WO x resistive memory elements for scaled Flash memories

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WOx resistive memory elements for scaled Flash memories S. Gorji Ghalamestani1, L. Goux1, D.E. Díaz-Droguett2, D. Wouters1,3, J. G. Lisoni1,4 1

IMEC, Leuven, Belgium Materials Science Labs, Faculty of Physics, Pontificia Universidad Católica de Chile, Chile 3 Electrical Engineering Dept. (ESAT), Katholieke Universiteit Leuven 4 Physics Dept., FCFM, Universidad de Chile, Santiago, Chile 2

ABSTRACT We investigated the resistive switching behavior of WOx films. WOx was obtained from the thermal oxidation of W thin layers. The parameters under investigation were the influence of the temperature (450-500 °C) and time (30-220 s) used to obtain the WOx on the resistive switching characteristics of Si\W\WOx\Metal_electrode ReRAM cells. The metal top electrodes (TE) tested were Pt, Ni, Cu and Au. The elemental composition and microstructure of the samples were characterized by means of elastic recoil detection analysis (ERD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray reflectivity (XRR). Electrical measurement of the WO x-based memory elements revealed bipolar and unipolar switching and this depended upon the oxidation conditions and TE selected. Indeed, switching events were observed in WOx samples obtained either at 450 °C or 500 °C in time windows of 180-200 s and 30-60 s, respectively. Pt and Au TE promoted bipolar switching while unipolar behavior was observed with Ni TE only; no switching events were observed with Cu TE. Good switching characteristics seems not related to the overall thickness, crystallinity and composition of the oxide, but on the W6+/W5+ ratio present on the WOx surface, surface in contact with the TE material. Interestingly, W6+/W5+ ratio can be tuned through the oxidation conditions, showing a path for optimizing the properties of the WO x-based ReRAM cells. INTRODUCTION There has been a tremendous development in the flash memory technology over the past three decades which has introduced higher density and lower cost devices to the market. However, conventional charge based flash memories are approaching the scaling limitation of 22 nm lateral features [1]. This limitation indicates the importance of an alternative non volatile memory to replace current flash for future technology. One alternative candidate is resistive random access memory (ReRAM), which stores the information based on resistance changes. In recent years, ReRAM has attracted much interest since it uses materials, which can be easily integrated in standard CMOS technology. Candidates for ReRAM devices are oxides where the active element can be either a binary metal oxide like NiOx, HfOx and TiOx or perovskites-like compounds such as SrTiO3 [2]. In this paper, we have prepared WOx-based ReRAM memory cells, where WOx was obtained by the ex-situ oxidation of 50nm sputtered W films. Resistive switching is investigated using Si\W\WOx\Metal structures. In particular, the role of the top electrode (TE) on the resistive switching characteristics of the WOx films was eval

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