Effect of Individual Layer Thickness on the Self-Organized Wrinkle Formation in a Metal Polymer Bilayer Thin Film

It is well-known that when a thin layer of metal is thermally deposited on top of a polymer thin film (particularly a cross-linked elastomer thin film), self-organized wrinkles form due to the thermal stresses generated at the metal polymer interface due to mismatch between the coefficients of thermal expansion of the two individual layers. It has been extensively reported that the wavelength of the wrinkles (λS) varies linearly with the metal film thickness (hM), which is consistent with the linear elastic buckling theory. However, in this paper, we show that in addition to hM, the polymer (bottom) layer thickness (hFP) and the elasticity of the polymer film (vis-à-vis the amount of cross-linker percentage, CL) in the Sylgard 184 film also influence both the wavelength (λS) and amplitude (aS) of the wrinkles, when a thin aluminum (Al) layer is thermally deposited over a spin-coated, fully cross-linked polydimethylsiloxane (PDMS) Sylgard 184 thin film. We show that both λS and aS show nonlinear and nonmonotonic dependence on hFP as well as CL. The work sheds new insight into how the thermal stress-mediated deformation and growth of the instability patterns at the metal-polymer interface are influenced by the presence of a rigid substrate underneath the bottom polymer layer. Based on careful atomic force microscopy, we also capture stress relaxation-mediated partial flattening of the wrinkles in the films with high CL due to stress relaxation. The work unleashes the potential of creating patterns on demand by modulating different parameters of the metal polymer thin bilayer system with possible modulation of wettability and optical properties.