Verlag des Forschungszentrums Jülich
JUEL-3435
Heinen, Dirk
In-situ TEM-Untersuchungen des durch thermische Spannungen und Elektromigration induzierten Materietransporte in Al-Leiterbahnen
109 S., 1997
Diluted Aluminum alloys are commonly used as interconnect material in integrated
electronic devices. Thermally-induced mechanical stresses in these devices which
are caused by thermal mismatch between the underlaying Si-substrate and the
surrounding passivation are of special interest because degradation effects such
as stress induced voiding or electromigration(EM)-damage are closely related to
the stresses in the lines.
Electromigration-driven mass transport in "near-bamboo" Al-lines, which consist
mostly of "blocking grains", is an important topic of research on ULSI-metallizations.
Because the most easy diffusion path, i.e. grain boundaries parallel to the line,
is suppressed in bamboo-like Al-grain structures other paths have to be considered.
One path is EM-driven intragranular diffusion in Al-lines. In this experiment,
inert gas-filled voids with a mean diameter of about 10nm, which were created after
gas implantation and annealing of the Al-lines, serve as indicators of mass
(or vacancy) transport. The EM-tests which were done by in-situ observations
in a transmission electron microscope (TEM) reveal no intragranular void motion
over a period of more than 100h at current densities of 1-1.75MA/cm² and
temperatures of 150-225°C. This leads to an estimation of the maximum void
diffusion velocity which was compared with calculated values of surface and
volume diffusion controlled void motion, respectively. This leads to the result
that surface diffusion has to be suppressed because it should have been observable
in the TEM. In contrast to that volume diffusion could have been possible but the
velocity of this process was below the resolution of the TEM.
The other examined path of diffusion was the behavior of dislocations in Al-lines
under an applied EM-force. Motion and buckling of dislocations which was dependent
on the current direction was observed and explained as a reaction of the stress
gradient due to EM-induced mass transport.
A further point of interest was the observation of relaxation of thermally induced
stresses within the lines by plastic deformation of the Al due to dislocation
movement. The behaviour of dislocations during thermal cycling has been examined.
The observed formation of dislocation lines in or near the interface behind a
onward moving curved dislocation confirms a theoretical model which describes stress
relaxation caused by dislocation glide.
The third topic of this thesis was the collection of detailed statistical data
of stress- and electromigration-induced void formation and growth in passivated
Al-lines by in-situ TEM experiments. Experimental parameters such as grain size,
relaxation time and temperature and electromigration current density were varied.
Some important results are: Void nucleation takes place exclusively at the
passivation side walls in a very narrow temperature range about 50°C below the
annealing temperature TA (450°C ( TA ( 550°C) on cooling. The growth rate of the
relative total void volume in the lines is very dependent on time and temperature
during isothermal relaxation consistent with previous data evaluated from
macroscopic stress and strain measurements. Although nearly all pre-existing voids
change their shape and/or position during an electromigration test the number of
voids and their total volume is maintained in excellent agreement with recent
X-ray measurements during electromigration testing showing no changes in average
strain or stress in the lines.
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