Thick electrodes for high frequency high Q tunable ferroelectric thin film varactors

Abstract

Thin film barium strontium titanate (BST) shows great promise for voltage tunable dielectric devices for use at RF and microwave frequencies. An MOCVD process has been developed for production of BST, resulting in films with very low losses (as low as 0.002–0.004) and tunabilities over 50% at low operation voltages. With these values of BST loss, overall device quality factors at RF (100 MHz+) frequencies are primarily limited by losses in the thin metal electrodes, such as Pt, normally used for ferroelectric thin films. The bottom electrode in parallel plate capacitor structures is particularly challenging, since it must provide a good growth surface for BST and be stable at high (>600 °C) growth temperatures in an oxidizing atmosphere yet have high conductivity and compatibility with Si or SiO₂/Si substrates. These challenges have previously prevented use of Pt thicknesses over 0.1–0.2 urn. Our solution to this problem, involves combinations of adhesion layers at the Pt/SiO₂ interface and embedded stabilization layers to make functioning Pt bottom electrodes as thick as 2 μm. Devices with dielectric Q factors over 150 at 100 MHz (tan δ ～ 0.006 as measured and modeled by S-parameters) and overall device Q factors over 50 at 30 MHz are described. We have also inserted these devices into tunable filters, achieving tunabilities of 50% and low insertion losses (0.3 dB) at RF frequencies.     

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.     

Complementary split rings resonators (CSRRs): Towards the miniaturization of microwave device design

In this paper, it is demonstrated that complementary split rings resonators (CSRRs), a new type of planar resonators recently introduced by some of the authors, are key elements for the miniaturization of microwave devices implemented in planar technology, such as filters and diplexers. These devices essentially consist on a host transmission medium (microstrip line) to which the CSRRs are electrically coupled, and additional microstructure in order to achieve the required device performance. From the analysis and numerical simulations of the equivalent circuit model of the basic device cell, as well as electromagnetic simulations of actual structures, it is confirmed that CSRRs are useful resonators for the synthesis of planar microwave filters where dimensions, out-of-band performance and bandwidth can be simultaneously optimized. The simultaneous fulfillment of these aspects is a relevant advantage of the proposed structures. The possibility to implement band pass filters with wide bandwidth and high performance, the small electrical size of CSRRs, as well as their potential applications in other microwave devices, make these particles of actual interest in microwave engineering.     

Design and Development of Ferroelectric Tunable Coplanar Waveguide Components for Ku- and K-Band Applications

In the past several years, we have demonstrated electrically tunable microstrip components such as resonators, filters, diplexers, and couplers based on conductor/ferroelectrics/dielectric two-layered structure. Recently, we are focusing our efforts on tunable coplanar waveguide (CPW) components in collaboration with NASA Glenn Research Center, Cleveland, OH. The advantages of CPW components include higher dielectric tunability compared to microstrip structure, ease of shunt connections, and ease of testing. To date, we have modeled several CPW structures with ferroelectric thin-film to study the effect of inserting the ferroelectric thin-film on electric tunability, attenuation and dispersion. Also, we have designed and fabricated devices such as resonators, and filters. The ferroelectric tunable CPW filters were tunable by more than 3% at bias voltage levels of - 100V. The BSTO based CPW filters offer higher sensitivity parameter as well as lower loss parameter compared to BSTO tunable 2-pole microstrip filters.     

High-Q tunable dielectric resonator filters

Tunable dielectric resonator filters can potentially address wireless and satellite applications that require very high Q values (4,000 and up) with a limited tuning range (less than 15%). Such high Q requirements cannot be met by any other known non-superconductor tunable filter technology at the present time. The intent of this paper is to provide newcomers and end users with the current status and prospective of using dielectric resonators for tunable filters. It is an enabling technology for high-Q tunable filter applications. A key challenge, however, is to increase the tuning range without degrading the Q value. While several techniques have been reported to demonstrate the feasibility of tuning dielectric resonators, the tunable dielectric resonator filter technology is still in its infancy. Very limited research effort has been dedicated to explore the potential for improving the tuning range. Most of the work reported thus far has focused on the use of TE_01delta modes and standard shape resonators demonstrating a narrow tuning range. We believe that the tuning range can be increased while maintaining reasonably high Q values by exploring the use of other modes and by the use of non-standard-shape dielectric resonators.     

Compartmental analysis of dielectric absorption in capacitors

This paper proposes a method of studying and modeling the dielectric absorption in capacitors. Because of dielectric absorption, the voltage on a charged capacitor partially recovers after momentarily shorting its terminals. The magnitude of this voltage recovery depends mainly on the dielectric material. Dielectric absorption causes errors in some analog applications based on charging and discharging of capacitors, such as sample-and-hold circuits, integrators and active filters. Designing compensation circuits based on models of the dielectric absorption can reduce these errors. This paper presents an analytical method to build a mathematical model of the dielectric absorption, and an equivalent electrical circuit. The method is based on compartmental analysis theory, mostly used in medicine and biology to study the kinetics of substances in biological systems     

Active integrated antennas

Even with microwave techniques, however, signal losses in materials and decreased gain and power from solid-state devices become significant obstacles to creating low-cost, high-frequency wireless systems. Perhaps the most dramatic effect occurs when a circuit component becomes a significant fraction of a wavelength. At this point it may begin to function well as an antenna. For microwave and mm-wave signals, this can occur with circuits that are only centimetres in size. With conventional circuit techniques, this radiation may cause drastic signal losses, spurious coupling between circuit elements, and radio interference with other. However, with new techniques, it is possible to create circuits that use these effects to advantage. Known as active integrated antennas, these circuits have sparked interest as possible solutions to problems in designing the next-generation wireless systems. Active integrated antennas are a combination of solid-state devices and circuits with printed antenna structures. They comprise integrated radio-system elements that are fabricated using inexpensive printed-circuit techniques     

Dielectric characteristics of spin-coated dielectric films using on-wafer parallel-plate capacitors at microwave frequencies

Dielectric properties of spin-coated dielectric insulators suitable for high-speed device fabrication are investigated. Complex dielectric permittivities and tangential losses of two polyimides, bisbenzocyclobutene (BCB), and a spin-on-glass (SOG) were extracted from the measured microwave reflection coefficient, S/sub 11/, of parallel-plate capacitors over a frequency range of 50 MHz to 40 GHz. A model for the dielectric permittivity as a function of frequency is developed based on measured data with a minimum square error of less than 10/sup -4/ between measured and modeled microwave reflection coefficients. A circuit model for the pad capacitance is obtained based on geometrical and physical considerations. The relationship between the dielectric loss and its thickness is considered. Experimental results are fitted to Debye and Cole-Cole models.     

Filter technologies for wireless base stations

Phenomenal growth in the telecommunication industry in recent years has brought significant advances in filter technology as new communication systems emerged, demanding more stringent filter characteristics. In particular, the growth of the wireless communication industry has spurred tremendous activity in the area of microwave filter miniaturization and has been responsible for many advances made in this field. The filters that are currently being used in wireless base stations can be divided into two main categories: coaxial cavity resonator filters and dielectric resonator (DR) filters. While coaxial cavity filters have limited quality factor (Q) values, they offer the lowest cost design and are still being widely employed, particularly in wide bandwidth applications. With increased demands for high performance wireless systems, dielectric resonator filters are emerging as the baseline design for wireless base stations. Over the next five years, dielectric resonator filters are expected to have a significant share of the overall wireless base station filter market. High-temperature superconductor (HTS) filters are also expected to have a share of this market, particularly for systems, which have very stringent requirements for out-of-band interference. In this article, we begin by reviewing the main filter requirements, highlighting the technologies that are being currently employed. Emerging filter technologies that have the potential to replace the existing technologies are then described.     

Analysis of Hybrid Dielectric Plasmonic Waveguides

Future ultracompact photonic integrated circuits (PICs) will rely on high-index-contrast dielectric materials, which permit a strong confinement of the optical field in the diffraction limit as well as low propagation losses. This is the case of PICs implemented on a silicon-on-insulator (SOI) platform. To achieve confinement beyond the diffraction limit, plasmonic waveguides (based on metal–dielectric interfaces) have been recently proposed. This new kind of waveguide provides a strong enhancement of the field in the metal–dielectric interface, which is of paramount importance for nonlinear functionalities or sensing. Plasmonic waveguides can also be built on SOI wafers. Thus, it can be reasonably thought that high index contrast as well as plasmonic waveguides can coexist in future ultradense PICs. In this paper, a theoretical and numerical study on the performance of several dielectric and plasmonic waveguides is presented. Thanks to their plasmon-coupled supported modes, ultracompact devices as hybrid ring resonators can be devised and integrated with silicon photonic circuits.      

Microwave Tunable Properties of Ni-Doped (Ba0.5Sr0.5)TiO3 Thin Films Grown by Pulsed Laser Deposition

(Ba_0.5Sr_0.5)TiO₃ (BST) thin films were deposited by pulsed laser deposition (PLD) and investigated as a function of Ni dopant concentration in low and high frequency regions. In low frequency region (<10 MHz), the Ni-dopant concentration in BST films has a strong influence on the material properties including dielectric and tunable properties. Ni-doped (≤3 mol%) BST films showed denser, smoother morphologies and smaller grain sizes than those with 6 and 12 mol% Ni. Dielectric constant and loss of 3 mol% Ni-doped BST films were about 980 and 0.3%, respectively. In addition, tunability and figure of merit of 3 mol% doped BST films showed maximum values of approximately 39% and 108, respectively. In high frequency region (>1 GHz), the frequency tunability range at center frequency of undoped BST and 3 mol% Ni-doped BST coplanar waveguide (CPW) resonators showed 102 and 152 MHz, respectively at 30 V dc bias. The Ni-doped BST thin films are possible in applications of microwave tunable capacitors.     

Performance Enhancement of Tunable Bandpass Filters Using Selective Etched Ferroelectric Thin Films

The inclusion of voltage-tunable barium strontium titanate (BSTO) thin films into planar band pass filters offers tremendous potential to increase their versatility. The ability to tune the passband so as to correct for minor deviations in manufacturing tolerances, or to completely reconfigure the operating frequencies of a microwave communication system, are highly sought-after goals. However, use of ferroelectric films in these devices results in higher dielectric losses, which in turn increase the insertion loss and decrease the quality factors of the filters. This study explores the use of patterned ferroelectric layers to minimize dielectric losses without degrading tunability. Patterning the ferroelectric layers enables us to constrict the width of the ferroelectric layers between the coupled microstrip lines, and minimize losses due to ferroelectric layers. Coupled one-pole microstrip bandpass filters with fundamental resonaces at ～7.2 GHz and well-defined harmonic resonances at ～14.4 and ～21.6 GHz, were designed, simulated and tested. For one of the filters, experimental results verified that its center frequency was tunable by 528 MHz at a center frequency of 21.957 GHz, with insertion losses varying from 4.3 to 2.5 dB, at 0 and 3.5 V/ w m, respectively. These data demonstrate that the tuning-to-loss figure of merit of tunable microstrip filters can be greatly improved using patterned ferroelectric thin films as the tuning element, and tuning can be controlled by engineering the ferroelectric constriction in the coupled sections.     

A design of waveguide elliptic filter based on resonant diaphragms with a complex aperture

A novel approach to the synthesis of waveguide band-pass elliptic filters on the transverse metal resonant diaphragms with the complex aperture is proposed. The aperture of the diaphragm is considered to be a rectangular window with two L-shaped metal ridges. For the complex conductivity of the diaphragm, we solve the corresponding electrodynamic problem. We also construct and investigate an equivalent circuit of the resonant diaphragm containing a series-parallel LC circuit. The geometry of the diaphragm aperture is found to be corresponding to the parameters of the equivalent circuit. We construct a third-order elliptic filter and provide a computer simulation of the diaphragms satisfying the amplitude-frequency characteristics of its resonators. We show that some resonators can be located on one complex resonant diaphragm, which then reduces the length of the filter. The band-pass filter for a rectangular waveguide WR137 is developed and manufactured. The central frequency of the filter is 6.6 GHz, the passband width 7.5 % with return loss of −20 dB, and the total length is 34 mm.     

Analysis of unbounded and bounded circuits considering finite substrate extent and inhomogeneous dielectric layer

A generalized method of lines algorithm is presented for characterizing unbounded and bounded circuits. Finite substrate extent and inhomogeneous dielectric layers are rigorously considered in this field‐based model. Radiating properties of unbounded regular and irregular microstrip patch resonators and arrays are studied with emphasis on effects of mutual coupling and finite dielectric extent on complex resonant frequencies. In addition, unbounded loss effects for microstrip open‐end and 90° angular bend deposited on finite substrate as well as chip‐to‐chip discontinuities are also investigated. Our developed algorithm incorporates an absorbing boundary condition using the Padé approximation to simulate any potential radiation and leakage losses for resonator structures while an improved lossy absorbing boundary condition (LABC) that can handle both propagating and evanescent waves is used to determine the unbounded effects for waveguiding structures. Results indicate interesting properties of the finite extent of dielectric substrate on resonance and radiation characteristics, and also on unbounded radiation and leakage losses. Copyright © 2000 John Wiley & Sons, Ltd.     

大型水轮发电机定子线棒介质损耗测量探讨

绝缘介质损耗是反映高压电气设备绝缘性能的一项重要指标.介损的测量值直接影响对绝缘性能的判断.本文结合三峡工程左岸、右岸及地下电站的定子线棒出厂试验实际情况,就定子线棒介损测量的有关问题进行分析.探索适合水轮发电机定子线棒的介质损耗测量的方法.     

Passive microwave receive filter networks using low-Q resonators

In this article, it has been shown that under certain circumstances, i.e., in a receiver, it is possible to allow microwave filters to have significant passband insertion loss. Systems analysis of various receiver configurations shows that acceptable noise figures and intercept points may be obtained in receivers containing lossy filters, i.e., ones with low-Q resonators. Thus, the engineering problem is how to develop design techniques for filters that have low-Q resonators, but with acceptable response in terms of selectivity and pass-band flatness. A review of several techniques for bandpass and bandstop filters is presented.     

Effective dielectric constant of two phase dielectric systems

Various theoretical approaches are implemented for electrical characterization of two phase material systems. Most of these approaches do not include possibility of percolation of the dispersed material. In this article brick wall model is extended on the systems exhibiting percolation. Transition from brick wall geometry to corresponding equivalent circuits provides final equations for effective dielectric constant. Comparison of theoretical results with experimental data shows that developed equations provide good fitting of effective dielectric constant to experimental values. Dielectric constants of both phases and percolation threshold calculated from the fitting match the corresponding experimental values of the phases.     

14 GHz Tunable Filters Base on Ferroelectric Films

Two new types of microwave four-pole tunable waveguide filters containing planar (Ba,Sr)TiO₃ film ferroelectric capacitors are presented. The frequency tuning of the filters is due to the variation of ferroelectric capacitance with applied dc voltage. The parameters of the K_u-band filters are the following: 1 dB bandwidth is ～0.9%, the range of tuning is 350 MHz and 260 MHz, insertion losses are not more than 4.7 dB and 3 dB respectively.     

Filler effect of low loss dielectricity in printed circuit board material

Dielectric properties of low-loss dielectric materials are investigated with variation of silica filler which is known to be general filler in PCB composite. With comparison of dielectric losses of various filler materials in use of BCB resin, it could be known that crystalline cristobalite was superior to other crystalline or amorphous silica-base materials and reduced dielectric losses in the composite with resin. And dielectric properties of composite with the variation of filler quantity showed that amorphous silica and quartz increased dielectric loss as their quantities increased, while cristobalite increased little. As quantity of crystalline cristobalite phase increases in cristobalite/quartz intermediates, dissipation factor decreases.