EMI shielding characteristics of permalloy multilayer thin films

High permeability magnetic films used for shielding or absorption of electromagnetic interference (EMI) are commonly built in a multilayer stack to maximize the permeability by eliminating or reducing the number of domain walls. The choice of spacer material is driven by the fact that electrically insulating spacer materials reduce eddy currents which cause incident waves to be reflected and not absorbed. Design of frequency specific EMI absorbers or shields requires knowledge of the rf permeability of magnetic multilayer films. We examine the complex, rf permeability spectra and low frequency magnetic field shielding characteristics of DC planar magnetron sputtered Fe₁₉Ni₈₁ multilayer films as a function of spacer material, number of layers, and thickness of the spacer and magnetic layer. The permeability spectra are measured using a permeameter over the frequency range 5 MHz to 100 MHz. The spacer materials examined include several metals and metal alloys, the corresponding oxides, silicon, and silicon nitride. Spacer layers made of Si₃N₄ result in permeability spectra with the highest permeability magnitudes, while copper spacers result in the highest shielding levels. For a fixed number of multilayer pairs and constant spacer layer thickness making the magnetic layer thicker tends to increase the permeability magnitude, and lower the resonance frequency. We find that for electrically insulating spacer layers the same characteristic permeability spectra can be obtained by either increasing the number of layers or increasing the thickness of the magnetic layer