Stress and Mechanical Constants Characterizations of Phase-change SbTe-alloys: Influence of the Film Thickness and Subst
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Stress and Mechanical Constants Characterizations of Phase-change SbTe-alloys: Influence of the Film Thickness and Substrate J. G. Lisoni1, T. Gille1,2, L. Goux1, R. Delhougne3, N. Jossart1, K. Attenborough3, and D. J. Wouters1 1 PT/MEMORY, IMEC, Kapeldreef 75, 3001 Heverlee, Leuven, Belgium 2 ESAT/INSYS, KULeuven, Kasteelpark Arenberg 10, Leuven, 3001 Heverlee, Belgium 3 Research, NXP Semiconductors, Kapeldreef 75, 3001 Heverlee, Leuven, Belgium ABSTRACT We have characterized the in-film stress and the mechanical constants of a growthdominated phase-change SbTe-alloy, a material beneficially used in the line-cell phase change memory architecture. The influence of the thickness of the films (12-120 nm) and of the substrate used for its deposition (Si, Si\SiO2 and Si\SiO2\SiC) was studied. The characterizations were carried out on both amorphous and crystalline films. The crystallization temperature was determined by the resistance changes as a function of the temperature. The mechanical characteristics of the films were measured by the wafer curvature method. We observed that the mechanical behavior of these films was strongly dependent on their thicknesses and on the substrate material. For the thinnest amorphous films, the in-film stress tended to be highly compressive (with the largest compressive stress for films on SiC), while it tended to be very low (fully relaxed films) for films thicker than 60 nm. The amorphous films furthermore did not reveal any stress relaxation in time-dependent stress measurements. Therefore, it was not possible to quantify the viscosity of the SbTe-alloy. This result maybe related to a lower defect state in the very thin films used, or to a blocking of the defects by enhanced reactivity of these thin SbTe-alloy films with their surroundings, both resulting in the absence of relaxation in the amorphous state. Finally, the coefficients of thermal expansion (CTE) of the amorphous and crystalline SbTe-alloys were similar, 1-3x10-6 K-1. These values were comparable to the CTE of the Si substrate, clearly indicating that for thin films the Si substrate dominates their thermal expansion behavior. INTRODUCTION Chalcogenide alloys have attracted much attention in recent years due to their possible application in phase-change non-volatile memories (PCM). The principle of PCM is based on the joule-heating induced reversible switching between crystalline (low resistance) and amorphous (high resistance) states. The switching goes along with thermal expansion during heating, volume changes at melting and crystallization, as well as microstructural modifications that all might generate high levels of stress during operation. In particular, it has been argued that the limited durability of PCM is caused by irreversible mechanical deformations. Therefore, it is of major importance to characterize the mechanical parameters of these materials and the stresses in thin films in order to gain a deeper understanding of the mechanical reliability aspects in this new type of memory. Only two art
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