Atomic force microscopy studies on self-organized wrinkles in constrained metallic films deposited on silicone oil substrates

Silicone oil droplets with varied sizes (micrometer to millimeter) were sprayed onto a clean glass slide and then Cr films were deposited on the droplets by DC-magnetron sputtering. Self-organized wrinkles induced by thermal stress have been investigated using atomic force microscope. It is found that the Cr films are limited by the droplet edges and thus possess constrained edges, which can well control the wrinkle morphologies: the wrinkles may exhibit radiated strips perpendicular to the edge. The wavelength and amplitude will decrease when approaching to the constrained edge. Morphologies of the wrinkles are closely related to the film thickness and oil drop size. As the film thickness increases, the wrinkles first form near the edge, and then propagate to the central region. The wavelength at the droplet center is almost unchanged for different oil sizes, but the amplitude first increases and then decreases with increasing film thickness. When the film thickness is fixed, the wavelength and amplitude may increase accordingly with increasing droplet size. Further studies show that the top surface of the silicone oil is modified to form a polymer layer during the deposition due to the bombardment from high-energy particles and heat radiation from the sputtering source. Morphologies of the wrinkles and amplitude evolutions are then explained in detail.