Authors:
Manuel Heidenreich | Ernst-Abbe-Hochschule Jena
Wanja Gitzel | Technische Universität Hamburg
Arne Jacob | Technische Universität Hamburg
Johannes Schur | Technische Universität Ilmenau
Jens Mueller | Technische Universität Ilmenau
Beate Capraro | Fraunhofer IKTS Hermsdorf
Jörg Töpfer | Ernst-Abbe-Hochschule Jena | Germany
Future satellite communication technologies require LTCC multilayer modules with integrated magnetic microwave components for applications in the Ka-band (26-40 GHz). Sc-, In-, and (Zn/Sn)-substituted Ba ferrites BaMexFe12-xO19 exhibit significant potential as self-biasing microwave components, e.g. for circulators. Static magnetic measurements show that both the saturation magnetization and coercivity decrease with concentration of Me substituents indicating a reduction of the anisotropy field with substituent concentration. The ferromagnetic resonance frequency of the ferrites, determined by reflectance measurements in U- and K-band (30-60 GHz) waveguide, shows a systematic reduction with x for Sc-, In-, and Zn/Sn substitution. For Sc and x = 0.5, a resonance is observed in the Ka-band at 30 GHz. For Zn/Sn-substituted ferrites, the FMR of 30 GHz is found at x = 0.25.
Sintered substituted ferrite samples were integrated into LTCC multilayer microwave modules as drop-in bulk components into LTCC cavities and subsequent post-firing. The precise geometry of the cavities and ferrite bulks is critical, and was achieved through laser machining. Different circulators designs were fabricated and tested. Before integration, the ferrite bulks were pressed as a slurry in a magnetic field and sintered. The c-axis orientation of the ferrite particles was characterized using X-ray diffraction and Lotgering factor calculations as well as pole figure measurements. Direction-dependent static magnetic hysteresis measurements, performed with a VSM, confirm the magnetic anisotropy. As another option, cofiring of the ferrite with the LTCC microwave multilayer module was performed. The sintering temperature of the ferrites was lowered to 900°C using sintering additives to allow for cofiring with the LTCC tapes. The ferrite layers were integrated as thick films through screen-printing of a ferrite paste onto the LTCC layers. Anisotropic ferrite films were obtained through drying of the printed ferrite films in a magnetic field. Another integration technique relies on tape casting of ferrite tapes and application of a magnetic field during green tape drying. The magnetic anisotropy of the printed films and ferrite tapes was tested using XRD and magnetic measurements. Cofiring of ferrite pastes or tapes with the LTCC multilayer assembly at 900°C results in LTCC microwave architectures with integrated ferrite layers. The potential of such oriented hexagonal ferrite layers as integrated self-biased ferrite layers in microwave components, i.e. circulators, is evaluated.