Memory retention of doped SbTe phase change line cells measured isothermally and isochronally
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Memory retention of doped SbTe phase change line cells measured isothermally and isochronally J.L.M. Oosthoek1, B.J. Kooi1, K. Attenborough2, G.A.M. Hurkx2, D.J. Gravesteijn2 1 Materials Innovation Institute M2i and Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands. 2 NXP-TSMC Research Center, Kapeldreef 75, 3001 Leuven, Belgium. ABSTRACT Doped SbTe phase change (PRAM) line cells produced by e-beam lithography were cycled for at least 100 million times. The memory retention of the PRAM cell was measured both isothermally and isochronally which showed excellent agreement. An activation energy for growth of 1.7 eV was found (after 100 million cycles) for both measurements. Similar isothermal and isochronal measurements were performed on PRAM cells produced by optical lithography which yielded activation energies of 3.0 eV and 3.3 eV, respectively. Our results show that the same phase-change material can show large differences in retention behavior depending on the way the cells are produced. INTRODUCTION Currently, Flash memory is the technology of choice for non-volatile memory applications. Although it already exceeds the expectations that were foreseen in the past, it is expected that down-scaling will become increasingly difficult. Phase-change random access memory (PRAM) is a potential candidate to replace Flash memory in the near future. PRAM expected to be scalable with the next generations of lithography, it requires less lithographic steps, it has a much higher writing speed and is more energy efficient to program.1-3 The information carrier in a PRAM cell is a nano-sized resistor made from phase-change material. Information is stored as a difference in electrical resistance between the amorphous and crystalline phase which is typically three orders of magnitude. It can be switched electrically between the two phases reversibly. By supplying a short high energy (RESET) pulse the crystalline resistor is melted and, when cooled sufficiently fast, quenched into the amorphous phase. By supplying a longer but lower energy (SET) pulse the temperature is raised above the glass temperature but below the melting temperature. This allows crystal growth into the amorphous mark, which can fully crystallize in the order of a hundred nanoseconds.1-2 The amorphous phase is meta-stable by nature. Over time the cell returns to the crystalline phase (SET state) by spontaneous crystal growth and nucleation of the phase-change material which is by all means an undesirable effect. These processes, like most phase transformations, are thermally activated, because the underlying physical phenomena such as atomic jump frequencies, viscosity, diffusion, nucleation and growth are thermally activated.4 This thermal activation is generally considered to be of Arrhenius type. Currently, in many applications so-called fast-growth type phase-change materials are employed, where the crystallization process only depends on growth and not on nucleation. One
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