In general, diffusion coefficient is greater through less restrictive structural regions . For instance, the diffusion coefficient through surface is greater than that through grain boundary, while it is greater through grain boundary than through single crystal as indicated in Figure 2014a.
Figure 2014a. Self-diffusion coefficients for silver (Ag) depend on the diffusion path. Adapted from 
As shown in Figure 2014b (a), in contacts in Si ICs, a silicide is formed by depositing a metal onto Si, followed by heating the sample. The Al above the silicide can migrate through the silicide along the grain boundaries of the silicide, resulting in formation of Al/Si contacts as shown in Figure 2014b (b). This migration can be enhanced by the high compressive stress in the Al induced by its high CTE than that of Si (CTEAl>> CTESi) at high temperatures.
Figure 2014b. (a) Silicides and (b) Formation of Al/Si contacts.
 J. H. Brophy, R. M. Rose, and J. Wulff, The Structure and Properties of Materials, Vol. 2: Thermodynamics of Structure, John Wiley & Sons, Inc., New York, 1964.