This work reports the effect of the heater electrode of a bilayer structure on phase-change memory ͑PCM͒ devices. The large amount of Joule heat generated from the high-resistive single-layer heater degraded the PCM device, whereas the bilayer heater composed of two kinds of different materials improved the device reliability. The PCM device employing a 2.0 m⍀ cm SiGe heater in contact with a GeSbTe phase-change material exhibited a much smaller programming current than that employing a conventional 0.5 m⍀ cm TiN heater. However, the open-mode failure, where the cell resistance was stuck at a high value, frequently occurred in the PCM device with the SiGe heater. Transmission electron microscopy revealed that the open-mode failure resulted from the physical separation of the upper tungsten electrode from the GeSbTe layer. The reliability problem was completely removed by inserting a 0.1 m⍀ cm TiN layer between the GeSbTe and SiGe layers. In addition, we found that the TiN layer in a GeSbTe/TiN/SiGe multilayer structure prevented the diffusion of Si and Ge atoms originating from the SiGe layer. The improved reliability with the bilayer heater is attributed to the moderate temperature rise in the GeSbTe layer preserving the attachment of the tungsten electrode to the GeSbTe layer.The phase-change memory ͑PCM͒ is considered one of the promising candidates for emerging nonvolatile memory. While most memory devices based on the floating-gate architecture are confronting physical limitations with scaling down beyond 45 nm, the PCM device is anticipated to overcome the scaling problem because the very small volume of a phase-change material is sufficient to attain a high on-off resistance ratio. Moreover, the programming current required to induce a phase transition in the phase-change material decreases as the device is scaled down. 1-3 When the programming current passes through a PCM device, Joule heating effect takes place and causes the phase transition. It is commonly believed that a high-resistive heater in contact with the phase-change material provides the additional heat accelerating the structural change. 4,5 Karpov et al. 6 recently presented this different explanation for the role of the high-resistive heater: the main source of Joule heat for the phase transition is not the high-resistive heater but the phase-change material, and the heater only leads to suppress thermal conduction through a thermal sink. However, because the thermal conductivity is proportional to the electrical conductivity ͑i.e., the WiedemannFranz law͒, this explanation does not preclude the merit of the highresistive heater.The reliability is an important factor that should be guaranteed to commercialize a memory device. There are two kinds of reliability degradation in the PCM device. One is the short-mode failure and the other is the open-mode failure, which are thought to be closely related to the interaction between the phase-change material and the adjacent substances including the high-resistive heater. 7,8 We found that the pro...
The electrical properties of a split-gate-type flash cell are investigated and optimized by junction engineering to obtain a high reliability. Phosphorus implantation is conducted to form a cell source junction, and the following three different anneal conditions change voltage coupling ratio between the source and the floating gate. As the ratio increases, it is observed that program characteristic is improved and endurance property is degraded, which matches well with simulation result. Therefore, cells in the pure N2 group are considered to be optimized cells. Optimized cells guarantee 105 cycle endurance, and show excellent program disturbance and bake retention properties.
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