Wet-process surface modification of dielectric polymers: Adhesion enhancement and metallization

For many electronic applications, the surface of a dielectric polymer must be modified to obtain the desired surface properties, such as wetting, adhesion, and moisture barrier, without altering the bulk properties. This paper reviews wet- process modifications of dielectric polymer surfaces and also presents unpublished results related to fluorinated polyimides. In a typical wet process, a substrate is immersed in or sprayed with a chemical solution, rinsed with a solvent to remove the excess reagents, and then dried if necessary. Wet processing can provide greatly enhanced adhesion and reliability of the adherate (top) layer to the modified polymer surface (adherend). We discuss a) the wet-process modification of various polymers (e.g., PMDA-ODA, BPDA-PDA, 6FDA-ODA, PTFE, PCTFE); b) polyimide/polyimide and PCTFE/glass adhesion, and c) the surface chemistry and the adhesion at a fluorinated polyimide (6FDA-ODA) surface. Entanglement of polymer chains plays an important role in polyimide/polyimide adhesion, while chemical reactions are the major contributors to PCTFE/glass adhesion strength. The metallization of dielectric substrates often requires surface pretreatments or conditioning by wet processes to sensitize a polymer surface for deposition of a metal seed layer. After seeding, a thick layer of a conducting metal (e.g., Cu) is deposited by electroless or electrolytic plating. Unlike dry or high-vacuum processing of polymer surfaces, the chemistry of a wet-processed polymer surface can be well characterized and often defined at a molecular level. A relationship can be established between a polymer's surface chemical (or morphological) structure and its surface properties such as adhesion and metallization.