Stress-induced void

FIGURE 11.18 Stress-induced void formed in Cu via interconnect, Cu is separated from barrier film (TaN) (from Ref 30). 

Park BL. Mechanism of stress-induced voids in multilevel Cu interconnects. IITC. Jun 2002. 

Bower, A. F. and Freund, L. B. (1993), Analysis of stress induced void growth... 

J.T. Yue, W.P Funsten, R.V. Taylor, Stress induced voids in aluminum interconnects during IC processing, in Proceedings of the 1985 International Reliability Physics Symposium, IEEE, 1985, p. 126. 

P.A. Finn, A.S. Mack, PR. Besser, TN. Marieb, Stress-Induced void formation in metal lines, MRS Bulletin 18 (12) (1993) 26. 

The Kirkendall effect alters the structure of the diffusion zone in crystalline materials. In many cases, the small supersaturation of vacancies on the side losing mass by fast diffusion causes the excess vacancies to precipitate out in the form of small voids, and the region becomes porous [11], Also, the plastic flow maintains a constant cross section in the diffusion zone because of compatibility stresses. These stresses induce dislocation multiplication and the formation of cellular dislocation structures in the diffusion zone. Similar dislocation structures are associated with high-temperature plastic deformation in the absence of diffusion [12-14]. 

FIGURE 17.29 Stress-induced pitting. The copper grain growth makes copper to reduce its volume slightly and leave some voids. It is visible in the middle of a long copper line. 

Case a stress-induced formation of fibrillated crazes. The weak rubber particles act as stress concentrators. Crazes are formed starting from the particle-matrix interface around the equatorial region of particles. The voids inside the crazes initiate a stress concentration at the craze tip, which propagates together with the propagating craze therefore, the crazes reproduce the stress state necessary for their propagation. Cavitation inside the rubber particles is not necessary, but it enables a higher stress concentration and easier deformation of the particles. 

Cavitation was also identified as an active mechanism in systems where a mbberlike phase (particles or interphase) is susceptible to implode under the effect of the hydrostatic stress induced by the applied tension. Fond (36) has recently revisited the critical conditions under which this form of damage becomes energetically favorable. In the case of core-shell rubber-toughened PMMA, he ascribed the extensive whitening under tension at room temperature to the profuse formation of voids in the mbber shell of the toughening particles. 

The mechanically induced void aperture can be related to the current effective normal stress, tr, using Barton-Bandis hyperbolic normal closure model according to... 

To determine the influence of stress on the microstructure, TEM investigations are in progress. First results on the average void diameter and the void concentration seem to show that the stress-induced swelling is a result of the accelerated void nucleation rather than the accelerated void growth. But this result needs further confirmation. 

Stresses induced by electromigration, in conjunction with those produced by thermal fluctuations, collectively lead to diffusion of vacancies and inelastic deformation processes which cause stress-voiding and slit cracking in metal interconnects (Sanchez et al. (1992) and Joo and Thompson (1997)). Such failure processes are also strongly influenced by the crystallo-... 

This phenomenon was attribnted to fibre breakage and void formation in the interior of the composite. Unlike in tungsten-wire-reinforced copper composites, in which interfacial sliding was suggested as the major deformation mechanism, in the copper/carbon composite void formation and growth were identified as the predominant mechanisms to relax the internal stress induced during thermal cycling. 

Eom et al., 2001). As a result, it may cause premature composite failure due to stress-induced cracks, debonding and delaminations, insufficient fiber—matrix bonding, warpage, and void formation. 

The delivery of heat into the material prior to its entering the die is accomplished via Radio Frequency (RF) preheating, induction heating or by conventional conductive heating of the resin. Desirable characteristics such as increased process rates and the reduction of thermal stresses induced by gradient of temperature and less content of voids can be achieved by the heating methods listed over more conventional approaches (8). 

FIG. 28 Photomicrograph depicting micro structural fracture created due to the Kirkendall eifect within the lead-free solder and Ag-33Pd metallization layer on a ceramic chip capacitor. Note (1) the thick intermetallic compound layer, (2) the Kirkendall voiding between the metal and ceramic capacitor, and (3) the stress-induced cracking in the intermetallic layer. 

In practice, thermal cycling rather than isothermal conditions more frequently occurs, leading to a deviation from steady state thermodynamic conditions and introducing kinetic modifications. Lattice expansion and contraction, the development of stresses and the production of voids at the alloy-oxide interface, as well as temperature-induced compositional changes, can all give rise to further complications. The resulting loss of scale adhesion and spalling may lead to breakaway oxidation " in which linear oxidation replaces parabolic oxidation (see Section 1.10). 

M. Dadfamia, B. P. Somerday, P. Sofronis, and I. M. Robertson, On the Small Scale Character of the Stress and Hydrogen Concentration Fields at The Tip of an Axial Crack in Steel Pipeline Effect of Hydrogen-Induced Softening on Void Growth, Int. J. Mater. Res., to appear (2008). 

Fond et al. [84] developed a numerical procedure to simulate a random distribution of voids in a definite volume. These simulations are limited with respect to a minimum distance between the pores equal to their radius. The detailed mathematical procedure to realize this simulation and to calculate the stress distribution by superposition of mechanical fields is described in [173] for rubber toughened systems and in [84] for macroporous epoxies. A typical result for the simulation of a three-dimensional void distribution is shown in Fig. 40, where a cube is subjected to uniaxial tension. The presence of voids induces stress concentrations which interact and it becomes possible to calculate the appearance of plasticity based on a von Mises stress criterion. 

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