Abstract

A laser spallation technique to measure the tensile strength of thin film interfaces is introduced. In this technique, a laser-generated stress wave of nanosecond duration in the substrate spalls off (completely removes) a coating deposited on it’s the substrate’s front surface. The threshold laser energy is converted into the tensile stress (strength) at the failure site (usually the interface) by using an optical interferometer. Because of the ultra-short duration of the stress wave loading, all plastic deformation processes that usually accompany the coating decohesion event are suppressed such that the measured value can be regarded as fundamental or intrinsic to the material system (including the defects, if any). Application of this technique to test planar as well geometrically heterogeneous interfaces in IC’s, substrates, and packages is demonstrated. The technique is used to quantify the degrading effects of moisture and in-situ temperature rise on the tensile strength of a polyimide/Si3N4/Si interface system whose strength was systematically degraded by exposing the samples to controlled humidity (50-70% RH) conditions for varying duration (12-96 hrs) and temperatures (30°C-150°C). These measurements of strength degradation can now be used to predict device reliability from a fundamental standpoint in conjunction with simulations capable of predicting time-dependent stress concentrations, moisture accumulation, and temperature rise at critical interfaces during processing and service environment in actual systems.

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