Evaluation of fracture strain of aluminium film by multilayer bulge method

As semiconductors become more capacitive and sophisticated, 3D packaging technology is gaining increased attention. Since multiple chips are stacked in such configurations, the wiring structure becomes more complex, leading to stress concentration. In addition to thermal stress, mechanical vibrations may also be applied, raising concerns about potential wiring failure. Aluminum and copper are commonly used as wiring materials. Traditionally, measuring the fracture strength of thin films required preparing a single-layer film specimen. However, because thin films are not self-supporting, special specimens and testing equipment were necessary.To address this, we have developed a multilayer bulge method. In this method, the test specimen is supported by a silicon substrate approximately 1 mm thick, which improves handling and simplifies the testing apparatus. In this study, the multilayer bulge method was applied to evaluate the fracture strain of aluminum films, yielding the following results. The fracture strain of the aluminum film was estimated to be between 3.3% and 4.2%. This value is approximately one-tenth of the fracture strain of high-purity (99.99% or 99.9999%) aluminum with a thickness of 1 mm. This reduction is presumed to be due to the thin film structure and the presence of an underlying layer, which inhibits dislocation growth.