An ultra-thin phase-change bridge (PCB) memory cell, implemented with doped GeSb, is shown with <100µA RESET current. The device concept provides for simplified scaling to small crosssectional area (60nm 2 ) through ultra-thin (3nm) films; the doped GeSb phase-change material offers the potential for both fast crystallization and good data retention.

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The multi-level operation of WO X based RRAM has been investigated. Improvement of our WO X process has produced an extended linear R-V region for our devices. By adding an electrical forming process and a program-verify algorithm we have demonstrated stable 2-bit/cell operation, with potential for 3-bit/cell. The reliability of the MLC operation has been examined and very stable high temperature retention, robust read disturb immunity and initial cycling endurance of > 1,000 times have been demonstrated.Keywords-resistive random access memory; RRAM; MLC; tungsten oxide; program verify algorithm; reliability I.INTRODUCTION Recently, resistance-based memory has attracted much attention for high-density applications because of its small cell size, simple structure, high speed, low power consumption, and potential for 3D stacking [1]. WO X based RRAM requires only one extra mask, no new equipment and no new material from the standard CMOS process and is thus especially attractive [2]. In addition, WO X devices have demonstrated the possibility for MLC operation [3]. However, past efforts had very little margin because the R-V curves were very steep and provided no operational plateau. We have improved the WO X process significantly and thus have achieved an R-V curve with extended linear range. This has increased the resistance and voltage window by ~ 10X, thus allowing stable MLC operation.II. DEVICE FABRICATION Fig. 1 shows the cell structure and cross sectional TEM image of graded WO X based resistive memory. The fabrication process flow was approximately the same as previously reported [2, 3] but we have shrunk the plug from 0.5um to 0.17um and refined the W CMP and other processes. The down stream plasma oxidation was done in a mixture of nitrogen and oxygen at 265 o C, for 1600s.III. MULTI-LEVEL CELL OPERATION A modest forming step (4V 50ns) is used to reduce the programming voltage [4]. Before the forming step, relatively high voltage is needed to reset the device, but after the forming process the reset voltage is reduced (Fig. 2). The hysteresis loop after the forming step is shown in Fig. 3. The resistance is increased by applying a positive voltage, and is decreased by applying a negative voltage. Fig. 4 shows the readout resistance at different programming voltages at 85 o C. It is interesting to note that between 1.5V and 3V the resistance increases smoothly and linearly with the applied voltage both at room temperature and at 85 o C. This represents an ideal characteristic