Progressive cracking of a multilayer system upon thermal cycling

This article considers progressive cracking upon thermal cycling of a thin multilayer on a thick substrate. The prototypical system comprises a thin e lastic layer of a dielectric material above another thin metal interconnect layer attached to a silicon substrate. Residual stresses exist because of the thermal expansion misfit and due to deposition. Putative fabrication fl aws are presumed to be present in the dielectric. When activated by residua l stresses these flaws can induce cracks that channel along the dielectric. Conditions that induce yielding of the metal upon initial cooling are show n to exacerbate this phenomenon. Moreover subsequent thermal cycling may in duce ratcheting, wherein cracks develop progressively due to repeated yield ing of the metal layer The roles of initial stress, cyclic temperature ampl itude, and interconnect yield strength in these phenomena are investigated using finite element models which explicitly account for cyclic yielding. T he most deleterious situation is found to be that wherein the entire metal layer reaches yield at some stage during the temperature cycle. Several sce narios relevant to semiconductor devices are considered.