Thermo-Mechanical Stresses in Laminated Polymer Films on Silicon Substrates

Abstract

Thermo-mechanical stresses in MCM-D substrate are important reliability and fabrication issues. The differences in coefficient of thermal expansion (CTE) between substrate, polymer, and metal leads to complicated stress fields in multilevel interconnect structures. While the majority of reports in the literature have focused on spin-coated polyimides, this paper mainly focuses on laminated polymer dielectrics. This study uses material sets representing typical MCM-D structures to monitor the stress level in the polymer films. The substrate deflection caused by composite stresses during fabrication and thermal cycling test is determined by a curvature measurement technique. A simple analytical model which predicts a stress contribution from each individual layer during MCM-D substrate fabrication is proposed and developed by computer simulation as well. The composite stress or bowing in multilayer structures is due to the summation of each individual layer contribution. From a thermo-mechanical stress viewpoint, Ultem® thermoplastic and the epoxy thermoset adhesive exhibit quite different behaviors. In case of the epoxy thermoset adhesive, the amount of bowing increases as number of layers increases and its behavior can be predicted by an elastic model. However, in the case of the Ultem® thermoplastic adhesive, bow values become stabilized as number of layers increases because of the stress relaxation effect of the thermoplastic adhesive during lamination process above T_g of the Ultem® thermoplastic.