Investigation on Ultra-high Density and High Speed Non-volatile Phase Change Random Access Memory (PCRAM) by Material En
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0918-H05-05-G06-05
Investigation on Ultra-high Density and High Speed Non-volatile Phase Change Random Access Memory (PCRAM) by Material Engineering E.G. Yeo, L.P Shi, R Zhao, and T.C. Chong Data Storage Institute, 5 Engineering Drive 1, Singapore, Singapore, 117608, Singapore
ABSTRACT In this paper, ultra-high memory density and high speed non-volatile phase change random access memory (PCRAM) was investigated by material engineering. The melting point, crystallization point and activation energy of crystallization of the Bismuth (Bi) doped Germanium-Antimony-Tellurium (GeSbTe) compound was measured using differential scanning calorimetry (DSC) and compared to other GeSbTe ternary compounds. It was observed that the melting temperature of Bi-doped GeSbTe was lower than that of GeSbTe. On the other hand, its activation barrier was found to be reduced, which in turn increased the speed of crystallization of Bi-doped GeSbTe. Bi-doped GeSbTe was then used as a phase change material in the fabrication of PCRAM devices. The properties of PCRAM fabricated using this material were then compared to those using GeSbTe, with emphasis on the programming current required. The results obtained revealed that lower programming current of up to 40% has been achieved for PCRAM with Bi-doped GeSbTe compared to those with other GeSbTe compounds. Bi-doped GeSbTe also has low RESET current and fast speed of crystallization with scaling, making it a suitable material for high speed, ultra-high density PCRAM fabrication in the future. INTRODUCTION There is currently a growing demand for non-volatile memories with a growing handphone and consumable electronics market. However, current memory technology cannot satisfy this demand. Hence, there is a search for the next generation emerging non-volatile memory technology, which include ferroelectric RAM (FeRAM), magneto resistive RAM (MRAM) and phase change RAM (PCRAM). PCRAM is regarded as the most promising. It has near ideal memory qualities such as non-volatility, fast switching speed, high endurance of more than 1013 read –write cycles [1], non-destructive read, direct overwriting and long data retention time of more than 10 years [2]. The one advantage that distinguishes it from the rest is the unique scaling advantage of having better performance with smaller sizes. The limit to which PCRAM can be scaled is lithography limited. Thus, it offers the biggest potential of achieving ultra-high memory density. Temp PCRAM are based on chalcogenide materials, which have the unique characteristic to effect a RESET pulse reversible phase change when heated or cooled, Melting pt switching between stable amorphous and crystalline SET pulse states by electrical pulses or light. To SET the Cryst pt memory, a long low amplitude pulse is applied, switching it from the amorphous, high resistance state to crystalline, low resistance state. The SET pulse duration must be sufficiently long such that Time crystallization can take place. Nucleation and crystal Figure 1 Effect of RESET and SET pulses on the
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