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

Pertinent simulations of micro-electromechanical systems using powerful com puter-aided design tools depend on a systematic characterization of the ele ctrical, optical, magnetic, and mechanical properties of the different mate rials 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 s tresses arising during nickel electroplating used in the LIGA technique. X- ray analysis and, in particular the sin(2) Psi method allow a local charact erization of the structural properties of the material as well as the resid ual stress tensor. The experimental results obtained on our nickel plates o btained with a nickel sulphamate electrolyte show that the structural prope rties and the residual stresses are tightly linked to the nature of the sub strates used for the depositions (silicon and copper), the densities of cur rent, the amount of NiCl2 in the bath and the thickness of the layers. In o rder to obtain the intrinsic stresses, the thermoelastic contribution to th e 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 f abricated by LIGA. (C) 2000 Elsevier Science S.A. All rights reserved.