ADHESION OF COPPER AND CHROMIUM FILMS TO POLYIMIDE STUDIED BY STATIC SIMS OF THE INTERFACE

The adhesion of evaporated copper films to polyimide (PI) substrates is of paramount importance in the electronics industry where such systems are used in circuit boards. This adhesion is notoriously poor, and various processes have been developed to improve the peel strength. One of these processes is the evaporation of a very thin layer of chromium between the copper and the polyimide. The underlying mechanism for the difference between copper and chromium adhesion not fully understood. The authors have attempted to elucidate the interfacial chemistry by using the static SIMS, in order to obtain structural molecular information. The results suggest a chemical modification of the PI at the interface by the metals. Chromium seems to be more reactive than Cu. It appears that the metal attacks the polymer at the inside functionality. Rupture of some bonds in the planar imide structure could lead to the formation of more mobile polar groups with a better interaction with the metal. Increase of the unsaturation might be interpreted as a result of cleavage of aromatic rings.