Verlag des Forschungszentrums Jülich
JUEL-3528 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 tri-axial
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 (( 105 A/cm2)) 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 (FEM) 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°) the observed stress relaxations are
very small because these temperatures are to low for the thermally activated processes.
The stress relaxation increases with maximum values at temperatures between 150° and
about 225°. In this range the change of the hydrostatic stress was about 4(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.
Roths, Christoph
Mechanische Spannungen und Elektromigration in passivierten Leiterbahnen
163 S., 1998
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