Thin Capacitors Research Articles

Superior and ultrafast energy storage performance of relaxorantiferroelectric HfO2-based supercapacitors

Compared to electrochemical energy storage, dielectric thin film-based capacitors possess the advantages of higher voltage stability and higher break-down voltage as well as lower leakage current etc. Since HfO2 films are compatible to microelectronic process and its ferroelectricity is strategically important in memory device, the realization of their excellent energy storage performance will broaden their applications in microelectronic devices. In this study, we utilized Al3+ dopant, known for its smaller ionic radius compared to Hf4+, to induce lattice disorder in the Hf0.5Zr0.5O2 films, facilitating transformation of the Hf0.5Zr0.5O2 film from ferroelectric to antiferroelectric, and ultimately to a superparaelectric-like relaxation antiferroelectric behaviors. We have experimentally demonstrated that, by introduction of an appropriate Al doping level of x = 4.13%, the Hf0.5Zr0.5O2 thin films with a relaxor antiferroelectric characteristic can achieve a corresponding recoverable energy density of more than 100 J cm−3, an efficiency of more than 80%, and an effective enhancement of the dielectric strength to more than 6 MV cm−1. Compared to HfO2-based amorphous/crystalline films reported previously, this is a record-high energy density in atomic layer deposited HfO2/ZrO2 ternary doped oxides, and beyond that, we also demonstrate their superfast charging/discharging as a capacitor. Targeting at high-speed supercapacitor applications in integrated circuit, HfO2-based dielectric may win the competition with perovskite oxides in terms of dielectric breakdown strength and charging/discharging speed etc. Our fundamental understanding of the physics behind also enriches the knowledge of materials science and dielectric physics.

Interlayer coupling-induced controllable negative differential thermal resistance in graphene/h-BN van der Waals heterostructure

The van der Waals (vdW) heterostructures employing graphene and hexagonal boron nitride (h-BN) have emerged as a typical system for building emergent two-dimensional devices, such as atomically thin transistors or capacitors. Herein, we study the nonlinear thermal transport in such vdW heterostructure by non-equilibrium molecular dynamics simulations. The results show that an obvious negative differential thermal resistance (NDTR) phenomenon can be observed under small temperature bias when the interlayer coupling becomes stronger. The vibrational spectra analysis manifests that the phonon filtering mechanism induced by interlayer coupling greatly hinders the interfacial thermal transport. To obtain the optimum conditions, the dependence of NDTR on the system length, lateral width, external temperature, and defect density is taken into account. Our findings extend the phonon filtering mechanism to thermal information processing.

LiNO3-Based Polymer Electrolytes for Solid Electrochemical Capacitors

Solid-state, thin, and flexible electrochemical capacitors (ECs) are promising power sources for wearable electronics such as smart textiles and medical sensors. One of the key enablers for safe and high performance solid electrical double layer capacitors (EDLCs) is aqueous-based neutral pH polymer electrolytes (NPPEs) [1-2]. NPPEs containing chloride [3-5] or sulfate salts [6-8] as ion conductors, have been demonstrated high ionic conductivities with wide cell voltage window (>1.5 V, beyond the typical limit of aqueous-based systems). While nitrate solution can also offer similarly wide potential window (demonstrated in liquid electrolytes [9]), their polymer electrolytes may offer several additional advantages: (i) better compatibility with wide-range of polymers from its chaotropic nature, (ii) good thermal stability from deep eutectic temperature with water, and (iii) good water-retaining ability from hygroscopic nature that allows for higher retention than sulfates while maintaining better structural integrity than chlorides.In this study, we aim to: (i) develop a class of high performance LiNO3 based NPPEs, (ii) investigate the underlying material characteristics of the NPPE that support good electrochemical performance, and (iii) demonstrate its application in solid EDLC devices using carbon-based electrodes. The polymer electrolytes utilizing either polyacrylamide or poly(vinyl alcohol) host with various amount of LiNO3 have been systematically studied for their ionic conductivities and performance in solid capacitive devices. While increasing the salt content can lead to higher ionic conductivity, the mechanical properties may be compromised from excessive water absorption. The optimized electrolytes exhibited a high ionic conductivity (>20 mS cm-1) at ambient, relatively high conductivity retention at sub-zero temperatures, and long shelf-life (>30 days). These electrolytes maintained well-hydrated ions and would enable solid-state double layer capacitors, without any separator.

A Further Theoretical Study of Capacitive Pressure Sensors Based on Thin Film Elastic Deflection and Parallel Plate Capacitor: Refined Closed-Form Solution and Numerical Calibration.

The capacitive pressure sensor based on thin film elastic deflection and a parallel plate capacitor uses a non-conductive elastic annular thin film centrally connected to a conductive, rigid, flat, concentric-circular thin plate as a pressure sensing unit. On application of pressure, the non-conductive thin film deflects elastically, which in turn moves the conductive thin plate (as a movable upper electrode plate of the parallel plate capacitor) towards the lower electrode plate, resulting in a change in the capacitance of the capacitor. Therefore, the applied pressure can be determined by measuring the capacitance change, based on the closed-form solution for the elastic behavior of the annular thin film under pressure. Such capacitive pressure sensors are more suitable for large-sized sensors such as those used for building-facade wind pressure measurements, etc. In this paper, a further theoretical study of such capacitive pressure sensors is presented. The newly presented, more refined closed-form solution can greatly reduce the output pressure error under the same input capacitance, in comparison with the previously presented closed-form solution. A numerical example of how to use the resulting closed-form solution to numerically calibrate input–output characteristics is given for the first time. The variation trend of pressure operation ranges and input–output characteristics with important parametric variations, which can be used for guiding the design of such capacitive pressure sensors, is investigated.

Low Temperature Performance of Solid Electrochemical Capacitors: Effects of Electrolytes Additives and Electrode Design

Solid-state, thin, and flexible electrochemical capacitors (ECs) can provide energy storage for next-generation wearable electronics such as smart textiles and medical sensors. One of the enablers for high performance and safe electrical double layer capacitors ECs is neutral pH polymer electrolytes due to their wide operating potentials, good ionic conductivity and benign chemical nature [1-3]. Nonetheless, the operation temperature is often limited to above the freezing point of water. For example, a polymer electrolyte based on Na2SO4 ion conductor and polyacrylamide (PAM) host has shown a promising performance at ambient (good shelf-life >30 days and wide 1.8 V window with activated carbon [4]), but its performance degraded drastically at sub-zero conditions [5]. Furthermore, it is of practical importance to understand the effect of high surface area porous carbon materials (e.g. carbon nanotube (CNT) [6-7] or activated carbon (AC) [7-9]) on solid EC at low temperatures, which has not been extensively characterized. To ensure robustness of EC devices operating under winter/sub-arctic and outer space conditions, it is necessary to develop proper polymer electrolytes and the right match of electrode/electrolyte for low temperature ECs.In this study, the performances of solid EC cells constructed with carbon electrodes and Na2SO4-PAM-based electrolytes were assessed at ambient and low temperatures. The objectives were to (i) improve the performance of Na2SO4-PAM electrolytes using anti-icing additives and (ii) elucidate the key characteristics of electrodes (i.e. pore structure and electrode loading/thickness) affecting cell performance at low temperatures.To improve the low temperature performance of the Na2SO4-PAM electrolyte, several anti-icing additives (e.g. dimethyl sulfoxide/DMSO or ethylene glycol/EGly) have been investigated and shown promising improvement to the ionic conductivity of the electrolyte at low temperatures. Leveraging the polymer electrolyte with these additives, capacitive CV profiles were attained at -10 ˚C (Fig. 1a), much more pronounced than their binary baseline. The cell performance with the DMSO-containing ternary electrolyte was then evaluated using either CNT or AC electrodes at various carbon loadings (Fig. 1b). Although solid cells with AC electrodes demonstrated higher capacitance at ambient, those with CNT electrodes retained more capacitance at low temperature. This is likely due to better ion accessibility into CNT electrodes with relatively more open pores. Meanwhile, AC electrodes with lower carbon loading have higher capacitance retention at low temperature due to better electrode utilization in a thinner electrode. These observations will provide the guideline for designing the next generation of ECs that are suitable for low temperature applications.

A review of neutral pH polymer electrolytes for electrochemical capacitors: Transitioning from liquid to solid devices

Abstract The development of neutral pH polymer electrolytes has enabled high-performance solid-state, thin, and flexible electrochemical capacitors (ECs) to provide power for future consumer electronics and Internet-of-Thing devices. Notwithstanding their promising prospect, there is still some lack of understandings or disconnections from fundamental science to practical applications of these electrolytes. In this review, we provide an overview of state-of-the-art studies on ECs with neutral pH electrolytes in both liquid and solid configurations. Starting from the fundamental studies on the voltage window and ion conduction of salt species in liquid solution to polymer electrolytes, key considerations in developing neutral pH polymer electrolytes are discussed. The performance of the polymer electrolytes along with their enabled solid symmetric and asymmetric EC devices, as well as some enhanced functionalities are presented. The future directions for research on neutral pH polymer electrolytes are proposed, expected to provide reference for further enriching the fundamental knowledge and improving the device performances.

Enhancement of ESD performances of Silicon Capacitors for RFID solutions

Abstract Radio-Frequency IDentification devices such as smart cards and RFID tags are based on the presence of a resonant tuned LC circuit associated to the RFID Integrated Circuit (IC). The use of discrete capacitor, external to the IC gives greater flexibility and design freedom. In the race of miniaturization, manufacturers of RFID devices always require smaller electronic components. To save space and in the same time improve performances, capacitors are exposed to height and volume constraints. In the same time, the capacitor has to withstand ESD stresses that can occur during the assembly of the device and during operation. Murata has developed a unique thin capacitor technology in silicon. This paper reports the development of a range of low profile capacitors with enhanced ESD performances. The manufacturing process optimization and the design adjustments will be presented here. The process was optimized by taking into account the main electrical parameters: leakage current, breakdown voltage, capacitance density, capacitance accuracy, Equivalent Series Resistance (ESR) and Self-Resonant Frequency (SRF). The dielectric stack was defined in order to integrate up to 330pF in 0402 case. The process architecture, based on accurate planar capacitor with thick dielectric will be discussed. With this architecture there is no constraint to reach low thickness, such as 100μm or even lower. The ESD threshold of each Silicon Capacitor was investigated with design variations associated to Human Body Model measurements. A Single Project Wafer (SPW) was founded with 36 different capacitor designs. Design modulations specifically addressed the orientation and position of the contacts openings. Special care was taken to maximize the width of the contact holes and metal tracks. A mosaic approach, constructed out of a massive network of parallelized elementary cells was also implemented, so that the charges of the ESD pulse do not concentrate at the same place, leading to electrical failure. Examples of defects due to ESD stress will be shown with failure analysis cross-sections and ways to enhance the ESD threshold by design will be illustrated.

Inter-digital capacitors with aerosol-deposited high-K dielectric layer for highest capacitance value in capacitive super-sensing applications

Inter-digital capacitors (IDCs) with aerosol-deposition (AD) high-k dielectric layer were compared via simulation and measurements of bare IDCs and AD IDCs at room temperature and subjected to a post-annealing process for realizing capacitive super-sensing applications. IDCs with thin AD films can provide higher capacitive intensity and improvements for other dielectric performances. Therefore, IDC patterns with AD high-k dielectric layers were fabricated by varying the finger widths and gap. Moreover, we analyzed the layer microstructure design patterns using simulations and experiments with AD BaTiO3 as-deposited IDCs and IDCs subjected to annealing at 500 °C. These three different IDCs were measured using an impedance analyzer; furthermore, the AD BaTiO3 films were evaluated using X-ray diffraction, atomic force microscopy, and traveling electron microscopy. The results for the IDCs with the AD BaTiO3 film show the highest capacitance when compared with other thin layer capacitors, which is expected to be useful in realizing super-sensing applications in the future.

MnO2 thin film electrodes for enhanced reliability of thin glass capacitors

J. Am. Ceram. Soc., 98[10], 3270–3279. https://doi.org/10.1111/jace.13774 (2015). In the published article above, the first sentence in the Acknowledgment should be replaced with the following: This material is based upon work supported by the National Science Foundation, as part of the Center for Dielectrics and Piezoelectrics under Grant Nos. IIP-1361571 and 1361503.

Defect mediated van der Waals epitaxy of hexagonal boron nitride on graphene

Van der Waals heterostructures comprising of hexagonal boron nitride and graphene are promising building blocks for novel two-dimensional devices such as atomically thin transistors or capacitors. However, demonstrators of those devices have been so far mostly fabricated by mechanical assembly, a non-scalable and time-consuming method, where transfer processes can contaminate the surfaces. Here, we investigate a direct growth process for the fabrication of insulating hexagonal boron nitride on high quality epitaxial graphene using plasma assisted molecular beam epitaxy. Samples were grown at varying temperatures and times and studied using atomic force microscopy, revealing a growth process limited by desorption at high temperatures. Nucleation was mostly commencing from morphological defects in epitaxial graphene, such as step edges or wrinkles. Raman spectroscopy combined with x-ray photoelectron measurements confirm the formation of hexagonal boron nitride and prove the resilience of graphene against the nitrogen plasma used during the growth process. The electrical properties and defects in the heterostructures were studied with high lateral resolution by tunneling current and Kelvin probe force measurements. This correlated approach revealed a nucleation apart from morphological defects in epitaxial graphene, which is mediated by point defects. The presented results help understanding the nucleation and growth behavior during van der Waals epitaxy of 2D materials, and point out a route for a scalable production of van der Waals heterostructures.

A Miniaturized Reconfigurable CRLH Leaky-Wave Antenna Using Complementary Split-Ring Resonators

Composite Right-/Left-Handed (CRLH) Leaky-Wave Antennas (LWAs) are a class of radiating elements characterized by an electronically steerable radiation pattern. The design is comprised of a cascade of CRLH unit cells populated with varactor diodes. By varying the voltage across the varactor diodes, the antenna can steer its directional beam from broadside to backward and forward end-fire directions. In this paper, we discuss the design and experimental analysis of a miniaturized CRLH Leaky-Wave Antenna for the 2.4 GHz WiFi band. The miniaturization is achieved by etching Complementary Split-Ring Resonator (CSRR) underneath each CRLH unit cell. As opposed to the conventional LWA designs, we take advantage of a LWA layout that does not require thin interdigital capacitors; thus we significantly reduce the PCB manufacturing constraints required to achieve size reduction. The experimental results were compared with a nonminiaturized prototype in order to evaluate the differences in impedance and radiation characteristics. The proposed antenna is a significant achievement because it will enable CRLH LWAs to be a viable technology not only for wireless access points, but also potentially for mobile devices.

A pressure-induced bending sensitive capacitor based on an elastomer-free, extremely thin transparent conductor

An elastomer-free, extremely thin, and transparent pressure-induced bending sensitive capacitor is achieved by using a combination of silver nanowires and polyvinyl butyral.

A Variable Nano Capacitor Behaved Arising from Lipid Bilayers of DMPC

Bio-membranes are thin capacitors and because the membrane has a dynamic behavior and in the various positions has different area and thickness, the capacitance of the membrane is not fixed and causes a variable capacitor. In this study, we have shown the model of a microscale dielectric membrane capacitor including DMPC structures. We have shown that Quantum effect has appeared in a small free space of the membrane thickness due to number of phospholipids layers. In addition, we have specifically investigated the quantum and coulomb blocked effects of different thickness. Furthermore, the electron density profile models, electron localization function (ELF) and local information entropy have been applied as a formal way to studies the interaction of protein with lipid bilayers in cell membrane.

Transfer Methods of PZT Thin Capacitor to Flexible Polymer by Self‐Assembled Monolayers

SUMMARYThis paper describes the fabrication method that a lead zirconate titanate (PZT) thin film capacitor, which is formed on a Si substrate by metal organic decomposition (MOD), can be easily transferred to flexible polymer (polydimenthylsiloxane; PDMS) without additional chemical contamination. The thin film capacitor is composed of sandwich structure of PZT between two metal layers. To embed the capacitor structure in the polymer substrate by dry peel‐off process, three kinds of self‐assembled monolayers (SAMs) were investigated for adhesion or antiadhesion layer between metal, polymer, and Si substrate, respectively. Also, the MOD's annealing process with SAMs for PZT lamination was investigated. As a result, the PZT capacitors of 1 μm thickness with a perovskite structure were successfully transferred and embedded in PDMS substrate. These results will be useful for fabricating MEMS devices and integration technology of electrical device to a polymer substrate such as capacitors in three‐dimensional integration or for flexible electronics.

Design Considerations of High-Current Density Capacitors Micromachined for Si Interposers

Many demanding power system requirements usher motivations for high-density, miniaturized capacitors capable of quickly sourcing large quantities of current. Various efforts achieved high capacitive density (∼500 nF/mm2) at D.C., but many applications promote an interest in transient and high-frequency (HF) characteristics. Thin capacitors (100–640 μm) were micromachined using several industry standard Si MEMS processes resulting in large surface area, high-density capacitive storage devices (7.34 nF/mm2). An evaluation of high-speed (1 GHz), energy storage devices and their respective fabrication technologies has been closely compared by considering each capacitor design's transient ring-down characteristics. Although their capacitive density remains ∼10 nF/mm2, large amounts of current (∼100 A/ns) were sourced while retaining desirable characteristics at high frequencies. These devices have been optimized as embedded passives and demonstrate compatibility with Si interposer technology.

MoP-36 DRY PEEL-OFF METHOD OF TRANSFER PZT THIN CAPACITOR TO FLEXIBLE POLYMER USING SAMs

This paper describes a new transfer method to realize wearable devices with embedded capacitors providing high permittivities and thinner structures. To miniaturize passive elements, such as capacitor on flexible substrate[1], lead zirconate titanate (PZT) thin films are formed on a Si substrate by MOD (metal organic decomposition) method, then can be easily transferred into a polymer (polydimethylsiloxane, PDMS or polyimide) without any chemical contamination. The thin film capacitor is composed of sandwich structure of PZT between two metal electrodes. To embed the capacitor layers in the polymer substrate, three kinds of self-assembled monolayers (SAMs) are investigated for adhesion or anti-adhesion layer among metal, polymer and Si substrate, respectively. As modifying the MOD's sintering process for PZT layers, thin capacitor in 1-μm-thick with perovskite structure is successfully transferred and embedded in a PDMS substrate.

Novel hybrid PVA–InZnO transparent thin films and sandwich capacitor structure by dip coating method: preparation and characterizations

In this paper, a very simple and cost effective dip coating method to obtain novel hybrid PVA–InZnO transparent thin films and sandwich capacitor structures for future transparent device applications.

Applications of electrostatic capacitance and charging

The capacitance of an arbitrarily shaped object is calculated with the same second-kind integral equation method used for computing static and dynamic polarizabilities. The capacitance is simply the dielectric permittivity multiplied by the area of the object and divided by the squared norm of the Neumann-Poincaré operator eigenfunction corresponding to the largest eigenvalue. The norm of this eigenfunction varies slowly with shape thus enabling the definition of two scale-invariant shape factors and perturbative calculations of capacitance. The result is extended to a special class of capacitors in which the electrodes are the equipotential surfaces generated by the equilibrium charge on the object. This extension allows analytical expressions of capacitance for confocal spheroidal capacitors and finite cylinders. Moreover, a second order formula for thin constant-thickness capacitors is given with direct applications for capacitance of membranes in living cells and of supercapacitors. For axisymmetric geometries, a fast and accurate numerical method is provided.

Scalable Generation of Strictly Monodisperse Droplets by Transverse Electrohydrodynamic Excitations

We report a scalable approach to generate strictly monodisperse microdroplets by introducing transverse periodic perturbations on microliquid jets. The fringe field of a thin capacitor, driven by sinusoidal signals, induces periodic electrohydrodynamic (EHD) stress and “chops” the liquid jet at precise and adjustable intervals. When the driving signal is near the 50% of the Rayleigh frequency, liquid jets exhibit highly ordered breakup. We have shown that for the same jet diameter, the monodispersed droplet diameter can be changed by a factor of 1.75, corresponding to more than five times in change of droplet volume. Further, we demonstrate the scale-up of this approach by chopping multiple jets using a single set of EHD exciters. Copyright 2013 American Association for Aerosol Research

Electric double-layer capacitance between an ionic liquid and few-layer graphene

Ionic-liquid gates have a high carrier density due to their atomically thin electric double layer (EDL) and extremely large geometrical capacitance Cg. However, a high carrier density in graphene has not been achieved even with ionic-liquid gates because the EDL capacitance CEDL between the ionic liquid and graphene involves the series connection of Cg and the quantum capacitance Cq, which is proportional to the density of states. We investigated the variables that determine CEDL at the molecular level by varying the number of graphene layers n and thereby optimising Cq. The CEDL value is governed by Cq at n < 4, and by Cg at n > 4. This transition with n indicates a composite nature for CEDL. Our finding clarifies a universal principle that determines capacitance on a microscopic scale, and provides nanotechnological perspectives on charge accumulation and energy storage using an ultimately thin capacitor.