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Resistive Non-Volatile Memories Characteri-zation by Conductive Atomic Force Micros-copy (C-AFM) in Ultra-High Vacuum Environment

Abstract : Memories are the fundamental for any electronic system we interact with in our daily life and are getting more and more important day by day in our present era. The growing functionalities and performance of the electronic products such as digital cameras, smart phone, personal computer, solid state hard disk and many more, need continues improvement of its features. Floating gate-based Flash technology is the main NVM technology used extensively in market these days. Nevertheless, Flash technology presents many problems making further scaling impossible. In this context, there are many other memory technologies emerging and interest in new concepts and materials to go beyond the Flash technology is growing. Resistive non-volatile memories based on two terminal devices, in which an active material is sandwiched between two electrodes have been investigated. The main idea of using this kind of structure and material is to use a specific physical mechanism allowing to switch it between two different resistive states for information storage. For example, in oxide based random-access memory (OxRAM), a conductive filament is grown inside the oxide layer, linking the two electrodes. By creation and disruption of this filament, two different resistance states can be generated. Another example is the phase change random-access memory (PCRAM), in which a chalcogenide material with the ability to change its phase between a high resistive amorphous and a low resistive crystalline state is used. Over the last few years OxRAM has been widely investigated due to many advantages like good scalability, long data retention time, fast read & write speed and low power consumption. The main benefit is that it is compatible with Back-end of line fabrication. In MIM structures for OxRAM, forming and disruption of the nanometer sized conductive filament is commonly accepted as the physical phenomenon for the switching, but still a debate is going on to understand the nature and characteristics of the conductive filament. Also, many studies have been done to evaluate the scaling capability of OxRAM and PCRAM. Hence, in this thesis work we studied mechanisms related to the conductive filament based resistive switching at nanoscale. To do the electrical characterization, a new technique using conductive atomic force microscopy (C-AFM) in ultra-high vacuum is proposed. The impact of different AFM tip materials (which is used as top electrode), different bottom electrode materials and the compliance current effect in two different regimes (in nA and in µA) are investigated. It is found that in the case of HfO2 based OxRAM, the filament is formed by Ti diffusion from the bottom electrode through the oxide layer. The results are in good agreement with device characteristics and could be reproduced by modeling. Also, phase transition in phase change materials for PCRAM is investigated for Ge2Sb2Te5 (GST-225) and Ge rich GST. It was found that the phase transition from amorphous to crystalline is possible at nanoscale. Finally, the threshold for GST-225 is observed at values nearer to those observed on devices than former observations with standard C-AFM.
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Amit Kumar Singh. Resistive Non-Volatile Memories Characteri-zation by Conductive Atomic Force Micros-copy (C-AFM) in Ultra-High Vacuum Environment. Micro and nanotechnologies/Microelectronics. Université Grenoble Alpes, 2019. English. ⟨NNT : 2019GREAT033⟩. ⟨tel-02482433⟩

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