Surface tension-driven assembly of metallic nanosheets at the liquid-air interface: Application to highly laminated magnetic cores

This paper presents a fabrication technique to develop highly laminated structures comprising stacked thin films, in which the structures are based on surface tension-driven assembly at the liquid-air interface. When multiple metallic films are removed from a liquid solution, there is a surface tension-driven coalescence and self-alignment of the wetted films, resulting in thick metallic microstuctures comprised of many layers of metallic nanosheets after evaporation of the liquid. If the liquid contains a dissolved material, each sheet can further be coated with the material prior to assembly. Based on this technique, we developed laminated structures comprising hundreds of nanoscale layers of alternating metallic film and non-conducting polymer. Electroplated Co₄₄Ni₃₇Fe₁₉ (Cobalt-Nickel-Iron alloy) and a commercial Novec 1700 solution (3M, Minnesota) were utilized for the metallic film and the liquid solution, respectively. However, the assembly process inherently allows a variety of materials to be exploited. Theoretical analysis and experimental results were compared, demonstrating a critical gap between the metallic films, below which capillary force is sufficient for driving self-assembly of the films. As an exemplary application of this technique, highly laminated magnetic cores comprising 600 layers of 500 nm thick CoNiFe that are insulated by 100 nm thick polymer were prepared. A 15-turn toroidal inductor with the fabricated magnetic core exhibited a constant inductance of 2.5 μH up to 30 MHz with a quality factor over 70 at 1 MHz.