In a series of articles, we consider the effects of the microstructure and the associated anisotropy of Sn grains on electromigration in lead-free solder bumps. In this article, we carried out experiments to characterize microstructures of tin-based solder bumps and to investigate their effects on electromigration-induced failure. Accelerated electromigration tests and electron backscatter diffraction (EBSD) tests are performed on tin-based solder bumps in wafer-level chip-scale packages (WLCSPs). EBSD analysis, together with the results of time to failure from accelerated electromigration tests, provides valuable information on the effects of the microstructure on electromigration-induced failure that can be used for predictive modeling. For Ni-based metallization, we find that tin self-diffusion is the dominant degradation mechanism and a strong effect of the grain orientation in the solder on the time to failure.

In this and subsequent papers, we consider the effects of the microstructure, in particular grain orientations, and the associated anisotropy of Sn grains on electromigration-induced degradation in lead-free solder bumps. This paper investigates the effect of the anisotropy of Sn grains on current-driven self-diffusion and the resulting atomic flux divergence that is associated with material depletion at the cathode side of the bump and ultimately failure. Finite-element thermoelectric simulations of a section of a chip scale package and a refined submodel are carried out assuming single-crystal and bi-crystal solder bumps. Anisotropic material properties of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -Sn are used in the simulations. By varying crystal orientation of the solder bumps, the effect of grain orientation on the current density, temperature, and atomic flux divergence is studied and discussed. In the future papers, we will consider the coupled mechanical response including stress-driven diffusion.

Wearable sensors can provide important human physiology and activity data and therefore have promising applications in healthcare, entertainment, and security. Here, we report the design, fabrication, and characterizations of a thin silicon film sensor for wearable sensor applications. Temperature, light, and strain sensing capabilities of the thin film of doped silicon, fabricated using a controlled spalling process, were fully characterized. An n-doped silicon thin-film sensor prepared with the spalling technology exhibited a temperature coefficient of -0.44%/degrees C. The sensor also showed excellent light sensin response in an illumination range from 110 to 1710 cd/mm(2). The ratio of the electrical resistance changes over the applied forces was measured to be around 0.6%/N.

Mechanisms of electromigration (EM) damage in Cu interconnects through various CMOS nodes are reviewed. Pure Cu and Cu alloy interconnects that were used down to 14 nm node can no longer satisfy the electrical current used for 10 nm node and beyond in high-performance ICs. Cu interconnects with a metal cap should be used. Cu interface diffusivity with EM activation energy of 1.6 eV was found to be the dominate EM factor in Cu lines with a Co liner and cap. The median lifetime of 7 or 10 nm node Cu with TaN/Co liner and Co cap is predicted to be over ten thousand years at 140°C with 1.5×10 7 A/cm 2 . However, the resistivity size effect and the difficulty of scaling barrier/liner layer without defects can limit the Cu BEOL roadmap below the 7 nm node.

In this paper, a finite element based simulation approach for predicting the effect of microstructure on the stresses resulting from electromigration-induced diffusion is described. The electromigration and stress-driven diffusion equation is solved coupled to the mechanical equilibrium and elastic constitutive equation, where a diffusional inelastic strain is introduced. Here, the focus is on the steady state, infinite life case, when the current-driven diffusion is balanced by the resulting stress gradient. The effect of the crystal orientation in Sn-based solder joints on the limiting current density for an infinite life is investigated and compared to experimental observations in the literature. The effect of the grain structure for Al interconnect lines on the dominant diffusion path and estimates for the effective charge number for two different diffusion paths in Al interconnects determined by matching simulations to experimental measurements of elastic strain components in the literature are also presented.

A finite element-based simulation approach is used to predict stress evolution resulting from electromigration-induced diffusion. A diffusion-mechanical coupled model is developed where the electromigration and stress-induced diffusion is coupled to the mechanical equilibrium with an introduction of the electromigration induced inelastic strain. A crystal plasticity constitutive model is developed to capture the plastic anisotropy of Sn. The problem is solved using a staggered approach at each time step. Simulations are conducted on a simplified geometry of a single crystal Sn cylinder and stress evolution is obtained with different current densities and Sn crystal orientations.

The elastic and plastic anisotropy of β-Sn has an important effect on the stress state, and hence the electromigration induced degradation in Sn-based solder joints. A crystal plasticity model is developed to capture this effect. Calibration of this model is done by using the available literature data. Future refinement of the slip parameters is needed to optimize the model. The model is implemented in a finite element framework and is used to simulate the response of single crystal β-Sn to electromigration induced strains. This work can be used as a basis for future development of coupled models of electromigration and stress induced degradation in Sn-based solders.

This paper studies the microstructure effects on electromigration in lead-free solder joints in wafer level chip scale package (WL-CSP). It is an extension of an earlier isotropic model [1]. The three dimensional finite element model for solder joints is developed and analyzed in ANSYS®. A sub-modeling technique combined with an indirect coupled electrical-thermal-mechanical analysis is utilized to obtain more accurate simulation results in solder bumps. Four representative microstructures of the solder bumps are modeled and anisotropic elastic, thermal and diffusion property data are used. The results obtained from the four representative microstructures are compared with each other. The microstructure effects on electromigration are drawn from the plots of the atomic flux divergence (AFD) and the time to failure (TTF) with respect to microstructure parameters.