Effects of misfit dislocation and film-thickness on the residual stresses in epitaxial thin film systems: Experimental a
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thin film system involving dissimilar materials, the residual stresses and microstructural defects are inevitable due to the misfits of lattice structures and thermal properties of the materials. Unfortunately, the relationship between the stresses and interface defects is still unclear to date. This article aims to clarify such an important relationship by a finite element (FE) analysis incorporating the dislocation distribution from high-resolution transmission electron microscopy. Layer removal and Raman spectroscopy were also conducted to explore the film-thickness effect. It was found that that residual stress variation in a thin film system is caused by the coupled effect of lattice-thermal misfits and discrete interfacial dislocations, that the residual stresses are dependent on the film thickness, and that it is particularly important to identify the correct density of interface dislocations for an accurate residual stress calculation by a FE analysis.
I. INTRODUCTION
To increase the capacities of IC chips, a heteroepitaxial thin film is often deposited to a dissimilar substrate. However, the disparity of the material properties can bring about substantial residual stresses in such a thin film system. When the residual stresses are sufficiently large, the electronic properties of an IC can be considerably affected.1 Lattice and thermal mismatches between the dissimilar material layers are the two origins of residual stress generation in heteroepitaxial thin film systems. Previously, theoretical predictions of residual stresses were based on the disparate lattice parameters and coefficients of thermal expansion (CTEs), using the theory of elasticity. Such predictions, however, are valid for certain range of film thicknesses when the lattice misfit can be entirely accommodated by elastic strain. Beyond a critical thickness,2,3 the residual stresses are partially relieved by the formation of crystalline defects such as twins4,5 and dislocations.6,7 The calculation of the stress relief is however not as simple as the previously thought because the interface defects are discrete and not uniform, and their interactions are complex. That is why quantitative relationships between residual stresses and experimentally measured lattice defects have never been established. A heteroepitaxial silicon-on-sapphire (SOS) thin film system has a high lattice mismatch strain of up to 5.9% in Si [100] and 14.1% in Si [010] directions. The residual a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.304 J. Mater. Res., Vol. 27, No. 21, Nov 14, 2012
stresses measured by the x-ray diffraction method8 are much smaller than the theoretical predictions when both lattice and CTE mismatches are considered. This indicates a significant stress relief due to lattice defects. Some early studies using transmission electron microscopy (TEM) showed that there is a high density of lattice defects,9–15 of which microtwins are predominant.9,10 This partly mitigates the compressive residual stresses in the Si
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