The present invention provides a diffusion barrier useful in an integrated circuit, which serves to prevent the migration of material from a conductive layer to the underlying substrate and further provides improved adhesion of the conductive layer to the substrate. The diffusion barrier comprises a polymer which is a polyelectrolyte, having both cationic and anionic groups along its backbone chain. Preferred polyelectolyte barriers are polyethyleneimine (PEI) and polyacrylic acid (PAA). Other polyelectrolytes may be used, such as those that contain SH--OH-- aromatic groups, or those that can interact with either the metal or the adjacent layers via covalent interactions and cross-linking (e.g., POMA, PSMA). The polymeric layer may be applied in two coatings, so that the amine side chains contact the dielectric (e.g. silicon) substrate and the acidic groups are adjacent to the overlying metallic interconnect (e.g. copper). The diffusion barrier may be made thin, preferably less than 5 nm thick, which is advantageous in devices having high aspect ratios.