Evolution of stresses in passivated and unpassivated metal interconnects
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Evolution of stresses in passivated and unpassivated metal interconnects A. Gouldstone, Y-L. Shen,a) S. Suresh, and C. V. Thompson Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 3 March 1997; accepted 9 February 1998)
This paper discusses computational simulations of the evolution of stresses and deformation in unpassivated and SiO2 -passivated Al lines on Si substrates. The finite element model accounts for elastic-plastic deformation in the Al lines during etching, passivation, and subsequent thermal cycling, by recourse to a generalized plane strain formulation within the context of a unit cell with appropriately constrained boundary conditions. The effects of different controlled variations in thermal history, and in the width, height, spacing, and yield behavior of the Al lines are analyzed; all these factors are seen to have potentially strong effects on the evolution of stresses within the lines. The predictions of the computations presented in this work are amenable for direct comparisons with experiments of curvature evolution along and perpendicular to the lines upon patterning, passivation, and thermal loading. The predicted stresses in metal interconnects can be directly used for reliability modeling purposes.
I. INTRODUCTION
The reliability of modern integrated circuits depends strongly on their thermomechanical performance. One of the most prominent damage mechanisms in metal interconnects is the formation of voids. The incorporation of several levels of metallization and passivation in a device exposes the interconnects to multiple thermal cycles, in each of which the temperature excursion can be as much as 400 ±C. High tensile stresses evolve in the metal lines upon cooling from the passivation temperature, as a consequence of the thermal contraction mismatch with the surrounding dielectric, passivation, and substrate materials. The triaxial tensile stresses that arise within the lines facilitate void formation and growth, which can eventually cause failure of the interconnects.1,2 In addition to this direct stress-voiding problem, electromigrationinduced voiding, which is engendered by the divergence of momentum transfer from electrons to atoms, is also strongly influenced by the stresses in the conductor lines.3–5 A thorough understanding of the evolution of stresses in metal interconnects is, therefore, essential for design of integrated circuits, for process development, and for reliability analyses. Numerical modeling using the finite element method provides a straightforward procedure for the systematic study of the evolution of thermal stresses and deformation in metal interconnects during processing and subsequent service. It has also become a necessity a)
Present address: Department of Mechanical Engineering, The University of New Mexico, Albuquerque, New Mexico 87131.
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J. Mater. Res., Vol. 13, No. 7, Jul 1998
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