Precipitation of Si from the Al metallization of integrated circuits

The Al metallization of integrated circuits is known to dissolve 1/2% or more Si, but the ultimate location of this Si has been uncertain. An electron microprobe operated at low beam energy so as to penetrate only the upper portion of the metallization was used to follow the movement of dissolved Si on cooling after the "forming" heat treatment. Dissolved Si substantially less than a diffusion length from the substrate was found to regrow there; elsewhere the Si forms precipitates in the Al matrix, preferentially near the free surface of the Al.