X-ray characterization of residual stresses in electroplated nickel used in LIGA technique

Pertinent simulations of micro-electromechanical systems using powerful computer-aided design tools depend on a systematic characterization of the electrical, optical, magnetic, and mechanical properties of the different materials involved in the fabrication of microdevices. Among these properties, we focused our attention on the residual stresses, which may be responsible for the degradation of the mechanical performances of micro-actuators and sensors. We report in this paper a detailed investigation of the residual stresses arising during nickel electroplating used in the LIGA technique. X-ray analysis and, in particular, the sin2 Ψ method allow a local characterization of the structural properties of the material as well as the residual stress tensor. The experimental results obtained on our nickel plates obtained with a nickel sulphamate electrolyte show that the structural properties and the residual stresses are tightly linked to the nature of the substrates used for the depositions (silicon and copper), the densities of current, the amount of NiCl2 in the bath and the thickness of the layers. In order to obtain the intrinsic stresses, the thermoelastic contribution to the global stresses is estimated with a finite element method and subtracted to the experimental values. In the final part of this paper, we present the effect of residual stresses in the mechanical behavior of a microgripper fabricated by LIGA.