Fracture strength evaluation of submicron-thick molybdenum films using a multilayer bulge test incorporating elasto-plastic behavior

As semiconductors become higher capacity and more complex, 3D packaging technology is gaining attention. In this structure, multiple chips are stacked, leading to complex wiring structures and stress concentration. In addition to thermal stress, the impact of external vibration is also a concern, creating a risk of wire breakage. Molybdenum is attracting attention as a candidate material for future wiring due to its characteristic of exhibiting extremely little increase in resistivity during thin-film formation. Traditionally, measuring the fracture strength of thin films required single-layer test specimens. Since the thin film could not stand independently, this necessitated specialized test specimens and equipment. To address this challenge, we developed a multilayer bulge method. This method constructs the area surrounding the test specimen using a silicon substrate approximately 1 mm thick, making handling easier and enabling simplification of the test equipment. In this study, we evaluated the fracture strength of a 220 nm thick molybdenum thin film using the multilayer bulge method, which accounts for elastoplastic deformation. The results revealed that the fracture strength of the molybdenum thin film is estimated to be 2.6 GPa, approximately 3 to 5 times higher than that of bulk molybdenum.