Microwave phase shifters have been fabricated from (YBa2Cu3 O7-δ or Au)/SrTiO3 and Au/BaxSr1−xTiO3 films on LaAlO3 and MgO substrates. These coupled microstrip devices rival the performance of their semiconductor counterparts at Ku- and K-band frequencies. Typical insertion loss for room temperature ferroelectric phase shifters at K-band is ≈5 dB. An experimental and theoretical investigation of these novel devices explains the role of the ferroelectric film in overall device performance. A roadmap to the development of a 3 dB insertion loss phase shifter that would enable a new type of phased array antenna is discussed.

We report the performance of 16-element phased array antennas operating at 30 GHz and ambient temperature. These antennas use BaxSr1−xTiO3(BST)-based phase shifters to produce the beam steering. Ferroelectric phase shifters offer advantages over current semiconductor and ferrite devices including faster switching speeds and lower costs. Also, ferroelectric phase shifters offer higher power handling capability than semiconductor devices and also have high radiation resistance. We made phase shifters from laser-ablated epitaxial BST films as well as from polycrystalline BST-oxide composite films. Although neither the devices nor the materials themselves are fully optimized, phase shifters have shown > 360° of phase shift with < 350 V DC bias (E < 9 V/µm) and ∼8 dB insertion loss. With ferroelectric phase shifters incorporated, antennas show radiation patterns with central-lobe half-power widths of ∼13° and side lobe intensities down by more than 10 dB. Using the phase shifters, the central lobe can be shifted, or “steered,” by ±18° in either direction. These results demonstrate a first step toward a prototype steerable antenna for 20–30 GHz satellite communications as well as other applications.

We have used a coplanar waveguide structure to study broadband electrodynamic properties of single-crystal and thin-film strontium titanate (STO), and thin-film barium strontium titanate (BSTO). We have implemented low-frequency capacitance (100 Hz - 1 MHz), swept-frequency transmittance (45 MHz - 4 GHz), and time-domain transmittance (dc - several GHz) measurements to determine effective refractive index (or, dielectric constant), and dissipation factor (or, loss tangent) as a function of dc bias (up to 4×106 V/m) and temperature (20 - 300 K). The STO samples used superconducting electrodes and were designed to operate at cryogenic temperatures, whereas BSTO samples used normal conducting electrodes and exhibited optimal performance around room temperature. By using nearly identical electrode geometries for all devices, we were able to conduct a direct comparative study among them, and investigate not only single-crystal vs thin-film, but also cryogenic vs room-temperature applications.

Periodically loaded line phase shifter circuits using voltage tunable BaSrTiO3 (BST) parallel plate capacitors have been demonstrated at X-band. The first such phase shifter circuit was capable of 100° of phase shift with an insertion loss of 7.6 dB at 10 GHz. Subsequently, the monolithic fabrication procedure was refined resulting in an improved phase shifter circuit with 200° of phase shift and an insertion loss of 6.2 dB at 10 GHz. In addition to promising loss performance (32°/dB) at 10 GHz, the circuits reported here have several desirable features such as moderate control voltages (20 V), room temperature operation, and compatibility with monolithic fabrication techniques.

SrTiO3 (STO) thin films are promising for a variety of applications requiring tunability. We describe the growth and characterization of STO thin films including their dielectric properties. We also present attempts at reducing the loss tangent of these films, and we discuss their integration with high-temperature superconductor (HTS) microwave filters for trimming purposes.

The low frequency dielectric properties of epitaxial SrTiO3 thin films deposited on LaAlO3 are presented. The films were deposited using radio-frequency magnetron sputtering from stoichiometric targets in an Ar/O2 atmosphere. For the first time, the effects of in situ ozone annealing during the early stages of deposition were explored. X-ray diffraction results indicated that the ozone treatment resulted in more symmetric and sharper diffraction peaks (2 Θ- FWHM decreased from 0.17° to 0.10°). In addition, the peaks for the ozone treated samples were shifted in 2 Θ towards values approaching the bulk value. Rutherford backscattering measurements showed Sr/Ti ratios of 1:1 for these samples, indicating these peak shifts are not due to compositional variations. The dielectric constant of the ozone treated samples increased from 275 at room temperature to 1175 at 22 K (measured at 100 kHz). The effective loss tangent of the device remained between 1 × 10−4 and 1 × 10−3 down to 100 K, where it began to increase. The tunability was also measured. The ozone treated sample showed tunability of 46%, 43% and 38% at 22 K, 40 K and 60 K, respectively. Finally, similar measurements were completed at 1 MHz, indicating a minimal dependence of these properties on frequencies in this range.

Precise control of Ba1−xSrxTiO3 (BST) film composition is critical for the production of high-quality BST thin films. Specifically, it is known that nonstoichiometry greatly affects the electrical properties of BST film capacitors. We are investigating the compositionmicrostructure- electrical property relationships of polycrystalline BST films produced by magnetron sputter-deposition using a single target with a Ba/Sr ratio of 50/50 and a (Ba+Sr)/Ti ratio of 1.0. 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. The crystalline quality as well as the measured dielectric constant, dielectric tunability, and electrical breakdown voltage of BST films have been found to be strongly dependent on the composition of the BST films, especially the (Ba+Sr)/Ti ratio. We discuss the impact of BST film composition control, through film deposition and process parameters, on the electrical properties of BST capacitors for high frequency devices.

Microwave systems for communications and radar require control of propagation of the rf signal. Devices that accomplish this function include phase shifters, isolators and circulators, and tunable filters. In many instances, these devices are magnetic and are based on the variable permeability of electrically insulating ferrimagnetic oxides (ferrites). Recent advances in microwave ferrite devices have featured superconductor circuitry that promises to virtually eliminate insertion losses due to rf surface resistance. Lower conduction losses allow the use of small lightweight microstrip configurations in place of traditional bulky waveguide structures. For operation at cryogenic temperatures ferrimagnetic spinels and garnets will require chemical alteration to realize the full potential of these devices. Challenges include the reduction of magnetocrystalline anisotropy to optimize switching energies and speeds, and the elimination of fast-relaxing impurities in the magnetic garnets that can increase magnetic losses and degrade resonator Q factors at low temperatures.

Thin ferromagnetic films have narrow ferromagnetic resonance bandwidths, and large RF permeabilities that are attractive for tunable high Q device applications. They can also be processed at low temperatures and integrated with most RF and microwave materials. At VHF/UHF frequencies, where discrete devices can be realized, magnetic loss tangents (Q−1) less than 10−3 can be obtained, if the effects of spin resonance and eddy current losses are reduced. The material figure of merit requires a tradeoff of the permeability, and Q. The highest FOM is obtained by maximizing the magnetization, anisotropy, resistivity, and minimizing the magnetic loss. This paper discusses the interplay of these properties, and the fill factor and magnetic bias required for tunable inductors with Q>1000 in the ∼50-500 MHz range. The most promising candidates for these applications are shown to be reactively sputtered nanocrystalline films. The large resistivity, magnetization, and thickness of these films can be used to attain large film permeances, and increased shape anisotropy fields from patterned film objects. The latter eliminates the need for using a large bias magnet to achieve Q's ∼1000 at VHF/UHF operating frequencies.

In an effort to develop a magnetic biasing layer for potential applications in integrated devices, we have grown thin films of Y3Fe5O12 with increased uniaxial anisotropy by doping with varying Co2+, concentration. To compensate for the charge differential between Co2+ and Fe3+, Ge4+ and Ce4+ are substituted for Fe3+ and y3+, respectively. These garnet films, prepared using pulsed laser deposition on (110) oriented Gd3Ga5O12 substrates, exhibit excellent crystallinity as determined from X-ray diffraction and Rutherford backscattering spectroscopy. The addition of Co2+ in Y3Fe5O12 films enhances the in-plane uniaxial anisotropy over an order of magnitude, depending on composition.

Thick films of scandium-substituted BaScxFe12−xO19 (x=0, 0.4, 0.6) were deposited by pulsed laser ablation deposition onto A-plane (1120) sapphire (Al2O3) substrates to yield highly oriented films having the crystallographic c-axis ([0001]) in the film plane. Selected films were characterized by x-ray diffraction, electron microscopy, magnetometry and torque magnetometry measurements. These structural and magnetic measurements show that dense films have excellent in-plane uniaxial anisotropies and good square loop behaviors. Such films will be useful for planar latched phase shifters that can be integrated into microwave integrated circuits.

Among other magneto-optic materials bismuth substituted yttrium iron garnet (BixY1−x)3 Fe5O12 has the highest Faraday rotation effect in visible region. Completely substituted bismuth iron garnet Bi3Fe5O12 (BIG) films have been grown by pulsed laser deposition technique onto (111) (NdGd)3(ScGa)5O12 single crystal with lattice constant of 12.623 Å. X-ray diffraction proves epitaxial film quality. VSM measurements yield the saturation magnetization 4πMs, = 1100 G and coercive field is about 50 Oe. Ferromagnetic resonance (FMR) method reveals perpendicular magnetic anisotropy in fabricated films. Angular measurements of FMR give the constants of uniaxial and cubic anisotropy in BIG film: Ku = 5.9×104 erg/cm3 and K1= - 6.95×103 erg/cm3. The Faraday rotation has been found to reach - 7.8 deg/µm at 630 nm.

We have demonstrated magnetically tunable superconducting filters consisting of microwave circuits coupled to ferrite substrates in monolithic structures using both niobium at 4 K and YBCO at 77 K. A three-pole 1% bandwidth filter with 10-GHz center frequency, 1-dB insertion loss, and greater than 10% tunability has been demonstrated. Operation of the ferrite in the partially magnetized state prevents degradation of the surface resistance of the superconductor. This technology has the potential for compact, light weight, multipole filters with very low insertion loss, <0.5 dB, and very rapid tunability, <1 µs, due to the low inductance of the magnetic circuits. We also discuss the use of lightly coupled ferroelectric films to introduce trimming of the individual resonators in the magnetically tuned filters. The materials issues that limit performance and means to improve present performance are discussed.

We report on the fabrication and characterization of single-crystal films of yttrium iron garnet (YIG) for heterogeneous integration onto growth-incompatible substrates. The process entails the implantation of energetic helium ions into the material, resulting in the formation of a fast etching sacrificial layer. Separation from the growth substrate is followed by an annealing step, yielding a ferromagnetic resonance (FMR) linewidth of 0.70 Oe from a virgin sample of linewidth 0.55 Oe. Magnetization measurements show a distinct softening of the in-plane response to an applied field.

(Ba,Sr)TiO3 (BST) thin films have been deposited by pulsed laser deposition (PLD) onto single crystal Y3Fe5O12 (YIG) substrates with/without an MgO buffer layer. The structure and microwave properties of BST films have been investigated as function of substrate orientation and O2 deposition pressures (5-800m Torr). The orientation of BST film is varies with the deposition conditions. The dielectric constant, loss tangent, and change in dielectric constant with an applied electric field have been measured at room temperature using interdigitated capacitors at 0.1 – 20 GHz. Polycrystalline BST films have a high tunability (∼40%) with a dc bias field of 67kV/cm and a dielectric Q (=l/tanδ) between 30 and 40, while (001) oriented BST films have a lower tunability (∼20%) but higher dielectric Q (∼50). A coplanar waveguide transmission line was fabricated from a (001) oriented BST film on (111)YIG which exhibited a 17° differential phase shift with an applied dc bias of 21 kV/cm. An equivalent differential phase shift was achieved with a magnetic field of 160Guass.

There has been significant interest recently in use of BaSrTiO3 (BST) thin films for integrated capacitors; these devices have benefits for high frequency operations, particularly when high levels of charge or energy storage are required. We discuss the electrical properties of BST thin films grown by metalorganic chemical vapor deposition (MOCVD) which make them suitable for these applications, as well as the impact of processing conditions such as growth temperature on specific film properties. We have also examined addition of Zr in amounts ranging up to 20% to the BST films. X-Ray diffraction indicates that the Zr is incorporated into the BST lattice. Voltage withstanding capability, leakage and dielectric constant of the thin films have been measured as functions of deposition temperature and Zr content. Addition of Zr to BST films increases breakdown voltages by as much as a factor of two, to approximately 2 MV/cm, raising their energy storage density values to levels approaching 30 J/cc. Charge storage densities of above 60 fF/µm2were also obtained.

Bismuth pyrochlore ceramics have modest temperature coefficients of capacitance, good microwave properties, and can be prepared at relatively modest temperatures (∼900 – 1100 °C). This work focuses on the preparation and characterization of thin films in this family for the first time. A sol-gel procedure using bismuth acetate in acetic acid and pyridine, in combination with zinc acetate dihydrate and niobium ethoxide in 2-methoxyethanol was developed. The solution chemistry was adjusted to prepare (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 and Bi2(Zn1/3Nb2/3)2O7 films. Solutions were spin-coated onto platinized Si substrates and crystallized by rapid thermal annealing. In both cases, crystallization occurred by 550 °C into the cubic pyrochlore structure. (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 films remained in the cubic phase up to crystallization temperatures of 750 °C, while the structure of the Bi2(Zn1/3Nb2/3)2O7 thin films is dependent of the firing temperature: cubic below 650 °C and orthorhombic above 750 °C. A mixture of cubic and orthorhombic structures is found at 700 °C. The resulting BZN films are dense, uniform, and smooth (rms roughness of < 5 nm). Cubic bismuth zinc niobate films show dielectric constants up to 150, a negative temperature coefficient of capacitance, TCC, (∼ - 400 ppm/°C), tan δ < 0.01, and a field tunable dielectric constant. Orthorhombic films showed smaller dielectric constants (∼80), low tan δ (< 0.01), positive TCC, and field independent dielectric constants. TCC could be adjusted to new 0 ppm/°C using a mixture of orthorhombic and cubic material.

Heterostructures consisting of (001) LaAlO3 or (111) SrTiO3 substrates, SrRuO3 metallic oxide bottom electrodes and Pb[Yb1/2Nb1/2]O3-PbTiO3 ferroelectric films were deposited by pulsed laser deposition. The combination of oxidic perovskite-type materials results in highly <001>- or <111>-heteroepitaxial capacitors with well-defined and homogeneous columnar microstructures. Most of the films show room temperature dielectric constant greater than 1500 associated with low dielectric loss (tgδ < 4%) and exhibit saturated hysteresis loops with remanent polarizations up to Pr ∼ 50µC.cm−2. It was found that the ferroelectric characteristics and, in particular, the fatigue phenomena greatly depend on the orientation and crystalline quality of the as-grown films. The stabilization of the <001>-orientation enhances the fatigue resistance and a good endurance up to 1011 cycles has been determined for highly <001>-textured samples. In contrast, fatigue tests performed on strongly <111>-oriented capacitors have revealed poor fatigue performances, with a progressive decrease in the switchable polarization by ac voltage cycling.

Na0.5K0.5NbO3 films have been grown onto polycrystalline Pt80Ir20 substrates at two different regimes: high (∼ 400 mTorr) and low (∼ 10 mTorr) oxygen pressure. Films grown at high oxygen pressure have been found to be single phase and highly c-axis oriented. The concept of discriminated thermalization has been developed to explain the dynamics of the laser ablation process and to find reliable pulsed laser deposition (PLD) processing conditions. The phenomenon of self assembling of Na0.5K0.5NbO3 films along [001] direction has been observed. On the other hand, films grown at low oxygen pressure have been found to be mixed phase of ferroelectric Na0.5K0.5 NbO3 and paraelectric potassium niobates. Superparaelectric behavior has been observed in these films: 5% tunability at electric field of 100 kV/cm, losses as low as 0.003 and excellent stability to the temperature and frequency changes.

Barium strontium titanate (BSTO) films were synthesized by the pulsed laser deposition technique (PLD) on silicon substrates at room temperature. The BSTO film synthesis took place at constant laser energy, 500 mJ, and partial oxygen pressure of 3, 15, 30, 45, 100 mTorr. All films were post annealed at 750 °C in a tube furnace in an oxygen atmosphere. The microstructure, crystallinity and lattice constant of the BSTO films were studied with the aid of scanning electron microscopy (SEM), photon tunneling microscopy (PTM) and Glancing Angle X-ray Diffraction analysis (GAXRD). The hardness and modulus of elasticity of the films were studied with the aid of a nanohardness indenter. The film stoichiometry was determined with the aid of Rutherford Back Scattering (RBS). The results of this research will be combined with the results of our previous work [1] on the effect of substrate temperature on the microstructure and mechanical properties of the BSTO films in order to construct a structural zone model (SZM) of the BSTO films synthesized by PLD.