Advanced TEM Techniques for Mechanisms and Reliability Analysis in State-of-the-Art OxRAM and PCM Non-Volatile Memory Devices
Authors: Lefèvre Gauthier , Guillaume Nicolas , Serra Anna-Lisa , David Sylvain , Vallée Christophe , Bernier Nicolas , Bourgeois Guillaume , Charpin-Nicolle Christelle , Beugin Virginie , Cyrille Marie-Claire , Navarro Gabriele 
Published: December 2020

Memory devices play an essential role in today’s electronics, with increasing requirements in terms of density and reliability. The incremental appearing of new and different applications in the market, open the door to cutting-edge memory technology featuring innovative storing mechanisms and improved performances. In particular, Phase-Change Memory (PCM) and OxRAM (metal oxide resistive memory) are promising candidates for next generation of non-volatile memory (NVM) [1]. They represent a very attractive solution to design downscaled ultra-low power devices for new applications such as neuromorphic systems or in-memory computing. Such NVM are both based on an electrically switchable resistance, however, their operation mechanisms are completely different, exactly like the reliability issues from which they can be affected. High-resolution imaging, i.e. Transmission Electron Microscopy (TEM) coupled with Convergent Beam Electron Diffraction (CBED) and Energy Dispersive X-ray Spectroscopy (EDX), in state-of-the-art ultra-scaled NVMs becomes essential for a complete understanding of mechanisms behind the functionality and the failure of these new technologies. In this work, we considered for our investigation both OxRAM and PCM cells integrated in the Back-End-Of-Line (BEOL) of the fabrication of LETI Memory Advanced Demonstrator (MAD) based on 130 nm CMOS technology.

OxRAM operations rely on the formation and dissolution of one or multiple conducting filaments in the active oxide of the device. This intrinsically stochastic phenomenon is considered the main responsible for OxRAM performances variability, thus the understanding of the origin of its evolution and degradation along the cell cycles, can enable new ways for the device engineering and improvement. TEM investigation of HfO2-based OxRAM failures along endurance tests were performed. In order to avoid extrinsic results belonging to experimental design, we considered a population of dozen OxRAM devices (each one composed by a selecting transistor and a resistive memory, or “1T1R”, and based on a Ti/HfO2(10nm)/TiN stack) for each level of endurance. We show here in TEM/EDX analyses, the evolution of the device from its pristine state towards different stages of its lifetime, up to the final failure achieved at 107 cycles. We clearly evidence the switching mechanism, represented by a metallic migration from the electrodes in the active oxide. In particular, EDX cartographies highlight this diffusion phenomenon happening in the entire volume of the insulator. Finally, we show how the failure of the cell is likely related to a gradual titanium enrichment of the oxide layer, hindering the dissolution process of the filaments.

PCM operations rely on the reversible phase transition of a chalcogenide material between the amorphous and the crystalline phase, which occurs upon current-induced Joule heating. The reliability of the programming operation and its energy consumption therefore depends on the thermal efficiency of the device [2]. In this perspective, we applied an optimized SiC based encapsulation in PCM devices enabling programming current reduction and improved data retention [3]. We considered in our experiment Ge-rich GeSbTe based devices, featuring high temperature stability required by automotive applications. Thanks to TEM analyses, we demonstrate the higher uniformity of the heating process during the programming operation achieved by optimized SiC encapsulation with respect to standard SiN. EDX analyses highlight the homogeneity of the active volume involved in the phase-change transition. We coupled these results with CBED spectra, to evidence the germanium segregation and its more uniform distribution in the active volume of SiC based PCM cells. This result matches and helps validating the hypothesis based on our TCAD simulations here presented.

This work presents the reliability analysis of state-of-the-art NVM devices performed by advanced TEM techniques. We provide demonstration of the OxRAM device failure, to be found in the gradual electrode diffusion in the whole active material. Finally, we show how the higher thermal efficiency of a Ge-rich GeSbTe PCM device, based on an optimized SiC encapsulation, is supported by a higher homogeneity of the active volume, evidenced by both EDX and CBED analyses.

[1] Yoshio, Nishi. Advances in Non-Volatile Memory and Storage Technology / Edited by Yoshio
Nishi. Elsevier Science, 2014.

[2] F. Xiong et al., "Towards ultimate scaling limits of phase-change memory," 2016 IEEE International Electron Devices Meeting (IEDM), pp. 4.1.1-4.1.4.

[3] A. L. Serra et al., "Outstanding Improvement in 4Kb Phase-Change Memory of Programming and Retention Performances by Enhanced Thermal Confinement," 2019 IEEE 11th International Memory Workshop (IMW), 2019, pp. 1-4.