Thermal Expansion of Phase-Change Random Access Memory Cells
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THERMAL EXPANSION OF PHASE-CHANGE RANDOM ACCESS MEMORY CELLS J.M. Li1, L.P. Shi1, H.X. Yang1,2, K.G. Lim1, X.S. Miao1, H.K. Lee1, and T.C. Chong1 1 Data Storage Institute, DSI Building, 5, Engineering Drive 1, Singapore, 117608, Singapore 2 Dept. of Electrical and Computer Engineering, National University of Singapore, 5, Engineering Drive 1, Singapore, 117608, Singapore ABSTRACT Three-dimensional finite element method (FEM) is used to solve the thermal strain-stress fields of phase-change random access memory (PCRAM) cells. Simulation results show that thermal stress concentrates at the interfaces between electrodes and phase change layer and it is significantly larger than that within the phase change layer. It has been found that the peak thermal stress is linearly related to the voltage of electrical pulse in the reset process but once amorphous state is produced in the cell, a nonlinear relationship between thermal stress and electrical power exists. This paper reported the change of thermal stress during set process. It was found that the stress decreases significantly due to the amorphous active region during set processes. INTRODUCTION The phase-change random access memory (PCRAM) has been demonstrated as a promising non-volatile memory (NVM) because it possesses versatile advantages such as high scalability, fast access time, long endurance, good data retention and well-matched complementary metal-oxide semiconductor (CMOS) technology. Compare with other candidates for next-generation nonvolatile memories such as magnetic random access memory (MRAM) and ferroelectric random access memory (FRAM), the high scalability is the most unique feature of the PCRAM which is thus considered as one of the best NVM candidates at 45 nm and beyond. PCRAM technology is based on chalcogenide materials (phase-change materials) which have reversible electrical switching phenomena originally reported by Ovshinsky in the late 1960s [1]. The data storage is implemented with reset defined as a process from crystalline state (low-resistance) to amorphous state (high-resistance) or set process (from amorphous state to crystalline state) by conducting an electric pulse. Thermal engineering is the key challenge for PCRAM technology because the properties and performance of PCRAM cells depend on the thermal conduction, the phase transition and thermal expansion. Kolobov and coworkers [2] studied the pressure-induced amorphization of Ge2Sb2Te5 which indicated that thermal expansion may influence the performance of PCRAM cells. For the simulation of thermal expansion of PCRAM cells, the electric-thermal and thermal-mechanical modeling have to be considered. Another important factor would be the dynamic process of active area generation for estimating the dynamic properties of PCRAM cells. The issues related to physical fundamentals have been investigated by simulation individually [3-7]. Practically, we have developed the computer-aided design and analysis software for the PCRAM cell design. Based on the PCRAM cell design s
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