Forming-free Resistance Random Access Memory Using Ta 2 O 5 /TaO x Bi-layer Prepared by Magnetron Sputtering Method
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Forming-free Resistance Random Access Memory Using Ta2O5/TaOx Bi-layer Prepared by Magnetron Sputtering Method Natsuki Fukuda, Kazunori Fukuju, Isamu Yogosawa, Kazumasa Horita, Shin Kikuchi, Yutaka Nishioka, and Koukou Suu Institute of Semiconductor and Electronics Technologies, ULVAC, Inc. 1220-1 Suyama, Susono, Shizuoka, 410-1231, Japan ABSTRACT This paper describes proposal of ReRAM switching mechanism, development of production tool for ReRAM sputtering and improvement in TaOx-ReRAM switching characteristics. We propose that a ReRAM-cell has stack a structure in which an oxygen vacancy supply layer (TaOx) and an oxygen accumulation layer (Ta2O5) sandwiched by the top and bottom electrodes. Resistance change of the ReRAM-cell is caused by the oxygen vacancies migrating between the TaOx and the Ta2O5 layers by applied voltage. This prediction corresponded to the experimental facts. The thickness of Ta2O5 film sputtered by a mass production tool had good uniformity (±1.0%) and excellent stability (±1.0%). Also the sheet resistance uniformity (1σ) of TaOx film had 3.6%. By examining the sputtering conditions, the ReRAM-cell having a Ta2O5/TaOx bi-layer operated in less than 100μA with a forming-free and had excellent endurance property to 1010 cycles at 50nsec. INTRODUCTION Flash Memory used for SSDs and smart phones, etc. is scaling limit at 20nm node and below for decreased number of stored electron. Resistance Random Access Memory (ReRAM) is one of the candidates for next generation non-volatile memory, because it shows high speed and low power operation in comparison with other non-volatile memories. Materials for ReRAM are classified into perovskite oxides and binary oxides. The binaryoxides such as NiOx [1], TiOx [2] and HfOx [3] are simple structures, lower preparation temperature and compatible with conventional CMOS processing. Therefore the binary oxides have many advantages for mass production compared with the perovskite oxides. However, the switching characteristics of the binary oxide ReRAM-cell had three disadvantages; Large-voltage initialization called a forming, large operation current, and short endurance property. In this study, we report and proposal of resistive switching mechanism and development of production tool for ReRAM sputtering. In addition, for the purpose of improving switching characteristics, we examined sputtering conditions of the Ta2O5/TaOx bi-layer structure for ReRAM-cell. EXPERIMENT Ta2O5 thin films were sputtered on 8inch-Si substrate by multi chamber type mass production tool (ULVAC, CERAUS ZX-1000 [4], [5], [6]) in this experiment. Thickness and refractive index were evaluated by spectrum ellipsometer. Ta2O5/TaOx bi-layer thin films were prepared by the magnetron sputtering method on 8inch-Pt/Si substrate. 50μm-diameter Pt top
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electrodes were deposited on the surface of Ta2O5 layer by a DC magnetron sputtering method using a shadow mask. Switching properties of TaOx-ReRAM cell were measured by semiconductor device analyzer (B1500A, Agilent). DISCUSSION Proposal of
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