Creep-controlled diffusional hillock formation in blanket aluminum thin films as a mechanism of stress relaxation
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Hillock formation, a stress-induced diffusional relaxation process, was studied in sputter-deposited Al films. The grain sizes in these films were small compared to those in other sputter-deposited Al films, and impurities (O, Ti, W) were incorporated during the preparation of the films. Stress and hardness measurements both indicate that the Al films were strengthened by the small grain size and incorporated impurities. We observed a new type of hillock in these Al thin films after annealing for 2 h at 450 °C in a forming gas ambient. The hillocks were composed of large Al grains created between the substrate and the original Al film with its columnar grain structure, apparently by diffusion from the surrounding area. By modifying the boundary conditions of Chaudhari’s hillock formation model [P. Chaudhari, J. Appl. Phy. 45, 4339 (1974)], we have created a new model that can describe the experimentally observed hillocks. Our model seems to explain the experimentally observed abnormal hillock formation and may be applied to other types of hillock formation using different creep laws.
I. INTRODUCTION
With its many good properties, aluminum is still used widely as an interconnect material in integrated circuit devices. During integrated circuit fabrication, interconnect materials go through many thermal cycles. Stresses are developed in aluminum during thermal cycling due to the thermal mismatch between the aluminum and the surrounding rigid materials. These stresses may be relaxed in many different ways, such as by dislocation glide/climb, void/hillock formation, and, in some cases, grain growth. Stress relaxation mechanisms in metallic films depend on grain size, crystallographic texture, interface structure, defects, and compositon. Hence the controlling stress relaxation mechanism is expected to depend on the film deposition technique and on the deposition conditions. Many types of hillocks in Al films have been reported as possible mechanisms of stress relaxation.1–7 In this study, we observed a new type of hillock, which consists of large aluminum grains created between the substrate and the original Al film with its columnar grain structure. The hillocks apparently grow by diffusion from the surrounding area. Hillock formation seemed to be the dominant stress relief mechanism. We modeled the
a)
Present address: Department of Materials Science and Engineering, Stanford University, Stanford, California 94305. J. Mater. Res., Vol. 15, No. 8, Aug 2000
experimentally observed hillocks in the Al films by modifying the boundary conditions in Chaudhari’s hillock formation model,1 allowing the creep resistance of the Al film on top of the hillock to suppress the rate of growth of the hillock. The model can describe various kinds of hillocks by using different creep laws. The motivation for this work was to understand stress-induced diffusional processes such as hillock formation. We expect this to be useful in modeling the mechanical behavior in integrated circuit backend structures. II. EXPERIMENTAL DETAILS
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