Effect of Stresses in Thin Films on Defect Nucleation
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A. S. NANDEDKAR C A S A Engineering, 15 Dartantra Dr., Hopewell Junction, NY 12533, U. S. A. ABSTRACT
Computer simulations were used to investigate the effect of stresses on the energy of defects in Ge, Ge7%Si' 9 3o (referred to as alloy) films on Si and encapsulated aluminum thin films. In Ge films, misfit dislocations were energetically favorable over coherent interfaces. Although the alloy films had lower energy compared to Ge films, they also formed dislocations at film thicknesses greater than 25 - 30 A. The energy of vacancy insertion at the film surface, which nucleates a misfit dislocation, increased with tensile stress. In encapsulated aluminum, agglomeration of vacancies leads to formation of voids. Simulations indicate that the activation energy of vacancy migration increased with compressive stresses. Thus compressive stresses in aluminum may prevent formation of voids, while tensile stresses in Ge film may prevent formation of misfit dislocations. INTRODUCTION Thin film technology is used routinely in manufacturing semiconductor devices. For example a Ge film deposited over Si may be used to fabricate individual devices. Aluminum thin films are used to interconnect devices. The performance and reliability of the semiconductor devices will be affected by the properties of the thin films. These are in turn dependent upon structural defects present in the film. Dislocations introduce traps and recombination centers for charge carriers. Voids in aluminum may grow large enough to break the continuity of the film and hence the interconnect. Therefore it is of importance to understand how these defects are formed and how their formation may be controlled. Furthermore, formation of the defects can best be controlled during their nucleation. Since the critical size of nucleus can be of the size of a few angstroms (1), computer simulation is a useful tool to study their energetics and configuration. In Ge/Si films, the strain energy in the compressed film that results from lattice mismatch, can be reduced by formation of misfit dislocations. One way to reduce the lattice mismatch and strain energy is to use an alloy of GeSi (such as GeS.i for the film. Another 9 3o/o) way would be to control nucleation of the misfit dislocation. Earlier studies have demonstrated that the misfit dislocation is nucleated by introduction of vacancies in the top layer of the film (2). In encapstulated aluminum thin films, a void is more likely to be nucleated at grain boundaries, dislocations or surface (1). Migrating vacancies are trapped at these sites thus forming a void. Stresses in the film affect migration of vacancies and hence the time required to form the void. In this communication, we report further atomistic studies in the Ge/Si and Al thin films. The main objective of this study is to investigate the effect of stress on vacancy insertion and migration in thin films. METHOD AND MODEL A commercially available computer software package, ADESH (Atomistic DEfect Simulation Handler; Vendor: CASA Engineering, Hopewell Junctio
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