Surface potential change with droplet formation and Cu watermark growth by electrochemical oxidation resulting therefrom

To reveal the mechanism of watermark formation on a Cu film, the Volta potential in a droplet area on Cu was evaluated using the scanning Kelvin-probe method. The droplet area on Cu exhibited an upward convex potential profile, indicating that the electrochemical reactions were more active in the areas around the droplet than at the center of the droplet. On the other hand, Si exhibited a profile converse to that of Cu: electrochemical reactions were more active at the center of the droplet area than in the areas around it. These evaluations revealed that the pinning and ring-shaped watermark formation at the droplet area on a Cu film resulted from the said Volta potential profile.