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

摘要

The possibility of using residual stresses favorably as a means of self-assemblingMEMS during material deposition is experimentally investigated. Two atomic forcemicroscope cantilevers are placed in contact at their free ends. Material is isothermallyelectroplated onto one (the deposition) cantilever, but no material is deposited onto theother (spring) cantilever. The deposited layer contains residual stresses that deform thedeposition cantilever. The deposition cantilever in turn deforms the spring cantilever,thereby doing work in the spring cantilever and proving that the two structures can selfassembleduring deposition processing. An insoluble nickel electroplating process and anall-sulfate nickel solution are used for the deposition. The deflection of the selfassembledcantilevers is measured in-situ as a function of the deposited thin filmthickness through the optical method of atomic force microscopy.The experimental results are compared to an analytical model which consists ofEuler-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 theintrinsic strain during the electroplating. Closed-form solutions are not possible, butnumerical solutions are plotted for the cantilever deflection and work on the springcantilever as functions of the deposition thickness.