The influence of stress on the magnetic hf-properties of amorphous ferromagnetic thin films

The development and integration of ferromagnetic thin film materials in new high frequency applications for the microelectronic- and microsystem-technology is a demanding challenge in the field of applied materials research. For the realisation and optimisation of such devices a detailed understanding how stress, internal film stress or applied external stress in conjunction with the magnetostriction of the thin films, is effecting their magnetic hf-properties is important. Therefore in this report the fabrication of amorphous ferromagnetic films with different positive and negative magnetostriction coefficients as well as the influence of stress on the hf-permeability is studied in detail. The negative magnetostrictive CoB and positive magnetostrictive FeCoBSi films investigated here are prepared by rf-magnetron sputtering. A special measurement set-up is realised to measure the hf-permeability of thin films up to frequencies in the GI-Iz-range and to compare the influence of applied stress on the hf-properties. The obtained results reveal that the internal film stress has to be well controlled to deposit films with low residual film stress and clear in-plane anisotropy. For CoB films optimised with respect to that fact frequencies up to 1.4 GHz are reached, while for optimised FeCoBSi films cut-off frequencies of 2.3 GHz are measured. The change of permeability and cut-off frequency is analysed as a function of the deflection of a bending cantilever beam. Considering the frequency shift by applying tensile stress the cut-off frequency is increasing for the CoB films and decreasing for the FeCoBSi films and contrary when compressive stress is applied. For the FeCoBSi films the permeability decreases for tensile and compressive stress, while for the CoB films the permeability increases for compressive and decreasing for tensile stress. The figure of merit (FOM) which represents an indicator for the sensing capability reaches high values up to 2000 for CoB and 2500 for FeCoBSi films. Summarised, this report illustrates that amorphous magnetostrictive ferromagnetic films are a suitable material to realise remote sensing strain sensor elements operating at high frequencies in the GHz-range.