3-D Finite Element Simulation of a Phase-change Random Access Memory Cell with a Self-insulated Structure
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1108-A11-04
3-D Finite Element Simulation Of A Phase-change Random Access Memory Cell With A Self-insulated Structure Ke Sun, Wen Feng, Jae Young Lee, Biyun Li and Ya-Hong Xie Department of Materials Science and Engineering, University of California at Los Angeles, Box 951595, Los Angeles, California 90095-1595 ABSTRACT In this paper, we proposed a phase-change random access memory (PCRAM) cell with a self-insulated structure (SIS), which is expected to have better thermal efficiency than the conventional structures. 3-D finite element simulation is used to study the most power consuming RESET process for both SIS and conventional normal bottom contact (NBC) cells driven by a MOSFET. Instead of programming current, power consumption is investigated to give a more fundamental comparison between the two structures. Thermal proximity effect for both kinds of cells is directly analyzed by simulating a 3×3 device array. The potential slow-quenching issue of SIS is also discussed.
INTRODUCTION Phase-change random access memory (PCRAM), as one of the most promising candidates for next generation memory technology, has been intensely investigated for the past decade [1-4]. PCRAM operation is based on Joule-heating-induced reversible phase transformation between the amorphous and polycrystalline states of chalcogenide materials, with Ge2Sb2Te5 (commonly known as GST225) being the most commonly studied due to the combination of an adequate melting temperature and fast crystallization process. During the RESET operation, a relatively short and intense current pulse melts the phase change material and a subsequent rapid quenching locks the melted volume in the high-resistance amorphous phase; During the SET operation, a relatively long and less intense pulse heats the cell to an intermediate temperature between the melting temperature Tm and the glass transition temperature Tg, which transforms the amorphous GST225 into the low-resistance polycrystalline phase. PCRAM has been shown to possess many desirable properties as a non-volatile memory device, such as a simple structure, fast switching speed, low cost, high endurance, long data retention time, extraordinary scalability and complete compatibility with CMOS technology. For PCRAM, same as for all non-volatile memory systems, low power operation is one of the most sought after performance metrics, if not the most important one. Efforts in reducing power consumption generally fall into two directions: (1) modify the electrical properties of the phase change material itself, such as increasing the resistivity of GST alloy by doping of nitrogen or oxygen [5-6]; (2) modify the device
cell architecture to improve the power efficiency or the ‘confinement’ of the local Joule heating. Besides the widely-used conventional normal bottom contact (NBC) structure, a lateral edge contact structure is proposed, which has much less power consumption but a more complex cell construction and limited lateral scalability [7-9]. Hwang et al reported a confined structure which is similar to t
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