  Monolithic three-dimensional integration of memory and logic circuits could
dramatically improve performance and energy efficiency of computing systems.
Some conventional and emerging memories are suitable for vertical integration,
including highly scalable metal-oxide resistive switching devices (memristors),
yet integration of logic circuits proves to be much more challenging. Here we
demonstrate memory and logic functionality in a monolithic three-dimensional
circuit by adapting recently proposed memristor-based stateful material
implication logic. Though such logic has been already implemented with a
variety of memory devices, prohibitively large device variability in the most
prospective memristor-based circuits has limited experimental demonstrations to
simple gates and just a few cycles of operations. By developing a
low-temperature, low-variability fabrication process, and modifying the
original circuit to increase its robustness to device imperfections, we
experimentally show, for the first time, reliable multi-cycle multi-gate
material implication logic operation within a three-dimensional stack of
monolithically integrated memristors. The direct data manipulation in three
dimensions enables extremely compact and high-throughput logic-in-memory
computing and, remarkably, presents a viable solution for the Feynman grand
challenge of implementing an 8-bit adder at the nanoscale.
