Thermal wrinkling of nanoimprinted SU-8 with masked UV-exposure

When wrinkling can be provided in a controlled way, this has potential for a lot of applications, e.g., in optics or microelectromechanical systems. This paper reports on localized wrinkles generated with an SU-8 layer on silicon; the surface is hardened by a deep-ultraviolet (UV) treatment (172 nm); the size of the wrinkling area is defined by masked UV-exposure. In addition, the SU-8 surface is provided with topography via capillary force lithography beforehand. To identify the interaction between masked UV-exposure and deep-UV treatment several issues are addressed; the size of the wrinkling area, the deep-UV treatment time, and the dose for the masked exposure. Different types of wrinkling are observed, an undulated type, a parallel type, and a zig-zag type. To define a clear wrinkling area an exposure dose of at least 160 mJ/cm² is advised with a 5 μm-thick SU-8 layer. Then, the wrinkles orient in parallel to the edges of the wrinkling area. The impact of topography on wrinkling within a defined wrinkling area is investigated by applying different types of prepatterns, lines, holes, and V-grooves. Typically, the geometry of the prepattern is smaller than the geometry of the wrinkling area. When the prepattern is unidirectional as in the case of lines, an anisotropic response of wrinkling is observed, which can be understood on the basis of a corrugated surface, featuring a weaker and a stronger direction. When wrinkling is controlled by the choice of adequate boundary conditions, this provides an easy method to cover the SU-8 surface with largely regular wrinkling patterns.