Thermo-mechanical characterization of passive stress sensors in Si interposer

Passive stress sensors have been integrated in a silicon interposer test vehicle to investigate thermo-mechanical stress in a typical 2.5D system. The present sensors are integrated in a rosette-shape consisting of eight oriented copper serpentines acting like strain gauges. An innovative design allows theoretically the calculation of a partial stress tensor, including three planar and one out-of-plane components. Electrical measurements at wafer level, combined to FIB/SEM cross-sections, revealed a strong impact of elaboration processes on the structures electrical characteristics. Numerical simulations using finite element analysis were built to evaluated the theoretical sensitivity of copper serpentine to mechanical strains. Finally a dedicated four-point bending tool coupled with a four-terminal resistance measurement setup was fabricated to extract experimentally the values of sensors sensitivity factors. Preliminary results depicted in this paper highlight a sensitivity to stress of distinctly oriented resistors. Several identified sources of data dispersion are inherent to the present measurement configuration and prevent a reliable calculation of strain gauges so-called “gauge factors”.