Measurement Methods And Analysis Of Pulsed I-V Characteristics For Phase-Change Memory Cells

Phase-change random access memory (PCRAM) has many advantages such as low power consumption, high operational speed, high scalability, good stability, compatibility with the CMOS technology, multi-value storage and so on. It’s regarded as the most competitive novel nonvolatile semiconductor memory in the next generation of mainstream memory. The research on the electrical properties of PCRAM cells has been a hot topic in the field of semiconductor memory. The current-voltage (Ⅰ-ⅤV) characteristic is the most important one of the electrical parameters. Normally, we explore the Ⅰ-Ⅴ characteristics of PCRAM cells through a direct current (DC) sweep which is step-like. Energy accumulation effect occurs in the testing procedure because the thermal energy generated by every historical increased step voltage partly remains in the cell and accumulates. In fact, the effect happens not only in DC Ⅰ-Ⅴ, but also in pulsed Ⅰ-Ⅴ, resistance-current and resistance-voltage (R-Ⅰ/R-Ⅴ). The direct consequence of this phenomenon adversely affects the intrinsic characteristics of PCRAM cells. Experimental results show that most of the samples will be damaged after multiple Ⅰ-Ⅴ tests. Therefore, the study on the self-heating behavior and the energy accumulation effect of PCRAM cells is very meaningful. A short pulsed Ⅰ-Ⅴ measurement method is proposed.The phase-change material of the samples is Ge2Sb2Tes (GST). The pulsed Ⅰ-Ⅴ characterization testing system of PCRAM cells is setup with semiconductor characterization system4200-SCS, high frequency oscilloscope DPO70064and self-designed PCB. A high-speed MOS transistor, BNC connectors, switches, PCRAM chips and so on are all integrated in PCB. By adjusting the switches, the system can not only measure pulsed Ⅰ-Ⅴ characteristics, but also the general electrical properties of PCRAM cells. The thickness of the GST layer under study is25nm,75nm, and150nm, respectively. By comparing the difference between DC Ⅰ-Ⅴ curves and pulsed Ⅰ-Ⅴ curves, the threshold voltages of pulsed Ⅰ-Ⅴ are much higher. It implies the existence of self-heating and energy accumulation. Pulse widths and pulse periods are found to be indispensable parameters to describe pulsed Ⅰ-Ⅴ characteristics of PCRAM cells Assume that the heating of the active region causes the change of the electronic barrier and the electronic activity, the physical model dominated by the self-heating in PCRAM cells is proposed.