Andrei Vorobiev

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.

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/.

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%.

_{x}{{\hbox{Sr}}_{1-x}}{\hbox{TiO}}_{3}

Parallel-plate Au(Pt)∕Ba0.25Sr0.75TiO3∕(Pt)Au thin film varactors were fabricated on high resistance Si substrates and characterized at dc, rf, and microwave frequencies. In the frequency range 10–45 GHz the varactors show relatively low losses, with loss tangent less than 0.025 at 45 GHz. Due to the thick and highly conductive Pt/Au electrodes the metal losses are less than 10%. However, the loss tangent of the ferroelectric film is still three to five times higher than that in Ba0.27Sr0.73TiO3 single crystal. The analysis of the dc field dependences of loss tangent and permittivity in a wide frequency range shows that these additional losses are mainly due to the charged defects. Extrapolation of measured low frequency (1 MHz) loss tangents to the microwave region using the power law ω1∕3 is in good agreement with experiment. The dc current through the varactor is found to be controlled by Schottky emission and Poole-Frenkel mechanisms depending on the polarity. The Poole-Frenkel mode is associated with...

Films

This paper reports on compact tunable true-time delay lines based on ferroelectric (Ba/sub 0.25/Sr/sub 0.75/TiO/sub 3/) varactors integrated on high-resistivity silicon. The delay lines are based on lumped elements, physically implemented as synthetic coplanar-strip lines. An approximate analytical design procedure, exactly valid for /spl omega//spl rarr/0, is proposed. The physical size of the fabricated delay lines is 2.0/spl times/0.33 mm/sup 2/, including bias pads. Measurements are performed from room temperature (RT) down to 80 K. The measurements reveal ultrawide-band characteristics for both group delay and insertion loss. At RT, the absolute group delay is /spl tau//sub /spl delta//(RT, 0 V) /spl ap/70 ps with tunability of 20% under 20-V dc bias, the insertion less than 3.5 dB, and the reflection loss better than 12 dB below 20 GHz. At 145 K, the absolute group delay is increased to /spl tau//sub /spl delta//(145 K, 0 V) /spl ap/100 ps with a tunability of 50% under 20-V applied bias. At 7 GHz, the insertion loss is 3 dB, resulting in figures-of-merit of 0.03 dB/ps and 50 ps/mm. The leakage current at RT is less than 0.1 /spl mu/A.

Microwave loss mechanisms in Ba0.25Sr0.75TiO3 thin film varactors

Electrically tunable solidly mounted thin film bulk acoustic resonators based on Ba0.25Sr0.75TiO3 and BaTiO3 films are fabricated and measured in wide dc bias voltage and temperature ranges. At room temperature, the tunability of the series and parallel resonances for the Ba0.25Sr0.75TiO3 resonator are 1.7% and 0.3%, respectively, for 15V bias voltage applied over the 350nm thick ferroelectric film (43V∕μm). The electromechanical coupling coefficient increases with dc bias up to 3.7% at 15V. The measured tunability and coupling coefficient are limited partly by the quality of the used films. Potentially, they may be substantially increased for high quality films allowing application of higher dc fields. The resonator quality factor is approximately 100. The measured resonator response is in good agreement with available models based on the electromechanical equations describing the ferroelectric film under applied dc and ac electric fields. Measurements of the resonance frequencies of the Ba0.25Sr0.75TiO3...

Ultrawide-band tunable true-time delay lines using ferroelectric varactors

Correlations between microstructure and Q-factor of tunable solidly mounted Ba(0.25)Sr(0.75)TiO(3) (BSTO) thin film bulk acoustic wave resonators are studied using analysis of test structures prepared at different growth temperatures of the BSTO films varying in the range 450-650 degrees C. The observed changes in the Q-factor with growth temperature are correlated with related changes in microstructure, including the grain size, texture misalignment, interfacial amorphous layer, surface roughness, and deterioration of the Bragg reflector layers. The correlations are established through analysis of corresponding extrinsic acoustic loss mechanisms, including Rayleigh scattering at localized defects, acoustic attenuation by amorphous layer, generation of the shear waves leaking into the substrate, waves scattering by surface roughness, and resonance broadening by local thickness variations. It is shown that the waves scattering by surface roughness at the BSTO film interfaces is the main loss mechanism limiting the Q-factor of the BSTO thin film bulk acoustic wave resonators

Field and temperature dependent parameters of the dc field induced resonances in BaxSr1-xTiO3-based tunable thin film bulk acoustic resonators

The tunable solidly mounted Ba0.25Sr0.75TiO3 (BSTO) thin film bulk acoustic wave resonators (TFBARs) with improved Q-factor are fabricated and characterized. The BSTO films are grown by magnetron sputtering at temperature 600 degrees C and extremely low sputter gas pressure 2 mTorr using on-axis configuration. The measured TFBARs Q-factor is more than 250 and mechanical Q-factor is more than 350 at 5 GHz resonance frequency. The improvement in the Q-factor is associated with reduction in the BSTO film grain misorientation. The latter is responsible for generation of shear waves leaking through the Bragg reflector and corresponding acoustic loss

Correlations between microstructure and Q-factor of tunable thin film bulk acoustic wave resonators

Tuneable Film Bulk Acoustic Wave Resonators discusses FBAR need, physics, designs, modelling, fabrication and applications. Tuning of the resonant frequency of the FBARs is considered. Switchable and tuneable FBARs based on electric field induced piezoelectric effect in paraelectric phase ferroelectrics are covered. The resonance of these resonators may be electrically switched on and off and tuned without hysteresis. The book is aimed at microwave and sensor specialists in the industry and graduate students. Readers will learn about principles of operation and possibilities of the switchable and tuneable FBARs, and will be given general guidelines for designing, fabrication and applications of these devices.