Development and optimisation of thin soft ferromagnetic Fe–Co–Ta–N and Fe–Co–Al–N films with in-plane uniaxial anisotropy for HF applications

Abstract

Thin ferromagnetic Fe–Co–Ta–N and Fe–Co–Al–N films with strong in-plane uniaxial anisotropy were successfully prepared by RF reactive magnetron sputtering and subsequent vacuum field annealing. The magnetic properties of as-deposited and field annealed films were investigated in order to optimise the deposition and annealing conditions. The films exhibit good soft magnetic properties and an excellent thermal stability because of their nanocrystalline microstructure. The critical temperature of degradation of the magnetic properties is around 500 °C; therefore, the films can be integrated in a standard semiconductor CMOS process. After a 1-h heat treatment at 400 °C, the following magnetic properties were achieved: saturation polarisation J s=1.2 T, coercive field μ 0 H c = 0.3 mT , electrical resistivity ρ =2×10 −6 Ω m, anisotropy field μ 0 H a = 3.5 – 5 mT . This results in a ferromagnetic resonance frequency of f r=1.8–2.1 GHz and initial permeability of μ low = 400 – 300 . The influence of intrinsic material parameters, such as saturation polarisation, uniaxial anisotropy field, electrical resistivity and film thickness, on HF permeability was analysed. The frequency-dependent permeability spectra were measured up to 4.5 GHz with a strip line permeameter.