Plasma treatment and surface analysis of polyimide films for electroless copper buildup process

We report here a detailed characterization of the surface chemical states and morphology of polyimide (PI) films following modifications by plasma treatment and electroless copper deposition. NH3 and Ar plasma treatments have been successfully used to achieve morphological and chemical modification of the PI surface so that electroless copper plating can occur. The adhesion strength of the electroless copper to the PI surface was measured and correlated with the plasma-induced chemical and physical modifications of the PI surface. The NH3 plasma causes primarily chemical changes to the PI surface through creation of nitrogen moieties (i.e., - N=C <) on the surface. The Ar plasma treatment brings about mainly physical changes to the surface (i.e., surface roughening). The combined-plasma treatment (Ar plasma followed by NH3 plasma) combines the desirable chemical and physical effects of each treatment, yielding a PI surface with higher roughness for physical anchoring of the copper and surface bonding sites (nitrogen and oxygen sites). During the electroless copper surface activation step with tin chloride and palladium chloride, tin bonds mainly with the oxygen on the surface, whereas palladium reacts with tin chloride as well as with the surface nitrogen. A direct relationship has been observed between surface palladium concentration and the abundance of the - N=C < sites on the surface. This suggests that the nitrogen radicals created during the NH3 plasma are incorporated into the surface and serve as bonding sites for the palladium. In the subsequent electroless Cu deposition, there was a direct correlation between the palladium surface concentration and Cu coverage. The adhesion strength of the electroless copper to the PI correlated well to the surface modifications and plasma treatment conditions. For the first time, a specific bonding configuration on the PI surface is shown to promote adsorption of palladium, which in turn promotes covalent bonding with Cu. The relative importance of surface roughness and chemical bonding on the adhesion strength is discussed. (c) 2005 The Electrochemical Society.