Crystallization-induced Stress in Phase Change Random Access Memory
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1137-EE05-10
Crystallization-induced Stress in Phase Change Random Access Memory M. H. Li1, J. M. Li1, L. P. Shi1, H. X. Yang1,2, T. C. Chong1,2, and Y. Li3 Data Storage Institute, (A*STAR) Agency for Science, Technology and Research, DSI Building, 5 Engineering Drive 1, Singapore 117608 2 Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, 117576 3 Department of Materials Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 1
ABSTRACT Switched phase change material in Phase Change Random Access Memory (PCRAM) is confined within a solid surrounding. As a result of mechanical properties and microstructure differences between the crystalline and the amorphous phases, strains and stresses are generated and may degrade the performance of PCRAM devices. This paper investigated the crystallization-induced stress in phase change Ge2Sb2Te5 (GST) nano film. The electric-thermal and thermo-mechanical simulation results show that the increases of both of the Young’s modulus and Coefficient of Thermal Expansion (CTE) are responsible for the stress generation upon crystallization. The XRD studies correlate the strains and stresses with the lattice deformation in crystalline GST films. INTRODUCTION The phase change random access memory (PCRAM) has been demonstrated as a promising non-volatile solid state memory due to its great performances such as fast access speed, good data endurance, excellent scaling ability, well-matched complementary metal-oxide semiconductor (CMOS) technology, and so on. In PCRAM, the active material is multicomponent chalcogenides, such as Ge2Sb2Te5 (GST), whose markedly different phase resistances are used as the two logic states. Switching from the high resistive amorphous to low-resistive crystalline phase can be triggered by external voltages. The data storage includes RESET process defined as the phase transition from crystalline phase to amorphous phase and SET process defined as the phase transition from amorphous phase to crystalline phase [1-2]. During the phase change obvious volume variation (6% in GST case [3]) has been detected due to the structure difference between the amorphous and crystalline phases, resulting in strains and stresses which may cause defects and degrade the performance of PCRAM devices. This problem is predicted to be more serious as the device scaling. The phase change-induced stress is an important issue for PCRAM device development. It has found that amorphization can be induced when ~ 15 GPa pressure is applied on a crystalline GST material [4]. On the other hand, the yield stress of crystalline GST increases with its thickness decreasing [5]. The surrounding layers like electrodes and isolators in PCRAM cell may influence the generation and relaxation processes of the phase change-induced stresses [6]. It has been reported that both of the Young’s modulus and coefficient of thermal expansion (CTE) differ between the amorphous and the crystalline phases [7]. In this
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