device-to-system codesign process for the two proposed 3-D DCIM architectures at the projected 5 angstrom (A5) complementary FET (CFET) technology node: 1) 3-D DCIM based on 8T DCIM bit cell and 2) lookup-table (LUT)-based 3-D DCIM. A novel A5 CFET-based 8T DCIM bit cell (6T SRAM +2T AND gate) is proposed to improve total footprint and latency over the conventional 10T DCIM bit cell, and its functionality is verified through technology computer-aided design (TCAD) simulation. For macro- and system-level evaluation of the proposed 3-D DCIM architectures, an extended NeuroSim V1.4 framework is developed, the first compute-in-memory (CIM) benchmark framework enabling CIM simulation at the A5 CFET technology node. We demonstrate that the proposed 3-D DCIM with 8T DCIM bit cell at the A5 CFET technology node can achieve$8.2\times $improvement in figure of merit (FOM) (=TOPS/W$\times $TOPS/mm2) over the state-of-the-art 3-nm FinFET-based DCIM design. The LUT-based 3-D DCIM design is additionally proposed to achieve further power consumption reduction from the 8T DCIM bit-cell-based 3-D DCIM. LUT-based 3-D DCIM achieves a 44% reduction in energy consumption compared to the conventional 10T DCIM bit-cell-based 3-D DCIM. Our findings suggest the significant implications for technology scaling below 1 nm in high-performance DCIM design.