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.
Static and dynamic analysis of failure locations and void formation in interconnects due to various migration mechanisms
Abstract To avoid time-consuming long-term stress tests, reliability of metallizations is determined by accelerated stress tests with high-applied current densities under high-temperature conditions. During these tests temperature gradients and mechanical stress will occur, and the main failure mechanism in the metallization will be migration effects like electro-, thermo- and stressmigration. For reliability prediction of interconnects and via structures, numerical methods (as the finite element method FEM) can give a better understanding of the local heating and current crowding as well as mechanical stress. Mismatch of thermal expansion coefficients of the different materials leads to mechanical stress. Thermal–electrical–mechanical behavior can be calculated with the FE program ANSYS. With user programs the stress gradient as well as mass flux divergence distribution including electro-, thermo- and stressmigration can be calculated (Frac. & Duc. vs. Brit. Beh.—Theo., Modelling and Experiment. Symp. Boston, 1999). Concentration gradients are not considered in this calculation. The location of the maximum mass flux divergence determines the failure location in the static case. Simulation of dynamical behavior can be done by deleting elements with maximum divergence values in the model step-by-step (Proceedings of EuroSimE April, 2002). Out of this, void formation in the metallization as well as time to failure can be calculated. Static and dynamic simulations can be verified by measurements and in situ investigations in a scanning electron microscope (SEM). With the static analysis the weakest link in metallization structures can be identified. With this simplified model it is possible to determine the failure location as well as the void formation in the metallization structure. Also a design optimization is possible with the help of the simulation.
Cutaneous myiasis caused by the housefly, Musca domestica
Summary We report a case of cutaneous myiasis by the housefly. Musca domestica.