The piezoresistance of aluminum alloy interconnect structures

The effects of applied strain on the resistivity of Al thin film metallization interconnects have been measured with a novel methodology that uses thermal expansion mismatch to produce the strain. The interconnect volumetric strain is induced by thermal cycling of passivated and unpassivated interconnects between ≈70 and 373 K. The coefficient of piezoresistivity, defined as dρ/dεv, where ρ=resistivity and εv=volumetric strain, is determined by properly accounting for the degree of interconnect constraint and thermal expansion mismatch strain induced during temperature changes. The volumetric strains are calculated for unpassivated and passivated lines of varying thickness and width. A model which incorporates the geometrical and piezoresistance effects on the measured interconnect resistance during temperature changes is described. The coefficient of piezoresistivity is calculated by a fitting procedure which provides an accurate and consistent fit for both unpassivated and passivated interconnects of different geometries and different strain states. The measured coefficient dρ/dεv is 2.0×10−5 Ω cm in tension, similar to earlier results in bulk Al samples measured in compression but significantly higher than values recently measured in Al interconnects. The application of the calibrated coefficient of piezoresistivity for the measurement of electromigration-induced stresses in novel interconnect test structures will be described.