Finite Element Modelling of Electroformation and Switching Mechanisms in TaOx-based Memory Device

As human civilization has entered its 21^st century, the enhancement of computational power is high in demand and has presented great challenges for researchers in different fields of study. Thanks to its capability of processing-in-memory (PiM), transition metal oxide based Resistive Random Access Memory (ReRAM) devices become one of the most promising candidates for accelerating artificial intelligence algorithms and lowering power consumption. The performance of ReRAM devices, such as cyclability and variability, still falls short of the necessary requirements. Most of these issues are strongly related to the ion migration in the electroforming and switching process. The work described here is to advance a fundamental understanding of ion motion during electroforming and switching in TaO_x ReRAM devices. In this thesis, a metalinsulator-metal (MIM) inverted via structure using TaO_x material was fabricated, tested, and characterized to gain insight into ion motion in this system.

The thesis comprehensively examines the relationship between the electrical conductivity of TaO_x and temperature as well as composition by the transmission line method (TLM). Subsequently, the thesis offers direct evidence of substantial lateral redistribution of cations through in-operando scanning thermal microscopy (SThM) and atomic force microscopy (AFM). A localized compressive stress buildup in the filament area during electroformation was evidenced and attributed to the aggregation of Ta ions. In the memory switching process, the switching is achieved by vertical ion drifting by the applied electric field. The hottest spot is always found at the resistive gap in the filament area. Lastly, a finite element model (FEM) considering Fick’s diffusion, thermodiffusion, and stress-induced diffusion was developed. The established model effectively reproduced most electrical and compositional device characteristics during the electroformation process. The thesis concludes that the ion motion during the electroformation process in TaO_x-based ReRAM is dominated by the thermodiffusion of cations towards the center, with stress playing a crucial role as a limiting factor in device evolution.