The strength limits of ultra-thin copper films [Elektronische Ressource] / vorgelegt von Guillaume Wiederhirn

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• Physik

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• Publié par: universitat_stuttgart
• Publié le: 01 janvier 2007
• Nombre de lectures: 28
• Langue: English
• Poids de l'ouvrage: 6 Mo

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Max-Planck-Institut für Metallforschung
Stuttgart

The strength limits of ultra-thin copper films

Guillaume Wiederhirn
Dissertation
an der
Universität Stuttgart

Bericht Nr. 199
Juli 2007

THE STRENGTH LIMITS OF
ULTRA-THIN COPPER FILMS


Von der Fakultät für Chemie der Universität Stuttgart
zur Erlangung der Würde eines Doktors der
Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung



Vorgelegt von

GUILLAUME WIEDERHIRN

aus Colmar, Frankreich




Hauptberichter: Prof. Dr. phil. Eduard Arzt
Mitberichter: Prof. Dr. rer. nat. Gerhard Dehm
Tag der mündlichen Prüfung: 02.07.2007


Institut für Metallkunde der Universität Stuttgart
und
Max-Planck-Institut für Metallforschung Stuttgart


Stuttgart, Juli 2007





















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Abstract


Guillaume Wiederhirn
THE STRENGTH LIMITS OF ULTRA-THIN COPPER FILMS

Max Planck Institute for Metals Research and
Institute of Physical Metallurgy, University of Stuttgart, 2007
212 pages, 97 figures, 15 tables

Abstract
Elucidating size effects in ultra-thin films is essential to ensure the performance and reliability
of MEMS and electronic devices. In this dissertation, the influence of a capping layer on the
mechanical behavior of copper (Cu) films was analyzed. Passivation is expected to shut down
surface diffusion and thus to alter the contributions of dislocation- and diffusion-based plasticity
in thin films.
Experiments were carried out on 25 nm to 2 µm thick Cu films magnetron-sputtered onto
amorphous-silicon nitride coated silicon (111) substrates. These films were capped with 10 nm
of aluminum oxide or silicon nitride passivation without breaking vacuum either directly after
Cu deposition or after a 500 °C anneal. The evolution of thermal stresses in these films was
investigated mainly by the substrate curvature method between -160 °C and 500 °C.
Negligible differences were detected for the silicon nitride vs. the aluminum oxide passivated
Cu films. The processing parameters associated with the passivation deposition also had no
noticeable effect on the stress-temperature behavior of the Cu. However, the thermomechanical
behavior of passivated Cu films strongly depended on the Cu film thickness. For films in the
micrometer range, the influence of the passivation layer was not significant, which suggests that
the Cu deformed mainly by dislocation plasticity. However, diffusional creep plays an
increasing role with decreasing film thickness since it becomes increasingly difficult to nucleate
dislocations in smaller grains. Size effects were investigated by plotting the stress at room
temperature after thermal cycling as a function of the inverse film thickness. Between 2 µm and
200 nm, the room temperature stress was inversely proportional to the film thickness. The
passivation exerted a strong effect on Cu films thinner than 100 nm by effectively shutting
down surface diffusion mechanisms. Since dislocation processes were also shut off in these
ultra-thin films, they exhibited purely elastic behavior in the measured temperature range. Their
lack of plasticity was confirmed by in-situ TEM analysis, which revealed the presence of sessile
parallel glide dislocations during thermal cycling. The stress plateau reported for films thinner
than 100 nm was attributed to the fact that the thermal strain applied was insufficient to induce
yielding. The highest stress value of 1.7 GPa measured at -150 °C is therefore a lower limit for
the actual flow stress since even at this high stress the films remained elastic.


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