Modeling the effect of etch holes on ferromagnetic MEMS

Ferromagnetic MEMS typically require etch holes to reduce the time required to release the micromechanical structure during the sacrificial-layer etch. However, the size and density of the etch holes strongly alters the coercivity of such perforated ferromagnetic films. In this study, varying the etch hole size from 100 μm squares to 5 μm squares and the etch hole spacing from 100 μm to 5 μm can nearly double the coercivity of electroplated Co-Ni-P films. A model for the dependence of coercivity on etch-hole size and density is presented that separates the contributions of surface and bulk pinning density and compares well with experimental results. In addition, the ratio of surface to bulk pinning density can be quantified with this experimental procedure and for our Co-Ni-P films is approximately equal to 2