Thin film BaxSr1-xTiO3 ku- and k-band phase shifters grown on MgO substrates

Abstract

We report measurements of gold circuits fabricated on four Ba_xSr_1-xTiO₃ ferroelectric films doped with 1% Mn grown on MgO substrates by laser ablation. Low frequency (1 MHz) measurements of σ_T and tanδ on interdigital capacitors are compared with high frequency measurements of phase shift and insertion loss on coupled microstrip phase shifters patterned onto the same films. The variation in temperature of both high and low frequency device parameters is compared. Annealed with amorphous buffer layer and unannealed films are compared. Room temperature figures of merit of phase shift per insertion loss of up to 58.4°/dB at 18 GHz and 400 V dc bias were measured.     

Ferroelectric phase shifters and their performance in microwave phased array antennas

Abstract

Recent advances in the development of Barium Strontium Titanate ferroelectric composition has made possible reasonable performance of ferroeletric phase shifters to frequencies up to 10 GHz. These material improvements, coupled with phase shifter circuit design changes have resulted in phase shifts greater than 360 degrees with less than 6 dB insertion loss. In particular additives to the BaxSr1-_xTiO₃ composition have been shown to exhibit a consistent electrical phase shift verses DC potential over parameters of temperature and humidity. These ferroelectric material improvements and circuit design changes, included with the development of multiple ferroelectric phase shifters makes possible the fabrication of a low cost electronic scanning antenna. A single four element phase shifter was used with a one dimensional linear antenna array which was constructed on three layers and used an aperture coupled distribution technique. Individual elements of this multiple four element phase shifter were evaluated with respect to uniformity phase shift and insertion loss. The four element antenna was fed by four ferroelectric phase shifters and the phase shifters are corporately fed by the microwave source. The ferroelectric phase shifters are controlled via a dedicated microcontroller which calibrates out element phase variations and provides a real time scan capability for the antenna assembly.     

Design of Tunable Balanced Amplifier Using Ferroelectric Materials

The non-linear electric field dependence of ferroelectric thin films can be used to design frequency and phase agile components. It is well known from the literature that ferroelectric-based tunable microwave component can easily be integrated into conventional microstrip circuits. These components are particularly attractive for broad-band and multi-frequency applications. However, most efforts have focused on the careful characterization of tunable passive devices while active components have been occasionally reported. In this work, simulated results from a full wave electromagnetic simulator are obtained to show the tunability of a quadrature hybrid made of Ba _x Sr_{1 ? x} TiO₃ (BSTO) ferroelectric thin film on MgO. The S-parameters of this passive structure are estimated for different bias conditions for the BSTO and used in the simulation of tunable balanced amplifier. Simulations of this type of amplifier are shown with and without matching networks.     

Simulation of Single-Stage Tunable Amplifier Using Ferroelectric Materials

The non-linear electric field dependence of ferroelectric thin films can be used to design frequency and phase agile components. Tunable components have traditionally been developed using mechanically tuned resonant structures, ferrite components, or semiconductor-based voltage controlled electronics, but they are limited by their frequency performance, high cost, high losses, and integration into larger systems. In contrast, the ferroelectric-based tunable microwave component can easily be integrated into conventional microstrip circuits and attributes such as small size, light weight, and low-loss make these components attractive for broadband and multi-frequency applications, many of these components are essential elements in the design of a microwave sensor and/or circuit. It has been reported that with a thin ferroelectric film placed between the top conductor layer and the dielectric material of a microstrip structure, and the proper DC bias scheme, tunable components above the Ku band can be fabricated. Components such as phase shifters, coupled line filters, and Lange couplers have been reported in the literature using this technique. In this work, simulated results from a full wave electromagnetic simulator are obtained to show the tunability of a single stage amplifier. Input and output matching networks are simulated on a ferroelectric thin film to control the frequency response of the amplifier.     

CHARACTERIZATION OF FERROELECTRIC FILMS UP TO 60 Ghz: APPLICATION TO PHASE SHIFTERS

ABSTRACT

Paraelectric Ba_0.5Sr_0.5TiO₃ films 0.3 μm thick have been deposited by sol-gel on c-axis sapphire substrates. They have been investigated from 1 kHz to 60 GHz using coplanar waveguide transmission lines and interdigitated capacitors. The dielectric constant ε_r is around 300 and the loss tangent is 0.16 at 50 GHz. The tunability is constant with frequency with a mean value of 42%. Analog phase shifters were subsequently fabricated. A 180° phase shift was obtained at 60 GHz with a 17 V bias. The maximum value of phase shift per decibel of insertion losses (at 0 V) is 13°/dB at 30 GHz with a bias of 30 V.     

Microwave Properties of Mn Doped (Ba1 - x,Srx)TiO3Thin Films for Tunable Phase Shifter

Ferroelectric Mn doped Ba_0.5Sr_0.5TiO₃ (Mn-BST) films with/without BaTiO₃ (BT) buffer layer have been grown on (001) MgO substrates by a pulsed laser deposition to investigate electrical tunability at microwave frequencies. Structural properties and surface morphologies of the films were investigated using an X-ray diffractometer and a scanning electron microscope, respectively. Microwave dielectric properties of Mn-BST thin films with BT buffer were studied for reduction of dielectric loss and improvement of electrical tunability. Distributed analog phase shifters have been designed and fabricated on Mn-BST films with/without BT buffer layer to understand microwave dielectric properties. The differential phase shift of the phase shifter fabricated on Mn-BST film was 22° at 10 GHz with 80 V of applied dc bias voltage. In comparison, phase shifter fabricated on Mn-BST/BT multilayers exhibit 41° of differential phase shift at the same condition. This suggests that a BT buffer layer is for microwave tunable device applications. The phase shifter fabricated on Mn-BST/BT multilayers exhibit a low insertion loss (S₂₁) of ?1.1 dB, and a low return loss (S₁₁) of ?14 dB with a bias voltage of 80 V.     

Improved Temperature Stability of Microwave Properties in Tunable Devices Using Substituted Ba 1− x Sr x TiO 3

An improvement in ferroelectric device technology has become possible through optimization of the properties of the ferroelectric materials used to make the devices. The improvements discussed herein impact true time delay line and phase shifter performance by enhancing control over dielectric constant and extending the temperature range of tunability and device operation. Figures of merit were obtained for substituted-Ba 1 m x Sr x TiO 3 materials which have lower, less varying dielectric constant and adequate, less varying tunability at 1 MHz over the mil spec ( m 50 °C to 100 °C) range. For the sample whose figure of merit varies the least, dielectric constant and losses at 20 GHz and room temperature are reported. Dielectric constant and losses at 1 kHz are discussed.     

Synthesis and characterization of (BaxSr1−x)Ti1+yO3+z thin films and integration into microwave varactors and phase shifters

Abstract

Precise control of composition and microstructure is critical for the production of (Ba_xSr_1?x)Ti_1+yO_3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high frequency devices. Here we review recent results on composition-microstructure-electrical property relationships of polycrystalline BST films produced by magnetron sputter deposition, that are appropriate for microwave devices such as phase shifters. Films with controlled compositions were grown from a stoichiometric Ba_0.5Sr_0.5TiO₃ target by control of the background processing gas pressure. It was determined that the (Ba+Sr)/Ti ratios of these BST films could be adjusted from 0.73 to 0.98 by changing the total (Ar+O₂) process pressure, while the O₂/Ar ratio did not strongly affect the metal ion composition. Film crystalline structure and dielectric properties as a function of the (Ba+Sr)/Ti ratio are discussed. Optimized BST layers yielded capacitors with low dielectric losses (0.0047), among the best reported for sputtered BST, while still maintaining tunabili-ties suitable for device applications. These BST films were used to produce distributed-cir-cuit phase-shifters, using a discrete periodic loading of a coplanar waveguide with integrated BST varactors on high-resistivity silicon. Phase shifters yielding 30 degrees of phase shift per dB of insertion loss were demonstrated at 20GHz.     

Barium strontium titanate and non-ferroelectric oxide ceramic composites for use in phased array antennas

Abstract

A ceramic ferroelectric phase shifting device has been demonstrated using Ba_1?xSr_xTiO₃ (BSTO) ceramics. ¹ As part of an effort to optimize the device performance, various composites of BSTO and other nonelectrically active oxide ceramics have been formulated. In general the composites have reduced dielectric constants, ε′, where ε = ε′-iε″ and reduced loss tangents, tan δ, compared to BSTO. The low dielectric constant and low loss tangent reduce the overall impedance mismatch and insertion loss of the device. In addition, the tunability (change in the dielectric constant with applied voltage) is maintained at a relatively high level (15% with an applied electric field of 1.5 V/μm) for dielectric constants of 200. The combination of electronic properties of these materials offer substantially higher operating frequencies, 10 GHz and above. The microstructures including grain size and phase analysis have been examined using SEM and EDX. X-ray diffraction has been used to identify the presence of any secondary phases formed in the composites. The analysis of the phase formation and compositional variations will be related to the electronic properties of the materials.     

High quality BaxSr1-xTiO3 films grown by mocld and novel ferroelectric/ferrite structures for dual-tuning microwave devices

Abstract

Excellent single crystal Ba_xSr_1-xTiO₃ (BST) films were grown on LaAlO₃ substrates using the metal-organic chemical liquid deposition (MOCLD) method. Very low losses (tanδ ∽0.002-0.008) were measured from these films at 400 KHz. Biaxially oriented BST films were successfully grown on polycrystalline YIG substrates using both MOCLD and pulsed laser deposition methods with biaxially oriented MgO and YSZ buffer layers. The dielectric losses of the films range from 0.005 to 0.015 while 25% of dielectric constant change was observed with 40V bias voltage up to 10 MHz. Both the dissipation and dielectric constant of the films remained nearly constants over a wide temperature range (77 K to 380 K). A dual-tuning microwave coplanar phase shifter using a BST film grown on a MgO buffered polycrystalline YIG substrate was fabricated. A significant phase shift was observed in GHz frequency range when an electric bias or a magnetic field was applied to the device.     

Ferroelectric tunable coplanar waveguide components for Ku- and K-band and applications

Abstract

The main objective of this research is to study the effect of inserting a Barium Strontium Titanate (BSTO) ferroelectric tuning layer in coplanar waveguide (CPW) and conductor-backed CPW (CBCPW) components. The modeled components include CPW and CBCPW transmission lines (with and without a dielectric filling between the center conductor and the ground planes). We have modeled the characteristic impedance (Z₀), effective dielectric constant (ε_eff), attenuation and dispersion as a function of circuit geometry and the ferroelectric thin-film's dielectric properties over the 10–20 GHz frequency range. We found that the presence of a ferroelectric layer between the transmission line and the ground planes improves the percentage change in ε_eff by almost two-fold with respect to a CPW deprived of this layer. This result is significant, as one could obtain larger frequency tunability with relatively lower applied fields compared to regular CPW or microstrip lines.     

Dielectric properties and tunability of BST and BZT thick films for microwave applications

Abstract

Dielectric properties and tunability of Ba_0.60Sr_0.40TiO₃ (BST) and BaZr_0.25Ti_0.75O₃ (BZT) thick films and bulk ceramics have been investigated as a function of temperature (90 K - 320 K) in the kHz region. Thick films show compared to bulk ceramics low permittivity and a very broad ferroelectric phase transition which leads to low temperature dependence of tunability. Tunability of 30% can be achieved with an external field of 2 kV/mm. Measurements in the microwave region adumbrate that the dielectric losses of BZT are about three times higher than that of BST thick films.     

Dielectric and Mechanical Losses in (Ba,Sr)TiO3 Systems

In the application of tuneable microwave devices of ferroelectric (BaSr)TiO₃ systems the two critical parameters needed for optimal device performance are high tunability and low dielectric loss. The dielectric loss of the materials is strongly dependent on microstructure. This paper is concerned with an investigation of the variation in the dielectric and mechanical losses in Ba _x Sr_{1 – x} TiO₃ systems (x = 0.5, 0.6, 0.7 and 1.0) with microstructure (grain sizes from 1 m to 50 m). The magnitude of the loss peak and sharpness of the anomaly in the dielectric constant/elastic modulus observed for the phase transitions in Ba _x Sr_{1 – x} TiO₃, depend not only on the composition and but also on the grain size. A relaxation peak has been observed in large grain material, which is indication of interactions between different configurations of domain walls and the diffusion of oxygen vacancies in the domains.     

A comparison of mocld with PLD BaxSr1−xTiO3 thin films on LaAIO3 for tunable microwave applications

Abstract

Historically, tunable dielectric devices using thin crystalline Ba_xSr_1-x TiO₃ (BST) films deposited on lattice-matched substrates, such as LaAlO₃, have generally been grown using pulsed laser deposition (PLD). Highly oriented BST films can be grown by PLD but large projects are hampered by constraints of deposition area, deposition time and expense. The Metal-Organic Chemical Liquid Deposition (MOCLD) process allows for larger areas, faster turnover and lower cost. Several BST films deposited on LaAlO₃ by MOCLD have been tested in 16 GHz coupled microstrip phase shifters. They can be compared with many PLD BST films tested in the same circuit design. The MOCLD phase shifter performance of 293° phase shift with 53 V/μm dc bias and a figure of merit of 47°/dB is comparable to the most highly oriented PLD BST films. The PLD BST films used here have measured XRD full-width-at-half-maxima (FWHM) as low as 0.047°. The best FWHM of these MOCLD BST films has been measured to be 0.058°.     

Structural and Electrical Properties of Ba(ZrxTi1-x)O3 Films Prepared by RF-magnetron Sputtering Using Metal Targets

Ba(Zr_xTi_1-x)O₃ (BZT) thin films with different Zr contents were deposited on (100)MgO and (100)Pt/(100)MgO substrates by RF-magnetron sputtering using metal targets. The BZT thin films had a single perovskite phase with only (001)/(100) orientation. In all cases, the ratio of Ba/Ti was stoichiometry and BZT films possess a dense microstructure. The grain size was decreased and BZT thin films showed ferroelectric-to-paraelectric properties with increasing Zr content. At room temperature, the tunability of nearly 30% was achieved at 1 MHz; meanwhile, a relatively low dielectric loss was obtained. These results indicated that we succeeded in depositing high-quality and potential tunable ferroelectrics.     

Widening of Operational Temperature Range of Microwave Ferroelectric Tunable Devices

The ways to solve a problem of providing stable operation of microwave ferroelectric phase shifters for steerable antennas in a wide temperature range are briefly considered. A ferroelectric phase shifter based on cascaded tunable ferroelectric planar capacitors is properly described. The connection of the tunable capacitors in series with transmission line sections allows obtaining a voltage tunable phase shift, which practically does not depend on temperature in the range 250–350 K. Such phase shifters can find a wide application in phased-array antennas.     

Ceramics Materials Based on (Ba, Sr)TiO3 Solid Solutions for Tunable Microwave Devices

Structure and electrical properties at radio frequencies as well as within the 3.5–35 GHz frequency range have been investigated for ceramic samples of the (1–y)(Ba_xSr_{1 – x})TiO₃ · yMgO (BSM) system where x = 0.4–0.6; y = 0.15–0.30. For the compositions studied the bulk ferroelectrics were synthesized with the dielectric constant of 400–600 and high tunability coefficient. We indicated that the quality factor of the samples was in the range of 100–1000 within the frequency band of 3.5–35 GHz. The phase correlations and unit cell constants of the perovskite phase of the BSM samples were studied. The low loss factor and high tunability of the bulk material allowed us using the BSM ferroelectric ceramic layer for tunable accelerating structures of the Argonne Dielectric Wakefield Accelerator and for high power switches design and development for the future linear colliders.     

Magnetron sputter-deposited multilayer (BaxSr1x)Ti1+yO3+z thin films for passive and active devices

Abstract

High permittivity (Ba_xSr_1?x)Ti_1+yO_3+z(BST) thin films are being investigated for integration into charge storage dielectrics and electric-field tunable elements for high frequency devices. For the latter application, it is desirable to have BST capacitors with high tunability and low losses. Therefore, we investigated the use of multilayer BST thin films consisting of very low dielectric loss BST/electrode interfacial layers ((Ba+Sr)/Ti = 0.73) sandwiching a high tunability, high permittivity primary BST layer ((Ba+Sr)/Ti = 0.9). BST capacitors with multiple layers of controlled composition can be effectively produced insitu by magnetron sputter deposition, using a single stoichiometric target and controlling the layer composition by changing the total process gas (Ar+O<₂) pressure. The layered BST film capacitors exhibit simultaneous low loss (tan Δ = 0.005), high tunability (76%), high charge storage energy density (34 J/cm³), low leakage, and high dielectric breakdown (>2.8 MV/cm).     

Effect of dc biasing on YBCO/STO/LAO tunable microstrip filters

Abstract

One of the critical design aspects in ferroelectric tunable microstrip filters is choosing the right bias configuration, for large tunability as well as to maintain the filter's passband characteristics. This work is based on strontium titanate (STO) ferroelectric thin-film based tunable microstrip filters for cryogenic temperature applications. Large tunability factors have been demonstrated in YBCO/STO/LAO two-layered microstrip filters when operated at or below 77 K. The effect of the dc electric field (primarily responsible for tuning) and critical design parameters such as the insertion loss, frequency tunability, return loss, and bandwidth of superconductor/ferroelectric/dielectric microstrip tunable K-band microwave filters is discussed in this work.     

Deposition and characterization of low-loss epitaxial non-linear dielectric thin films for microwave devices

Abstract

Oxide ferroelectric thin films for frequency-tunable microwave devices, in which the dielectric constant of the non-linear dielectric is varied by application of electric fields, have been deposited using PLD. We have fabricated single phase epitaxial Ba_0.6Sr_0.4TiO₃ and KTaO₃ thin film capacitors for applications at 300K and 77K, respectively. Single phase KTaO₃ films were obtained only with excess potassium source in the target along with KTaO₃ perovskite phase. The films have been characterized for structure and morphology by X-ray diffraction and AFM. The dielectric properties were measured in the low frequency range from 100 kHz to 10 MHz, using interdigitated capacitors. Low loss tangents (0.002 at 300K) were observed for highly oriented Ba_0.6Sr_0.4TiO₃ films. The importance of low losses for various devices is discussed and the dielectric constants, loss tangents and tunability of these films are reported in this paper.