fF Μm Research Articles

Fabrication and electrical properties of flexible polymer-based MIM capacitors of high-k nanolaminate dielectrics of HfO2–SnO2–TiO2 with ultrathin Al2O3 insertion layer by plasma-enhanced atomic layer deposition

Abstract Flexible metal–insulator–metal (MIM) capacitors of high-k nanolaminate HfO2–SnO2–TiO2 thin films were fabricated on several polymer substrates of polyethylene terephthalate, polyimide and epoxy resin at 80 °C by plasma-enhanced atomic layer deposition. The electrical properties were optimized by adjusting the sub-cycle ratio of Hf: Sn: Ti to 6: 5: 4. In order to reduce the leakage current density of flexible capacitors, the ultrathin Al2O3 layer varying from 0.5 to 1.5 nm was inserted to form Al2O3/HfO2–SnO2–TiO2/Al2O3 stacking capacitors. The effect of the Al2O3 insertion layer thickness and the super-cycle number of HfO2–SnO2–TiO2 on the capacitance density, leakage, and quadratic voltage linearity was investigated. Under optimal processing, flexible MIM capacitors could stand 40 000 bending cycles at curvature radius of 8.2 mm, indicative of better electrical stability. Moreover, compared with the polymer-based HfO2–SnO2–TiO2 capacitors, the introduction of 1 nm Al2O3 ultrathin layer greatly decreases the leakage current density by 4 orders of magnitude (10−8 A cm−2) with relative lower voltage linearity (350–540 ppm V−2), but the capacitance density also declines (∼3 fF μm−2) simultaneously. Despite this, the method of inserting Al2O3 ultra-thin layer is still an effective method to improve the electrical performances of polymer-based HfO2–SnO2–TiO2 nanolaminate capacitors for flexible electronics.

High capacitance density highly reliable textured deep trench SiN capacitors toward 3D integration

We developed high capacitance density and highly reliable Si deep trench capacitors with textured surface and SiN dielectric film. The developed capacitor consists of parallel unit cells with 14.3 μm depth textured surface trench capacitors, using Si wafer process compatible to 3D integration, realizing high scalability and versatility. Various fabrication conditions were experimented with to optimize the electrical characteristics. As a result, over 230 fF μm−2 capacitance density and 9.0 V breakdown voltage were achieved. Regarding reliability, it has been confirmed that SiN dielectric film leads to below 10−9 A cm−2 leakage current density at 1 V and the predicted lifetime of over 50 years at 3.3 V. For low voltage applications, higher capacitance density is available by using thinner SiN dielectric films.

Highly flexible, stable and transparent capacitors with enhanced performances by composite electrodes of AZO and metallic nanomeshes

Transparent and flexible capacitors with composite electrodes consisting of ZnO:Al(AZO) and Cr–Au nanomeshes and dielectric ZrO2 films were directly prepared on plastic substrates. The transparent composite electrodes exhibited an ultralow sheet resistance (5.6 Ω·square−1) and good flexural properties. ZrO2 thin films were grown by atomic layer deposition to serve as dielectric layers. The capacitors showed a higher capacitance density (17.8 fF μm−2) than that capacitors with AZO electrodes (9.8 fF μm−2) while maintained an extra-low leakage current density (1.3 × 10−9 A cm−2 at 1 V). Additionally, the quality factor was increased from 9.6 to 80.5 at a frequency of 1 MHz with the incorporation of Cr–Au nanomeshes, which greatly reduced the power consumptions. More importantly, the capacitors with the composite electrodes exhibited excellent bendability, good cycling stability, and relatively high transparency. It is, therefore, a promising route for application in developing transparent, flexible, high-performance and low-cost integrated circuits.

Ballistic Quantum Transport of Sub‐10 nm 2D Sb2Te2Se Transistors

Abstract2D materials, due to their ultrathin dangling‐bond‐free configuration and excellent electrostatic control, have potential as possible Si‐alternative building blocks for future competitive devices. Exploring 2D electronic materials with suitable band gap and high mobility is key to designing ultra‐scaled devices. The electronic properties and device performance of sub‐10 nm 2D Sb2Te2Se are studied by ab initio quantum‐transport simulations. The results show that 2D Sb2Te2Se, a stable system, possesses an indirect bandgap of 1.01 eV and high electron mobility exceeding 103 cm2 V−1 s−1. Through comparing with the ballistic quantum transports of Sb2Te2Se, the n‐metal‐oxide‐semiconductor field‐effect transistor (MOSFET) exhibits better device performance than p‐MOSFET. In particular, not only can the on‐current of n‐MOSFET with gate length Lg = 9 nm reach as high as 1660 µA µm−1, but its other figures of merit including gate capacity (0.72 fF µm−1), delay time (0.31 ps), and power dissipation (0.37 fJ µm−1) also satisfy the International Technology Roadmap for Semiconductors 2020 requirements. Therefore, 2D Sb2Te2Se could become a potential electronic material for next‐generation nanodevices.

High-quality-factor flexible and transparent capacitors with Cr–Au nanomeshes as bottom electrodes

Flexible and transparent capacitors have been fabricated on polyethylene naphthalate (PEN) substrates. The Cr–Au nanomeshes, ZrO2, and aluminum-doped zinc oxide (AZO) films were prepared in sequence as bottom electrodes, dielectric layers and top electrodes. A high quality factor of 82 was obtained, nearly 4.9 times larger than that ever reported, which directly indicates much lower power consumption. The flexible capacitors can normally work at a bending radius of 7 mm without any loss of electrical performance, and even maintain electrical characteristics after cyclical bending tests. Moreover, a high capacitance density of 12.3 fF μm−2 and an ultralow leakage current of 1.5 × 10−8 A cm−2 at 1 V have been achieved. Transmittance measurements indicate that the capacitors represent an average optical transmittance of over 50% at a visible region. This is, therefore, a dramatic step toward real applications in flexible and transparent integrated circuits.

Fully Transparent and Flexible Capacitors with ZrO2 as the Dielectric Layer

AbstractTransparent and flexible capacitors with ZrO2 as the dielectric layer are fabricated directly on polyethylene naphthalate (PEN) substrates by an in situ atomic layer deposition method. The hybrid AlZnO and silver nanowires serve as the bottom electrode. The capacitors exhibit a high capacitance density of 15.2 fF µm−2 and an ultralow leakage current (1.0 × 10−9 A cm−2 at 1 V). Bending tests reveal that the capacitors can be outwardly bended for up to 1000 times at a radius of 7 mm without any deterioration in electrical performance. Both the top and bottom electrodes of such capacitors are based on ZnO materials, which can effectively reduce the manufacturing cost and promote the mass production process.

Transparent capacitors with hybrid ZnO:Al and Ag nanowires as electrodes

Transparent conducting films with a composite structure of AlZnO–Ag nanowires (AgNWs) have been prepared by atomic layer deposition. The sheet resistance was reduced from 120 to 9 Ω when the AgNW networks were involved. Transparent capacitors with Al2O3–TiO2–Al2O3 dielectrics were fabricated on the composite electrodes and demonstrated a capacitance density of 10.1 fF μm−2, which was significantly higher than that of capacitors with AlZnO electrodes (8.8 fF μm−1). The capacitance density remained almost unchanged in a broad frequency range from 3 kHz to 1 MHz. Moreover, a low leakage current density of 2.4 × 10−7 A cm−2 at 1 V was achieved. Transparent and flexible capacitors were also fabricated using the composite electrodes, and demonstrated an improved bendability. The transparent capacitors showed an average optical transmittance over 70% in the visible range, and thus open the door to practical applications in transparent integrated circuits.

Flexible Self-Aligned Double-Gate IGZO TFT

In this letter, flexible double-gate (DG) thin-film transistors (TFTs) based on InGaZnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and fabricated on free standing plastic foil, using self-alignment (SA) are presented. The usage of transparent indium-tin-oxide instead of opaque metals enables SA of source-, drain-, and top-gate contacts. Hence, all layers, which can cause parasitic capacitances, are structured by SA. Compared with bottom-gate reference TFTs fabricated on the same substrate, DG TFTs exhibit a by 68% increased transconductance and a subthreshold swing as low as 109 mV/dec decade (-37%). The clockwise hysteresis of the DG TFTs is as small as 5 mV. Because of SA, the source/drain to gate overlaps are as small as ≈ 1 μm leading to parasitic overlap capacitances of 5.5 fF μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Therefore a transit frequency of 5.6 MHz is measured on 7.5 μm long transistors. In addition, the flexible devices stay fully operational when bent to a tensile radius of 6 mm.

Graphene oxide-based flexible metal–insulator–metal capacitors

This work explores the fabrication of graphene oxide (GO)-based metal–insulator–metal (MIM) capacitors on flexible polyethylene terephthalate (PET) substrates. Electrical properties are studied in detail. A high capacitance density of ∼4 fF µm−2 measured at 1 MHz and permittivity of ∼6 have been obtained. A low voltage coefficient of capacitance, VCC-α, and a low dielectric loss tangent indicate the potential of GO-based MIM capacitors for RF applications. The constant voltage stressing study has shown a high reliability against degradation up to a projected period of 10 years. Degradation in capacitance of the devices on flexible substrates has been studied by bending radius down to 1 cm even up to 6000 times of repeated bending.

Design and fabrication of a high-density multilayer metal–insulator–metal capacitor based on selective etching

This paper presents a novel and cost-effective method for fabricating high-density multilayer metal–insulator–metal (MIM) integrated capacitors. To eliminate the usage of numerous photolithography steps when parallel stacking multiple capacitors layers, a unique process has been developed based on depositing the MIM layers onto a substrate with two protruding pillars, polishing down the pillars to expose the multilayer cross sections and then selectively etching the metal layers on each pillar to form the alternating capacitor plate electrodes. For demonstration purpose, only capacitors with two dielectric layers were fabricated, and the measurement results were verified by a compact analytical model together with finite element simulations. With 200 nm thick silicon nitride/oxide dielectric layers, a capacitance density of 0.6 fF µm−2 was achieved, which can be easily increased by scaling down the layer thicknesses and/or stacking more layers. A low equivalent series resistance (ESR) of 300–700 mΩ was measured, and the self-resonance frequency was above measurement limits (>100 MHz). Further design optimization shows that the ESR can be reduced to below 80 mΩ, while the operation frequency extended to above 2.6 GHz.

Controlled deposition of new organic ultrathin film as a gate dielectric layer for advanced flexible capacitor devices

This letter describes a new organic (1-bromoadamantane) ultrathin film as gate dielectric, which was successfully deposited by sol–gel spin-coating process on a flexible polyimide substrate at room temperature. The metal–insulator-metal (MIM) device with organic (1-bromoadamantane) ultrathin (10 nm) film as gate dielectric layer operated at gate voltage of 5.0 V, showing a low leakage current density (5.63 × 10−10 A cm−2 at 5 V) and good capacitance (2.01 fF μm−2 at 1 MHz). The chemical structure of the 1-bromoadamantane layer was investigated by Fourier transform infrared spectrometer. The excellent leakage current density and better capacitance, probably due to the presence of polar, non-polar, low-polar groups, and bromine atoms in ultrathin film. Practical properties of the film in MIM capacitor such as dielectric constant as well as bending result of leakage current density and breakdown voltage have been better related to such fundamental adhesion nature over flexible substrate. This permits estimation of the properties of new dielectric in thin film form and short lists of the best materials for low loss and good capacitance flexible capacitors could be drawn up in future.

Enhanced capacitance ratio and low minimum capacitance of varactor devices based on depletion-mode Ga-doped ZnO TFTs with a drain-offset structure

Depletion-mode Ga-doped ZnO thin-film transistors (TFTs) with a drain-offset configuration were fabricated employing a plasma-enhanced chemical vapour deposited Si3N4 gate insulator and a metal organic chemical vapour deposition grown Ga-doped ZnO channel layer. For comparison, conventional TFTs with drain-to-gate overlap were also fabricated. Capacitance–voltage characteristics of gate-to-drain capacitors and current–voltage characteristics of TFTs were measured. Although drain-offset TFTs exhibit poor device characteristics compared with conventional TFTs, these TFTs show better Cmax/Cmin ratio and Cmin values. The Cmax/Cmin ratio is as large as 71.83 and Cmin is as small as 0.3 fF µm−1 normalized for channel width, demonstrating the potential of these devices as varactors for circuit applications. Their better varactor performance is ascribed to the presence of a very small capacitance in the drain-offset region. The effect of drain-offset length variation on TFT and gate-to-drain capacitor performance is also reported.

Flexible metal–insulator–metal capacitors on polyethylene terephthalate plastic substrates

Realization of flexible electronic devices on polyethylene terephthalate flexible plastic substrates is reported. Metal–insulator–metal capacitors have been fabricated using radio frequency sputtered deposited thin films of Nb2O5 on the flexible substrate. Good electrical characteristics have been obtained in terms of quadratic voltage coefficient of capacitance, high capacitance density (∼ 11 fF µm−2), high dielectric constant (∼37) and low dissipation factor (<0.1). Besides, in repetitive bending test, the devices show excellent electrical stability and high mechanical flexibility due to the high ductility of niobium and low-temperature processing used in this study.

Frequency-dependent dielectric response of HfTaOx-based metal–insulator–metal capacitors

Frequency dependence of dielectric response of HfTaOx-based metal–insulator–metal (MIM) capacitors (sandwich structure; with dielectric layer of different thickness) fabricated with Pt as top and bottom electrodes has been reported. A high capacitance density of ∼8.3 fF µm−2 (at 1 kHz) and a dielectric constant of ∼ 21 have been achieved. Experimental results show that both the dielectric constant and loss tangent decrease with increasing frequency and reach a minimum at ∼50 kHz, and then increase sharply. Modeling of loss tangent and dielectric constant has been attempted and the results agree well with the reported results. From frequency-dependent ac conductivity analyses, it is shown that the hopping-type conduction mechanism is responsible for the decrease in resistance with frequency.

Mechanosensitive channel properties and membrane mechanics in mouse dystrophic myotubes

Muscular dystrophy is associated with increased activity of mechanosensitive channels (MSCs) and increased cell calcium levels. MSCs in patches from mdx mouse myotubes have higher levels of resting activity, compared to patches from wild-type mice, and a pronounced latency of activation and deactivation. Measurements of patch capacitance and geometry reveal that the differences are linked to cortical membrane mechanics rather than to differences in channel gating. We found unexpectedly that patches from mdx mice are strongly curved towards the pipette tip by actin pulling normal to the membrane. This force produces a substantial tension (approximately 5 mN m(-1)) that can activate MSCs in the absence of overt stimulation. The inward curvature of patches from mdx mice is eliminated by actin inhibitors. Applying moderate suction to the pipette flattens the membrane, reducing tension, and making the response appear to be stretch inactivated. The pronounced latency to activation in patches from mdx mice is caused by the mechanical relaxation time required to reorganize the cortex from inward to outward curvature. The increased latency is equivalent to a three-fold increase in cortical viscosity. Disruption of the cytoskeleton by chemical or mechanical means eliminates the differences in kinetics and curvature between patches from wild-type and mdx mice. The stretch-induced increase in specific capacitance of the patch, approximately 80 fF microm(-2), far exceeds the specific capacitance of bilayers, suggesting the presence of stress-sensitive access to large pools of membrane, possibly caveoli, T-tubules or portions of the gigaseal. In mdx mouse cells the intrinsic gating property of fast voltage-sensitive inactivation is lost. It is robust in wild-type mouse cells (observed in 50% of outside-out patches), but never observed in mdx cells. This link between dystrophin and inactivation may lead to increased background cation currents and Ca2+ influx. Spontaneous Ca2+ transients in mdx mouse cells are sensitive to depolarization and are inhibited by the specific MSC inhibitor GsMTx4, in both the D and L forms.

Atomic-layer-deposited Al2O3-HfO2 laminated and sandwiched dielectrics for metal–insulator–metal capacitors

Metal–insulator–metal (MIM) capacitors with atomic-layer-deposited Al2O3–HfO2 laminated and sandwiched dielectrics have been fabricated and electrically compared for analog circuit applications. The experimental results indicate that the laminated dielectrics exhibit much better leakage and breakdown characteristics than the sandwiched ones while maintaining higher capacitance densities and acceptable voltage linearity. In respect of the 1 nm Al2O3 and 10 nm HfO2 laminated dielectric, the resulting capacitor offers an extremely low leakage current of 2.4 × 10−9 A cm−2 at 8 V and a breakdown electric field of ∼3.3 MV cm−1 at 125 °C together with a capacitance density of ∼3.1 fF µm−2 and voltage coefficients of capacitance of 100 ppm V−2 and −80 ppm V−1 at 100 kHz. The superiority of the laminated dielectrics correlates with inhibition of HfO2 crystallization, discontinuity of the grain boundary channels from the top to the bottom and changes of the dielectric electronic properties due to the bonding and polarization effects at the multi-interfaces.

Microwave dynamics and electromagnetic properties of YBa 2Cu 3O 7 step-edge junctions on MgO substrates

In this paper, we report the successful fabrication and current-voltage characteristics of the epitaxial c-axis YBa 2Cu 3O 7 step-edge junctions (SEJs) on MgO substrates. The junctions were fabricated by patterning the laser deposited YBa 2Cu 3O 7 thin films on MgO substrates with ionmilled steps. Integer Shapiro steps were observed in the current-voltage characteristics up to 1 THz under 72 GHz external microwave irradiation. Additional subharmonic Shapiro steps have also been found in some junctions with small step height and film thickness ratio, which may be explained by the inhomogeneous YBa 2Cu 3O 7 grain boundary interface formed at the edges of the substrate step. Self-induced electromagnetic resonance (Fiske mode) peaks have been observed in the current-voltage curves of the SEJs with large step height and film thickness ratios. The Swihart velocity and the effective shunting capacitance have been estimated to be 6.9 × 10 6 m s −1 and 55 fF μm −2 at 4.2 K, respectively, from the voltage of the observed Fiske resonance. The shunting capacitance obtained from the electromagnetic resonance compares well with that estimated from the McCumber constant β c .

Loss mechanisms in fine-grained ferroelectric ceramic thin films for ULSI memories (DRAMs)

Ferroelectric thin films can now be prepared by a variety of techniques to have dielectric constants of 800, loss tangents of 0.01, and leakage currents as small as 1 nA cm −2, which means that a capacitance of 120 fF μm −2 can be achieved at 60 nm thickness (30 fF cell −1 in a 0.25 μm 2 256 Mbit DRAM). It is significant that these properties are maintained up to 2 or 3 GHz, permitting operation of memories at very high clock rates (greater than 100 MHz). High-frequency roll-off of dielectric constants and loss tangents for barium strontium titanate fine-grained ceramics are compared with those for lead zirconate-titanate and discussed in terms of grain size and processing. The low loss tangents for very fine-grained material (40–100 nm) are surprising in view of earlier work on strontium titanate. Our data support the Neumann-Hofmann theory [ Ferroelectrics, 134 (1992) 201] of surface-layer dominated loss.

Erratum to: Effect of deposition temperature on dielectric properties of PECVD Ta2O5 thin film

Tantalum oxide film formation by plasma-enhanced chemical vapour deposition (PECVD) using TaCl5 as a source material was examined. The effects of deposition temperature on the formation, structure and electric properties of the Ta2O5 film were investigated for Al/Ta2O5/ p-Si (MTS) capacitors. The deposition rate and refractive index increased with increasing deposition temperature. It was found that the structure of Ta2O5 deposited by PECVD was amorphous as-deposited. However, crystalline δ-Ta2O5 of hexagonal structure was formed by a 700 °C, 1 h heat treatment in argon. Capacitance and relative dielectric constant of the PECVD Ta2O5 were found to be 2.54 fF μm−2 and 23.5, respectively. The PECVD films obtained in this study have higher dielectric constants and remarkably better general film characteristics than those obtained by other deposition methods.

Effect of deposition temperature on dielectric properties of PECVD Ta2O5 thin film

Tantalum oxide film formation by plasma-enhanced chemical vapour deposition (PECVD) using TaCl5 as a source material was examined. The effects of deposition temperature on the formation, structure and electric properties of the Ta2O5 film were investigated for Al/Ta2O5/ p-Si (MTS) capacitors. The deposition rate and refractive index increased with increasing deposition temperature. It was found that the structure of Ta2O5 deposited by PECVD was amorphous as-deposited. However, crystalline δ-Ta2O5 of hexagonal structure was formed by a 700 °C, 1 h heat treatment in argon. Capacitance and relative dielectric constant of the PECVD Ta2O5 were found to be 2.54 fF μm−2 and 23.5, respectively. The PECVD films obtained in this study have higher dielectric constants and remarkably better general film characteristics than those obtained by other deposition methods.