ReRAM – Resistive RAM

ReRAM stores information via the electrical resistance of a thin metal-oxide film embedded between two electrodes – a structure often referred to, due to its simplicity, as a “MIM structure” (metal-insulator-metal). In its as-fabricated initial state, this oxide film is largely insulating and homogeneous. By applying a sufficiently high voltage in the so-called forming step (performed once, at the cell's first activation), oxygen vacancies within the oxide lattice – or, in other material systems, metal ions from one of the electrodes – migrate along the electric field and arrange themselves into a thin, conductive path: a so-called filament, often only a few nanometers in diameter. This filament locally bridges the insulating layer and drastically lowers the cell's resistance at that spot, while the rest of the oxide film remains insulating.

Physically, this is an electrochemical process: the oxygen vacancies act as positively charged defects in the oxide lattice (since a negatively charged oxygen ion is missing at that lattice position) and can be moved in a targeted way by an electric field, much like ion migration in a solid-state electrolyte. This property – resistance change through ion movement rather than charge storage – fundamentally distinguishes ReRAM from flash and places it close to memristive devices.

Filament formation: initial state, set, and reset

Creating or strengthening the conductive filament is called the set operation and switches the cell from the high-resistance to the low-resistance state (“1”). By applying a voltage of reversed polarity, the filament can be partially dissolved again at its weakest point, as the oxygen vacancies migrate back and a thin insulating gap forms locally within the filament – the reset operation, which returns the cell to the high-resistance state (“0”). Notably, reset typically does not dissolve the entire filament but only a short section of it – an energetically more favorable and faster process than rebuilding the filament from scratch.

Since both operations, set and reset, rely purely on the movement of ions or vacancies within the solid and cause no mechanical or electrical stress on a tunnel oxide as flash does, they are in principle faster (switching times in the nanosecond range have been demonstrated in research results) and gentler on the cell than Fowler-Nordheim tunneling in flash.