A grain size limitation inherent to electroplated copper films

A study of changes in the Avrami exponent a during secondary grain growth in thin electroplated Copper layers shows an almost linear change between 1 and 3 as the thickness of the layer increases from 0.55 to 1.5 /spl mu/m, followed by a stabilization around /spl alpha/=3 for thicker films. The occurrence of values below /spl alpha/=2 for a process that should be two dimensional at least, can only be understood in terms of a model where the nucleated secondary grains can never grow beyond a pre-determined final grain size. Using this model, the complete experimentally obtained dependence can be quantitatively reproduced in numerical calculations, indicating that there exists a size limitation for secondary grains that is inherent to the use of modern electroplating baths.