Thermal analysis of nonvolatile and non rotation phase change memory cell
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Thermal analysis of nonvolatile and non rotation phase change memory cell L.P. Shi, T.C. Chong, J.M.Li, H.X.Yang, and J.Q. Mou Data Storage Institute, DSI Building, 5 Engineering Drive 1, Singapore 117608; Email: [email protected] ABSTRACT In this paper, a three-dimensional finite-element modeling is performed for the analyses of Chalcogenide Random Access Memory (C-RAM), a non-rotation nonvolatile phase change memory cell. The thermal effect generated by an incident electric pulse was mainly discussed. Thermal performances of the cell as a result of electrical and geometrical variations were quantified. Current density distribution, temperature profiles, temperature history, heating rate, cooling rate, and heat flow characteristics were obtained and analyzed. The study is useful for the failure analysis of the C-RAM. 1. Introduction Chalcogenide Random Access Memory (C-RAM), also known as Ovonic Unified Memory (OUM), is an inexpensive nonvolatile semiconductor memory with the potential to be highperformance, high endurance, and low power [1,2]. This novel semiconductor memory is based on the phase change of chalcogenide alloy materials. The key to the chalcogenide materials is the switching property between amorphous state and crystalline state by electrical pulses. In the crystalline state, the materials have a low electrical resistance, while in the amorphous state, they have a high resistance. In C-RAM, data storage is implemented by a thermal induced phase-change between the amorphous and crystalline states in a chalcogenide phase change thin film. Thus, it is important to investigate the thermal influence of factors such as cell structure, on the performance of the CRAM. In this paper, the influence of factors such as thickness of different layers, cell radius, angle of sidewall on the thermal performance is mainly addressed. 2. Thermal modeling A three-dimensional thermal finite-element modeling is performed on C-RAM cells. The model takes into consideration of the thermal effect generated by the incident electric pulse. Fig.1 shows the cross section of a gyratory C-RAM cell structure for analysis using finite element method (FEM). In this structure, TiW and TiWN were used as electrodes, ZnS-SiO2 as dielectric layers and GeSbTe as a phase change layer. There are two functions of dielectric layers and electrode layers: (1) mechanical protection, including protection from humidity and prevention of thermal damage to the substrate; (2) controlling thermal conditions of the phase change layer. From the thermal point of view, the dielectric film takes the role of isolating the heat from neighbor units. The electrodes are also used to keep a suitable heat diffusion velocity, thus attaining the desired temperature, heating rate and cooling rate. The phase change layer is the heat generation source because of its resistance. In this simulation, Assume that properties of the cell materials are assumed to be
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Cell diameter
Fig.1. Cross section of the simulated C-RAM cell structure. indepe
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