Experimental study of the residual stress-induced self-assembly of MEMS structures during deposition

Abstract

The possibility of using residual stresses favorably as a means of self-assembling MEMS during material deposition is experimentally investigated. Two atomic force microscope cantilevers are placed in contact at their free ends. Material is isothermally electroplated onto one (the deposition) cantilever, but no material is deposited onto the other (spring) cantilever. The deposited layer contains residual stresses that deform the deposition cantilever. The deposition cantilever in turn deforms the spring cantilever, thereby doing work in the spring cantilever and proving that the two structures can selfassemble during deposition processing. An insoluble nickel electroplating process and an all-sulfate nickel solution are used for the deposition. The deflection of the selfassembled cantilevers is measured in-situ as a function of the deposited thin film thickness through the optical method of atomic force microscopy. The experimental results are compared to an analytical model which consists of Euler-Bernoulli beam theory that is modified to account for moving boundaries as the material is deposited. The model accounts for the through-thickness variation of the intrinsic strain during the electroplating. Closed-form solutions are not possible, but numerical solutions are plotted for the cantilever deflection and work on the spring cantilever as functions of the deposition thickness.