Tunable Ferroelectric Capacitors Research Articles

A tunable high-Q flexible ferroelectric film capacitor for GHz RF applications

With the development of flexible electronics, flexible tunable capacitors with high dielectric constant, high tunability, and low loss become important components in modern communication. A tunable Ba0.6Sr0.4TiO3 capacitor for radio frequency devices is prepared on a flexible substrate. The capacitor has a dielectric constant greater than 1000, capacitance tunability n exceeding 70%, and minimum loss tangent lower than 0.03 at 500 kHz. The Q factor can be above 103 at the GHz frequency range. These parameter values (especially the Q factor) have been comparable to the rigid substrate tunable capacitor. Meanwhile, the minimum radius of curvature of the flexible tunable capacitor can be 3 mm with stable electrical performances. The flexible tunable capacitor retains its mechanical and electrical stability after 24 000 high-frequency bending cycles, which provides potential uses in bendable, collapsible tunable capacitors at the radio frequency range.

Development of Tunable Ferroelectric Ceramic Capacitors.

Ferroelectric perovskite ceramics with a high dielectric constant, low loss, high tunability, and high electric breakdown are ideal for nonlinear transmission lines (NLTLs) to generate radio frequency (RF) signals at high-power levels. To achieve the required properties, a comprehensive study of the material phase transitions and the optimal ratio adjustment between the chemical elements in the perovskite crystal structure is required. The advancement of this solid-state technology is the most promising optimization for NLTLs in developing high-power (>100 MW) devices with high tunability (>60%) and high repetition rate (>1 kHz) for soliton generation. The barium strontium zirconium titanate (BSZT) ceramic compositions were synthesized and characterized to maximize material tunability. The composition Ba0.97Sr0.03Zr0.2Ti0.8O3 exhibited a high permittivity (>12200), low loss tangent (< 0.01), and an exceptional tunability of the order of 79% at an electric field of 10 kV/cm near the phase temperature transition at 300 K. Ferroelectric ceramic is an outstanding material with promising characteristics for producing RF signals in an NLTL, and here, the BSZT is considered for this application.

Tunability Investigation in the BaTiO3-CaTiO3-BaZrO3 Phase Diagram Using a Refined Combinatorial Thin Film Approach

Tunable ferroelectric capacitors, which exhibit a decrease in the dielectric permittivity under an electric field, are widely used in electronics for RF tunable applications. Current devices use barium strontium titanate (BST) as the tunable dielectric, but new applications call for tunable materials with specific performance improvements. It is then of crucial importance to dispose of a large panel of electrically characterized materials to identify the most suited compound for a given set of device specifications. Here, we report on the dielectric tuning properties of Ba1−xCaxTi1−yZryO3 (BCTZ) thin films libraries (0 ≤ x ≤ 30% and 0 ≤ y ≤ 28.5%) synthesized by combinatorial pulsed laser deposition (CPLD). An original CPLD approach allowing reliable and statistical ternary phase diagrams exploration is reported. The effects of Ca and Zr content on tunability, breakdown voltage and dielectric losses are explicated and shown to be beneficial up to a certain amount. Compounds close to (Ba0.84Ca0.16)(Ti0.8Zr0.2)O3 exhibit the highest figures of merit, while a zone with compositions around (Ba0.91Ca0.09)(Ti0.81Zr0.19)O3 show the best compromise between tuning ratio and figure of merit. These results highlight the potential of BCTZ thin films for electrically tunable applications.

A tunable ferroelectric based unreleased RF resonator

This paper introduces the first tunable ferroelectric capacitor (FeCAP)-based unreleased RF MEMS resonator, integrated seamlessly in Texas Instruments’ 130 nm Ferroelectric RAM (FeRAM) technology. The designs presented here are monolithically integrated in solid-state CMOS technology, with no post-processing or release step typical of other MEMS devices. An array of FeCAPs in this complementary metal-oxide-semiconductor (CMOS) technology’s back-end-of-line (BEOL) process were used to define the acoustic resonance cavity as well as the electromechanical transducers. To achieve high quality factor (Q) of the resonator, acoustic waveguiding for vertical confinement within the CMOS stack is studied and optimized. Additional design considerations are discussed to obtain lateral confinement and suppression of spurious modes. An FeCAP resonator is demonstrated with fundamental resonance at 703 MHz and Q of 1012. This gives a frequency-quality factor product f cdot Q = 7.11 times 10^{11} which is 1.6× higher than the most state-of-the-art Pb(Zr,Ti)O3 (PZT) resonators. Due to the ferroelectric characteristics of the FeCAPs, transduction of the resonator can be switched on and off by adjusting the electric polarization. In this case, the resonance can be turned off completely at ±0.3 V corresponding to the coercive voltage of the constituent FeCAP transducers. These novel switchable resonators may have promising applications in on-chip timing, ad-hoc radio front ends, and chip-scale sensors.

Atomically interface engineered micrometer-thick SrMoO3 oxide electrodes for thin-film BaxSr1-xTiO3 ferroelectric varactors tunable at low voltages

In the field of oxide electronics, there has been tremendous progress in the recent years in atomic engineering of functional oxide thin films with controlled interfaces at the unit cell level. However, some relevant devices such as tunable ferroelectric microwave capacitors (varactors) based on BaxSr1−xTiO3 are stymied by the absence of suited compatible, very low resistive oxide electrode materials on the micrometer scale. Therefore, we start with the epitaxial growth of the exceptionally highly conducting isostructural perovskite SrMoO3 having a higher room-temperature conductivity than Pt. In high-frequency applications such as tunable filters and antennas, the desired electrode thickness is determined by the electromagnetic skin depth, which is of the order of several micrometers in the frequency range of a few gigahertz. Here, we report the pulsed laser deposition of a fully layer-by-layer grown epitaxial device stack, combining a several micrometers thick electrode of SrMoO3 with atomically engineered sharp interfaces to the substrate and to the subsequently grown functional dielectric layer. The difficult to achieve epitaxial thick film growth makes use of the extraordinary ability of perovskites to accommodate strain well beyond the critical thickness limit by adjusting their lattice constant with small shifts in the cation ratio, tuned by deposition parameters. We show that our approach, encompassing several orders of magnitude in film thickness scale whilst maintaining atomic layer control, enables the fabrication of metal-insulator-metal (MIM) varactors based on 50–100 nm thin BaxSr1−xTiO3 layers with high tunability above three at the Li-ion battery voltage level (3.7 V).

Tunability, dielectric, and piezoelectric properties of Ba(1−x)CaxTi(1−y)ZryO3 ferroelectric thin films

Tunable ferroelectric capacitors, which exhibit a decrease of the dielectric permittivity (ϵ) under electric field, are widely used in electronics for RF tunable applications (e.g., antenna impedance matching). Current devices use barium strontium titanate as the tunable dielectric, and the need for performance enhancement of the tunable element is the key for device improvement. We report here on libraries of Ba0.97Ca0.03Ti1−xZrxO3 thin films (0 ≤ x ≤ 27%) with a thickness of about 130 nm deposited on IrO2/SiO2/Si substrates using combinatorial pulsed laser deposition allowing for gradients of composition on one sample. A total of 600 capacitors on a single sample were characterized in order to statistically investigate the dielectric properties. We show that the tunabilty is maximum at intermediate compositions, reaching values up to 60% for an electric field of about 400 kV cm−1. We attribute the high tunability in the intermediate compositions to the paraelectric-ferroelectric phase transition, which is brought down to room temperature by the addition of Zr. In addition, the piezoelectric coefficient is found to be decreasing with increasing Zr content.

Loaded-line ferroelectric phase shifter based on Mn-doped (Ba,Sr)TiO3 thin films for Ka-band systems

This paper presents on the microwave properties of a loaded-line ferroelectric phase shifter based on (Ba, Sr)TiO3 (BST) thin films for Ka-band applications. In order to reduce the dielectric loss of the BST thin film and its enhance a dielectric tunability, we optimized the deposition of Mn-doped (Ba0.5, Sr0.5)TiO3 (Mn-BST) thin films on MgO substrates by using pulsed laser deposition (PLD). The crystallinity, microstructure and thickness of the thin films were investigated by using an X-ray diffractometer and a scanning electron microscope, respectively. We designed and fabricated a tunable ferroelectric capacitor loaded coplanar waveguide (CPW) type phase shifter to enhance the tuning and the loss characteristics of a conventional CPW ferroelectric phase shifter. The fabricated loaded-line phase shifter showed a low return loss of −15 dB in the frequency band from 25 GHz to 35 GHz. The differential phase shift was 293°, and the insertion loss decreased from −11.3 dB to −1.4 dB with increasing dc bias voltage from 0 to 80 V at 35 GHz.

Tunable Patch Antenna with BST Capacitors

Recently there is increasing interest in developing tunable circuits to implement multiband communication systems. These include tunable filters, tunable amplifiers and antennas. Tunable ferroelectric capacitor fabricated with thin film barium strontium titanate (BST) gives the opportunity to fabricate these tunable circuit modules. In this paper, we present the results of tunable patch antenna with tunable ferroelectric capacitors. The patch antenna in the frequency range of 2–3 GHz was fabricated on FR-4 substrate. The parallel plate ferroelectric capacitors were fabricated on sapphire substrates. The performance of the patch antenna was studied with ferroelectric capacitors attached to the coplanar wave guide as well as radiating edge. The performance of the antenna was simulated using ADS and compared with experimental results the radiation pattern of the antennas is studied in an anechoic chamber.

Adaptive Electronics with Ferroelectric Capacitors

Tunable ferroelectric capacitors such as barium strontium titanate(BST) operating in the paraelectric region offer a tunability of 50%, within a applied voltage of ± 6V up to 10 GHz. They can be successfully used for a variety of adaptive frequency circuits for communication systems on a variety of substrates. These include tunable filters, tunable multiband power amplifiers and voltage controlled oscillators.

Spread Spectrum Clock Generation Using Ferroelectric Capacitors

In this paper, a spread spectrum clock generator (SSCG) has been implemented with tunable ferroelectric capacitor based voltage controlled oscillator (VCO). Simple sinusoidal waveform modulation results in a uniform spread in the energy with respect to the central frequency of the oscillator. The performance of this SSCG is compared with the SSCG implemented using conventional p-n varactor tuned VCO.

Analysis of distortion in tunable ferroelectric capacitors.

The harmonic distortion in ferroelectric capacitors is measured using a parallel resonant circuit approach. The measured results have been compared with simulated results based on the nonlinearities in capacitance versus the voltage characteristics of ferroelectric capacitors.

Analysis of distortion in tunable ferroelectric capacitors

The harmonic distortion in ferroelectric capacitors is measured using a parallel resonant circuit approach. The measured results have been compared with simulated results based on the nonlinearities in capacitance versus the voltage characteristics of ferroelectric capacitors.

AN EFFECTIVE PASSIVATION FILM STACK FOR THIN FILM BST CAPACITORS

ABSTRACT Thin film voltage tunable ferroelectric capacitors on various substrates are promising for use in high power microwave and other RF systems. Benefits of ferroelectric capacitors include miniaturization and integration with other passive devices. Commercialization of ferroelectric components requires compliance with certain industry standards for reliability. For ferroelectric capacitors the requirement to pass the THB test requires a silicon nitride layer in the back end of the process (after interconnect metalization). However the nitride layer deposition process results in a drastic increase in the leakage of the BST capacitors. This effect is attributed to the exposure of the ferroelectric material to atomic hydrogen which is a by-product of the PECVD deposition process. This paper presents results of different passivation materials for ferroelectric capacitors and for a barrier layer which preserves the performance of ferroelectric capacitors during a PECVD nitride deposition. This allows qualification of ferroelectric capacitors to industry reliability standards.

Reliability study of tunable ferroelectric capacitors

In this paper, the reliability of tunable ferroelectric thin film capacitors has been addressed. The long term stability of tunable capacitors based on Ba0.3Sr0.7TiO3 films deposited by chemical solution deposition onto an alumina substrate was investigated by monitoring their tunable performance as a function of the number of ON-OFF switching cycles and as a function of time. The conditions of civil aeronautic applications were chosen to plan the lifetime experiments. More than 300×106 of switching cycles with an amplitude of 150 V and frequency of 1 kHz were successfully applied to the investigated capacitors without damaging them. Moreover, no degradation in the C-V characteristic was observed after the application of 150 V dc voltage during 15 h. Additionally, the effect of illumination of the ferroelectric film by ultraviolet light on the tunable performance of the capacitors has been investigated. A positive effect of such illumination on the C-V characteristics of ferroelectric capacitor has been found.

Tunable Ferroelectric Capacitor-Based Voltage-Controlled Oscillator

Ferroelectric capacitors made from Ba(1-0.5)Sr0.5TiO3 (BST) are applied as varactors in tunable, high-frequency circuit applications. In this context, a voltage-controlled oscillator (VCO) has been designed and implemented using discrete RF bipolar junction transistor (BJTs) and tunable ferroelectric capacitor. The designed VCO has a tuning range from 205 MHz to 216.3 MHz with a power dissipation of 5.1 mW. The measured phase noise is -90 dBc/Hz at 100 kHz and -140 dBc/Hz at 1 MHz offset.

Low Cost, Broadband Tunable Ferroelectric Filters for JTRS Cluster 5 Applications

Tunable ferroelectric ceramic capacitors have shown great promise in reducing the size, weight, and power (SWAP) of traditional varactor-based devices. Ferroelectric devices fabricated from Barium Strontium Titanate (BST) have been demonstrated in the laboratory and are just now becoming available for commercial narrowband applications. This paper presents some of the tradeoffs in designing a small form factor (SFF), easily manufacturable, broadband tunable filter utilizing high volume, low-cost BST devices. Development status for filters that cover the 225 MHz to 2.7 GHz frequency range with superior narrowband performance, low in-band insertion loss, and high out-of-band rejection that is directly applicable to the stringent requirements of Joint Tactical Radio System (JTRS) Cluster 5 Software Defined Radio (SDR) is also presented. These Manufacturing Readiness Level (MRL) 4 filters currently provide a 50% reduction in size over traditional approaches, and future short-term plans promise further reductions in unit production cost (UPC) and SWAP.

Low-loss microelectrodes fabricated using reverse-side exposure for a tunable ferroelectric capacitor application

Narrowly spaced thick microelectrodes are fabricated using a self-aligned multiple reverse-side exposure scheme for an improved quality-factor tunable ferroelectric capacitor. The microelectrodes are fabricated on a functional substrate—a thin film ferroelectric (barium strontium titanate, BST; BaxSr1−xTiO3) coated sapphire substrate, which has an electric-field-dependent dielectric property providing tuning functionality, as well as UV transparency permitting an additional degree of freedom in photolithography steps. The microelectrode process has been applied to interdigitated capacitor fabrication, where a critical challenge is maintaining narrow gaps between electrodes for high tunability, while simultaneously forming thick electrodes to minimize conductor loss. A single mask, self-aligned reverse-side exposure through the transparent substrate achieves both these goals. A single-finger test capacitor with an electrode gap of 1.2 µm and an electrode thickness of 2.2 µm is fabricated and characterized. Tunability (T = 100 × (C0 − Cbias)/C0) of 33% at 10 V has been achieved at 100 kHz. The 2.2 µm thick structure shows improvement of Q-factor compared to that of a 0.1 µm thick structure. To demonstrate the scalability of this process, a 102-finger interdigitated capacitor is fabricated and characterized at 100 kHz and 1 GHz. The structure is embedded in a 25 µm thick epoxy resin SU-8 for passivation. A quality factor decrease of 15–25%, tunability decrease of 2–3% and capacitance increase of 6% are observed due to the expoxy resin after passivation. High frequency performance of the capacitor has been measured to be 15.9 pF of capacitance, 28.1% tunability at 10 V and a quality factor of 16 (at a 10 V dc bias) at 1 GHz.

A Tunable Active Phase Shifter Using a Thin Film Ferroelectric Capacitor

This paper describes an active phase shifter with a large amount of variable phase. We propose a design that has second-order all-pass network characteristics and that uses a tunable ferroelectric capacitor. The transmitted phase is changed by varying the capacitance of a ferroelectric capacitor. A computer simulation is presented that shows that the network, even with markedly non-ideal transistors, can provide a true all-pass response over the frequency band of interest (100 MHz–400 MHz). These simulated results demonstrate an analog tunability of about 200° with a gain variation of about 3 dB at 300 MHz—when using a Ba0.96Ca0.04Ti0.84Zr0.16O3 (BCTZ) capacitor with a tunability of 2:1. The simulation performed at 300 MHz because the physical layout of the real life circuit will be done mostly with the discrete components. As the self resonance frequency of most of the discrete components lies in the few hundreds of MHz range, our preferred frequency is a practical one to deal with. The simulation also predicts a flat band gain of approximately 10 dB with ± 2 dB of gain ripple.

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.

A new design of tunable ferroelectric capacitors in RF applications

Epitaxial SrTiO 3 films are deposited by on-axis magnetron sputtering on CeO 2 buffered r-cut sapphire. The ferroelectric films indicate low losses (e.g. tan δ≈0.002–0.004 at 300 K) and a large tunability at small electric fields. Different complex designs for ferroelectric capacities are prepared via dry etching. The resulting capacity data can be explained in terms of an analytic model for parallel plate capacitors. The quality factors K [J. Supercond. 12 (2) (1999) 325] of a new capacity design exceeds the requirement for most applications K>50 already for small applied voltages U dc≈18 V, which demonstrates the good properties of the new design in combination with the quality of our ferroelectric films.