Montesi, Luca; (2018) Novel electrical and chemical findings on SIOx-based ReRAM devices. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Existing non-volatile flash memory technologies are characterised by slow access time, high power consumption and a quickly approaching scaling limit. Filamentary resistive RAM (ReRAM) is an emerging type of storage device that relies on the electrically driven change in resistance of a thin film sandwiched between two electrodes. The active region is often a binary oxide that develops a restorable conductive filament thanks to the electrically driven movement of oxygen. This technology offers potential sub-10 nm scalability, nanosecond programming with direct overwriting (unlike FLASH) and an appealing sub pJ/bit power consumption (compared to nJ/bit of FLASH). In this thesis, metal-insulator-metal ReRAM devices with a TiN/SiOx/TiN structure are used. While other binary oxides have been used in the literature, SiOx must be used in its amorphous form allowing for easier fabrication, and is an extremely well-studied material as its CMOS compatibility dates back 40 years. Using the above devices, it was possible to observe data storage performance comparable to the one of other types of ReRAM. More interestingly, it was observed that the resistance states of this family of devices may be programmed using nanosecond pulses of identical magnitude, possibly leading to simple programming circuits. Consequently, it is shown that this programming method may also be used to gradually increase or decrease the device resistance state as well as have devices enter states that relax over time. These types of behaviour mean that SiOx devices may be used in neuromorphic networks that require components whose behaviour resembles the one of the neuronal synapsis or the mammalian brain’s forgetting process. The literature reports on endurance-hindering electrode deformation phenomena during the operation of oxide-based ReRAM devices. A residual gas analyser (RGA) was used to detect that oxygen species are emitted during operation and therefore confirmed that such phenomena are caused by oxygen emission. Using SIMS (secondary ion mass spectroscopy) analysis on devices switched in atmospheres containing isotopically labelled oxygen, it was observed that, under deformed regions, it is possible to find incorporated atmospheric oxygen. Additionally, reducing atmospheric pressure had negative impact on device reliability. SiOx-based filamentary ReRAM is a strong candidate in the search for alternatives to flash memory. Moreover, these devices display behaviour that may be useful in applications trying to emulate the mammalian brain. Having observed device dependence on its atmosphere, endurance issues may now be addressed using electrodes capable of either adsorbing oxygen without bubbling or letting it go through without cracking.

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