Low-power flexible resistive random access memory via initiated chemical vapor deposition

Abstract

Resistive Random Access Memory (RRAM) is a two-terminal structure in which an insulating film is inserted between metal electrodes. When a high voltage is applied with the current compliance set to the insulating film, a path through which the current flows is formed inside the insulating film. The RRAM writes and erases via changes of the resistances caused by opening and closing the path while switching the voltages. The RRAM is a non-volatile memory that is very easy to increase memory density due to its very simple structure and can operate at a relatively low voltage. As the absolute amount of information increases exponentially, it is attracting attention as a next-generation memory suitable for the Internet of Things (IoT) era, which requires low power in a battery-limited situation.

On the other hand, initiated Chemical Vapor Deposition (iCVD) uses a gas phase unlike a solution process, thus it is possible to deposit an ultrathin polymer film with both high purity and quality. Therefore, it is regarded as a suitable process to realize polymer-based integration circuits. However, due to the mechanism of the iCVD process, the deposited polymer has very good insulating properties. For this reason, a strong electric field must be applied to the polymer insulating layer to cause electroforming, giving rise to a problem that it is difficult to control the formed current path. Meanwhile, a method of forming a current path with metal diffusions on a polymer insulating layer using copper as an upper electrode has been reported. This report also has a limitation that it is difficult to apply directly to a low-power device because the resistances in the on-state are very low.

In this study, the RRAM based on a polymer thin film formed by copolymerizing the small number of monomers called 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3D3) with monomers called 2-Hydroxyethyl Methacrylate (HEMA) was fabricated and its electrical properties were evaluated. Even under a strong bending radius of 3 mm (1.6 % of tensile stress), the resistances in the on-state were rather high, in the order of tens of thousands of $\Omega$, showing that it is very suitable for low-power devices. In addition, excellent Off characteristics ensured an overall On / off ratio of more than $10^3$. The on/ off states of $10^4$ sec or more were maintained and the number of operations over $10^5$ cycles was recorded. Entirely, the fabricated RRAM showed the applicability as a next-generation memory device to realize the Internet of Things (IoT) very well.