Thermal Diffusion Characteristic And Testing System Research For Phase Change Memory Cell

Phase change memory is considered to be the most competitive one of new non-volatile semiconductor memories. With size scaling and increasing operation speed of phase-change random access memory, the accumulated heat in devices during pulse interval result in the rise of temperature during erase-write process. Experiment results have shown that threshold voltage is affected by the number of applied pulse, in addition, it is affected by the rise of ambient temperature or the temperature rise caused by self-heating effect. And heat diffusion during pulse interval and the temperature changes would directly affect the internal temperature of the device. So investigating thermal diffusion characteristic during pulse interval is important for high speed erase-write operation of PCM and improvement in the low power and operation speed. The study in this article is focused on thermal diffusion characteristic based on experimental and HSPICE simulation methods. Besides, a set-up is designed to test phase-change memory cell based on DSP.First, a simplified cell model of phase change memory was proposed to analyze the gradual phase change inside the phase change layer. Secondly, we set up a testing system combines Keithley Model 4200-SCS, and applied RESET pulse sequence to a PCM cell. The cell resistance would increase gradually if the pulse parameters were appropriate, which indicated that the volume of amorphous region inside a phase change layer was becoming bigger and bigger. The heat capacity and thermal resistance were extracted based on the simplified model and the thermal diffusion equation. Thirdly, we took heat accumulation effect into consideration for the HSPICE model, and simulated the cell resistance-pulse number characteristics by changing accumulated temperature and heat capacity respectively. The results shown that, when the cell resistance gradual changed under pulse sequence, the heat capacity decreased gradually, the thermal resistance increased slowly and accumulated temperature showed a trend of logarithmic function.An I-V test system of PCM cell was designed based on DSP chip. The measurement results on Ge2Sb2Te5 of our system and 4200-SCS is similar, this indicated the feasibility of our system. But there is still a lot of room for improvement.