Micromolding of Ni-P with Reduced Ferromagnetic Properties for 3D MEMS

This paper presents the development of nickel phosphorus (Ni-P) micromolding for the manufacturing of a 3D electrostatic energy harvesting microsystem. Ni-P alloy exhibits weak ferromagnetic properties beyond 10-12 wt% phosphorus content. Deposits were prepared at different current densities (−10 to −150 mA/cm²) and concentration of phosphorous acid in the electrolyte (0-20 g/l). It was found that the deposition rate decreases when phosphorus content increases in the deposit. The final process leaded the choice of a H3PO3 concentration of 5 g/l to reach a 0.1 μm/min deposition rate for phosphorus content higher than 13 wt%. Mechanical, electrical and magnetic properties of the Ni-P films were investigated on 1 mm² and 1 cm² square deposit and confirmed the suitability of that material for the target MEMS. Comb patterns of micromolded Ni-P have been realized on a 2-inch wafer, leading to a 10 μm thick deposit containing 13.5 wt% in P, which is, at our knowledge, the first high phosphorus Ni-P micromolding involving electrodeposition growth for 3D MEMS applications.