Effect of annealling process on microstructure of Cu-TSV

Three-dimensional (3D) package technology is currently considered as one of the best solution to beyond Moore's Law and achieve miniaturization, high density and multi-functional of the device. Copper filled in through silicon via (Cu-TSV) technology with good electrical performance and high reliability, is a new type of 3D packaging technology. Cu-TSV has been widely used in integrated circuit technology due to its excellent interconnection performance. Residual stress could be generated in Cu-TSV during the manufacturing process which could be relaxed by annealing treatment. However, the Cu protrusion out of the TSV could also occurred during annealing process due to the large temperature variation and the severe mismatch of thermal expansion coefficient between copper (17.6×10-6/K) and silicon (2.8×10-6/K). This could be a potential threat to reliability of the back-end interconnect structure, since it might lead to cracking or even failure of the device. In this paper, the TSV samples were annealed at different temperatures and times to study the protrusion and microstructure of the copper. The heating temperatures were 250°C, 350 °C and 450°Cwith 30min, 60min and 90min, respectively. The protrusion behaviors and microstructure of different samples were further characterized and compared. The protrusion behaviors were characterized by Focus Ion Beam (FIB) and Confocal Laser Scanning Microscope (CLSM), which reveal that the annealing temperature and heating time have a significant effect on the protrusion. With the annealing temperature and time increase, the protrusion behavior becomes more and more obvious, however, the annealing time has little effect than annealing temperature on the protrusion. The cross section microstructure of TSV was also studied by Electron Backscatter Diffraction (EBSD). The results showed that the Cu grain size of all specimen increases but the grain orientation no significant changes after annealing. The grain size is positive correlation to the annealing temperature. The residual stress in Cu and Si around the TSV was measured using Raman spectrometer. This work could help to find out the optimized annealing condition which could further improve the reliability of TSV structure.