Application of a Laser Cutter to Pattern Wrinkles on Polymer Films

Wrinkling of thin films is a simple way to fabricate microscale topographical structures without the use of expensive lithographic processes or clean rooms. Such wrinkles have applications in surface wetting, adhesives, optics, and flexible electronics. It is possible to form wrinkles on a surface by heating a film stack of two or more layers featuring different Young's modulus and coefficient of thermal expansion (CTE). Wrinkles form over the entire surface due to the stresses arising from the mismatch in CTE between the layers. Here, we induce wrinkles only at specified locations of an aluminum/polystyrene film stack on a silicon wafer by delivering heat to targeted locations on the surface of the film via a laser beam from a conventional "laser cutter". Wrinkles form during irradiation within a circular spot defined by the laser without the need for additional heating. Rastering the laser beam across the surface forms various shapes such as circles, lines, and squares composed of individual wrinkles. The wrinkles have wavelengths ranging from 3.5 to 11.2 μm depending on the thickness of the metal and polymer films and laser power. Conventional theory predicts similar wavelength values, suggesting that they are driven by thermal expansion. The resolution of features formed by conventional laser ablation is closer to 50 μm; thus, wrinkling offers a way of patterning structures with "subablation"dimensions. With postheating, these wrinkles can extend into the pristine regions of the film with a direction and alignment seeded by the laser pattern. The discrete topographical patterns formed by the laser can alter the local mechanical, optical, and wetting properties for applications, including, antifouling surfaces, wettability gradients, and optics.