Constrained Diffusional Creep in Thin Copper Films
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Constrained Diffusional Creep in Thin Copper Films D. Weiss, H. Gao, and E. Arzt Max-Planck-Institut für Metallforschung and Institut für Metallkunde der Universität, Seestr. 92, D-70174 Stuttgart, Germany
ABSTRACT The mechanical properties of thin metal films have been investigated for many years. However, the underlying mechanisms are still not fully understood. In this paper we give an overview of our work on thermomechanical properties and microstructure evolution in pure Cu and dilute Cu-Al alloy films. Very clean films were produced by sputtering and annealing under ultra-high vacuum (UHV) conditions. We described stress-temperature curves of pure Cu films with a constrained diffusional creep model from the literature. In Cu-1at.%Al alloy films, Al surface segregation and oxidation led to a "self-passivating" effect. These films showed an increased hightemperature strength because of the suppression of constrained diffusional creep; however, under certain annealing conditions, these films deteriorated due to void growth at grain boundaries.
INTRODUCTION For many years, materials research has focused on the understanding of deformation mechanisms in thin metal films on stiff substrates (see [1] and [2] for an overview). Copper and aluminum films on silicon substrates have been of special interest, as these materials are used today for microchip metallization. The large difference in thermal expansion coefficients of the film and substrate materials can lead to mechanical film stresses up to several 100 MPa, which are considered a serious reliability issue in microelectronic industry. Basic research has been interested in two specific phenomena: First, thin-film yield stresses at low temperatures are often proportional to the inverse of the film thickness. This effect has been explained in terms of a dimensional constraint on dislocation motion [1]. Second, the particular shape of a stress-temperature curve under thermal cycling depends not only on the film material, but can also change substantially if the film surface is protected by a thin protection layer (so called passivation) [3]. In this paper we report on the dramatic effect of vacuum conditions during film synthesis on the thermomechanical behavior and the microstructure of pure Cu films. Films produced under UHV conditions supported much higher stresses at high temperatures than conventional films produced under high-vacuum (HV) conditions. Stress-temperature curves of the UHV-produced Cu films could be well described with constrained diffusional creep. We furthermore report on alloying effects in Cu-1at.%Al films. Al surface segregation and oxidation led to a "selfpassivating" effect. Stress-temperature curves were more "square" than the curves of pure Cu films and apparent signs of constrained diffusional creep were absent in these films. We also show that, under certain annealing conditions, large voids can grow at grain boundaries and grain-boundary triple junctions of the alloy films. A growth mechanism based on grain-boundary diffusional creep
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