Thermo-mechanical stresses in laminated polymer films on silicon substrates

Thermo-mechanical stress in MCM-D substrate is an important reliability and fabrication issue. The difference 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 coating of the polyimide to fabricate a dielectric layer, this paper mainly focuses on the lamination process. This study uses materials set representative of typical MCM-D technology to monitor the stress level in the thin film layers of polymer. The substrate deflection caused by composite stresses due to fabrication and thermal cycling is determined by a curvature measurement technique. A simpler analytical model which predicts a stress contribution from each individual layer during MCM-D substrate fabrication is proposed and verified by computer simulation as well. Initial stress or bending is intrinsic but becomes extrinsic upon thermal cycling. The composite stress or bending in multilayer structures is due to a contribution from each individual layer. In a thermomechanical stress viewpoint, Ultem(R) 1000 thermoplastic and the epoxy thermosetting adhesive exhibit quite different behaviors. While the epoxy thermosetting adhesive exhibits an elastic behavior predicted from the elastic analysis, the total thermo-mechanical stress tends to decrease when Ultem(R) thermoplastic is used as an adhesive.