Magnetic/semiconductor multilayer flip-chip-type tunable bandstop filter

A flip-chip-type MMIC architecture is used to build a wideband filter with ultrathin iron (Fe) films grown on compound semiconductor substrate (GaAs). Microwave and millimeter-wave components incorporating these ferromagnetic films possess the unique capability of having a resonance frequency that can be easily tuned by an externally applied magnetic bias. A bandstop notch filter is realized using this feature, with a wide tuning range of 10-40GHz being observed with bias fields up to 5000 Oe. Measured performance is compared to simulation based on a twodimensional model employing the full permeability tensor. Introduction Present systems employing microwave and millimeterwave technology require high operating frequencies and the circuit integration of a variety of electronic components to provide wideband performance is a necessity. The maturation of ferrimagnetic microwave devices such as yttrium iron garnet (YIG)-based filters demonstrates a continuing effort to develop magnetic thin-film structures capable of operating at high frequencies [1,2]. Tunability of operating frequency via an external magnetic bias has been the focus of our device research. One important limitation YIG-based applications present is a relatively low frequency tuning range with large bias field requirements. In this work we present structures having an iron (Fe) film grown by molecular beam epitaxy (MBE) on a compound semiconductor substrate (GaAs) [3], which possesses a saturation magnetization Ms that is more than an order of magnitude higher than that of its ferrimagnetic predecessors [4]. This characteristic in turn facilitates much higher operating frequencies – under the same bias condition – for devices incorporating such films [5-7]. The filter consists of a broadband transmission structure with a flip-chip overlay containing the magnetically active layer. The operating principle is straightforward: At the frequency of ferromagnetic resonance (FMR) electromagnetic energy is absorbed into electronic spin precession in the magnetic layer. This results in a passband outside of resonance and a sharp insertion loss peak at FMR. This notch frequency not only tunes readily with external bias but can also be combined in multiple bias configurations to yield desired stop bandwidth for microwave applications such as image frequency suppression. Device Operation A microstrip on GaAs substrate covered by iron film in flip-chip configuration constitutes a waveguide where the incoming signal is attenuated most strongly at the FMR frequency, which is in turn determined by the externally applied magnetic bias. The device schematic is shown in Fig. 1. Fig. 1 Microwave filter based on flip-chip-type MMIC configuration The elliptically polarized magnetic component of the signal contains two counter-rotating circularly polarized components. The component that rotates in the same direction as the electronic spin precession in the ferromagnetic film will lead to resonance absorption. Maximum coupling, and thus peak attenuation of microwave power, occurs at a frequency determined by the following relation [8]: ) 4 )( ( 0 0 s an an res M H H H H f π γ + + + = (1) The term 4pMs in Eq. (1) is the saturation magnetization of the iron film, which is equal to 22 KOe, while Han= 650 Oe and ? = 2.8MHz/Oe [3,5]. Fig. 2 Simplified two-dimensional model of multilayer wave guiding structure A simplified two-dimensional model shown in Fig. 2 is used to simulate filter response. The multilayer structure is bound by perfect electric conductors on top and bottom. The calculation involves finding longitudinal propagation constants and does not differ from the slab waveguide problem in electromagnetics, where successive application of boundary conditions yields propagation characteristics in the waveguide. In contrast to the slab waveguide, however, a magnetic layer characterized by an anisotropic permeability tensor must be employed. In a linear, homogeneous, and isotropic magnetic medium the curl of the magnetic field is given by the following: