Reaction Kinetics, Stress and Microstructure in Titanium-Copper-Titanium Thin Films
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ABSTRACT Interdiffusion, reactions and microstructure in Ti-Cu-Ti trilayer thin films deposited on silicon wafers have been investigated by Rutherford backscattering, and cross-sectional TEM techniques in the temperature range of 350 to 500 1C. The formation of intermetallic compounds was found to be symmetric at both interfaces in Ti-Cu-Ti film package. TiCu compound formed first at low temperature and was followed by TiCu3 at higher temperatures. The stress in Ti-Cu film has also been measured in-situ as functions of both temperature and annealing time by thin fused quartz bending-cantilever beam technique. The stress in metallic Ti-Cu film couple on fused quartz was tensile. The stress increased as both annealing time and temperature increased and followed a parabolic relationship with time due to the growth of the intermetallic compound TiCu 3 . It was possible to calculate stress in the TiCu. layer from the changes of stress in the bilayer thin film. The stress in TiCu 3 was computed to be at about 3 times larger than the stress in the unreacted Ti-Cu thin film couple.
INTRODUCTION Recent trends in VLSI are toward large circuit density and multilevel interconnections. During the fabrication and performance of the devices, thin films packages have to undergo large temperature excursions which may generate high stress due to interdiffusion, material reaction, or thermal mismatch between the films and Si substrate. High stresses in thin films can cause device performance and reliability problems. We have previously reported the temperature dependence of stresses in unilayer metallic thin films(Ti, Cr, Cu)(I), and polymeric films(2). The stress behavior among the various thin films was quite different. Notably, the intrinsic stresses were enhanced in Ti and Cr films during the initial heat treatment. On the other hand, stress in Cu films was relieved during thermal cycling by plastic deformation and mass transport. In polymeric films, flow at glass transition temperature, the thermal expansion, and cracking were detected. In this investigation, we report the temperature dependence of stresses in bilayer Ti-Cu thin metallic films on fused quartz substrate along with the material reaction kinetics and the accompanying microstructure changes in the trilayer Ti-Cu-Ti film package. The techniques employed for these studies were respectively, the bending-cantilever quartz beam, the Rutherford Backscattering and cross-sectional transmission electron microscopy.
EXPERIMENTAL PROCEDURES The in-situ temperature dependence of stress measurements was carried out by a cantilever bending beam technique similar to that reported in Ref. 3. The system consisted of a thin fused quartz cantilever used as a substrate and a thick reference quartz beam. Insitu measurements were made in a vacuum of 2 x 10-"Torr at temperatures in the range of 25-525 'C. The experimental procedure for in-situ stress measurements has all been de-
Mat. Res. Soc. Symp. Proc. Vol. 54. ,1986 Materials Research Society
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been described in Ref. I and ap
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