Spartak Gevorgian

CAD models for multilayered substrate interdigital capacitors

Conformal mapping-based models are given for interdigital capacitors on substrates with a thin superstrate and/or covering dielectric film. The models are useful for a wide range of dielectric constants and layer thicknesses. Capacitors with finger numbers n/spl ges/2 are discussed. The finger widths and spacing between them may be different. The results are compared with the available data and some examples are given to demonstrate the potential of the models.

Do we really need ferroelectrics in paraelectric phase only in electrically controlled microwave devices

Typical paraelectric materials (e.g., SrTiO/sub 3/, KTaO/sub 3/, Ba/sub x/Sr/sub 1-x/TiO/sub 3/, x 10 GHz) is of the order of 0.01 (at zero dc-bias field) at room temperature. Nevertheless, quite promising component and subsystem level devices are successfully demonstrated. Use of ceramic (bulk and thick film) ferroelectrics in tunable microwave devices, currently considered for industrial applications, offer cost reduction. In this paper, explicitly for the first time, we consider possibilities and benefits of using ferroelectrics in polar phase in electrically controllable microwave devices. Examples of using ferroelectrics in polar state (e.g., Na/sub 0.5/K/sub 0.5/NbO/sub 3/, SrTiO/sub 3/ in antiferroelectric phase) in electrically tunable devices are reported.

Silicon substrate integrated high Q-factor parallel-plate ferroelectric varactors for microwave/millimeterwave applications

Parallel-plate Ba0.25Sr0.75TiO3 (BST) varactors with a record high Q factor are fabricated on Si substrate. At 45 GHz the Q factor is about 40, and the tuneability at 25 V is more than 40% in the measured frequency range 0.045–45 GHz. The improvement in the Q factor is achieved by using a thick bottom electrode consisting of Pt (50 nm)/Au (0.5 μm) allowing us to reduce the microwave losses associated with metal layers. The BST films exhibit relatively high permittivity (150) at zero bias and high resistivity (1010 Ω cm) at fields up to 700 kV/cm.

CAD models for shielded multilayered CPW

Conformal mapping technique is used to obtain CAD oriented closed form analytical expressions for characteristic impedance per unit length capacitance and relative effective dielectric permittivity of top and bottom shielded multilayered coplanar waveguides. Analytical expressions are deduced for a wide verity of CPW structures. >

Conformal mapping of the field and charge distributions in multilayered substrate CPWs

Conformal mapping (CM) and partial capacitance techniques are used for analytical evaluation of charge/current and electric- and magnetic-field distributions in a multilayered substrate coplanar waveguide (CPW) in the quasi-TEM approximation. The results, compared with finite-element method simulations, show that the magnetic wall assumed at the dielectric-dielectric interfaces in CM is a good approximation for many practical cases. The method is applied to a CPW with a thin ferroelectric film used in tunable microwave devices.

dc field and temperature dependent acoustic resonances in parallel-plate capacitors based on SrTiO3 and Ba0.25Sr0.75TiO3 films: Experiment and modeling

Experimentally observed resonant absorption peaks in frequency dependent loss tangent in thin film parallel-plate SrTiO3 and Ba0.25Sr0.75TiO3 capacitors are explained by electrostriction and piezoelectric effects. A simple theory (linear approximation) is proposed to model the electric field and temperature dependences of the acoustic resonant frequencies, and the intensities of the microwave into acoustic transformations. The electric field dependence of the acoustic resonant frequencies may be used to develop tunable thin film bulk acoustic wave resonators (TFBARs) operating at frequencies up to 10GHz and above. The expected tuneability of the TFBARs is more than 5%.

Composite right/left handed transmission line phase shifter using ferroelectric varactors

A composite right/left handed (CRLH) transmission line (TL) phase shifter, using ferroelectric (Ba/sub 0.25/Sr/sub 0.75/TiO/sub 3/) varactors as tunable element, is presented for the first time. It is theoretically and experimentally demonstrated how the unique features of CRLH TLs, enables a differential phase shift with flat frequency dependence around the center frequency. The experimental prototype is a coplanar design integrated on a high resistive Si substrate. It includes four CRLH T-unit cells and has a physical length of 3850/spl mu/m. The ferroelectric varactors are realized in parallel plate version. Under 15-V dc bias applied over each varactor, the differential phase shift is flat around 17GHz and has an absolute value of 50/spl deg/.

GROWTH AND FIELD DEPENDENT DIELECTRIC PROPERTIES OF EPITAXIAL NA0.5K0.5NBO3 THIN FILMS

Na0.5K0.5NbO3 thin films have been grown on LaAlO3(001) substrates using rf magnetron sputtering from a Na and K enriched target. X-ray diffraction showed that the films are epitaxial with mosaic broadening as narrow as 0.044°. Interdigital Au finger electrodes were photolithographically defined on the film surfaces. The dielectric properties of these interdigital capacitors were measured at 1 MHz from room temperature down to 50 K. The capacitor showed 35% tunability at room temperature and a loss tangent of 0.007 without dc bias applied. The loss decreased further with increasing dc bias. For lower temperatures, the capacitance exhibited a broad maximum at ∼200 K, which is possibly linked to a phase transformation of the Na0.5K0.5NbO3 film.

HTS/ferroelectric devices for microwave applications

High Temperature Superconducting (HTS, e.g. YBCO) microwave devices based on bulk or thin film ferroelectrics (e.g. Strontium Titanate-STO) are studied theoretically and experimentally. YBCO/STO/YBCO parallel-plate resonators and based on bulk single crystal STO are for electrically tunable high power filters, phase shifters and other devices in the frequency band 0.5-2.0 GHz of advanced microwave communication systems. Thin film YBCO/STO Inter-digital Capacitors (IDC), Coplanar Waveguides (CPW) phase shifters/delay lines are also designed and studied experimentally for low microwave power applications. Modelling problems of these devices and microwave losses in ferroelectrics are also discussed.

Tunable Solidly Mounted Thin Film Bulk Acoustic Resonators Based on Ba

Electrically tunable solidly mounted thin film bulk acoustic resonators based on BaxSr1-xTiO3 films are reported for the first time. The films are acoustically isolated from the silicon substrate by a Bragg reflector stack. Applying DC bias induces piezoelectric effect and an acoustic resonance at approximately 4 GHz. Under 10 V applied DC bias the resonance frequency of the resonators based on Ba0.25Sr0.75TiO3 films is tuned 1.2% to lower frequencies. The Q-factor of these resonators is approximately 120, and the electromechanical coupling coefficient is 0.5%. The resonant frequency of the BaTiO3 based resonators shifts upwards 1.3% under 10 V DC bias, and the -factor is approximately 30, with an electromechanical coupling coefficient of 6.2%.