Thin Film Critical Residual Stress for Metal Pads on a Ceramic Substrate

Abstract

AbstractResidual stresses present in thin films evaporated on ceramic substrates, can lead to loss of adhesion of the metal features to the ceramic, or, between metal films in multi-level stacks. If the adhesion is good, ceramic cracking may occur around the metal features. Long term one must avoid stress corrosion cracking in the ceramic. These residual stresses are generated by a variety of causes. In general, defect incorporation during the deposition process and thermal expansion mismatch between thin film and substrate play major roles.In this paper, we describe a method for determining the residual stresses that may be detrimental, or more accurately, the residual stress induced load that may cause reliability problems. This method involves evaporating metal pads or various thicknesses onto the ceramic to the point of failure, and measuring the residual stress loading on the substrate. A direct measure of this loading is the stress intensity factor which takes into account both the residual stress and pad or film thickness. Thus, a critical “load” or stress intensity at which substrate cracking or thin film delamination occurs can be estimated. This approach can be used to select the optimum metal-thickness combination for various substrates.