Verlag des Forschungszentrums Jülich
JUEL-3601
X-ray diffraction technique is used in this work to measure the mean strain in an
array of parallel passivated lines. Different aluminum alloys, passivation materials
and passivation geometries are investigated during thermal cycles.
All samples show a strong [111]-texture that seems to be present independent of
the samples shape, i.e., in a blanket film or in patterned lines. The amount of
copper in the aluminum lattice has a small but noticeable influence on the texture
i.e. the texture formation is less distinct for an increasing amount of copper.
The strains and stresses in the metal lines during thermal cycling are mainly
influenced by the elastic behavior of the passivation but not by the alloy
composition of the metallization. A stiffer passivation induces higher thermal
stresses in the metallization than a softer. For example, the higher Young’s-modulus
of siliconnitride compared to siliconoxide and also an increasing passivation
thickness result in higher metal stresses. A planarized passivation behaves in
the cross-direction (which is now filled up with passivation material) much stiffer
than a passivation with a conformal shape.
Finite element method (FEM) calculations give information about the pure elastic
reaction of the metallization on temperature changes. By combining these results
and the measured x-ray data within the so-called theory of eigenstrains, data for
volumetric changes (voiding) and also for (volume preserving) plastic shear
deformation in the lines can be determined during thermal cycling.
The results of all different samples show only small but no significant variations
in the eigenstrains during the thermal cycles. Therefore, the plastic shear
deformation is nearly the same for all samples. No distinct correlation between
the voiding and the hydrostatic stress in the metal lines , shydr, could be found.
For most of the samples the voiding during cycling is nearly zero within the
experimental resolution. Especially, the sample with the lowest [sigma]hydr shows a
slightly higher amount of voiding compared to other samples with significantly
higher [sigma]hydr.
These results imply the supposition that other parameters than the metal [sigma]hydr
are mainly responsible for the amount of voiding damage in the metallization.
Different passivation materials are produced by different deposition techniques
and different chemistry resulting in different interfaces and possibly different
interface contaminations. It seems to be much more important to better control
these chemical parameters during passivation deposition because those factors may
reduce the void nucleation barrier at the metal-passivation interface which is
known as the exclusive void nucleation site.
Nielen, Heiko
Einfluß von Passivierung und von Legierungszusätzen auf die thermichen Spannungen von Leiterbahnen in integrierten Schaltkreisen
133 S., 1998
Mechanical stress and electromigration effects become of increasing concern
with continuing miniaturization. After processing at elevated temperatures, where
a nearly stress free state in the metallization is assumed, and cooling down to
room temperature high thermal strains remain in the metal lines due to the thermal
expansion mismatch between the metallization on the one hand side and the substrate
and the passivation on the other. Due to the relaxation of these stresses plastic
deformation such as creep flow and stress voiding is very important for reliability
concerns because it often leads to resistance increase or complete line failure e.g.
when the void diameter reaches a major part of the line dimensions.
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