Evaluation of residual stress in a transistor using micron scale strain sensors

Thermal and intrinsic stresses that occur during thin-film processing and assembly processes vary the residual stress in transistors. The residual stress causes the shift of electronic functions of dielectric and semiconductor materials and degrades their mechanical reliability. Therefore, it is important to measure the residual stress in transistor structures. In this study, we have measured the local distribution of the residual stress on a transistor formation surface of an LSI chip assembled by using a flip chip structure with an area-arrayed metallic bumps by applying strain sensor chips. It was found that the amplitude of the local residual stress distribution in the stacked chips reached about 250 MPa. This local stress distribution caused the local shift of electronic functions of transistors by about 10%. The residual stress in a transistor after thin-film processing was also measured by using these strain gauges. Transistor structures were formed on the gauges and the change of the stress was measured by removing the thin films by focused ion beams. It was confirmed that both the thin film processing-induced stress and the assembly-induced stress affect the final residual stress in a transistor structure.