Mechanical Reliability of Metal-Si Systems at different Length scales under Thermal Cyclic Loading

Abstract

Metal structures of nanometer to micrometer sizes in different configurations, such as thin films, thick coatings and 3D interconnects, are widely used for conducting electrical signals in several semiconductor-based devices, such as microelectronic chips, photovoltaics (PV), micro/nano-electromechanical systems (MEMS/NEMS), etc. In practice, the main challenge that confronts the structural reliability of these systems arises due to the wide difference in the coefficient of thermal expansion (CTE) of metal (Cu:17.5 × 10-6 K-1, Ag:19 × 10-6 K-1, Al: 23 × 10-6 K-1) and Si (2.7 × 10-6 K-1); This generates large thermal stresses during fabrication, post-fabrication annealing as well as thermal excursions during service. Thus, it is necessary to engineer the devices based on the understanding of the microscopic processes that lead to deformation and failure of these structures under cyclic thermal stresses to ensure their reliable functioning over the designed service life. In this regard, three metal-Si systems, namely Cu filled through Si vias (TSV), PV solar cell and thin metal film/particles on Si - all having different geometrical constraints as well as characteristic length-scales - were investigated in the course of this work. The first metal-Si system studied was 100 µm diameter Cu- filled TSV (Cu-TSV) which are used as the interconnects in the 3D microelectronic systems. In this segment of the study, the effect of TSV fabrication process and subsequent annealing (temperature range of 250 to 550 °C) and accelerated thermal cycling (temperature range of -50 to 150 °C) on its structural integrity was studied. During annealing and thermal cycling tests, grain boundary sliding induced non-uniform extrusion of Cu, Cu-Si reaction and nucleation of micro-cracks in Si were observed, whereas, during post-annealing aging at room temperature, slow growth of the micro-cracks in Si was observed. Electron back-scatter diffraction (EBSD) analysis of extruded grains and crystal plasticity (CP) simulations indicated only a minor role of elastic-plastic anisotropy in the non-uniform extrusion of Cu. The micro-crack nucleation in Si was observed as a result of the residual thermal stresses and the volumetric stress associated with the Cu-Si compound formation. The conclusion was supplemented by stress measurement using Raman spectroscopy. Subsequently, slow crack growth observed in Si was attributed to the oxidation of Si at the crack tip, which was confirmed by extended finite element method (XFEM) and contour integral based finite element analysis (FEA), electron-probe micro-analysis (EPMA) and aging of Cu-TSV samples inside high vacuum environment. The second system studied was commercial PV solar cell, which has flat metal coatings of a few tens of micrometer thickness on both the top surface and the bottom or backside of Si. To assess mechanical reliability, accelerated thermal tests were carried out on the PV solar cell samples (temperature range of -40 to 90 °C and -50 to 150 °C), which showed that Ag-Si interface (i.e., on top) is less susceptible to cracking as compared to Al-Si interface (i.e., at the bottom). Ideal geometry and microstructurally sensitive FEA showed that plasticity in the Al-Si eutectic layer precluded the deflection of the cracks into Si. Further, XFEM based analysis showed that the wavy interface was more susceptible to fracture as compared to the flat interface. This prediction was confirmed by performing thermal cycling tests on PV solar cell samples having a substantially less wavy interface between Al and Si, over a wider temperature range of -50 to 150 °C, which showed improved resistance to crack nucleation and growth. The third metal-Si system studied in this work was Cu nano- and micrometer-sized particles on the Si, which were formed via dewetting of 10-40 nm thick Cu films sputter deposited on Si. The kinetics of the dewetting of Cu thin films, which was used to fabricate Cu particles on Si, was investigated using in-situ annealing experiments carried out inside a scanning electron microscope (SEM) and phase-field modeling. It was observed that Cu dewetting is governed by the void nucleation at triple points and the fractal growth of voids through surface diffusion mechanism. Subsequently, thermal cycling, annealing and shock tests were carried out on the Cu nanoparticles-Si system using different heating-cooling ramp rates over the temperature range of -25 and 150 °C and 25 and 400 °C. With a faster rate, changes in shape and size of the Cu particles and re-dewetting of the residual Cu layer were observed, while a slow cooling rate did not induce any such effect, even if the annealing was performed at high temperature (e.g., 400 °C). It is understood that the heating-cooling rate controls the deformation of Cu particles, which is highly sensitive to length scale as well as strain rate. Nevertheless, the study performed on different systems revealed a strong effect of length scale, metal-Si boundary conditions and processes involved in the fabrication on the structural integrity of the metal-Si systems. The underlying mechanism responsible for the difference in the observed effects is a change in either the stress state or the length-scale sensitive failure of the materials or both. Overall, it is inferred that a smooth metal-Si interface with a stronger adhesion layer would enhance the structural reliability of the metal-Si system.