Investigation and Improvement of Verify-Program in Carbon Nanotube-Based Nonvolatile Memory

Abstract

Carbon nanotube (CNT)-based random access memory (NRAM) cells are measured to investigate cell program at different set current compliances and temperatures. Then, a physical model is proposed to explain the mechanism of cell resistance switching. Specifically, the changes in the NRAM cell tunneling current and resistance can be attributed to the variation of the distance between CNTs. An attraction force ( $F_{\mathrm {attraction}})$ , generated by electrical induction, reduces the distance, whereas a repulsion force ( $F_{\mathrm {repulsion}})$ , generated by phonon-induced temperature, increases the distance. It is proposed that the dominance of these two forces is reversed during set and reset programs, possibly due to the reduction of $F_{\mathrm {repulsion}}$ in set program. Finally, two verify-reset schemes are proposed to improve the NRAM cell verify-program performance. The first proposal, multiple-pulse reset demonstrates 23% program time reduction by skipping a cell resistance read between two successive reset pulses. The second proposal, gate-pulse reset is calculated to decrease more than 40% program energy by reducing bitline charge energy in array program.