Simulation of mass flux divergence distributions for an evaluation of commercial test structures with tungsten-plugs
The evaluation of via structures, metallisations as well as a miniaturisation of the layout dimensions lead to the determination of new test structures. The time for prediction of the reliability of such structures should be very short but also effective. For instance for fast reliability prediction, constant power or constant current tests can be used. Due to a change in fault mechanisms an extrapolation to working conditions is sometimes difficult. Simulations can help in prediction of the failure mechanism as well as the determination of the weakest link in the structures. In this study the void location as well as the mass flux divergence distribution of a conventional tungsten-plugged metallisation structure was investigated by finite element simulation (using ANSYS/spl reg/). A calculation routine is used for the determination of the mass flux divergences and the void formation in the model. The suitability of the simulation in test structure design will be shown. Based on the default models the overlap of the metallisation, the metallisation length, and width and the applied current density was varied. The stress free temperature was set to 450/spl deg/C. This temperature was taken from the last process step. The substrate temperature was set to 150/spl deg/C like the temperature in the reliability stress test. The mechanical stress and temperature and current density distribution as well as the influence of the different migration mechanism was determined. Using these results the void formation and its localisation was calculated and compared with the measurement.
Electromigration characteristic of SnAg/sub 3.0/Cu/sub 0.5/ flip chip interconnection
Electromigration is a reliability concern of microelectronic interconnections, especially for flip chip solder bump with high current density applied. This study shows that with the line-to-bump geometry in a flip chip solder joint, the current density changes significantly between the Al trace and the bump, while the current crowding effect generates more heat between them. This large Joule heating under high current density can enhance the migration of Sn atoms at the current entrance of the solder bump, and cause the void formation at the entrance point. The present study finds two kinds of electromigration failure modes at the cathode/chip side of the solder bump: the pancake-like and the cotton-like void. The experimental finding shows that the effects of polarity and tilting are key factors to observe in the electromigration behavior of SnAg3.0Cu0.5 solder bumps. Consequently, this study has designed a three-dimensional numerical model and a corresponding test vehicle to verify the numerical finding. The maximum current density is simulated through the finite element method (FEM) to provide a better understanding of local heat and current crowding. This study finds that the current crowding ratio is reduced linearly while the void formation is increased. Furthermore, it is concluded that there is a linear relationship between the growth of the intermetallic compound (IMC) layer and the applied current density at the anode/substrate side
Finite Element Analysis of Thermal-Mechanical Stress Induced Failure in Interconnects
In this work a study of the nature as well as an evalution of the thermal-mechanical stress in aluminum interconnects was carried out. A theoretical model discribes the atom flux which can be induced by the relaxation of the stress. Based on this theory an algorithm has been developed and integrated into the finite element simulation software. This algorithm allows the calculation of the mass flux divergence and prediction of the failure location before the damage occurs. For the verification of this algorithm an aluminum pad structure sputtered on thermal oxide layer was used. The failure location was correlated with in situ observation during the long term stress tests. Experimental results confirm that the observed structure degradations correspond with the simulations very well.