A Study of the Global and Local Cu Protrusion in Cu-Filled through Silicon Vias under Heat Treatment

Thermo-mechanical reliability problem of Cu-filled through silicon via (TSVs) has been considered as one of the crucial issues for TSV application. Due to the coefficient of thermal expansion (CTE) mismatch is large between Cu and the consistent materials, thus free expansion and shrinkage of Cu will be restrained when working temperature changes. The induced thermal stresses, including axial stresses and interfacial shear stresses, will lead to TSV-Cu protrusion. However, lots of problems are still unsettled about Cu protrusion, the observation and analysis of local Cu protrusion phenomenon is an outstanding problem that needs urgent attention. In order to comprehensively investigate the Cu protrusion phenomenon in Cu-filled TSVs, the mechanism of local Cu protrusion needs to be further studied. In this paper, the Cu-filled TSV interposer samples with blind TSV were utilized in heat treatment experiments to analyze Cu protrusion. According to the different heat treatment parameters, seven samples of TSV interposer were selected, one of which was used as the control sample. Using a tube furnace, six TSV interposer samples were heated to different peak temperatures (250 °C, 350 °C and 450 °C) by two heating modes (samples were heated to peak temperatures in furnace with the heating rate of 10°C /min, samples were placed in the furnace after furnace temperature had risen to peak temperatures). The occurrence of global Cu protrusion and local Cu protrusion was related to possible influence factors including thermal stress, grain growth and void formation. This research was expected to help optimizing the structure and technology design of TSVs and further improve thermo-mechanical properties of TSVs.