New insights into fracture of Si in Cu-filled through silicon via during and after thermal annealing

Abstract

Copper (Cu)-filled through silicon (Si) via (CF-TSV) acts as an electrical conduit between different layers of circuitry in the advanced microelectronic devices. Due to wide difference in the coefficient of thermal expansion between Cu and Si, large thermal stresses get generated in both Si and Cu during the inevitable thermal excursions taking place during fabrication of microelectronic devices and their regular operation. This study examines the fracture of Si wafer in a CF-TSV due to annealing over the temperature range of 250–550 °C and the post-annealing natural ageing. Nucleation of micro-cracks in Si in the vicinity of the Cu pillars were observed, which propagated at a constant velocity during post-annealing room temperature ageing, thereby substantiating delayed fracture in Si. Microstructural examination revealed that the micro-cracks nucleated as a result of formation of a Cu-Si compound during annealing, whereas the crack propagated due to the oxidation of Si at the crack tip. Consistently, the micro-cracks did not propagate when the sample was stored in high vacuum. Furthermore, Raman spectroscopy and finite element method were used to gain insights into stress generation due to Cu-Si reaction, and nucleation and propagation of the crack.