Multilayer nanogranular magnetic thin films for GHz applications

(CoFe)–Si–O/Si–O multilayer nanogranular magnetic thin films for GHz rf circuits were designed and fabricated on SiO2/Si substrates using inductively coupled rf sputtering. A multilayer structure consisting of a CoFe ferromagnetic mononanogranular layer covered with an atomic-order SiO2 amorphous phase and a nanometer-order insulative SiO2 amorphous layer was fabricated by repeatedly cosputtering CoFe alloy and SiO2 glass targets and the subsequent single sputtering SiO2 target. The optimized multilayer structured film (appropriate grain size of CoFe and Si–O layer thickness, i.e., CoFe mononanogranular layer: Si–O layer=6 nm: 1 nm) shows superb soft magnetism performance in high-frequency regions, such as a constant 200 of real permeability with 2.2 mΩ cm resistivity and 2.86 GHz self-resonant frequency.