Buckling, wrinkling and debonding in thin film systems

Thin films bonded to substrates commonly occur in semiconductor dielectric stacks. In these systems, many times, the mismatch in the coefficient of thermal expansion between the films and the substrate result in significant compressive stresses during processing. These compressive stresses may lead to instabilities such as buckling or wrinkling, possibly resulting in debonding of the films. In general, at the present time, the mechanics of buckling and wrinkling leading to debonding in thin film systems are not well understood. A further significant challenge to modeling these systems is that the conditions under which the instabilities occur depend on possible plastic deformation in metal films (in turn dictated by processing temperatures) as well as the presence of geometric features such as vias in the film stack. In this paper, we review analytical derivations of conditions under which buckling and wrinkling induced debond occur in thin film systems. We describe the film interface and (where appropriate) the compliant substrate using a bi-linear Cohesive-Zone fracture model. We utilize the developed theory to estimate interfacial fracture toughness of weakly bonded thin film systems through a newly proposed non-contact, thermally driven, patterned buckling delamination test. The proposed test does not need a weakened region of the interface to initiate crack, nor does it require mechanical loading; it relies on inducing a compressive stress due to heating of the film on a thick silicon substrate. The test is demonstrated on a model system consisting of Al/Su-8/Si stack. Finally, we review wrinkling of thin film systems, and analyze the impact of pattern features on the propensity of thin films to wrinkle sooner than predicted for blanket films.