Using µ-RAMAN Spectroscopy to Inspect Sintered Interconnects

To enable high operation temperature, sintering is the technology of choice for connecting the semiconductor die to the substrate for power electronic converters. Due to the mismatch of the coefficient of thermal expansion of die and ceramic based boards thermo-mechanical stress is created after sintering. Therefore, the mechanical properties, i.e. young's modulus and creep parameters, of the interconnect materials are essential for the reliability of the converters. µ-RAMAN spectroscopy (µ-RS) is a powerful tool to access the strain in the surface of semiconductors. In the present paper, the RAMAN shift is used to measure the strain in silicon test chips after bonding them to copper test boards. In the paper three different interconnect materials are investigated: eutectic AuSn soldering, Ag and Cu sintering. The AuSn interconnect is used as a benchmark because mechanical properties of eutectic AuSn solder including creep properties are well known. The packaging induced in plane stresses $\Delta \sigma (\Delta \sigma = {\sigma_{max}} - {\sigma_{min}})$ are measured for the interconnect materials: high stress is revealed for very dense almost void free Ag interconnects using silver paste (Δ $\sigma$=880MPa) which is closed to those observed for AuSn soldering (Δ $\sigma$=920MPa). Significant lower stress values are measured for copper flake-based interconnects (Δ $\sigma$=700MPa). It was observed that the stress distribution of well sintered interconnects is very similar to the distribution in eutectic AuSn solder. Deviations from the theoretical stress distribution identified by µ-RS indicate sinter process deviations. For instance, asymmetric stress distribution can be caused by inhomogeneous densification of the paste due to inhomogeneous paste printing or tilt in the bond tool. For AuSn soldering a Finite Element Model (FEA) was validated by the authors previously using µ-RS. To simulate the stress distribution for sintered interconnects parameter sets for creep models (Anand and Garafalo) were taken from literature. It must be noted that the published parameters vary extremely (several order of magnitudes) which is hardly plausible. Most parameter sets couldn't reproduce the experimental results measured by µ-RS at all. This might be attributed to the fact that sintered interconnect may have quite different properties. Using the Anand parameter (model 1) [1] the stress distribution and creep relaxation of the Ag interconnects could be modelled in Ansys mechanical work bench very well. The approach opens the door to use µ-RAMAN as a tool to access the mechanical properties of new sinter interconnects to develop reliable power electronic modules.