Planar Ferrite Materials for Millimeter-Wave Applications
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Planar Ferrite Materials for Millimeter-Wave Applications Stephen W. McKnight1,2, Steven A. Oliver2, Hoton How3, and Carmine Vittoria1,3 1 Department of Electrical and Computer Engineering Northeastern University, Boston, MA 02115 U.S.A. 2 Center for Electromagnetics Research Northeastern University, Boston, MA 02115 U.S.A. 3 EMA Corp., Boston, MA USA ABSTRACT Transferred-film technology provides a path to the integration of self-biased hexaferrite film devices into MMIC circuits. While progress is being made on the growth of thick oriented films of barium hexaferrite doped with aluminum and scandium by pulsed laser deposition or liquidphase epitaxy, we have demonstrated prototype devices using available yttrium-iron-garnet films and thinned bulk hexaferrites. Insertion losses less than 2dB and isolation greater than 20dB have been demonstrated, and theoretical modeling indicates that insertion losses less than 0.5 dB are accessible with optimized materials and through matching the active circuit components to the ferrite device. The prospects of accessing ferrite non-reciprocal, non-linear, and tunable functions at frequencies up to and exceeding 100 GHz through devices integrated onto microwave IC substrates without the use of DC biasing magnets are the driving force of this research. INTRODUCTION Magnetic oxide ferrites are widely used for reciprocal and non-reciprocal functions in microwave and millimeter-wave circuits including tunable phase-shifters, baluns, limiters, isolators, and circulators [1, 2, 3]. Techniques for the integration of planar ferrite devices into semiconductor integrated circuits have been sought for many years to supercede present hybrid technologies. Two paths have been investigated for fabricating thick ferrite films on semiconductor IC substrates: direct growth [4] and film transfer techniques [5]. Direct growth is hampered by the incompatibility of optimal ferrite fabrication temperatures (650-900 C) with semiconductor IC materials and processes. We have investigated processes to transfer ferrite films and fabricate devices on semiconductor IC substrates after growing the films on the substrates that best suited to producing high-quality, oriented films. Growth substrates are single-crystal materials that are lattice-matched to a selected orientation of the ferrite crystal, and growth techniques include pulsed laser deposition from pressed powder targets and liquid phase epitaxy. Ferrites investigated include yttrium-iron-garnet (YIG) films for X-band (8-12 GHz) and lower frequency application, and barium and strontium hexaferrite materials for millimeter-wave devices. Doping of barium hexaferrite with scandium or aluminum will allow the tuning of the ferrite materials for lower (BaScxFe12-xO19) or higher (BaAlxFe12-xO19) frequencies. Tuning ferrite film properties to match the properties required for ferrite device design promises to create a new generation of high-performance microwave/millimeter-wave devices. Transferring the ferrite films from their growth substrate to the microwav
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