In-process surface metrology for thin film flexible electronic devices

Recently, metal nanoparticle (NP) based ink solutions are widely used for fabricating flexible electronic devices due to many advantages such as low temperature process, simple and low-cost manufacturing, and compatibility with various flexible substrates. The electromechanical characteristics of metal NP thin films on flexible substrates are very important because they often operate under various mechanical loading conditions. Here, we report an improvement of electromechanical properties of silver (Ag) NP thin films by composite formation with short Ag nanowires (Ag NWs). We fabricated Ag NP-Ag NW hybrid thin films on the flexible polymer substrates, which presented more reliable electromechanical characteristics than the Ag NP thin film. From in situ scanning electron microscope images, we observed a bridging behavior of Ag NWs inhibiting the growth of cracks on the surface as well as in the inside of hybrid thin films. The improvement of electromechanical characteristics by composite formation with short Ag NWs could be also observed in the microscale line patterns, both in the static and cyclic tensile loading conditions.

This paper aims to present the simulation work and obtain optimized parameters for the development of drop-on-demand electrohydrodynamic jet (DoD E-Jet) to print control and stable micro-structures on a flexible insulating substrate. In this work, the novel comparison of three types of combination needle structures was developed based on the multiphase flow (liquid–air) technique, in order to achieve optimal printing conditions for a flexible PET substrate. According to simulation results, steel-quartz needle combination provides very unique compensations in the controllability and stability of E-Jet. Printing on a flexible substrate was challenging, but parameters used in simulation validate the possibilities for DoD E-Jet printing method. Optimize working parameters were achieved by the numerical simulation executed to generate developed and stable E-Jet morphology. In addition, various stable and uniform microscale droplets and structures were directly printed on a flexible polymer substrate with the help of collective impact of electrical force, viscous force, and internal pressure force throughout DoD E-Jet printing process. The results of numerical simulation and experimental work exhibited an excellent and promising E-Jet printing tool for flexible electronic systems.

By today, die bonding on flexible circuit boards is only possible with adhesive connections or can be realized in combination by rigid-flex technology via soldering. The flex technology has a big potential with regard to new application fields such as automotive interiors (e.g. new lighting concepts, integration of electronics as functional components of the equipment), autonomous driving (new camera and sensor concepts, radar, car-to-x communication), building integration (e.g. smart home integration of functional electronic assemblies in building walls) but also medical technology and textile electronics. The application areas and the reliability of assemblies with the use of adhesive technology minimizes the application areas of flex technology. Great opportunities to new reliable polymer-based electronics could, however, be made up by transferring a low-temperature silver sintering process from power electronics to the microelectronic field. But the transfer of this bonding process also contains the risk of electrochemical failure processes (e.g. due to residual moisture in flexible polymer substrates, Ag migration, acceleration of failure modes due to mobile ions). In our studies we analyzed electrochemical corrosion and migration processes at silver sintered flexible substrates. Therefore, in a first step, we investigated technical processes to produce reliable low temperature silver sintered systems at polyimide based flexible substrates. Afterwards the silver sintered systems were tested by biased humidity testing and salt spray testing. The materials, surfaces and reaction products were investigated by high resolution microstructural and physical methods like $\mu$ XRF and SEM-EDS. The results achieved by this work give a good overview for the possibilities and limits of pressureless silver sintered systems on flexible substrates.

Electrochromism refers to the phenomenon where optical changes in a material occur during electrochemical reactions under an applied voltage. Because of advantages such as high contrast and low operating voltage, electrochromic (EC) devices have been widely used in smart windows, switchable mirrors, optical shutters, and information displays.Such devices are usually fabricated on a rigid glass substrate to permit their use in automobile and building applications, which require long-term stability under climatic conditions. However, the use of soft polymer or textile substrates has increased the versatility of such technology, allowing the fabrication of novel flexible, stretchable, and wearable EC devices. Since the first flexible EC devices using WO3layers were developed in 1995 a wide variety of inorganic and organic materials has been investigated with respect to their compatibility with flexible substrates. However, most EC devices fabricated using these materials showed poor performances, especially in terms of coloration speed and long-term stability, which was mainly due to the electrical and mechanical properties of the flexible substrate. In addition, the large area and cost-effective roll-to-roll (RTR) coating process for transparent electrodes has been demonstrated for the fabrication of low cost flexible EC devices. To date, the most common transparent electrode for EC devices is indium-tin-oxide (ITO) due to its low resistivity, high transparency, and mature mass production via processes such as sputtering. Amorphous ITO(a -ITO) films can easily be coated on a flexible substrate using conventional DC sputtering at ambient temperatures, but the high sheet resistance and brittleness of thick a-ITO films prevents their use for flexible and transparent EC devices. Oxide-metal-oxide(OMO) multilayer films have been extensively reported in flexible optoelectronic devices as an alternative to a-ITOfilms. However, there are few reports regarding flexible and transparent EC devices fabricated using OMO multilayer electrodes. The development of a conductive layer with high electrical conductivity and stable mechanical flexibility is greatly needed for flexible EC device applications. In this paper, we report the electrochromic properties of poly(3-hexylthiophene) (P3HT) films coated on ITO/Ag/ITO (IAI) multilayers deposited on flexible PET substrates using an ambient temperature RTR sputtering system. By continuous sputtering of ITO/Ag/ITO layers on the flexible PET substrate, we realized highly transparent, flexible, and conductive electrodes for flexible and transparent EC devices. We characterized the coloration efficiency, response time, and long-term stability of the P3HT on the flexible substrate compared to the material prepared on glass substrates.

A facile process combined with inkjet printing, surface modification and electroless deposition to fabricate adhesion-enhanced copper patterns on flexible polymer substrates for functional flexible electronics

Probing the wear characteristics of graphene on flexible polymer substrates using a heated atomic force microscopy tip

Flexible electronics have been of great interest in the past few decades for their wide-ranging applications in health monitoring, human-machine interaction, artificial intelligence, and biomedical engineering. Currently, transfer printing is a popular technology for flexible electronics manufacturing. However, typical sacrificial intermediate layer-based transfer printing through chemical reactions results in a series of challenges, such as time consumption and interface incompatibility. In this paper, we have developed a time-saving, wafer-recyclable, eco-friendly, and multiscale transfer printing method by using a stable transferable photoresist. Demonstration of photoresist with various, high-resolution, and multiscale patterns from the donor substrate of silicon wafer to different flexible polymer substrates without any damage is conducted using the as-developed dry transfer printing process. Notably, by utilizing the photoresist patterns as conformal masks and combining them with physical vapor deposition and dry lift-off processes, we have achieved in situ fabrication of metal patterns on flexible substrates. Furthermore, a mechanical experiment has been conducted to demonstrate the mechanism of photoresist transfer printing and dry lift-off processes. Finally, we demonstrated the application of in situ fabricated electrode devices for collecting electromyography and electrocardiogram signals. Compared to commercially available hydrogel electrodes, our electrodes exhibited higher sensitivity, greater stability, and the ability to achieve long-term health monitoring.

Cold spray-based rapid and scalable production of printed flexible electronics

Mitigating early fracture of amorphous metallic thin films on flexible substrates by tuning substrate roughness and buffer layer properties

Organic light emitting diode (OLED) based displays are well suited for fabrication on flexible polymer substrates. In order to achieve the vision of a highly flexible, rollable display, significant progress must be made in understanding and overcoming the mechanical limitations of the component materials. Methodologies and metrics for evaluating thin film components as well as full devices on flexible substrates need to be developed and standardized. A new technique for bend testing is described here and compared to a more traditional technique. One thin film component of flexible OLEDs that must be very mechanically robust is the permeation barrier. It is desirable to have a rapid method to characterize the onset of cracking upon bending of a barrier-coated substrate. We have developed a convenient method that allows characterization of mechanical cracking of flexible permeation barriers. In addition, transparent conductors with mechanical properties superior to those of ITO are desired for improved flexibility of displays. We report dramatically improved mechanical properties of multilayer ITO-Ag-ITO films that also exhibit excellent electrical and optical properties. Finally, we show preliminary results from bend tests of complete OLED devices on polymer substrates. The results demonstrate that in addition to evaluating component materials, other factors must be considered for integrated devices, and further investigation on this subject is needed.

Formation and characterization of silicon films on flexible polymer substrates

The role of the applied potential to the electrodeposition process of ZnO onto flexible polymeric substrates was investigated. ZnO nanostructures were prepared on indium-tin oxide conducting flexible substrates, using the electrodeposition method at 70°C in a zinc nitrate electroplating bath for different deposition potential, Edep, which has been divided in three distinct regions named as A, B and C. Chronoamperometric analysis reveals that the nucleation growth mechanism is strongly affected by the deposition conditions. The analysis of the current–time transients indicates that the ZnO electrodeposition occurs by progressive three-dimensional nucleation with diffusion-controlled growth in region B, −1000 < Edep< −1200 mV. For −700 < Edep < −1000 mV (region A), it is assumed that the electrodeposition process is mainly controlled by the slowest kinetic electroreduction of dissolved oxygen and nitrate ions resulting a low nucleation rate. Furthermore, in region C, related to more negative applied potentials, the electrodeposition process is conditioned by the substrate stability. The kinetic parameters values obtained from the diffusion controlled model are in good accordance to the dimensionless analysis of the transients. X-ray diffraction analysis confirmed the formation of (002) textured ZnO structures prepared in regions B and C.

Fabricating metallic wire grating inside a polymeric substrate by insertion nanoimprint

Characteristics of micro DMFCs array fabricated on flexible polymeric substrate

This study focuses on developing an efficient large-area graphene transfer method that combines high-throughput and precise laser engraving, simple polymer casting, and water immersion to fabricate conductive graphene and biodegradable polymer-based implantable flexible electronic devices. The low-temperature treatment of graphene sheets on a glass substrate reduced graphene sheet roughness and increased hydrophobicity, enabling facile and high-efficiency (∼100%) large-area graphene transfer to a flexible polymer substrate. This method also benefited from differences in the work of adhesion at the graphene sheet/glass substrate and the graphene sheet/flexible polymer substrate interfaces. The transferred graphene sheets showed stability, structural integrity, and high conductivity (∼40 Ω/sq sheet resistance) under in vitro and in vivo mimicking conditions. The low-temperature-treated and laser-engraved conductive graphene patterns, transferred on a flexible and biodegradable polymer substrate, demonstrated in vitro cytocompatibility on different cells. Two flexible electronic devices (1─a graphene coil-integrated electrode cuff and 2─an interdigitated graphene cuff-integrated piezoelectric device) were fabricated using the developed method, and both demonstrated functionality and proof of concept by generating output voltages that can enhance cell/tissue regeneration. In addition, the ease of handling, ex vivo implantation, and feasibility of suturing were demonstrated by performing implantation surgeries on the pudendal nerve in cadaveric rats. Overall, this promising large-area graphene transfer method can be used to fabricate biodegradable, implantable devices that can serve as interfaces to stimulate cells and tissues for regeneration and repair.