Abstract
BackgroundCopper is the primary metal used in integrated circuit manufacturing of today. Even though copper is face centered cubic it has significant mechanical anisotropy depending on the crystallographic orientations. Copper metal lines in integrated circuits are polycrystalline and typically have lognormal grain size distribution. The polycrystalline microstructure is known to impact the reliability and must be considered in modeling for better understanding of the failure mechanisms.MethodsIn this work, we used Voronoi tessellation to model the polycrystalline microstructure with lognormal grainsize distribution for the copper metal lines in test structures. Each of the grains is then assigned an orientation with distinct probabilistic texture and corresponding anisotropic elastic constants based on the assigned orientation. The test structure is then subjected to a thermal stress.ResultsA significant variation in hydrostatic stresses at the grain boundaries is observed by subjecting the test structure to thermal stress due to the elastic anisotropy of copper. This introduces new weak points within the metal interconnects leading to failure.ConclusionsInclusion of microstructures and corresponding anisotropic properties for copper grains is crucial to conduct a realistic study of stress voiding, hillock formation, delamination, and electromigration phenomena, especially at smaller nodes where the anisotropic effects are significant.
Highlights
Copper is the primary metal used in integrated circuit manufacturing of today
Depending on processing conditions and orientation of the grains, some of the Depending on the grain boundary distribution, the polycrystalline structure can result either in a more reliable structure or a less reliable structure compared to a single crystal structure when only normal stress based delamination is considered
When we consider stress migration which is primarily driven by the hydrostatic stress gradient which typically have local maxima at the grain boundaries the polycrystalline structures might result in easier nucleation of voids along grain boundaries and growth of voids (Yang et al 2011; Zschech et al 2009; Sukharev et al 2009)
Summary
Copper is the primary metal used in integrated circuit manufacturing of today. Even though copper is face centered cubic it has significant mechanical anisotropy depending on the crystallographic orientations. With polycrystalline copper, for a given value of the hydrostatic stress, the normal stress at the Basavalingappa et al Mechanics of Advanced Materials and Modern Processes (2017) 3:6 interface where a delamination would take place is a function of the crystallographic orientation of the copper grains in contact with that surface (Lloyd et al 2006). This results in a bimodal or multi-modal failure distribution where the behavior is a sensitive function of the texture and geometry of the conductor in relation to the flux divergence. This has great importance when extrapolating test results with relatively small numbers of samples (dozens at most) to the case of a modern processor that may have hundreds of thousands of potential failure sites (Lloyd et al 2006)
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