Controlled Wrinkling of Gradient Metal Films

Controlled wrinkling is a rather simple method of fabricating surface topographies. The production process is based on the spontaneous formation of wrinkles upon compression of a hard film attached to a soft elastic substrate. Controlled wrinkling typically features large-scale wrinkled samples with a discrete wavelength and amplitude. In this report, we employ an approach utilizing linear metal layer thickness gradients for the controlled formation of gradient wrinkle patterns. The observed wavelength modulation was experimentally achieved by preparing layer thickness gradients of gold, chromium, and indium by physical vapor deposition in combination with a poly(dimethyl siloxane) elastomer substrate. In case of chromium and indium, a thin SiO _x surface layer was sufficient to ensure adhesion. However, in case of gold, an additional thin chromium adhesion layer was required. For the wrinkled gradient gold film, it was possible to tune the wavelength from 3.4 to 12.2 μm on a single substrate. The experimental data correspond well to the theoretical bilayer model from Stafford et al. Chromium has a significant higher Young's modulus and melting temperature than gold. However, chromium was successfully evaporated and gradient wrinkle patterns with wavelengths from 1.0 to 3.5 μm were realized. In contrast, indium has a considerable lower Young's modulus than gold and chromium, respectively. Consequently, lower wavelengths (0.6-1.0 μm) of the wrinkled gradient indium film were observed. These tunable wrinkled gradient metal films can be envisioned as components in sensors and optical and electro-optical devices.