Constitutive modelling of annealing behavior in through silicon vias-copper

The mechanical properties of electroplated copper (Cu) filled in through silicon vias (TSVs) play a key role in the thermo-mechanical reliability of TSV packages. Generally, it is necessary to anneal TSV-Cu after electroplating to stabilize its microstructure. Different annealing conditions could affect the microstructure and deformation behavior of TSV-Cu. Thermal and mechanical load during post-process and service life could often cause TSV-Cu reliability issues by its deformation, such as TSV-Cu pumping, interfacial delamination and back end of line (BEOL) cracking. To understand the deformation behavior of TSV-Cu, this article tries to obtain the constitutive equations of TSV-Cu under different annealing conditions. Nanoindentation tests and finite element method (FEM) inversion are used to establish a constitutive model of annealing behavior in TSV-Cu. The power law constitutive models are determined to describe the elastic and plastic behaviors. Strain rate and annealing soften effects are both found from the constitutive model. The microstructures of TSV-Cu are also characterized by electron backscatter diffraction (EBSD) to correlate with mechanical performance. Grain size increased with the increase of annealing temperature, and both hardness and yield strength follow the Hall-Petch relationship, which are meaningful for calculating the deformation of the TSV package and improving the thermomechanical reliability of the device.