Mechanische Spannungen und Elektromigration in passivierten Leiterbahnen

Aluminium and aluminium-alloys are commonly used as metallizations for electrical connections of electronic devices in integrated circuits. Two related effects can reduce the lifetime of these lines: thermally induced mechanical stress and electromigration. Mechanical stresses are imposed on the lines after cooling from processing temperature down to room temperature due to the different thermal expansion coefficients of the metal and the surroundings (Si substrate and the passivation). The relaxation of these triaxial stresses may induce voiding and subsequently damage the lines leading to complete failures in the worst case. Electromigration-damage can occur whenever the metal lines are stressed by very high DC-current-densities ($\geq$ 10$^{5}$ A/cm$^{2}$). Electromigration-driven mass transport divergencies induce local mechanical stresses in the metal lines which also may relax by voiding. The aim of this work is to determine the amount of stress relaxation during isothermal annealing of the metal lines with and without electromigration-current. A wafer curvature technique was used to mesasure the average mechanical stresses in the metal lines. Using finite element methode (FEAR) calculations and the formalism of eigenstrains estimates of the plasticity of the lines could be made seperately for shear relaxation at constant volume and volumetric relaxation by voiding. At first the stress relaxation in metal lines was investigated without an electromigration current. At low temperatures (50°C) the observed stress relaxations are very small because these temperatures are to low for the the finally activated processes. The stress relaxation increases with maximum values at temperatures between 150°C and about 225°C. In this range the change of the hydrostatic stress was about 4 $\cdot$ 10$^{-3}$ after 100h, i.e. there are already many voids nucleated in metal lines. With an electromigration current the measurements show nearly no additional changes in average stress in the lines although the electrical resistance of the lines rises and SEM investigations exhibit significant damage in the metallizations up to complete failure of lines. Constant stress means no change in the void-volume during the electromigration test, so that the observed damage is most probably a result of the redistribution of already existing voids created by relaxation of thermal stresses