X-ray Microbeam Diffraction Measurements in Polycrystalline Aluminum and Copper Thin Films
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X-ray Microbeam Diffraction Measurements in Polycrystalline Aluminum and Copper Thin Films L. E. Moyer1, G. S. Cargill III1, W. Yang2, B. C. Larson2, G. E. Ice2 1 Dept. Mat. Sci. and Engr., Lehigh University, Bethlehem, PA 18015, USA 2 Solid State Div., Oak Ridge National Lab., Oak Ridge, TN 37831, USA ABSTRACT Thermally induced residual strains in polycrystalline Cu and Al films on single crystal Si and glass substrates, respectively, have been examined on a grain-by-grain basis by x-ray microbeam diffraction. The crystallographic orientation and the deviatoric strain tensor, εij*, are determined for each grain by white beam Laue diffraction. From grain orientation mapping and strain tensor measurements, information is obtained about the distributions of strains for similarly oriented grains, about strain variations within single grains, and about grain-to-grain correlations of strains. This type of information may be useful in developing and testing theories for intergrain effects in strain evolution in polycrystals. INTRODUCTION Integrated circuit devices are typically connected by patterned lines of a passivated polycrystalline metal, usually copper or aluminum. As devices and interconnects continue to decrease in size, electrical performance and mechanical reliability become greater concerns. Thermal strains develop in interconnects during fabrication and during operation at elevated temperatures due to the difference in thermal expansion between interconnects and surrounding layers. These strains can lead to failures from voids and hillocks in interconnects. X-ray diffraction is a well-established technique for measuring average elastic strains in polycrystalline materials. Recent advances in third generation synchrotron sources and focusing optics now provide the means for grain-by-grain x-ray measurements [1-5]. X-ray microbeam strain measurements on polycrystalline Cu and Al films are described in this paper. EXPERIMENT The microdiffraction experiments were performed at the Advanced Photon Source, Argonne National Lab, on the UNICAT beamline. A schematic of the setup is shown in Fig.1. Kirkpatrick-Baez (K-B) focusing mirrors are used to obtain a sub-micron x-ray beam for microdiffraction. The sample is positioned on a translation stage at an angle of 45° from the incoming beam. A CCD detector located directly above the sample records the Laue diffraction pattern. A computer program developed at Oak Ridge National Lab [2] calculates the Figure 1. Schematic of setup for microbeam orientation matrix of each grain in the x-ray diffraction experiments (from B. C. Laue pattern. The program also measures the deviations of the Laue Larson et al., UNICAT and ORNL).
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spots from the unstrained positions to obtain a complete deviatoric strain tensor. A 1 µm thick polycrystalline Cu film with a (111) fiber texture on a (001) oriented single crystal Si substrate was investigated. The sample was heated to 400°C, held for 1 hour, and then was cooled to room temperature. Microdiffraction measurements were t
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