Temperature - Dependent Yield Properties of Passivated Aluminum Thin Films on Silicon Wafers

A systematic set of stress-temperature measurements using the laser-scanning wafer curvature technique is performed to examine the effect of passivation thickness and passivation material on stress evolution and yield properties of passivated Al films during thermal cycling for twenty cycles. A modification of Stoney's equation for multiple films on a substrate is utilized to separate the individual stress response of the passivated Al film from experimental measurements of the average stress in the combined film structure of a passivated Al film and the average stress in single films of the passivation materials. Measurements of the stress-temperature response in unpassivated Al films indicate that film strength increases with decreasing film thickness, in agreement with work by other authors. The addition of a passivation layer to an Al film is observed to alter the stresstemperature response in the Al film. These effects include: (i) suppression of the compressive stress drop observed in some unpassivated Al films during thermal cycling, and (ii) a significant increase in the number of thermal cycles required to attain saturation in the stress hysteresis loop, and hence, in microstructural evolution. A preliminary microstructural study shows 3 to 4 grains present through the thickness of a passivated Al film after twenty thermal cycles, indicating that grain growth is suppressed by a passivation layer and suggesting that current theories of the strength of thin films based on a dislocation loop spanning the film thickness may not be valid for passivated films prior to microstructural stabilization. The tensile and compressive flow stresses of passivated Al films are affected by the thickness and type of passivation and change with thermal cycling. The effects of hillock suppression and constrained grain growth in passivated Al films may contribute to the observed differences in flow stress from unpassivated Al films. Thesis Supervisor: Subra Suresh Title: R. P. Simmons Professor of Materials Science and Engineering