Observation of Dislocation DisAppearance in Aluminum Thin Films and Consequences for Thin Film Properties
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ABSTRACT Dislocation structures in Al-Cu thin films have been studied by transmission electron microscopy (TEM). We have observed that the contrast of interface dislocations disappears in the electron beam. We assume that the contrast dissolution is due to the spreading of the dislocation core at the crystalline/amorphous interface or due to a diffusive movement of the dislocation through the oxide. In any case, the relaxation is assumed to be controled by irradiation induced diffusion. As a consequence, the short range stresses and at least partly also the long range stresses of the dislocations relax. This relaxation changes the interaction force between dislocations and may thus significantly affect the mechanical properties of thin films. It is concluded that interaction between interface dislocations may not be responsible for the high temperature strength of aluminum films. INTRODUCTION Thin films on substrates can sustain a much higher stress than corresponding bulk materials. This "thin film effect" is especially strong in aluminum thin films at high temperature. It has been assumed that interface dislocations are to some extent responsible for the thin film effect [1,2]. The term "interface dislocations" is reserved in the context of the present paper for portions of lattice dislocations of the film which lie in the film/substrate or film/passivation
interface or very close and parallel to these interfaces. Interface dislocations are produced during plastic deformation when moving dislocations meet these interfaces. The properties of dislocations at interfaces are in many respects different from properties of dislocations within a crystal [3]. One particular property of boundaries with respect to dislocations is that they offer more possibilities for dislocation core relaxation by dissociation. This is due to the DSC lattice (displacement shift completed lattice) which has much smaller lattice constants than the adjacent crystals [4]. This is valid for grain boundaries as well as for phase boundaries. In the special case of a crystal/amorphous phase boundary, there are even fewer restrictions for the geometry of the dislocaton core and thus, there are more possibilies for core relaxation. In particular, due to the loss of crystallinity, the core may continuously spread along the interface. By core spreading, the character of a dislocation changes from a Volterra dislocation to a more general Somigliana dislocation [5,6]. If the Burgers vector of the interface dislocation has a component perpendicular to the interface, diffusion is required for the core spreading. Dislocation core spreading in high angle grain boundaries has been observed experimentally (e.g. [7,8]) and discribed theoretically (e.g. [9,10]). In the present paper we apply the concept of dislocations also to the amorphous phase which might look unusual at first sight. However, the concept of dislocations has been introduced by Volterra in the elastic continuum and thus does not require any internal structure [11].
149 Mat. Res. Soc. Symp. P
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