X-Ray Determination of Residual Stresses of Encapsulated Thin Aluminum Lines

Differences in thermal expansion coefficients between the dielectric layers, the substrate, the metallizations, and the circuit chips generate complex residual stress fields within multichip module (MCM) packages. For example, encapsulated interconnect lines in a typical MCM are under triaxial stress. Quantifying the triaxial stress is needed to understand critical issues such as stress relaxation, stress-induced voiding, as well as providing benchmarks for finite element calculations. Employing a novel sample design, triaxial residual stresses have been determined in thin aluminum lines encapsulated in benzocyclobutene dielectric layers by x-ray diffraction techniques. Significant differences are observed in the stresses if the lines are deposited on the Si substrate or on intervening polymer layers. The stress level is constant with intervening polymer layer thickness. The results suggest a fundamental difference between the strain transfer across the silicon oxide-aluminum interface relative to the silicon oxide-polymer interface.