Optimization of Passive Isolator Based On Barium Ferrite Sputtered Films
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Optimization of Passive Isolator Based On Barium Ferrite Sputtered Films M. Le Berre1, S. Capraro2, J.P. Chatelon2, T. Rouiller2, B. Bayard2, D. Barbier1, J. J. Rousseau2 1
Lab. Physique de la Matiere (LPM), UMR CNRS 5511, INSA de Lyon, 7 Av. Capelle, 69621 Villeurbanne Cedex, France. 2 DIOM, Univ. J. Monnet, 23 rue du Dr. Michelon, 42023 Saint-Etienne Cedex 2, France. ABSTRACT Ferrites have magnetic properties suitable for electronic applications, especially in the microwave range (circulators and isolators). Hexagonal ferrite, such as barium ferrite (BaFe12O19 or BaM), which have a large resistivity and high permeability at high frequencies are of great interest for microwave device applications. This contribution deals with BaM films, 1 to 36 microns thick, which were deposited under optimized conditions by RF magnetron sputtering. The films were then crystallized using a 800°C thermal annealing. Isolators were then realized using patterning of coplanar wave guides with standard lift-off technique. The slots and the central width were 300 µm wide, gold was used for the conductor lines. We evaluated the influence of various parameters on the device performances: the magnetic film thickness, the positioning of the magnetic film (CPW deposited onto the magnetic film or directly on the substrate) and the CPW metallic thickness. As standard design, the CPW were deposited on the top of the magnetic film. For the first design, transmission coefficients showed a non reciprocal effect, which reaches 7 dB per cm of line length at 50 GHz for a 26.5 µm thick BaM film. Both the insertion losses and the non-reciprocal effect measured increased with the magnetic film thickness with a saturation effect. In the second design where the CPW is deposited directly on the substrate, the BaM film was selectively wet etched prior to the metal deposition everywhere except between the conductors. In that case we measured that the non reciprocal effect reached high values for lower BaM thicknesses than in comparison to design # 1 and that the insertion losses also decreased. Moreover for the design # 2, also in relation with the localization of the BaM between the conductors, the non reciprocal effect improved with increasing conductor thickness as the interaction between the field lines created by the conductors and the magnetic film is then favored. INTRODUCTION Ferrite materials are used extensively in microwave devices such as isolators and circulators. Achieving the integration of such components is thus a major issue for current research. These components are based on the so-called gyromagnetic or ferrimagnetic resonance (FMR) of the ferrite which causes the non reciprocity of wave propagation [1, 2]. However, the polarization of these circuits is mainly achieved by permanent magnets which are incompatible with semiconductor technology. In this scope, the development of thin film based technologies could be a major step in the miniaturization of such components. C. P. Wen first realized a coplanar isolator in 1969 [3]. The i
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