Characterization of Young's modulus and residual stress gradient of MetalMUMPs electroplated nickel film

Abstract

Metal multi-user MEMS processes (MetalMUMPs) offered by MEMSCAP provide a 20 μm thick electroplated nickel film suitable for constructing micro RF tunable capacitors, RF inductors, relays, switches, etc. Currently the Young's modulus and the residual stress gradient of the MetalMUMPs nickel film have not been characterized. In this paper the resonance method is used to characterize the Young's modulus of the MetalMUMPs nickel film. The characterization results show that the nickel film has a Young's modulus of 155–164 GPa with an average of 159 GPa. A stress gradient induced free beam mechanism is proposed in this paper to characterize the residual stress gradient in the MetalMUMPs nickel film. Characterization results show that the residual stress in the electroplated nickel film has a gradient across the film thickness of −5.49 MPa/μm to −4.30 MPa/μm with the average of −4.72 MPa/μm. The residual stress change from the bottom surface to the top surface of the nickel film is −97.7 MPa. The Young's modulus and residual stress gradient of the MetalMUMPs nickel film obtained in this paper provide MetalMUMPs users an important reference for designing, optimizing and analyzing suspended nickel structures. The stress gradient induced free beam mechanism proposed in this paper provides a method of characterizing negative residual stress gradient in thin films without using trenches or through-wafer holes.