Stress Mapping near Simulated Defects in Thin Film Wiring Using X-Ray Microbeam Diffraction
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STRESS MAPPING NEAR SIMULATED DEFECTS IN THIN FILM WIRING USING X-RAY MICROBEAM DIFFRACTION Patrick Dehaven and Charles Goldsmith, IBM Analytical Services, Hopewell Junction, NY; and Sundar Kamath, IBM Technology Products, Hopewell Junction, NY.
ABSTRACT The stress distribution about a series of deliberately fabricated defects in a Cu/Ni/Au/Cr redundant metal stack deposited on a Cr/Cu/Cr serpentine has been studied via x-ray microbeam diffraction using the sin 2q, technique. Strong directional anisotropies in the stress were found for both the second level nickel and copper. The extent of nickel stress relaxation from the edge of a defect far exceeded that predicted by a simple metal on substrate model. The size of the defect did not appear to significantly influence the stress distribution in the second level metal; however, the data suggest that the linewidth may influence the magnitude of the stress along the length of a line when near a defect or other sharp discontinuity. INTRODUCTION One means of ensuring reliable interconnections on ceramic multichip carriers is to use redundant metallization. In this technique, two or more layers of metallization are deposited sequentially using separate photomasks. In principal, any defect in a given layer (such as missing metal) will be bridged by the redundant layer(s). However, there is a concern over the stress distribution near the edge of a defect. This is because large localized stresses can occur in the substrate near a sharp discontinuity due to so-called "edge forces".[l] The magnitude of these forces are dependent on the film geometry, residual stress in the film, and film thickness. Too great a force can lead to cracking in the substrate, and/or delamination of the film. It is therefore essential to understand the distribution of stresses around a defect or other sharp discontinuity in patterned metal films. Modeling is presently the most common means of determining these distributions. However, while modeling can show how stesses are distibuted, it cannot determine the absolute magnitude of these stresses. Consequently, it is necessary to have some experimental measure of the residual stress in the film. For polycrystalline materials, x-ray diffraction is the most widely used non-destructive means of stress measurement, and has a number of advantages over other means of stress measurement, such as wafer curvature. It is nondestructive, can be used with either blanket or patterned films, measures the strain in the film directly (so that one can work with a wide range of substrates having varying thicknesses), and is element specific (so that one can measure the contribution from each component in a multielement thin film stack). However, in conventional diffractometers, the area illuminated by the incident beam is on the order of several square millimeters. For fine (micron sized) patterned films, this means that one can obtain only an average stress over several features. This limitation can be overcome by the use of microbeam diffraction, in which the incid
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