Temperature Dependence of Stress Distribution in Depth for Cu Thin Films
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Temperature Dependence of Stress Distribution in Depth for Cu Thin Films Tokuji Himuro and Shinji Takayama Dept. of Systems and Control Engineering, Hosei University, 3-7-2, Kajino-Cho, Koganei, Tokyo, 184-8584, Japan ABSTRACT The stress distribution of (111) textured Cu films with depth was measured by using a GIXS method. We derived the equation to correct a scattering diffraction angle with depth, measured in the GIXS Seemann-Bohlin geometry, to obtain the actual scattering angle. It was revealed after the correction of the measured scattering angles that the internal stresses of (111) grains, on the whole, tend to increase almost linearly with increasing film depth from the free surface toward the substrate. It was suggested that these results were opposite to the results of the elastic calculation reported, and hence that a large stress relaxation took place during and/or after deposition and annealing. After annealing at various temperatures, these stress distribution profiles are almost unchanged, and are simply shifted uniformly in magnitude. INTRODUCTION The thermal stability of Al and Cu films, which are commonly used as an interconnect line in advanced ultra large-scale integrated (ULSI) circuits, is an important reliability issue of ULSI semiconductor devices. In particular, residual internal stresses in thin films has become more important in relation to stress migration in narrower interconnect lines. To clarify this, we investigated their temperature dependence of stress distribution with depth by employing a grazing incidence X-ray scattering (GIXS) method [1]. This GIXS method is well recognized as a powerful technique for the analysis of not only thin film structure and surface, but also the measurement of the stress distribution through the film thickness [2, 3]. In this scattering geometry, the X-ray beam is incident on the surface at a very small grazing angle near the total reflection angle range in thin film samples, so that the scattering vector is nearly inclined to the direction parallel to the sample surface. The scattered X-rays are collected by a detector in the direction parallel to the film surface along a circle at constant distance from the sample. Therefore, the penetration depth can be controlled by changing the incident angle near the total reflection angle and thereby, the structure and the interplanar spacing with depth can be measured. The schematic geometry of the GIXS method is shown in Fig. 1. However, we have realized that since in this geometry, the detector position is always fixed within the plane parallel to the sample surface (such as a Seemann-Bohlin geometry), the true takeoff angle from a scattering plane becomes underestimated as the incident angle increases. Thus, the rigorous correction should be
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made to get a true scattering angle in order to get the depth X-ray profiling of thin films. In this report, we have attempted to calculate this error correction factor and measured the depth stress distribution in textured (111) Cu thin films as a
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