Characterization of ionic migration on CoF substrate by ToF-SIMS

It is very important to investigate the polyimide (PI) surface condition and the metal residues between the electrodes of the chip on film (CoF) substrate in order to prevent poor insulation and short-circuiting due to the ionic migration. In this study, we report the correlation between PI surface chemistry and the insulation characteristics of CoF. We carried out the electrical resistance measurements using the high temperature and high-humidity bias test (HHBT) and the surface characterization using XPS and ToF-SIMS techniques. Both the increase of hydroxyl group by XPS and the decrease of the PI fragments by ToF-SIMS correlate with the decrease in the resistance. It is generally considered that carboxyl group is produced by hydrolysis of an imide group in the present study and this makes PI surface hygroscopic. Even though no metal residues were detected on the very surface between the electrodes, the resistance still went up after soaking in a potassium permanganate solution. This implies that the PI surface condition seems to affect the insulation characteristics of CoF as well as the metal residues stimulating ionic migration.