Raman piezospectoroscopic stress measurement and FEM modeling techniques were combined to study the stresses in metal trenches. A trench in a metal layer is one of the most frequenctly encountered structures at intermediate stages of mutilayer IC interconnect manufacturing. An aluminum film deposited at elevated temperatures developes tensile stresses during cooling due to its larger thermal expansion coeffficient than the silicon substrate. Further, if a reaction takes place between an Al layer and a Ti shunt layer, additional tensile stresses can develop due to the volume shrinkage associated wiht TiAl_3 formation. Because of the abrupt termination of the metal layer at the trench edges, large tensile stresses are induced in the trench due to load transfer, creating a potential reliability hazard. For Raman stress measurement purposes, special samples were amde with a Al/Ti bilayer metal stack deposited on silicon wafers ith a thin thermal oxide separation layer between the metal layer and the silicon substrate. When trenches were etched in the metal layer, the silicon substrate was directly accessible by the Raman optical microprobe. Measurement of stres sinduced frequency shift profiles indicated that the reaction of Ti and Al increased stresses in tenches significantly, and the stress level was higher for narrower trendches. FEM modeling was performed to calculate the frequency shift profiles using an effective Ti-Al reaction induced volume shrinkage as an adjustable parameterm. This is because the extent of stress relaxation in the TiAl_3 layer during or after the reaction process was unknown. By matching measured and calculated shift profiles, the effective volume shrinkage was determined and the stress fields in the trenches were obtained. The results showed that significant stress relaxation occurred in the TiAl_3 layer.
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