Synaptic Characteristics and Vector-Matrix Multiplication Operation in Highly Uniform and Cost-Effective Four-Layer Vertical RRAM Array

This study implements a highly uniform 3D vertically stack resistive random-access memory (VRRAM) with a four-layer contact hole structure. The fabrication process of a four-layer VRRAM is demonstrated, and its physical and electrical properties are thoroughly examined. X-ray photoelectron spectroscopy and transmission electron microscopy are employed to analyze the chemical distribution and physical structure of the VRRAM device. Multilevel capability, reliable endurance (>10⁴ cycles), and retention (10⁴ s) are successfully obtained. Synaptic memory plasticity, such as spike time-dependent plasticity, spike rate-dependent plasticity, excitatory post-synaptic current, paired-pulse facilitation, and long-term potentiation and depression is presented. Finally, the vector-matrix multiplication (VMM) operation is conducted on a 4 × 12 VRRAM array, according to the low resistance state ratio. It is ascertained that the accuracy drop, which can occur due to VMM error, can be limited to a decrease of less than 0.44% point. Utilizing the high-density, multilevel, and biological characteristics of VRRAM, it is possible to implement high-performance neuromorphic systems that require densely integrated synaptic devices.