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A new oxygen diffusion reliability model for a high-density bipolar ReRAM is developed based on hopping conduction in filaments, which allows statistical predication of activation energy. The filament in the active cells is confirmed by EBAC and TEM directly for the first time. With optimized filament size, a 256-kbit ReRAM with long-term retention exceeding 10 years at 85˚C is successfully demonstrated. IntroductionResistance random access memory (ReRAM), which has the advantages of high speed and low power consumption, shows major potential for use in embedded and stand-alone non-volatile memory applications [1][2]. Recently, the retention mechanism of ReRAM has attracted considerable attention. In particular, D.Ielmini et. al., introduced a size-dependent reliability model for unipolar ReRAM based on metallic filament conduction [3]. However, the physical reliability model for bipolar ReRAM with hopping conduction has up to now remained elusive. In this study, we developed an oxygen diffusion reliability model based on our experimental results. Experiment and discussionA test sample with Ir/Ta 2 O 5-δ /TaO x /TaN active cells was fully integrated using 0.18 μm CMOS technology [4]. Ta 2 O 5-δ /TaO x thin films were fabricated by reactive RF magnetron sputtering using a Tantalum target in O 2 atmosphere at 25°C.First, the filaments in the active cells (0.5 × 0.5 μm 2 ) were directly observed using the electron beam absorbed current (EBAC) method. The test samples with thin top electrodes were examined before and after the forming process. The bright and dark regions correspond to the low-resistance and high-resistance regions, respectively. The current image of the test sample before the forming process ( Fig. 1 (a)) shows no significant contrast, whereas the bright area, which corresponds to the local conduction path (the filament), is observed after the forming process ( Fig. 1 (b)). The cross-sectional structure of the active filament area was also characterized in detail using Transmission Electron Microscopy (TEM) and Electron Energy-Loss Spectroscopy (EELS), as shown in Fig. 1 (c) and Fig. 1 (d). The TEM image indicates that the filament extends from the top electrode (anode) to the middle part of the TaO x layer through the Ta 2 O 5-δ layer. The EELS O mapping image revealed that the filament includes an O-poor area, in which the oxygen concentration is lower than that of the TaO x layer. Both the TEM image and the EELS mapping image show that the filament area did not reach the bottom electrode. From these results, it is confirmed that the resistance-switching region is occurred in a filament localized at the interface near the anode.To clarify the conduction mechanism of the filament, the temperature dependence of the electrical conductivity for both the high resistance state (HRS) and the low resistance state (LRS) was studied at 300 -483 K. As shown in Fig. 2 (a), both HRS and LRS follow the same conduction mechanism, as their conductivity increases with increasing temperature. Detailed investigation ...

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We investigate, for the first time, the expansion of resistive random access memory (ReRAM) conductive filaments during pulse cycles, which may cause retention failure after cycling endurance. We find that filament size becomes larger gradually because of oxygen diffusion from the region surrounding a filament during reset operations. To achieve long-term use of ReRAM while avoiding filament expansion, it is the key to control both an electric power and a pulsewidth input at a switching operation. We successfully demonstrate good data retention even after endurance of 100-k cycles with an optimized reset pulse.