Effect of Dislocation Core Spreading at Interfaces on Strength of Thin-films
- PDF / 153,122 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 92 Downloads / 229 Views
Lin Zhang Avant! Corporation, 46871 Bayside Parkway, Fremont, California 94538
Huajian Gao Max-Planck-Institut fu¨r Metallforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany (Received 8 January 2002; accepted 25 April 2002)
Critical strain arguments are often used to model the thickness dependence of the strength of thin films on substrates. In these arguments, plastic deformation occurs when the stress in a film is high enough that the strain energy relieved by the introduction of a misfit dislocation is sufficient to generate the line energy of that misfit. Such models typically assume compact dislocation cores. However, experimental evidence suggests that, under certain circumstances, dislocation cores may spread out into the interface between the film and the substrate. If this happens, the energy of the misfit dislocation, and the critical stress needed for its propagation, will be lowered. In this paper, the effect of dislocation core spreading on the critical stress has been modeled. The effects of interface strength, film thickness, and misfit dislocation spacing are considered. I. INTRODUCTION
The well-established concept of a critical strain needed to form misfit dislocations at the interface between a stressed film and substrate1 has been used to explain a number of thin-film phenomena. The basic idea is illustrated in Fig. 1(a). In response to a biaxial stress imposed on the film by the constraint of the substrate, a threading dislocation (running from the film/substrate interface to the free surface) can move on its slip plane to reduce the strain energy. However, if the dislocation is confined to the film, a misfit dislocation must be deposited parallel to the interface as the threading segment moves in response to this stress. At the critical strain, the strain energy released per unit distance that the threading dislocation glides is equal to the energy per unit length required to form the misfit dislocation. If the applied strain is greater than the critical strain, the threading arm moves ahead, leaving a misfit dislocation behind to reduce the strain energy. If the applied strain is less than the critical strain, the threading arm moves back, eliminating the misfit dislocation, to reduce the total dislocation line energy. Nix,2 following earlier work by Freund,3 used this argument to predict that the flow stress of a metallic thin film on a substrate at room temperature would vary with the reciprocal of the film thickness in the absence of other strengthening mechanisms. More recently, Baker et al. have used this critical strain argument as one 1808
http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 7, Jul 2002 Downloaded: 13 Mar 2015
explanation for the early yielding behavior observed in thin films.4,5 The latter case is an example of threading dislocations moving back, against the applied stress, to remove misfit dislocation segments. The critical stress level at which a dislocation is stable (without moving) following such arguments depends on the strain energy per unit l
Data Loading...
