Resistive Random-access Memory - Mechanism

Mechanism

The basic idea is that a dielectric, which is normally insulating, can be made to conduct through a filament or conduction path formed after application of a sufficiently high voltage. The conduction path formation can arise from different mechanisms, including defects, metal migration, etc. Once the filament is formed, it may be reset (broken, resulting in high resistance) or set (re-formed, resulting in lower resistance) by an appropriately applied voltage. Recent data suggest that many current paths, rather than a single filament, are probably involved.

A memory cell can be produced from the basic switching element in three different ways. In the simplest approach, the single memory element can be used as a basic memory cell, and inserted into a configuration in which parallel bitlines are crossed by perpendicular wordlines with the switching material placed between wordline and bitline at every cross-point. This configuration is called a cross-point cell. Since this architecture can lead to a large "sneak" parasitic current flowing through non selected memory cells via neighboring cells, the cross-point array may have a very slow read access. A selection element can be added to improve the situation, but this selection element consumes extra voltage and power. A series connection of a diode in every cross-point allows to reverse bias, zero bias, or at least partial bias non selected cells, leading to negligible sneak currents. This can be arranged in a similar compact manner as the basic cross-point cell. Finally a transistor device (ideally a MOS Transistor) can be added which makes the selection of a cell very easy and therefore gives the best random access time, but comes at the price of increased area consumption.

For random access type memories, a transistor type architecture is preferred while the cross-point architecture and the diode architecture open the path toward stacking memory layers on top of each other and therefore are ideally suited for mass storage devices. The switching mechanism itself can be classified in different dimensions. First there are effects where the polarity between switching from the low to the high resistance level (reset operation) is reversed compared to the switching between the high and the low resistance level (set operation). These effects are called bipolar switching effects. On the contrary, there are also unipolar switching effects where both set and reset operations require the same polarity, but different voltage magnitude.

Another way to distinguish switching effects is based on the localization of the low resistive path. Many resistive switching effects show a filamentary behavior, where only one or a few very narrow low resistive paths exist in the low resistive state. In contrast, also homogenous switching of the whole area can be observed. Both effects can occur either throughout the entire distance between the electrodes or happen only in proximity to one of the electrodes. Filamentary and homogenous switching effects can be distinguished by measuring the area dependence of the low resistance state.