Self-Assembly Based on Chromium/Copper Bilayers

In this paper, we detail a strategy to self-assemble microstructures using chromium/copper (Cr/Cu) bilayers. Self-assembly was primarily driven by the intrinsic residual stresses of Cr within these films; in addition, the degree of bending could be controlled by changing the Cu film thickness and by introducing a third layer with either a flexible polymer or a rigid metal. We correlate the observed curvature of patterned self-assembled microstructures with those predicted by a published multilayer model. In the model, measured stress values (measured on the unpatterned films using a substrate curvature method) were utilized. We also investigated the role of two different sacrificial layers: 1) silicon and 2) water-soluble polyvinyl alcohol. Finally, a Taguchi design of experiments was performed to investigate the importance of the different layers in contributing to the stress-thickness product (the critical parameter that controls the curvature of the self-assembled microstructures) of the multilayers. This paper facilitates a deeper understanding of multilayer thin-film-based self-assembly and provides a framework to assemble complex microstructures, including tetherless self-actuating devices.