Low-loss Microwave Research Articles

Tunable Coplanar Waveguide Microwave Devices on MOCVD-SrTiO3 Thin Films

SrTiO 3 (STO) ferroelectric (FE) thin films associated with high temperature superconductors (HTSC) make a good compromise to realize electronically tunable microwave devices combining controllable dielectric properties of FE films with low loss microwave conductivity in HTSC. Up to now, STO thin films dedicated to this function were deposited by sputtering and pulsed laser deposition. In this study, we have explored the feasibility of tunable microwave devices made from STO thin films prepared by metalorganic chemical vapour deposition. We have characterized Au/STO/MgO coplanar waveguide structures to demonstrate tunability from 500 MHz to 35 GHz, in the 300 K to 60 K temperature range.

Microwave, magnetic, and structural properties of nanocrystalline exchange-coupled Ni11Co11Fe66Zr7B4Cu1 films for high frequency applications

Nanocrystalline structured films of the alloy Ni11Co11Fe66Zr7B4Cu1 were deposited by pulsed laser ablation deposition onto fused quartz substrates. A substrate temperature of 300°C was found to produce films consisting of body-centered-cubic (bcc) metallic grains suspended in an amorphous matrix. The bcc grain size ranged from 5–8nm for substrate temperatures up to 300°C. Measured values of coercivity were consistently below 4Oe for films having a saturation magnetization, as 4πMS, of ∼16kG with in-plane uniaxial anisotropy fields of 25to30Oe. The ferromagnetic resonance peak-to-peak derivative linewidth was measured to be 34Oe at a frequency of 9.61GHz. The zero magnetic field ferromagnetic resonance occurred at ∼2GHz. These films exhibit soft magnetic properties, high saturation induction, low microwave loss, and the structural stability desirable for thin film inductor applications.

Fabrication and characterization of anisotropic dielectrics for low-loss microwave applications

New magneto-photonic assembly designs for high-gain antennas require dielectrics with a significant anisotropy and low loss at GHz frequencies. This paper describes an approach to fabricate such dielectrics from ceramic laminates. These laminates consist of two ceramics with largely different permittivities and low dielectric losses. Alternating layers of commercially available α-Al2O3 and Nd-doped BaTiO3 were laminated using organic adhesives. Equivalent permittivity tensors and loss tangents were characterized using a resonant cavity-based approach, which was coupled with a finite-element method full-wave solver. Measured permittivity values were in good agreement with mean field predictions; a minimum loss tangent 1.1 × 10−3 was obtained when using one-component epoxy (Loctite®-3982) adhesive. Application of two-component epoxy (M-bond 610) adhesive results in a slightly higher loss but better mechanical properties and machinability. These laminates were used to demonstrate high gain in a prototype antenna with 6 misaligned anisotropic dielectric layers.

MOCVD-SrTiO3 thin film microwave coplanar tunable devices: modelling of varactors

SrTiO3 (STO) ferroelectric (FE) thin films associated with High Temperature Superconductor (HTSC) is a good compromise to realize electronically tunable microwave devices combining tunable dielectric properties of FE films with low loss microwave conductivity in HTSC. STO, which exhibits a perovskite structure, is suitable for epitaxial growth of YBaCuO films and has been widely studied to realize tunable components around 77 K. Up to now, STO thin films were essentially deposited by sputtering and pulsed laser deposition. In the framework of this study, we have explored the feasibility of microwave devices made from STO thin films prepared by metalorganic chemical vapour deposition (MOCVD). We have characterized Au/STO(250 nm thick)/MgO coplanar waveguide transmission lines and microwave variable capacitors (varactors) from 45 MHz up to 40 GHz, in the 300 K to 60 K temperature range. Dielectric characteristics of the STO films were extracted from measurements and studied as a function of frequency, electric field and temperature. The geometry of the interdigital capacitors (IDC) was chosen after evaluating their capacitances using an analytical theoretical model. We have used a de-embedding method to extract, from electromagnetic simulations using Sonnet® Software, the capacitance of the IDC alone.

Low-loss microwave interconnections by using polymeric based coplanar waveguides on low resistivity silicon substrates

In this paper a new approach to manufacture low losses coplanar waveguide (CPW) lines for microwave and millimeter wave signal processing is presented. A photolithographic process is performed by using SU-8 thick negative photo-resist on low resistivity silicon wafers, to obtain CPW lines elevated with respect to the substrate, in order to take advantages, in terms of propagation losses, from transmission line structures which are almost on-the-air.

Polarized left-handed extraordinary optical transmission of subterahertz waves

In this paper we design and measure a metamaterial polarizing device working in the sub-terahertz range. The polarizer is based on a modified version of our previous miniaturized Stacked Hole Array (SHA) structure, an arrangement that combines Extraordinary Optical Transmission (EOT) and Left-Handed Metamaterial (LHM) propagation even under Fresnel illumination. Here, we use a self complementary screen by connecting the holes of an EOT structure. Importantly, EOT remains and simultaneously total reflection is obtained for the orthogonal component. Moreover, by computing the dispersion diagram, we demonstrate that LHM propagation can be achieved for the principal polarization within the stop band of the orthogonal component, which propagates in other bands as a standard forward wave. Finally, we check our conjectures by measuring the transmission and reflection coefficients of screens milled on a low-loss microwave substrate. Measurements have been taken for 1 to 6 stacked wafers and they show clearly that the stack acts as a polarizer with left-handed characteristic. Our results open the way to design of novel polarization control metamaterials at Terahertz wavelengths.

Coupled loops at 20 GHz for stacked planar circuits

Differential planar coupled loops are examined as a method of integrating silicon electronics with passive elements on low-loss microwave laminates. Two test structures are examined which abut planar loops on a CMOS chip to similarly sized loops on a low-loss microwave laminate. The insertion loss of a pair of the 1000times1000 mum loops was measured to be approximately 3 dB at 20 GHz, and the loss between the 700times300 mum loops was measured to be approximately 6 dB at 20 GHz

Miniaturisation of planar microwave circuits by using resonant-type left-handed transmission lines

It is demonstrated that planar microwave circuits and components such as impedance inverters or power dividers, among others, can be compacted by using artificial left-handed transmission lines in their designs. Key to this size reduction is the possibility to control the electrical characteristics of these lines (namely electrical length and image impedance) over wide margins by means of a single cell structure. It consists of a microstrip line section with a series capacitive gap etched in the conductor strip and loaded with a complementary split rings resonator etched in the ground plane. A recently reported model of the artificial line is used as a first step in the design of the desired devices. To demonstrate the viability of the approach, several prototype device examples are provided, that is a 90° impedance inverter and several power dividers with different topologies. A 50% size reduction (as compared to conventional devices) has been achieved by implementing the devices in conventional low loss microwave substrates, but further levels of miniaturisation can be obtained if the devices are fabricated on advanced technologies such as low temperature co-fired ceramic or multi chip module-deposited, among others.

Characterization of Extremely Low Loss Dielectrics (Mg0.95Zn0.05)TiO3 at Microwave Frequency

The microwave dielectric properties and microstructures of (Mg0.95Zn0.05)TiO3 ceramics, prepared by a mixed oxide route, were investigated. The ceramics could be sintered at temperatures as low as 1250 °C and their microwave dielectric properties were found to be strongly correlated with sintering temperature. The quality factor (Q×f) of (Mg0.95Zn0.05)TiO3 increased with temperature up to 1300 °C and decreased thereafter. The decrease in Q×f was coincident with the observed abnormal grain growth. A maximum Q×f of 264000 GHz associated with an εr of 17.1 and a τf of -40.3 ppm/°C was achieved for the samples at 1300 °C/4 h. Thus, (Mg0.95Zn0.05)TiO3 is proposed as a very promising dielectric material for low-loss microwave applications.

Effect of oxygen vacancies on nonlinear dielectric properties of SrTiO3 thin films

Ferroelectrics (FE) are a class of nonlinear dielectrics which exhibit an electric field dependent dielectric constant. High temperature superconductors (HTSC) offer lower microwave surface resistance than conventional materials. There have been significant interest and effort in combining the low microwave losses of HTSC with the voltage control of FE dielectric constant er to implement high quality voltage tunable microwave devices, such as phase shifters, filters, delay lines, and tunable oscillators [1–4]. In such devices, it is desirable to have a large capacitance change ratio [tunability = (Cmax–Cmin)/Cmax] under a certain electric field range accompanied by a small dielectric loss. SrTiO3 (STO) thin films which posses a transition peak located at 60–70 K is one of the most interesting FE materials due to its high nonlinearity and reasonably low dielectric loss. A main focus on research for effective utilization of STO thin films is the development of materials with simultaneously optimized permittivity (absolute magnitude as well as its electric field dependence.) and low dielectric loss (tand). However, the tunability and dielectric loss in STO thin films have not been comparable to that of bulk materials. Comparing with STO single crystal, most STO thin films showed low dielectric constant, low tunability and large dielectric loss. Many reasons have been suggested to the degradation in dielectric properties of STO thin films. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D = a/c), the dead layer near interface, and the local polar regions near charged defects like oxygen vacancies are expected to degrade the dielectric properties of STO thin films [5–7]. Several researchers have investigated the effect of oxygen vacancies on dielectric properties of heteroepitaxially grown STO thin films [8, 9]. It is worthwhile to note that the study of oxygen vacancies on dielectric properties of hetero-epitaxial STO thin films is complicated because of the additional effect of film stress due to film-substrate crystal lattice mismatch and difference in thermal expansion coefficients. There have been no systematic studies on the influence of oxygen vacancies on dielectric properties of STO thin films. In this study, STO thin films were homoepitaxially grown on Nb doped STO (Nb:STO) substrate. No additional stress is caused by lattice constant mismatch and thermal expansion coefficient differences between films and substrate in the homo-epitaxial STO thin films. The effect of oxygen vacancies on dielectric properties of STO thin films are presented. STO thin films were deposited on Nb:STO substrates by pulsed laser deposition (PLD). KrF excimer laser radiation (Lamda Physik Compex 201) with repetition rate of 5 Hz was focused on a rotating target set in the chamber through a quartz window. The incident angle of laser beam to the target surface was fixed to be 45 . The output laser pulse energy was kept as 150 mJ which yielded a growth rate of 0.02 nm per pulse. The laser energy density was about 2 J/cm. X. Z. Liu B. W. Tao (&) Y. R. Li The National Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science & Technology of China, Jianshe Road, Chengdu 610054, China e-mail: bwtao@uestc.edu.cn

Unpatterned Ferroelectric Thin Film Measurements for Optimization of Perovskite Oxide Thin Film Based Microwave Devices

The Army's Future Force (FF) antennas require increased bandwidth for multimission communications; increased antenna gain for robust communications and range extension; increased mobility for high data rate on-the-move (OTM) communications; and undetectable antenna visual signatures for low probably of mission detection and increased survivability. Electronic scanning antennas (ESA's) are the key components for such advanced communication systems. ESA's provide rapid scanning capability, which enables modes such as multiple target tracking, track while scan, and sensor fusion operation. One of the major challenges, which must be overcome before such advanced ESA systems can be realized, is the development of affordable, low loss, high tunability, low power, lightweight, and high performance microwave frequency phase shifters. The enabling technology for such phase shifters is centered on the development of single-phase nanometer-scale constituent perovskite oxide thin films with enhanced dielectric, insulating, and microstructural properties. The Army Research Laboratory (ARL) has successfully designed, fabricated, characterized, and optimized, novel pure and acceptor doped Ba 0.6 Sr 0.4 TiO 3 (BST) based thin films on MgO substrates with excellent microwave material properties. Pure and 1-mol% Mg doped BST thin films were fabricated by the metalorganic solution deposition (MOSD) technique and the film's dielectric properties were characterized at microwave frequency via a tuned coupled-split dielectric resonator technique developed at NIST. Using this measurement technique, for first time ever, has allowed the true “singular” dielectric loss (tan δ) and permittivity (ϵ r ) of the BST-based films to be assessed and optimized without the influence of device design (radiative losses) and electrode metallization (conductor losses).

Improvement of Microwave Loss Tangent and Tunability of Ba0.55Sr0.45TiO3/MgO Composites Using the Heterogeneous Precipitation Method

Ba0.55Sr0.45TiO3/MgO composites were successfully prepared by the heterogeneous precipitation method and their structural, surface morphological, tunable properties, and dielectric properties at microwave frequency were systemically investigated. Compared with the sample prepared by the traditional solid‐state method, the sample prepared by the heterogeneous precipitation method exhibits a smaller grain size, more uniform microstructure, higher tunability, and lower microwave loss, and these properties are very beneficial to the development of the microwave tunable devices application. Moreover, the effects of La2O3 doping on the dielectric and tunable properties of BST/MgO composites are investigated. The result shows that the La3+‐doped sample has higher tunability and lower microwave loss than the undoped one.

Effect of the sintering temperature on the Ba(Zn 1/3Ta 2/3)O 3 dielectric properties

Preparation of dielectric materials with optimal properties for wireless applications requires special thermal treatments due to the difficult control of the cationic ordering. The BZT samples prepared by solid-state reaction were sintered at temperatures in the range 1400–1600 °C for 4 h. For compositional, structural and morphological characterization, XRD, SEM and EDX were employed. The order–disorder transition of the BZT material was analyzed. With the increase of the sintering temperature, a long-range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was observed. The dielectric parameters were measured in the microwave range and were correlated with morphological and structural properties. In order to improve the microwave properties, annealing treatment at 1400 °C for 10 h was performed. Sintering temperatures greater than 1550 °C are required for undoped BZT compositions, in order to obtain low microwave loss ( Qf > 100,000).

Oriented barium hexaferrite thick films with narrow ferromagnetic resonance linewidth

Hexagonal BaFe12O19 ferrite films, having thicknesses ranging from 200–500μm, were prepared by a screen printing process followed by sintering heat treatments. Structural, magnetic, and microwave measurements confirmed that the polycrystalline films were suitable for applications in self-biasing microwave devices in that they exhibited a large remanence (4πMr=3800G), high hysteresis loop squareness (Mr∕Ms=0.96) and low microwave loss. A derivative linewidth ΔH of 310 Oe was measured at 55.6 GHz. This represents the lowest ΔH measured in polycrystalline hexaferrite materials. ΔH can be further improved by reducing porosity and improving the c-axis orientation of grains in polycrystalline ferrite.

Effect of ZrO2 doping on the tunable and dielectric properties of Ba0.55Sr0.45TiO3/MgO composites for microwave tunable application

Undoped and ZrO2 doped (0.5, 1.0, 2.0, 3.0wt.%) Ba0.55Sr0.45TiO3/MgO composites were prepared by traditional ceramic processing. The ZrO2 doping effect on the structural, surface morphological, tunability and dielectric properties were systemically investigated. The result shows that the tunability of Ba0.55Sr0.45TiO3/MgO composites was improved by using ZrO2 dopant. The Ba0.55Sr0.45TiO3/MgO composite with 1.0wt.% ZrO2 dopant exhibits high tunability (17% at 2.5kV/mm), low dielectric constant (200) and a low microwave loss (0.005, 2.7GHz), which are suitable for microwave tunable application.

RF Micro-Electro-Mechanical Systems Capacitive Switches Using Ultra Thin Hafnium Oxide Dielectric

A π-type RF capacitive switch using about 45-nm-thick HfO2 dielectric layer was fabricated. High isolation performance was obtained in wide-band range when the switch was down-state. The isolation was better than -40 dB at the frequency range of 4–35 GHz. Particularly, the isolation was better than -50 dB in the frequency range of 8–12 GHz, i.e., X band. HfO2 showed excellent process compatibility with conventional microfabrication procedure. The 45-nm-thick HfO2 film was prepared using sputtering at room temperature so that it was feasible to be integrated into RF switch and other microwave circuits. The results of constant bias stressing showed that the ultra thin HfO2 had excellent reliability. The electric breakdown of HfO2 was observed, which had no apparent negative effects on the reliability of the dielectric. HfO2 dielectrics were attractive in the application of RF micro-electro-mechanical systems (MEMS) switch for new generation of low-loss high-linearity microwave circuits.

A STUDY ON FERROELECTRIC PHASE SHIFTER WITH PHOTONIC-BANDGAP (PBG) STRUCTURE

ABSTRACT In order to get the low-loss microwave tunable device, we adopted the photonic-bandgap (PBG) structure based on BST thin films on MgO (001) single crystal substrate. We designed and fabricated a microstrip ferroelectric phase shifter with PBG structure for microwave integrated circuit. The proposed multilayer PBG structure consisted of microstrip tunable interdigital (IDT) patterns atop BST thin film and two square defected areas in the ground metallic plane. To analyze microwave characteristics of the proposed multilayer PBG structure, finite-element-method (FEM) was used. The fabricated phase shifter shows 103° differential phase shift, 3.3 dB insertion loss, and return loss of better than 20 dB at 20 GHz with dc-bias variation from 0 V to 110 V.

High-Speed Coplanar Waveguide Based Submount for 40 Gbit/s Electroabsorption Modulator

A high-speed submount has been designed and fabricated for 40 Gb/s electroabsorption (EA) modulators. The submount contains a coplanar waveguide (CPW) for microwave signal feeding and a Ta2N thin-film resistor for impedance matching. The CPW transmission line is designed to ensure low microwave loss and reflection, and Ti/Cu/Ni/Au metal is adopted for electrode fabrication to guarantee good contact with the Ta2N thin-film. The typical reflection coefficient of fabricated submount is estimated to be lower than−21 dB up to 40 GHz. As a demonstration, a high-speed EA modulator was chip-level packaged using the high-speed submount, and the measured small-signal modulation bandwidth was over 40 GHz.

Pulsed laser-deposited single-crystal LiZn-ferrite films with low microwave loss

Lithium zinc ferrite (LiZnF) films were prepared for a wide range of deposition parameters. Bulk LiZnF rather than pure Li ferrite targets were used to mediate the mismatch problem between the film and the MgO substrate. Preparation conditions were optimized to give single phase epitaxial films with half power ferromagnetic-resonance (FMR) linewidths of as low as 57 Oe at 9.5 GHz. The films were deposited on (100)-plane MgO substrates with a KrF excimer laser at a wavelength of 248 nm. The minimum linewidth films with a thickness of 0.70 μm were obtained for an oxygen partial pressure of 400 mTorr, a substrate temperature of 800 °C, and a laser fluence of 1.7J∕cm2. X-ray diffraction pattern indicated a pure LiZn phase with local normal axis deviations below 0.05°. The FMR profiles were measured at 9.5 GHz with the static field perpendicular and parallel to the film plane, and as a function of rotation angle for in-plane fields. The data yielded a saturation induction 4πMs value of 2.730 kG, about the same as for a bulk, and cubic anisotropy, with a first-order cubic anisotropy energy constant K1 of about 7.33×103ergs∕cm3, a factor of five or smaller than that for a bulk LiZn ferrite.

Complex permittivity measurements of low-loss microwave ceramics employing higher order quasi TE0np modes excited in a cylindrical dielectric sample

Higher order quasi TE0np modes have been used for precise measurements of permittivity and dielectric loss tangent of low-loss dielectric ceramics versus frequency in the microwave frequency range. It has been shown that, using this technique, dielectrics having arbitrary permittivity value and dielectric loss tangent values in the range 10−6–10−2 can be measured with relative uncertainties below 0.5% for real permittivity and below 5% for dielectric losses. Several dielectric materials have been measured including single- crystal quartz, single-crystal sapphire and the low-loss microwave ceramics BMT, BZT and BZNT.