Hierarchical Multi-Mode Computing in Interlayer-Coupled 3D RRAM Crossbar Arrays

This work presents a series-stacked 2-deck RRAM crossbar array with hierarchical multi-mode operation enabled by a shared mid electrode. A 2 × 16 × 16 stacked crossbar array is fabricated, where the mid electrode electrically connects two vertically integrated resistive switching layers, simultaneously serving as the top electrode of the lower layer and the bottom electrode of the upper layer. This architecture enables bias-selective access to independent single-layer operation (1F and 2F) as well as electrically coupled serial operation (1F + 2F) within the same cell footprint. The shared mid electrode plays a key role in controlling voltage distribution and interlayer interaction, expanding the operational space beyond simple density scaling. Using an incremental step pulse with verify algorithm (ISPVA), all modes exhibit stable multilevel conductance modulation up to 6-bit resolution (64 states) with clear state separation and retention exceeding 10⁴ s, while maintaining endurance over 100 switching cycles. System-level evaluation using a VGG-based CNN for CIFAR-10 classification achieves inference accuracies of 93.37%, 93.38%, and 93.37% for the 1F, 2F, and 1F + 2F modes, respectively. The serial configuration also enables logic-in-memory functionality and demonstrates strong physical unclonable function (PUF) characteristics with near-ideal uniformity (∼50%) and high entropy.