Flexible Electronic Circuits Research Articles

Extraction of SnO Subbandgap Defect Density by Numerical Modeling of p-Type TFTs

Recently, p-type tin monoxide (SnO) thin-film transistors (TFTs) have gained interest for all-oxide complementary metal–oxide semiconductor (CMOS) circuits for flexible electronics and back-end-of-line integration with Si. However, most SnO TFTs demonstrated so far exhibit a low on– off-current ratio and limited field effect mobility. To understand and improve SnO TFT performance, it is necessary to quantitatively characterize the subbandgap defects in SnO and at its dielectric interface. In this work, we establish numerical models which provide this understanding. By concurrent fitting of simulated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text{D}}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text{GS}}$ </tex-math></inline-formula> and effective mobility versus <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text{GS}}$ </tex-math></inline-formula> to experimentally measured data, we can accurately extract SnO defect state profiles. Using the extracted simulation models, we then identify ways to improve SnO TFT performance. Specifically, a reduction in the density of shallow, bulk Gaussian acceptor states, is necessary to reduce the off-current, while reducing the contact resistance and interface donor-like tail state density can improve the SnO field effect mobility.

Electric and Dielectric Properties in Low-Frequency Fields of Composites Consisting of Silicone Rubber and Al Particles for Flexible Electronic Devices.

Understanding the electrical conduction and dielectric polarization properties of elastomer-based composites is important for the design of flexible and elastic electronic devices and circuits. Five samples were manufactured by mixing silicone rubber (RTV-530) with Al particles in different volume fractions, x equal to 0%, 0.5%, 1%, 2.5% and 5.1%. Using the complex impedance measurements, the electric modulus, M, the electrical conductivity, σ, and the dielectric permittivity, ε, over the frequency range 100 Hz–200 kHz were analyzed. The electrical conductivity spectrum, σ(f), follows the Jonscher universal law and the DC conductivity of the samples, σDC, increases from 2.637·10−8 S/m to 5.725·10−8 S/m, with increasing x from, 0 to 5.1%. The conduction process was analyzed in terms of Mott’s variable-range-hopping (VRH) model. The hopping distance of the charge carriers, Rh decreases with increasing x, from 7.30 nm (for x = 0) to 5.92 nm (for x = 5.1%). The frequency dependence of permittivity, ε(f) = ε′(f) − iε″(f), reveals a relaxation process with the maximum of ε″(f) shifting from 301 Hz to 385 Hz and values of ε′(f) increasing with the increase of x.

Flexible and quasi-isotropically thermoconductive polyimide films by guided assembly of boron nitride nanoplate/boron nitride flakes for microelectronic application

• A hierarchical “brick and plank” structure is fabricated by dopamine modified h -BN nanomaterials. • This special structure can be used to improve the performance of polyimide. • The modified PI films have quasi-isotropic thermal conductivity and high thermal stability. • The modified PI films have good flexibility and excellent electrical insulation. • The modified PI films show faster shape memory response time reducing 50 %. Currently, the application of polyimide (PI) based materials often demands mutual additional properties, such as high thermal conductivity, good flexibility, high thermal stability and low dielectric constant and loss, etc. . Herein, a well-aligned hierarchical “brick and plank” architecture by rational assembly of dopamine modified hexagonal boron nitride flakes and nanoparticles (PDA-BNF@BNNPs) which can effectively improve the thermal conductivity, thermal stability, electrical conductivity, dielectric properties and shape memory performance of pure PI. The alternating stacking of vertically arranged h -BNNPs and horizontally laid h -BNFs yield a quasi-isotropic thermal conductivity of PI films with a high through-plane thermal conductivity of 6.43 W/(mK) and an in-plane thermal conductivity of 11.85 W/(mK) because of the simultaneously vertical and horizontal phonon scattering enabled by the vertically arranged h -BNNPs and horizontally laid h -BNFs. Besides, this special structure also endues PI films high thermal stability of over 560 °C, prominent electrical resistance of over 1.40 × 10 -13 Ω•cm, and low dielectric loss of less than 0.02 in the high frequency. Moreover, the PDA-BNF@BNNP/PI films exhibit excellent flexibility and shape memory performance with faster recovery response time reduced over 50 % comparing with that of pure PI film. These outstanding properties combining with the facile and scalable fabrication process make PDA-BNF@BNNP/PI films are very promising in diverse electronic device applications, especially in flexible electronic devices or circuits requiring additionally high thermal conductivity and low dielectric loss.

Demonstration of durable electronic textiles via mechanically assisted highly adhesive printing of carbon nanotube-polymer composites on commercial fabrics

We demonstrate the highly adhesive and uniform printing of carbon nanotube (CNT)-polymer composites (CPCs) on diverse commercial fabrics for durable electronic textiles. The mechanically assisted printing (MAP) technique proposed in this work enables the uniform and faithful printing of conductive CPCs on fabrics with excellent adhesion against fabric deformation and stretching. We have systematically studied that the printability and conductivity of CPCs can be optimized by controlling the mixture ratio of the CNTs in an adequate polymer. We also present that various practical patterning methods, such as masked printing and pattern molding, can be applied to the MAP of specifically patterned CPCs to fabricate flexible electronic circuits that operate durably on fabrics. Many diverse industrial applications may benefit from this work, including but not limited to lightweight flexible electronics and sensors, wearable devices and gears, and functional textiles.

Damage of Flexible Ag Line Covered with Insulating Layer under High Density Current

The demand for flexible electronic circuits is increasing with the development of wearable devices. The downsizing of electronic circuits with the aim of miniaturization and high functionalization of electronic devices increases the electrical current flowing through the line. The increase in current density causes electromigration (EM), which is the migration of metal atoms due to electron’s collision and EM eventually leads to wire breakdowns. Therefore, clarifying the mechanism of EM damages and predicting the lifetime of electronic lines are important for evaluating the reliability of electronic devices. In addition, electronic line is usually coated with insulating layer to protect the line from the influence of external environment. Although high-density current loading tests have been conducted on flexible Ag nanoparticle lines in previous studies, the effects of insulating layer on the EM damages of the lines have not been fully clarified. In this study, high-density current loading tests were conducted to investigate the effect of the insulating layer on the EM behavior. In the experiments, Ag nanoparticle lines were fabricated on flexible substrates with and without insulating layer coating. And the characteristics of EM damages were evaluated under different density current loading.

Chemical activation of commodity plastics for patterned electroless deposition of robust metallic films.

A general approach to increase the adhesion of metal films to commodity plastic substrates using a metal-chelating polymer, polyethyleneimine, in conjunction with patterned electroless deposition is described. This general fabrication method is compatible with a diverse array of plastics and metals with properties applicable to flexible electronic circuits and electrochemical cells.

A Large-Size HfO2 Based RRAM Structure Suitable for Integration of One RRAM with One InGaZnO Thin Film Transistor for Large-Area Applications

This work aims at finding a HfO2-based resistive random-access memory (RRAM) structure suitable for the integration of one RRAM with one InGaZnO thin film transistor (TFT) for large-area applications such as flexible electronic circuits. One of the major concerns is that the compliance current (CC) required for the formation of stable and strong conductive filaments in the forming and set processes as well as the maximum current required in the reset process in a large-size RRAM should be lower than that of the maximum current a TFT can deliver. In this work, an ultrathin Al2O3 layer of 2 nm was inserted between the HfO2 switching layer and the reactive Ti layer of the top electrode in the RRAM with the structure of Pt (bottom electrode)/HfO2/Al2O3/Ti/TiN (top electrode). With the ultrathin Al2O3 layer, the forming voltage was greatly reduced, and the CC for stable forming and set operations and maximum reset current can reach a low current level that an InGaZnO TFT is able to provide, while the device-to-device variation of the forming operation and cycle-to-cycle resistance variations of the set and reset operations are improved significantly.

Resistive Switching Effects of Crystal‐Ion‐Slicing Fabricated LiNbO3 Single Crystalline Thin Film on Flexible Polyimide Substrate (Adv. Electron. Mater. 9/2021)

High-performance Flexible Memristor In article number 2100301 by Wenbo Luo, Xinqiang Pan, and co-workers, high-performance flexible memristor on polyimide substrate based on single crystalline LiNbO3 thin film using Crystal-ion-slicing technology is demonstrated. This flexible memristor which could be integrated with other flexible electronic devices and flexible circuits can become a promising candidate for memory and neuromorphic computing.

Tailoring Trap Density of States through Impedance Analysis for Flexible Organic Field‐Effect Transistors

AbstractThe selection of dielectric material impacts the dielectric/semiconductor (D/S) interface which plays a significant role in defining the device performance. Hence, investigation of the D/S interfacial defects and trap states is essential for improving the device performance and designing new semiconductor and dielectric materials for organic field effect transistors (OFET). Here, the trap density of states (DOS) at the interface is investigated by impedance spectroscopy (IS). OFETs are fabricated with three different dielectric combinations and the highest mobility is found to be 0.12 cm2 V−1 s−1. Detailed analysis of the semiconductor thin film and the D/S interface is performed by atomic force microscopy, photoluminescence, time‐resolved photoluminescence and is found consistent with the DOS analysis. This work validates that IS can be utilized as a prospective DOS analysis method for OFET applications. Finally, for evaluating the potential application of the device architecture toward developing flexible electronic circuit components, the device is fabricated on a flexible substrate and the mechanical stability is examined by subjecting the device to a strain of 2.5%. The device shows no significant degradation in operation, confirming its practical utility.

Over-molding of flexible polyimide-based electronic circuits

With the increasingly high demand for flexible microsystems, their embedding technologies have become more and more important. In this paper, we present the developments of over-molding technology to realize polyimide–copper (PI/Cu) electronic circuits embedded in plastics. The circuit consists of Cu interconnections on a PI substrate with assembled electronic components, which is inserted in a flat mold of a tensile specimen shape in an injection molding machine. This substrate is over-molded on one side with polyamide-6 (PA6). We have investigated two main properties of the circuits: the adhesion with the over-molded plastic and the effect of the over-molding on the electronics, measured after the molding process. Different Cu cladding foils were used in this study to optimize the adhesion between the foil and PA6. It was shown that the adhesion between the PA6 and the PI–Cu foil with acrylic as an adhesive layer is superior compared to the adhesive-less and the epoxy-based foils. Moreover, to check the electronics, 0 Ω resistors were assembled to evaluate the influence of over-molding on the electronics interconnections. The measurements of the acrylic foil were taken in three steps: before over-molding, after partial over-molding process without plastic injected, and finally after a complete over-molding cycle with plastic injection. The data presented showed that the fabrication process is suitable to integrate electronic components via over-molding and still have a functional electronic circuit on PI–Cu foil. Additionally, simulations were done to prove the same concept on a theoretical base, showing that the resistors should be functional after over-molding. Finally, we demonstrated our findings with a simple functional circuit where Near-field communication (NFC) etched antenna and a chip can wirelessly power assembled LEDs. Moreover, a long-term reliability test was performed and proved that such test structures could be used in long-term applications.

An introduction to flexible electronics: Manufacturing techniques, types and future

Flexible Electronics is an emerging technology in the fields of electronics. These are electronic circuits made on substrates which can be modified into various shapes. It can be rolled or twisted with minimal or no effect on its electronics components and its function. Flexible electronic components are usually made of a bi-layer of thin substrate which can be either plastic, metal or textile or any other suitable material with a second layer of active electronic component. Wide range of its application in solar cells, aerospace and medical fields have increased its demand by many folds. Flexible electronic circuits play a major role in components where density, availability of space, and weight of the material has to be taken into consideration. The following is the review of the current status of it the modern electronic industries, its manufacturing process, its application in various technological fields, and the role it will play in the future

Research progress in laser direct writing of flexible circuit

<p indent="0mm">At present, the flexible electronics industry is in an important transitional period, and various flexible electronic devices with novel properties and enhanced functions are constantly emerging, from limited flexibility to flexible electronic devices with shape adaptability and ductility. This has greatly stimulated demand for flexible electronic devices. The development of manufacturing technologies for flexible circuits with smaller feature sizes and better performance on larger-format substrates at lower costs has attracted increasing attention. Among various technologies, laser direct writing has proven to be an efficient and flexible manufacturing method that can produce large-area flexible electronic circuits. As a nonlithographic, nonvacuum, and online processing technology, laser direct writing has received considerable attention. It can be employed in the production of circuit electrodes of various substrates, including heat-sensitive flexible substrates, and it has huge application prospects in the production of flexible electronic and energy storage devices, sensors, and wearable electronic devices. On this basis, this article summarizes the latest development in the laser direct writing of flexible circuit (LDWFC) technology for manufacturing flexible circuits. It focuses on the material types and characteristics of the conductive inks suitable for LDWFC in the production of flexible circuits. The LDWFC processing technology is introduced in detail from four aspects: laser sintering technology, laser reduction technology, laser-induced modification technology, and laser auxiliary circuit manufacturing technology. In addition, the application of LDWFC in flexible energy storage devices, tenderness sensors, and flexible displays is introduced, and the development prospects of LDWFC technology in flexible circuit manufacturing are highlighted.

Cellulose Nanofiber Coatings on Cu Electrodes for Cohesive Protection against Water-Induced Short-Circuit Failures

Water is detrimental to electronic devices because it easily causes short circuits. The use of sealing to prevent water permeation is considered as the current conventional solution; however, the trend for developing flexible and stretchable electronic circuits has placed severe demands on waterproof technologies. This report describes a coating that protects circuits and prevents between-wire short circuits, even if the waterproof seals are damaged and the circuit becomes wet. We show that when Cu electrodes are coated with cellulose nanofibers (carboxylate content of 1.8 mmol/g), short circuits between the electrodes do not occur, even if the circuit is submerged in water for 24 h. The cellulose nanofibers accumulate at the anode because of electrophoresis, thereby forming a cohesive cellulose nanofiber layer that prevents short circuits between electrodes. Even if the cellulose nanofiber coating cracks because of external factors, the electrophoretic effect repairs the coating. This failure-containment mechanism is expected to be used in combination with existing waterproofing technology to dramatically improve the reliability of next-generation electronic devices under extreme operating conditions.

Electro-thermo-mechanical modelling of micro solar sails of chip scale spacecraft in space

This paper presents a novel design of micro solar sails for emerging lightweight chip scale spacecraft based on flexible electronic circuits. To acquire large deformation, the micro solar sails were designed to be bilayer beams that were able to be electro-thermally actuated by Joule heating. The concept design of the solar sail with high area-to-mass ratios allowed the solar sailing system, named as ChipSail, for efficient orbital transfer and attitude adjustment. The principle of solar sailing with ChipSail was illustrated, and the thickness of the two metals for the bilayer sails should be no more than 1 µm, so as to achieve the efficient solar sailing. Then, the fabrication and characterization of such bilayer microstructures for solar sails were introduced briefly. After that, the electro-thermal analysis of such solar sails deployed on the low earth orbit was carried out, and it was found that the balanced temperature of the sails under the effect of solar radiation and thermal reemission of the sails was 315.31 K, followed by electro-thermal modelling of the sails under the Joule heating. A nonlinear second order differential equation was derived, which allowed rapid prediction of the thermal distribution across the sail. Equivalence of the bilayer solar sail to a width-changing 980 µm long bilayer beam was proposed and validated by finite element analysis. Finally, the thermo-mechanical model on the bilayer sail was then established and solved numerically. Results showed that the maximum bending angle could reach to 94.05o by applying a voltage of 0.05 V across the sail. The electro-thermo-mechanical model laid a solid foundation for dynamic control of the configuration of the ChipSail for efficient orbital transfer and attitude adjustment in space.

Guest Editorial Circuits and Systems for Flexible Electronics

This special Section of the IEEE Open Journal of Circuits and Systems (OJCAS) is dedicated to the recent developments in circuits and systems for flexible electronics.

Fully-printed flexible n-type tin oxide thin-film transistors and logic circuits

We achieved fully-printed SnO2 based n-type thin-film transistors with high electrical performance and robust mechanical flexibility, promising for constructing low-cost, high-performance flexible electronic devices and circuits.

Aerosol jet printing on paper substrate with conductive silver nano material

Abstract Aerosol jet printing (AJP) is a promising method for micro-scaled digital additive manufacturing for printed electronics. It usually requires a high sintering temperature (280 °C) and a long sintering time (e.g., 12 h on a glass substrate) to guarantee a high conductivity of the printed structures, which has limited AJP to only a few types of substrates. Moreover, the printed metallic structure peels off from the substrate easily. In this paper, a procedure of AJP printing on cellulose fiber paper is proposed, which includes the use of cellulose fiber paper as the substrate, and a new sintering method, i.e., hot-air sintering with an optimized sequence. With the proposed approach, the sintering temperature is significantly lowered (80 °C), and the sintering time is considerably shortened (40 min). The printed structure has a measured sheet resistance of 1.13 × 10−2 Ω/m2 which is equivalent to a conductivity of greater than 106 S/m and close to that of the bulk silver (6.30 × 107 S/m), and it has good adhesion to the substrate. The determinant factors underlying the diffusion and curing processes of AJP, i.e., the properties of the substrate (cellulose fiber paper), the printing parameters, the sintering parameters, and the sequence were systematically investigated. The investigation is carried out through evaluation of the morphologies of the printed structures based on scanning electron microscope (SEM) images and through the study of the correlation between the morphology and the conductivity of the structures. Moreover, the proposed paper-based AJP electronics offer tremendous flexibility. They can be folded, bent, and pasted to any surface. This proposed cellulose-fiber-paper-based AJP opens a window for low-cost, eco-friendly, flexible, and high-resolution printed electronics. It will be an essential alternative fabrication approach for flexible electronic circuits, antennas, and electromagnetic-functioning surfaces, such as reconfigurable meta-surfaces.

Low-Voltage, Dual-Gate Organic Transistors with High Sensitivity and Stability toward Electrostatic Biosensing.

High levels of performance and stability have been demonstrated for conjugated polymer thin-film transistors in recent years, making them promising materials for flexible electronic circuits and displays. For sensing applications, however, most research efforts have been focusing on electrochemical sensing devices. Here we demonstrate a highly stable biosensing platform using polymer transistors based on the dual-gate mechanism. In this architecture a sensing signal is transduced and amplified by the capacitive coupling between a low-k bottom dielectric and a high-k ionic elastomer top dielectric that is in contact with an analyte solution. The new design exhibits a high signal amplification, high stability under bias stress in various aqueous environments, and low signal drift. Our platform, furthermore, while responding expectedly to charged analytes such as the protein bovine serum albumin, is insensitive to changes of salt concentration of the analyte solution. These features make this platform a potentially suitable tool for a variety of biosensing applications.

Assessing Current‐Carrying Capacity of Aerosol Jet Printed Conductors

Printed conductors are the main building blocks of printed flexible electronic circuits. With the advancement and growing demand for printed and flexible electronics, researchers are focused on assessing the mechanical reliability of interconnects. Even more important is, often, the electrical performance or the current‐carrying capacity of the interconnects. A mechanically reliable interconnect has limited use if it cannot meet the current‐carrying requirements. However, current‐carrying capacity curves and circuit design guidelines such as those available for conventional copper conductors have not yet been established for printed interconnects, i.e., there is a technological gap in this domain. The present work focuses on assessing the maximum allowed current through an aerosol jet printed silver nanoparticle‐based conductor on a 75 μm thick polyimide substrate. The temperature coefficient of resistance for the printed ink is experimentally determined. The effect of printed ink microstructure on the temperature coefficient of resistance is also investigated. Standard curves are generated that would guide designers to determine the trace width required for an allowed temperature increase. Furthermore, the effect of sintering temperature on the current‐carrying capability is evaluated.

Unraveling Structure and Device Operation of Organic Permeable Base Transistors

AbstractOrganic permeable base transistors (OPBTs) are of great interest for flexible electronic circuits, as they offer very large on‐current density and a record‐high transition frequency. They rely on a vertical device architecture with current transport through native pinholes in a central base electrode. This study investigates the impact of pinhole density and pinhole diameter on the DC device performance in OPBTs based on experimental data and TCAD simulation results. A pinhole density of NPin = 54 µm−2 and pinhole diameters around LPin = 15 nm are found in the devices. Simulations show that a variation of pinhole diameter and density around these numbers has only a minor impact on the DC device characteristics. A variation of the pinhole diameter and density by up to 100% lead to a deviation of less than 4% in threshold voltage, on/off current ratio, and sub‐threshold slope. Hence, the fabrication of OPBTs with reliable device characteristics is possible regardless of statistical deviations in thin film formation.