The Connection Between Electromigration Resistance and Thin-Film Adhesion and Their Degradation With Temperature

Abstract

Growth of voids driven by electromigration in current carrying metal thin films is a common mode of failure in microelectronic devices. Studies have shown that electromigration driven void growth rate in such devices is inversely related to the interface adhesion strength between the current carrying metal thin films and the passivation layers on top of the metal films. In this paper, we characterize the degradation in electromigration resistance with temperature in metallic thin films with passivation and argue on a general mathematical form for connecting the electromigration resistance to adhesion strength. Test structures with thin film Cu metal lines are designed and fabricated for performing electromigration experiments and characterizing the void growth rate. Tests are carried out on devices with Cu thin film having TiN and SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> as passivation layers, over a range of current densities and temperatures. The growth rates of the voids in the Cu lines undergoing electromigration are analyzed using classical models from literature. The experimental observations on void growth rate with increase in current density and temperature are validated by comparing the data to those from prior literature. The void growth is also simulated and the void interface is tracked using an explicit boundary tracking computational technique. It is observed that the contribution of interface adhesion strength to electromigration resistance decreases with increase in temperature. That is, the experiments suggest that the passivation layers cease to provide resistance to electromigration with increasing temperature. Based on the test results, we develop a general form of a mathematical model to describe the degradation in electromigration resistance with metal film adhesion to the passivation layer.