ReRAMs Look To Silicon For Silicon Compatibility

For such a critical material, silicon oxide is not especially well understood. The semiconductor industry certainly understands how to grow high quality oxides with high breakdown voltages, but what happens in less ideal situations? What does the introduction of microstructure do? If there are regions that are oxygen-rich or silicon-rich relative to the stoichiometric SiO₂ composition, how do they behave?

Hard breakdown of silicon oxide implies a catastrophic and permanent failure of the dielectric. In other materials, though, it’s possible to exploit reversible “soft” breakdown behavior to create a memory element — an ReRAM. Typically, the low resistance state in ReRAMs occurs when a conductive filament forms between the top and bottom electrodes of a capacitor. Applying a suitable RESET pulse dissolves the filament, restoring the high resistance state. Other research on ReRAMs has focused on transition metal oxides. In contrast, silicon-oxide-based devices are entirely CMOS-compatible. Silicon oxide deposition uses well known chemistry and mature equipment.

Several years ago, researchers at University College London wondered whether ReRAM behavior was possible in silicon oxide. They found that an applied pulse below the hard breakdown threshold could still break silicon-oxygen bonds. These soft breakdown events created “Frenkel pairs,” consisting of oxygen ion interstitials and oxygen vacancies.⁠[1]

Once Frenkel pairs form, an applied bias can drive the formation of a conductive filament, which a reverse bias can dissolve. That is, they can provide a basis for ReRAM switching behavior.

Having demonstrated the principle, Intrinsic Semiconductor Technologies, founded in 2017, is seeking to develop reliable commercial structures and recently secured funding to support the expansion of their engineering team. On one hand, Tony Kenyon, Intrinsic’s chief scientific officer, explained that ReRAM behavior depends on the controlled introduction of defects.

Maximizing device stability, meanwhile, requires precise control of the oxide layer and the interface to it. For example, (source) found that columnar oxide structures supported more consistent filament formation. Columnar growth, in turn, depends on the roughness of the underlying substrate. The company’s immediate focus, Kenyon said, is on co-optimization of device stability and programmability, with an eye on non-volatile memory applications.

Reference
1. Adnan Mehonic, et. al., “Silicon Oxide (SiO_x): A Promising Material for Resistance Switching?” Adv. Mater. 2018, 30, 1801187. https://doi.org/10.1002/adma.201801187