Ultra-thin Flexible Substrate Research Articles

Flexible Nanoscale Amorphous Oxide Transistors with a Gold-Assisted Transfer Method.

We present a new approach to achieve nanoscale transistors on ultrathin flexible substrates with conventional electron-beam lithography. Full devices are first fabricated on a gold sacrificial layer covering a rigid silicon substrate, and then coated with a polyimide film and released from the rigid substrate. This approach bypasses nanofabrication constraints on flexible substrates: (i) electron-beam surface charging, (ii) alignment inaccuracy due to the wavy substrate, and (iii) restricted thermal budgets. As a proof-of-concept, we demonstrate ∼100 nm long indium tin oxide (ITO) transistors on ∼6 μm thin polyimide. This is achieved with sub-20 nm misalignment or overlap between source (or drain) and gate contacts on flexible substrates for the first time. The estimated transit frequency of our well-aligned devices can be up to 3.3 GHz, which can be further improved by optimizing the device structure and performance.

Rapid and versatile, micro-patterning and functionalization of complex structures on ultra-thin and flexible polymeric substrates

Microscale patterning on flexible substrates is important in many applications such as in wearable sensors and microfluidics-based diagnostics, therefore low-cost fabrication methods which are scalable and amenable to mass production have attracted the interest of many companies and research groups. Dry film resists (DFRs) are commercially available materials with properties compatible with their implementation on flexible substrates to cover a wide range of applications, which also offer environmental and sustainability benefits due to the low waste generation compared to the liquid resists. However, there are limited detailed reports in the literature regarding the use of DFRs for the fabrication of microfluidic channels or other micropatterns (i.e., posts) on thin and flexible substrates. Herein we present in detail the fabrication of: a) microfluidic channels of width ranging from 50 μm up to 800 μm, and depth ranging from 30 μm up to 270 μm and b) square posts 80 μm × 80 μm in size and 30 μm in height. Particularly, our method enables the fabrication of ultra-deep microchannels (depth > 250 μm), highly ordered post arrays over large area (appr. 60 cm2), as well as complex designs with hierarchical scale features (80 μm posts inside 800 μm microchannels or micro-nanotexturing inside microchannels) on ultra-thin flexible substrates. To demonstrate the versatility of the method, three different DFRs were used on ultra-thin (30 μm), flexible, single-sided copper-clad polyimide substrates. It is also demonstrated that DFRs can be effectively modified using plasma etching to tune the surface wetting properties towards applications such as pumpless capillary action, where such functionality is required.

Materials, Device Structures, and Applications of Flexible Perovskite Light‐Emitting Diodes

AbstractThe flexible type of displays, which can alter their shape freely (e.g., bendable or foldable displays) according to situational demands, is under an intense spotlight due to applications in human‐friendly mobile electronics. Among various types of light‐emitting diodes (LEDs), meanwhile, perovskite‐based LEDs (PeLEDs) have garnered particular attention in recent years as next‐generation optoelectronic devices due to their exceptional optical characteristics, such as high efficiency (up to theoretical limits), high color purity, wide color gamut over the entire visible range, and ultrathin form factor. By integrating perovskite materials on an ultrathin flexible substrate with a proper encapsulation layer, the flexible PeLEDs, which can conform to various curved surfaces and be stably operated in an ambient condition, have been developed. Here, recent advances of the flexible PeLEDs are reviewed. First, light‐emitting materials and device structures for the PeLEDs are introduced. Then, research progress on the flexible PeLEDs, either with and without the encapsulation layer are summarized. Next, some representative application examples of the flexible PeLEDs are briefly discussed. Finally, this review includes a brief discussion on future prospects.

Multi-functional terahertz metamaterials based on nano-imprinting.

This paper reports a multi-functional terahertz (THz) metamaterial based on a nano-imprinting method. The metamaterial is composed of four layers: 4 L resonant layer, dielectric layer, frequency selective layer, and dielectric layer. The 4 L resonant structure can achieve broadband absorption, while the frequency selective layer can achieve transmission of specific band. The nano-imprinting method combines electroplating of nickel mold and printing of silver nano-particle ink. Using this method, the multilayer metamaterial structures can be fabricated on ultrathin flexible substrates to achieve visible light transparency. For verification, a THz metamaterial with broadband absorption in low frequency and efficient transmission in high frequency is designed and printed. The sample's thickness is about 200 µm and area is 65 × 65 mm2. Moreover, a fiber-based multi-mode terahertz time-domain spectroscopy system was built to test its transmission and reflection spectra. The results are consistent with the expectations.

Carbon Nanotube-Based Flexible Ferroelectric Synaptic Transistors for Neuromorphic Computing.

Biological nervous systems evolved in nature have marvelous information processing capacities, which have great reference value for modern information technologies. To expand the function of electronic devices with applications in smart health monitoring and treatment, wearable energy-efficient computing, neuroprosthetics, etc., flexible artificial synapses for neuromorphic computing will play a crucial role. Here, carbon nanotube-based ferroelectric synaptic transistors are realized on ultrathin flexible substrates via a low-temperature approach not exceeding 90 °C to grow ferroelectric dielectrics in which the single-pulse, paired-pulse, and repetitive-pulse responses testify to well-mimicked plasticity in artificial synapses. The long-term potentiation and long-term depression processes in the device demonstrate a dynamic range as large as 2000×, and 360 distinguishable conductance states are achieved with a weight increase/decrease nonlinearity of no more than 1 by applying stepped identical pulses. The stability of the device is verified by the almost unchanged performance after the device is kept in ambient conditions without additional passivation for 240 days. An artificial neural network-based simulation is conducted to benchmark the hardware performance of the neuromorphic devices in which a pattern recognition accuracy of 95.24% is achieved.

Flexible and Reconfigurable Frequency Selective Surface With Wide Angular Stability Fabricated With Additive Manufacturing Procedure

In this letter, a flexible and frequency-reconfigurable bandstop frequency selective surface (FSS) is studied theoretically and experimentally. The FSS patterns are fabricated on an ultrathin flexible substrate through screen printing, a low-cost, high-efficiency additive manufacturing methodology. With the embedded varactors biased from 0 to 20 V, resonant frequency tuning from 3.5 to 5.7 GHz was measured. Outstanding wide angular stability under different bias voltages has also been examined in theory and measurement by changing the incidence angle from 0° to 60°. As a demonstration of the flexible FSS in the conformal application, the FSS is tightly covered on a half-cylinder with a diameter of 10 cm, which agrees well with the case when placed on a flat surface. Furthermore, the unit cell has a size of λ/15 at 3.5 GHz, featured with miniaturization. The mechanical flexibility, frequency reconfigurability, and angular stability make it meaningful in applications of flexible/conformal covering for aiming objects, including electromagnetic scattering control, electromagnetic compatibility (EMC), etc.

Flexible Carbon Nanotube Synaptic Transistor for Neurological Electronic Skin Applications.

There is an increasing interest in the development of memristive or artificial synaptic devices that emulate the neuronal activities for neuromorphic computing applications. While there have already been many reports on artificial synaptic transistors implemented on rigid substrates, the use of flexible devices could potentially enable an even broader range of applications. In this paper, we report artificial synaptic thin-film transistors built on an ultrathin flexible substrate using high carrier mobility semiconducting single-wall carbon nanotubes. The synaptic characteristics of the flexible synaptic transistor including long-term/short-term plasticity, spike-amplitude-dependent plasticity, spike-width-dependent plasticity, paired-pulse facilitation, and spike-time-dependent plasticity have all been systematically characterized. Furthermore, we have demonstrated a flexible neurological electronic skin and its peripheral nerve with a flexible ferroelectret nanogenerator (FENG) serving as the sensory mechanoreceptor that generates action potentials to be processed and transmitted by the artificial synapse. In such neurological electronic skin, the flexible FENG sensor converts the tactile input (magnitude and frequency of force) into presynaptic action potential pulses, which are then passed to the gate of the synaptic transistor to induce change in its postsynaptic current, mimicking the modulation of synaptic weight in a biological synapse. Our neurological electronic skin closely imitates the behavior of actual human skin, and it allows for instantaneous detection of force stimuli and offers biological synapse-like behavior to relay the stimulus signals to the next stage. The flexible sensory skin could potentially be used to interface with skeletal muscle fibers for applications in neuroprosthetic devices.

Carbon nanotube dual-material gate devices for flexible configurable multifunctional electronics

Electronic devices with configurability to providing multiple functions are of great interests for their superior adaptability to the ever-changing and multifarious application scenarios. Here, we report flexible integrated circuits (ICs) possessing configurable functions constructed with dual-material gate (DMG) devices based on carbon nanotube thin films, which can serve as either transistors or diodes, on a 2-μm-thick parylene substrate. When configured as a transistor, the DMG device has great advantages over the normal-gated (NG) devices regarding the current on/off ratio (Ion/Ioff), the subthreshold swing (SS) and the drain-induced barrier lowering (DIBL) due to the regulated energy band distribution in channel area. When operating as a diode, a typical DMG device demonstrates a sufficient rectification ratio of 8×104 and a diode-on-current of over 26 μA. Scalable manufacturing of DMG devices was also demonstrated with great uniformity both in diode and transistor configurations. Finally, multifunctional integrated circuits, which can dynamically switch their function from rectifier to follower or from OR gate to voltage adder by changing controlling signals, were constructed. The functional-configurability, together with scalable manufacturing and the realization on ultrathin flexible substrates, will open up great opportunity for the future environmentally-adaptive system in the field of flexible electronics.

Room-temperature, printed, low-voltage, flexible organic field-effect transistors using soluble polyimide gate dielectrics

In this work, a room-temperature, printed, low-voltage, flexible organic field-effect transistor (OFET) has been successfully developed by utilizing 4,4′-(hexafluoroisopropylidene)diphthalic anhydride-3,5-diaminobenzyl cinnamate (6FDA-DABC) and diketopyrrolopyrrole-dithienylthieno[3,2-b]thiophene (DPP-DTT) as polymer insulator and semiconductor layers, respectively. Dielectric properties are systematically evaluated to investigate the room-temperature processability of 6FDA-DABC. In addition, the introduction of insulating polymer, polystyrene (PS), blends considerably improves the electrical characteristics of DPP-DTT-based OFETs. The operation voltage is successfully lowered to −5 V by reducing the gate dielectric thickness. OFETs based on DPP-DTT:PS annealed under various temperature conditions demonstrate the fully room-temperature processability. Finally, OFETs integrated with ultrathin flexible substrates exhibit excellent mechanical flexibility while maintaining device performance. This work provides a great freedom in the choice of plastic substrates for the development of flexible electronic applications.In this work, a room-temperature, printed, low-voltage, flexible organic field-effect transistor (OFET) has been successfully developed by utilizing 4,4′-(hexafluoroisopropylidene)diphthalic anhydride-3,5-diaminobenzyl cinnamate (6FDA-DABC) and diketopyrrolopyrrole-dithienylthieno[3,2-b]thiophene (DPP-DTT) as polymer insulator and semiconductor layers, respectively. Dielectric properties are systematically evaluated to investigate the room-temperature processability of 6FDA-DABC. In addition, the introduction of insulating polymer, polystyrene (PS), blends considerably improves the electrical characteristics of DPP-DTT-based OFETs. The operation voltage is successfully lowered to −5 V by reducing the gate dielectric thickness. OFETs based on DPP-DTT:PS annealed under various temperature conditions demonstrate the fully room-temperature processability. Finally, OFETs integrated with ultrathin flexible substrates exhibit excellent mechanical flexibility while maintaining device performance. This work provid...

Low-Temperature Solution-Processed Soluble Polyimide Gate Dielectrics: From Molecular-Level Design to Electrically Stable and Flexible Organic Transistors.

Aromatic soluble polyimides (PIs) have been widely used in organic field-effect transistors (OFETs) as gate dielectric layers due to their promising features such as outstanding chemical resistance, thermal stability, low-temperature processability, and mechanical flexibility. However, the molecular structures of soluble PIs on the electrical characteristics of OFETs are not yet fully understood. In this work, the material, dielectric, and electrical properties are evaluated to systematically investigate the chemical structure effect of aromatic dianhydride and diamine monomers on the device performance. Four soluble PIs based on 4,4'-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 5-(2,5-Dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, in which the monomeric precursors contain different backbones, side groups, and linkages, were employed to compare the chemical structure impact. The dielectric properties, which significantly affect the charge transport and crystallinity of OSC thin films, clearly depended on the soluble PI types as well as the surface energy and the thermal stability. Furthermore, the electrical characteristic measurement and parameter extraction of OFETs based on TIPS-pentacene revealed that the 6FDA-based soluble PIs, which lead to high field-effect mobility, near-zero threshold electric field, and outstanding electrical stability under bias stress, are the most promising gate dielectric candidates. Finally, low-temperature solution-processed OFETs are successfully integrated with ultrathin flexible substrates, and they exhibit no significant electrical performance loss after mechanical flexibility tests. This work presents a step forward in the development of soluble PI gate dielectrics for flexible electronic devices with high device performance.

(Invited) A New Approach to Multifunctional Piezoelectric Thin Films for Flexible Device Applications

The objective of this research is to demonstrate the novel concept of enhancing the sensing capability and the long-term reliability of thin film piezoelectric devices via a low cost and simple MEMS-less processed flexible structure with our unique ultra-thin conductive metal foil substrate. The flexible thin metal foil substrate can act as a substrate and electrode without any process temperature limitation up to 750 oC. Piezoelectric films (PZT-based and environmentally friendly lead (Pb)-free KNN-based systems) constructed on ultra-thin flexible substrate enhance the piezoelectric performance because of the contributions resulting from the biaxial strain imposed by the substrate. These films show further improved output voltage and sensing capacity due to increased flexure of the device in comparison with piezoelectric devices integrated with more rigid substrates. Moreover, this flexible piezoelectric structure has a high degree of freedom with no shape limitation and is easy to laminate on any substrates including PCBs if necessary. In this research, we also suggest the key solutions for solving the current degradation mechanisms such as depolarization, temperature, voltage, and stress limitations, and other aging effects by systematic investigation from materials to structure designs. The prediction and the solution for degradation mechanism and failure mode will be explained in simple and clear terms. Our experimental results for piezoelectric materials and flexible devices propose to achieve the robust piezoelectric sensors and actuators that overcome the limitations of current state-of-the-art devices. Our suggested concept is envisioned as a commercially viable structure since the devices can be constructed by simple and mass-producible process.

(Invited) Fully Formed Reverse Fabrication Techniques for Flexible Electronics

Recent progress in the development of the flexible and stretchable systems with various materials strategies establishes a fundamental basis for a high-performance electronics on the soft and ultrathin bendable elastomeric deformable substrates. The resulting enabled type of functionality cannot be reproduced using conventional wafer-based technologies. These types of electronics on the unusual soft substrates relies on the techniques of either top-down approaches or bottom-up approaches. These approaches allow assemblies of ultra-thin electronics onto various curvilinear shapes, with linear elastic mechanical responses to large strain deformations. However, such electronic devices fabricated using these techniques suffers from the poor device performances compared to that of the conventional rigid substrate based electronics. Here, we report a new technology in flexible electronics based on a new fabrication technique called, “Fully Formed Reverse Device Fabrication.” The key fabrication strategies and examples of various device characteristic allow integration of high-performance electronics with ultra-thin flexible substrates.

Ultrathin flexible piezoelectric sensors for monitoring eye fatigue

Eye fatigue is a symptom induced by long-term work of both eyes and brains. Without proper treatment, eye fatigue may incur serious problems. Current studies on detecting eye fatigue mainly focus on computer vision detect technology which can be very unreliable due to occasional bad visual conditions. As a solution, we proposed a wearable conformal in vivo eye fatigue monitoring sensor that contains an array of piezoelectric nanoribbons integrated on an ultrathin flexible substrate. By detecting strains on the skin of eyelid, the sensors may collect information about eye blinking, and, therefore, reveal human’s fatigue state. We first report the design and fabrication of the piezoelectric sensor and experimental characterization of voltage responses of the piezoelectric sensors. Under bending stress, the output voltage curves yield key information about the motion of human eyelid. We also develop a theoretical model to reveal the underlying mechanism of detecting eyelid motion. Both mechanical load test and in vivo test are conducted to convince the working performance of the sensors. With satisfied durability and high sensitivity, this sensor may efficiently detect abnormal eyelid motions, such as overlong closure, high blinking frequency, low closing speed and weak gazing strength, and may hopefully provide feedback for assessing eye fatigue in time so that unexpected situations can be prevented.

Flexible small-channel thin-film transistors by electrohydrodynamic lithography.

Small-channel organic thin-film transistors (OTFTs) are an essential component of microelectronic devices. With the advent of flexible electronics, the fabrication of OTFTs still faces numerous hurdles in the realization of highly-functional, devices of commercial value. Herein, a concise and efficient procedure is proposed for the fabrication of flexible, small-channel organic thin-film transistor (OTFT) arrays on large-area substrates that circumvents the use of photolithography. By employing a low-cost and high-resolution mechano-electrospinning technology, large-scale micro/nanofiber-based patterns can be digitally printed on flexible substrates (Si wafer or plastic), which can act as the channel mask of TFT instead of a photolithography reticle. The dimensions of the micro/nanochannel can be manipulated by tuning the processing parameters such as the nozzle-to-substrate distance, applied voltage, and fluid supply. The devices exhibit excellent electrical properties with high mobilities (∼0.62 cm2 V-1 s-1) and high on/off current ratios (∼2.47 × 106), and they are able to maintain stability upon being bent from 25 mm to 2.75 mm (bending radius) over 120 testing cycles. This electrohydrodynamic lithography-based approach is a digital, programmable, and reliable alternative for easily fabricating flexible, small-channel OTFTs, which can be integrated into flexible and wearable devices.

Analytical investigation on thermal-induced warpage behavior of ultrathin chip-on-flex (UTCOF) assembly

The serious warpage issues of ultrathin chip-on-flex (UTCOF) assembly induced by mismatched thermal stresses have greatly affected the mechanical stability and reliability of emerging ultrathin chip packaging technology. Currently, a theoretical prediction as a convenient and straightforward approach is still lacked for describing effectively the thermal-mechanical behavior of UTCOF during the adhesive curing and cooling process. In consideration of the adhesive thickness approximating to ultrathin chip and flexible substrate thickness, we develop a layerwise-model of ultrathin chip-adhesive-flex structure under plain strain condition, where the behavior of thick adhesive bonding can be described precisely through increasing the subdivided mathematical plies. Further, the analytical results show that the concave and convex forms of ultrathin chip warpage yield at the end of the curing and cooling process respectively. Meanwhile, the effects of its structure dimensions and material properties are also revealed for discussing a way to relieve the extent of ultrathin chip warpage. Additionally, in order to verify the validity of the theoretical prediction, we also introduce the corresponding numerical technique and experimental method. These results suggest that a kind of rigid and ultrathin flexible substrate such as metal foil should be adopted for small warpage of ultrathin assembly.

Skin‐Like Oxide Thin‐Film Transistors for Transparent Displays

Flexible transparent display is a promising candidate to visually communicate with each other in the future Internet of Things era. The flexible oxide thin‐film transistors (TFTs) have attracted attention as a component for transparent display by its high performance and high transparency. The critical issue of flexible oxide TFTs for practical display applications, however, is the realization on transparent and flexible substrate without any damage and characteristic degradation. Here, the ultrathin, flexible, and transparent oxide TFTs for skin‐like displays are demonstrated on an ultrathin flexible substrate using an inorganic‐based laser liftoff process. In this way, skin‐like ultrathin oxide TFTs are conformally attached onto various fabrics and human skin surface without any structural damage. Ultrathin flexible transparent oxide TFTs show high optical transparency of 83% and mobility of 40 cm2 V−1 s−1. The skin‐like oxide TFTs show reliable performance under the electrical/optical stress tests and mechanical bending tests due to advanced device materials and systematic mechanical designs. Moreover, skin‐like oxide logic inverter circuits composed of n‐channel metal oxide semiconductor TFTs on ultrathin, transparent polyethylene terephthalate film have been realized.

Fuzzy Control Scheme for Transportation of Ultra-Thin Flexible Substrates with Variable-Section

This paper proposes a fuzzy control scheme to achieve the high-precision and high-speed continuous transportation of ultra-thin flexible substrates with variable-section. A tension control model is presented based on the torque balance of pinches. In the model, the flexible substrate is regarded as a parallel spring-damp system, challenged by the dynamics associated with low-tension operation, besides the time-variation resulted by the variable-section. A fuzzy control algorithm is designed for the tension controller. Finally, the analysis in dynamic performance of the whole system is done using MATLAB toolbox. The simulation results show that the fuzzy control algorithm presented is effective to the tension even in transient state.

Rework of Lead-Free Area Array Packages Assembled on Ultrathin Flexible Substrates

The introduction of stacking technology has enabled the memory industry to cope with the continuous demand for higher performance and greater capacity. Among other variants of stacking, module-level stacking is gaining steady popularity and acceptance as an option to fulfill this need. One of the approaches for module-level stacking is to fold a double-sided flexible substrate-based assembly around a metal core. Carrying out rework on a flexible substrate poses several unique challenges. Moreover, the tightly spaced assembly of lead-free area array devices in module stacking further increases rework complexity. As a result, the rework process of area array devices assembled on flexible substrate needs to be carefully controlled. This paper describes the attempts to develop a process for reworking area array devices on ultrathin flexible printed circuit boards (PCBs) with a tight component-to-component spacing of 0.38 mm. The research addresses several different aspects related to the rework of area array components on an ultrathin flexible PCB. This paper has been divided into two sections. The first section deals with initial screening experiments which include designing a rework fixture, development of thermal profiles, and the selection of a site-redressing technique. The second section of this paper evaluates different reflow methods, namely a rework station and a convection oven. Locations of the rework devices as well as the options of component attachment methods are incorporated into the experimental design. Based on the experiments performed and results obtained, a recipe for successful rework is presented. The key concerns and observations related to the issues and challenges associated with the rework process are also presented.

Analysis of dynamic magnetics and high effective permeability of exchange-biased multilayers fabricated onto ultrathin flexible substrates

A detailed investigation of the influence of substrate thickness on the static and dynamic properties of exchange-biased FeCo/MnIr multilayers fabricated onto flexible substrates by sputtering technique was performed together with the analyses based on Landau–Lifshitz–Gilbert equation. Reducing the substrate thickness to lower than 3 μm significantly enhances the exchange bias field which is attributed to the deformation of the substrates creating more pinned antiferromagnetic spins at the interface. The effect of substrate thickness on effective magnetization, effective damping coefficient, frequency linewidth, ferromagnetic resonance frequency, and static permeability is also discussed. The observed difference between static and dynamic magnetic anisotropies is attributed to the contribution of rotatable magnetic anisotropy presumably arising from the changes in the antiferromagnetic domain which also results in the enhancement of coercivity. It is evidently suggested that by using ultrathin flexible substrates one can obtain high effective permeability which is useful for microwave applications.

Design and optimization of an ultra thin flexible capacitive humidity sensor

In this paper we present the design and fabrication of a humidity sensor on ultra thin (8 μm) flexible polyimide substrate. The ultra thin flexible substrate can be preserved also when a read-out electronic interface is integrated by using Polycrystalline Silicon Thin Film Transistors technology. The sensor device is a capacitor where a thin layer of [bis(benzo cyclobutene)] is used as a dielectric sensitive material between two metal electrodes. The electrode layout has been designed with the aid of numerical simulations in order to optimize the sensor performances. The fabricated sensor has shown sensitivity to relative humidity of 0.38%/RH% and a linearity of 0.996 in the range of 10–90 RH%. Furthermore, measurements regarding the sensor response time, different bending and bias voltage effects have been performed.