Passive isolators based on barium ferrite sputtered films

Ferrites have magnetic properties suitable for electronic applications, especially in the microwave range (circulators and isolators). Hexagonal ferrite, such as barium ferrite (BaFe12O19 or PF BaM), are of great interest for microwave device applications because of their large resistivity and high permeability at high frequencies. This contribution focuses on BaM films, 1 to 10 microns thick, which were deposited under optimized conditions by RF magnetron sputtering on alumina or silicon substrates. In order to crystallize the films that were amorphous after deposition, a post deposition annealing at 800degreesC was implemented. Optimized samples presented a good crystallization, a smooth surface and no cracks. The films were either randomly oriented or showed slight preferential orientations among the crystallographic planes (101), (200), (206), (102), (110) and (205) when the substrates were heated up to 400degreesC during the deposition. Ba, Fe and O depth profiles obtained by Secondary Ion Mass Spectroscopy (SIMS) showed that the films have a good in-depth uniformity. The magnetic properties of BaM films determined by VSM, showed that the optimized coercive force and the saturation magnetization reached 330 kA/m and about 500 mT respectively. These values are closed to that of the bulk BaM. Isolators were then realized using patterning of coplanar metallic lines with standard lift-off technique. The slots and the central width were set to 300 pm, gold was used for the lines. First results on transmission coefficients showed a non reciprocal effect, which reaches 3.3 dB/cm at 50 GHz. This proved that such a component behaves like an isolator in the 50 GHz band.