Impact of Material Crystallization Characteristics on the Switching Behavior of the Phase Change Memory Cell
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0918-H06-02-G07-02
Impact of Material Crystallization Characteristics on the Switching Behavior of the Phase Change Memory Cell Thomas Gille1,2, Ludovic Goux1, Judit Lisoni1, Kristin De Meyer1,2, and Dirk J. Wouters1 1 IMEC, Kapeldreef 75, Leuven, 3001, Belgium 2 ESAT/INSYS, KU Leuven, Kasteelpark Arenberg 10, Leuven, 3001, Belgium
ABSTRACT The impact of material crystallization characteristics on the switching behavior of phase change memory cells has been investigated using a finite element simulation. Both a conventional vertical cell and a horizontal line cell have been analyzed, using the widely used Ge2Sb2Te5 (GST) which is a nucleation dominated material for the vertical cell, and Ag5.5In6.5Sb59Te29 (AIST) which is a growth dominated material for the horizontal cell. Nucleation and growth models were implemented for both materials. Both RESET and SET program cycles were simulated. From these simulations, it was shown that the crystallization models gave realistic results for switching voltages, currents and switching times for the two different cell types. It is found that for GST, both nucleation (at lower voltages) and growth (at higher voltages) can play an important role in the crystallization. However, for AIST, crystal growth from non-amorphized crystal regions dominated over nucleation for all program conditions. The high growth rate of AIST moreover is shown to allow much shorter SET times in the line cell compared to that of GST in the vertical cell. INTRODUCTION Phase change memory (PCM) is a promising candidate for next generation non-volatile memories. The advantages of PCM are its scalability and low fabrication costs [1]. However, a drawback is its relatively long program time, i.e. a relatively long SET time (typically 100-200 ns [1]) required to allow full crystallization into the low resistive state. Different chalcogenide materials have been proposed for application in PCM cells. While differences in the material parameters as melting temperature Tm and crystallization temperature Tc directly affect important operation parameters as the current levels required for melting and crystallizing the material (RESET/SET) , we focus here on the impact of the crystallization mechanisms on the SET time. The conventional PCM cell is the so-called vertical cell [1], see figure1a. Typically, Ge2Sb2Te5 (GST) is hereby used as phase-change material. GST is generally characterized as a nucleation dominated material, with Tm= 894 K and Tc = 446 K [2]. Recently [3], a new horizontal “line” cell has been proposed (figure 1b), to be applied with a growth dominated material. In our analysis, we have selected AgInSbTe (AIST) as a model growth dominated material (with Tm= 810 K and Tc = 458 K [2].
Figure 1 : simulated cell configurations : a) vertical cell (cross-section), b) horizontal line cell (top and cross-sectional view). In general, however, domination of nucleation or growth may be dependent on the degree of undercooling (temperature) and this for a single material. As the temperature distribution duri
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