Residual stress investigations at TSVs in 3D micro structures by HR-XRD, Raman spectroscopy and fibDAC

In this paper the residual stress in single-crystalline Si around W-filled TSVs was determined experimentally by three methods with high spatial resolution and compared to one another. In contrast to Cu as TSV filler, W has the potential advantage of a lower CTE mismatch to Si resulting in lower thermally induced stress at the TSV-interface. As test layout a cross-sectioned double-die stack was used consisting of a top die with TSVs which is bonded by Cu-Sn Solid Liquid Interdiffusion Bonding (SLID) to the bottom die. Three different experimental methods have been used to determine mechanical stresses in silicon nearby tungsten TSVs - HR-XRD performed at a synchrotron beamline, microRaman spectroscopy and stress relief techniques put into effect by FIB milling. All methods possess, to a different extend, high spatial resolution capabilities. However they differ in their sensitivity and response to the particular stress tensor components relevant for the residual stress state nearby TSV structures. Stress measurements were performed on test samples with W-TSVs in thinned dies, which were SLID bonded to a thicker Si substrate die. The measurements captured stresses introduced by the W-TSV as well as by the wafer bonding process. A stress range from several MPa to hundreds of MPa could have been covered with a spatial resolution ranging from 100 nm to tens of microns. Measurement results were compared to one another and to simulated stresses from finite element analysis (FEA). All experimental methods show the influence of W and Cu-Sn-Bond in Si. The very high stress sensitivity for HR-XRD below 1 MPa could be shown. For small stress gradients the analysis of the peak position gives reasonable results and for larger stress gradients a profile analysis of the diffraction peak is more accurate. The results show that in intrinsic stress in W may have to be considered in FEA and more attention should be directed to the accuracy of the FE-modelled Cu-Sn SLID bond with respect to shrinkage during phase formation of Cu
₃
Sn.