Detailed analytical transmission electron microscopy investigations were performed on a well‐known diffusion barrier system for very‐large‐scale integration metallization. It will be demonstrated that interfacial reactions are of great importance for the barrier mechanism. Both Ti and TiN act as diffusion barrier for the semiconductor and the metallization, respectively. For an aluminum‐based metallization, TiN has a ‘‘spongelike’’ function due to its ability to absorb several amounts of aluminum at elevated temperatures and therefore inhibits diffusion towards the substrate. Ti acts for silicon as a compound forming barrier according to Nicolet’s classification [inTungstenandOtherRefractoryMetalsforVeryLargeScaleIntegrationApplicationsII, edited by E. K. Broadbent (Materials Research Society, Pittsburgh, 1987); pp. 19–26].