Ultrathin Polyimide Substrate Research Articles

Flexible microwave metasurface absorber with reconfigurable absorption intensity in ultra-wideband

This article proposes a flexible metasurface absorber (MA) with reconfigurable absorption intensity in an ultra-wideband spectrum. Due to the utilization of an ultrathin polyimide substrate, the MA is endowed with superior flexibility over previous rigid designs. The pattern of the MA consists of a fourfold symmetrical metal strip and four PIN diodes, which simultaneously activate multiple resonances to generate a wide operating band. By adjusting the voltage of the PIN diodes embedded in the strips, the MA can execute various stages of reconfigurable absorption intensity through varying states of ohmic loss. When the PIN diodes are off, MA is switched to a reflector. In addition, the results of the equivalent circuit simulation confirm the feasibility of the design. Finally, an MA sample is fabricated and measured, and good agreement is obtained between the measured and simulated results. The MA achieves a reconfigurable absorption intensity varies from −16.8 to 0 dB over an ultra-wide frequency range of 5.6–16.98 GHz, with relative bandwidth up to 100.8%. As expected, the designed MA features excellent characteristics both in plane and curved states.

Needle-integrated ultrathin bioimpedance microsensor array for early detection of extravasation

Extravasation is a common complication during intravenous therapy in which infused fluids leak into the surrounding tissues. Timely intervention can prevent severe adverse consequences, but early detection remains an unmet clinical need because existing sensors are not sensitive to leakage occurring in small volumes (< 200 μL) or at deep venipuncture sites. Here, an ultrathin bioimpedance microsensor array that can be integrated on intravenous needles for early and sensitive detection of extravasation is reported. The array comprises eight microelectrodes fabricated on an ultrathin and flexible polyimide substrate as well as functionalized using poly(3,4-ethylenedioxythiophene) and multi-walled carbon nanotubes. Needle integration places the array proximity to venipuncture site, and functional coating significantly reduces interface impedance, both enable the microsensors with high sensitivity to detect early extravasation. In vitro and in vivo experiments demonstrate the capability of the microsensors to differentiate various intravenous solutions from different tissue layers as well as identify saline extravasation with detection limit as low as 20 μL.

Experimental demonstration of ultrathin broken-symmetry metasurfaces with controllably sharp resonant response

Symmetry-protected resonances can be made to couple with free space by introducing a small degree of geometric asymmetry, leading to controllably sharp spectral response. Here, we experimentally demonstrate a broken-symmetry metasurface for the technologically important low millimeter wave spectrum. The proposed metasurface is fabricated on an ultrathin polyimide substrate, resulting in a low loss and flexible structure. Measurements inside an anechoic chamber experimentally verify the theoretically predicted sharp spectral features corresponding to quality factors of several hundreds. The demonstrated sharp response is also observed with the complementary structure, which responds to the orthogonal linear polarization (Babinet's principle). The designed metasurfaces can be exploited in diverse applications favored by a controllably sharp spectral response, e.g., filtering, sensing, switching, and nonlinear applications, in either reflection or transmission mode operation. More generally, the demonstrated fabrication process provides a generic platform for low-cost, large-scale engineering of metasurfaces with minimal substrate-induced effects.

Armalcolite Nanocomposite: A New Paradigm for Flexible Capacitive Humidity Sensor

In this paper, a unique Armalcolite-PDMS-based flexible capacitive humidity sensor is designed by using spin coating technique, which uses interdigitated gold (Au) electrode and ultrathin ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 80~\mu \text{m}$ </tex-math></inline-formula> ) flexible polyimide substrate. The structural and morphological properties of the Armalcolite nanocomposite are characterized by X-ray diffraction (XRD) and SEM. The electrical characterization of the Armalcolite-PDMS-based flexible capacitive humidity sensor is performed at 25 °C, followed by relative humidity ranging from 33% to 95% and frequency varying 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Hz to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> Hz. The experimental investigation reveals a fast response time (8.53 s) and recovery time (11.25 s). The developed sensor exhibits a reversible response with ultra-low hysteresis of 0.92%. The sensor response is recorded for 30 days to confirm its long-term stability. The sensing mechanism is studied using impedance plots obtained at different humidity environments. The excellent sensing performance of the developed sensor could be beneficial for the researchers working in the area of flexible humidity sensors.

Investigation of the Chemical Structure of Ultra-Thin Polyimide Substrate for the Xenon Flash Lamp Lift-off Technology.

Lift-off is one of the last steps in the production of next-generation flexible electronics. It is important that this step is completed quickly to prevent damage to ultrathin manufactured electronics. This study investigated the chemical structure of polyimide most suitable for the Xe Flash lamp–Lift-Off process, a next-generation lift-off technology that will replace the current dominant laser lift-off process. Based on the characteristics of the peeled-off polyimide films, the Xe Flash lamp based lift-off mechanism was identified as photothermal decomposition. This occurs by thermal conduction via light-to-heat conversion. The synthesized polyimide films treated with the Xe Flash lamp–Lift-Off process exhibited various thermal, optical, dielectric, and surface characteristics depending on their chemical structures. The polyimide molecules with high concentrations of –CF3 functional groups and kinked chemical structures demonstrated the most promising peeling properties, optical transparencies, and dielectric constants. In particular, an ultra-thin polyimide substrate (6 μm) was successfully fabricated and showed potential for use in next-generation flexible electronics.

Improving the performance of ultra-flexible perovskite photodetectors through cation engineering

High performance ultra-flexible lightweight perovskite photodetectors have promising applications on a variety of substrates, including those used on unmanned aerial vehicles. Herein, we report on a facile method to improve the performance of perovskite photodetectors on flexible ultrathin (50 μm) polyimide substrates by tuning the composition with different ratios of cations. In the presence of lead thiocyanate, the introduction of a certain amount of formamidinium could sufficiently enhance the quality of methylammonium lead triiodide perovskite film, with prolonged carrier lifetime. A perovskite photodetector with a response time of ton = 89 ms and toff = 47 ms is achieved with a very simple device architecture. In addition, our flexible photodetectors maintain good performance after rigorous folding and bending, demonstrating excellent mechanical properties, which could be very useful for a variety of applications.

Solution‐Processed, Photo‐Patternable Fluorinated Sol–Gel Hybrid Materials as a Bio‐Fluidic Barrier for Flexible Electronic Systems

AbstractReports have recently been published on ultrathin biofluid barriers, which enable the long‐term measurement of biological signals and exhibit conformability on nonlinear surfaces such as skin and organs. However, inorganic‐ and organic‐based barriers have process incompatibility and high water permeability, respectively. Siloxane‐ (inorganic) based fluorinated epoxy (organic) hybrid materials (FEH) are demonstrated for bio‐fluidic barrier and the biocompatibility and barrier performance for flexible electronic systems as solution‐processed oxide thin‐film transistors (TFTs) on 1.2 µm thick polyimide (PI) thin film substrate is confirmed. FEH thin film can be patterned as small as 10 µm through conventional photolithography. The fabricated solution‐processed indium oxide TFTs with FEH barriers exhibit durable performance over 16 h with no dramatic change of transfer characteristics in phosphate‐buffered saline (PBS) environment. Furthermore, to realize FEH barriers for flexible systems, the solution‐processed indium oxide TFTs with FEH barriers on ultrathin PI substrate are demonstrated subjected to compression test and successfully measure the electrical properties with no irreversible degradation during 1000 cycles of mechanical testing in PBS.

Flexible wideband power divider with high isolation incorporating spoof surface plasmon polaritons transition with graphene flake

In this paper, we present a design scheme of a wideband power divider structure with excellent flexibility and high isolation. The structure consists of a gradual transition from H-shaped spoof surface plasmon polaritons (SSPPs) transmission line (TL) to a pair of U-shaped SSPPs TLs on the ultrathin Polyimide substrate. A piece of graphene flake is loaded between the two U-shaped SSPPs TLs as an isolation resistive film. We experimentally demonstrate that the flexible broadband power divider can achieve equal power dividing with limited loss, good matching and high isolation covering 6.8–11.2 GHz under the flat, severely bent and twisted conditions.

A 340-GHz thin film polarisation converter

ABSTRACTThis paper introduces a 340-GHz polarisation converter. The polarisation converter consists of a bilayer metamaterial, which is processed by the flexible thin film lithography method. Two-dimensional metamaterial offer such advantages over traditional quarter-wave-plates as compactness, low profile and simple fabrication process, but the insertion loss is comparably high. In order to lower the insertion loss, the 10-μm ultrathin polyimide substrate and a bilayer structure separated by a quarter wavelength air impedance match layer are adopted. After the horn antenna’s far-field test by vector network analyser and the Gaussian beam test based on Terahertz time-domain spectroscopy (THz-TDS), the results show that in the band of 325–350 GHz, the transmission loss is less than 2 dB, and the axial ratio is less than 3 dB; besides, simulation and test insertion loss performance difference is less than 1 dB, which indicates that the method we adopt is effective. In addition, based on transmission line theory, we analyse its working principle and the influence from the fabrication error.

Highly flexible, foldable, and rollable microsupercapacitors on an ultrathin polyimide substrate with high power density

The design and functionality of extremely flexible, foldable, and rollable microsupercapacitors (MSCs) with in-plane interdigital electrodes that consist of single-walled carbon nanotube (SWCNT) networks on an ultrathin polyimide substrate are demonstrated through experiments and finite element simulations. The all-solid-state MSCs can be reversibly bent, folded, and rolled purely elastically without degradation of their electrical performance. The simulation results confirm that the deformation in bent, folded, and rolled MSCs is purely elastic. The high power density (1125 W cm–3) and small time constant (1 ms) of the present MSCs are comparable to those of aluminum electrolytic capacitors. The MSCs operate at scan rates of up to 1000 V s–1, are characterized by a volumetric capacitance of 18 F cm–3 and an energy density of 1.6 mWh cm–3, and exhibit superior electrochemical stability with 96% capacity retention even after 100,000 charge/discharge cycles. The developed MSCs demonstrate high potential for integration in flexible and wearable electronic systems.

Flexible Film Bulk Acoustic Wave Filters toward Radiofrequency Wireless Communication.

This paper presents a flexible radiofrequency filter with a central frequency of 2.4 GHz based on film bulk acoustic wave resonators (FBARs). The flexible filter consists of five air-gap type FBARs, each comprised of an aluminum nitride piezoelectric thin film sandwiched between two thin-film electrodes. By transfer printing the inorganic film structure from a silicon wafer to an ultrathin polyimide substrate, high electrical performance and mechanical flexibility are achieved. The filter has a peak insertion loss of -1.14 dB, a 3 dB bandwidth of 107 MHz, and a temperature coefficient of frequency of -27 ppm °C-1 . The passband and roll-off characteristics of the flexible filter are comparable with silicon-based commercial products. No electrical performance degradation and mechanical failure occur under bending tests with a bending radius of 2.5 mm or after 100 bending cycles. The flexible FBAR filters are believed to be promising candidates for future flexible wireless communication systems.

Letter : Solution-processed flexible zinc-tin oxide thin-film transistors on ultra-thin polyimide substrates

In this letter, solution-processed flexible zinc-tin oxide (Z0.35T0.65O1.7) thin-film transistors with electrochemically oxidized gate insulators (AlOx:Nd) fabricated on ultra-thin (30 µm) polyimide substrates are presented. The AlOx:Nd insulators exhibited wonderful stability under bending and excellent insulating properties with low leakage current, high dielectric constant, and high breakdown field. The device exhibited a mobility of 3.9 cm2/V · s after annealing at 300 °C. In addition, the flexible device was able to maintain the electricity performance under various degrees of bending, which was attributed to the ultra-thin polyimide substrate.

Fully Printed Foldable Integrated Logic Gates with Tunable Performance Using Semiconducting Carbon Nanotubes

The realization of large‐area and low‐cost flexible macroelectronics relies on both the advancements in materials science and the innovations in manufacturing techniques. In this study, extremely bendable and foldable carbon nanotube thin film transistors and integrated logic gates are fabricated on a piece of ultrathin polyimide substrate through an ink‐jet‐like printing process. The adoption of a hybrid gate dielectric layer consisting of barium titanate nanoparticles and poly(methyl methacrylate) has led to not only excellent gating effect but also superior mechanical compliance. The device characteristics show negligible amount of change after up to 1000 cycles of bending tests with curvature radii down to 1 mm, as well as very aggressive folding tests. Additionally, the electrical characteristics of each integrated logic gate can be tuned and optimized individually by using different numbers of carbon nanotube printing passes for different devices, manifesting the unique adaptability of ink‐jet printing as a digital, additive, and maskless method. This report on fully printed and foldable integrated logic gates represents an inspiring advancement toward the practical applications of carbon nanotubes for high‐performance and low‐cost ubiquitous flexible electronics.

Flexible Sensors Based on Low-Temperature Polycrystalline Silicon Thin Film Transistor Technology

Flexible sensors are gaining increasing interest in a number of applications, including biomedical, food control, domotics and robotics, having very light weight, robustness and low cost. In order to improve signal-to-noise ratio, integration of readout electronics is crucial and several technologies are available for the fabrication of thin film transistor (TFTs) based circuits on flexible substrates. Among these technologies, the low temperature polycrystalline silicon (LTPS) is particularly attractive, since LTPS TFTs show excellent electrical characteristics, good stability and offer the possibility to exploit CMOS architectures. The different aspects for the direct fabrication of LTPS TFTs on polymer substrates are reviewed and the specific fabrication process adopted on ultrathin polyimide (PI) substrates is described in some detail. In particular, LTPS TFTs were fabricated on ultra-thin PI substrates, deposited by spin-coating on a Si wafer, and excimer laser annealing was used to crystallize the Si. Characteristics electrical parameters are: field effect mobility up to 50 cm2/Vs; threshold voltage of 7 V; on/off ratio >105 . Then, as examples of flexible sensors, we present a tactile sensor for robotic applications and a pH sensor for biomedical applications.The tactile sensors were designed according to the Piezoelectric Oxide Semiconductor Field Effect Transistor (POSFET) structure, enabling sensing and signal conditioning at same site. In this way, it is possible to improve signal to noise ratio and hence the force sensitivity. In our LTPS POSFET devices we adopted as piezoelectric film the polyvinyledenedifluoride –trifluoroethylene P(VDF-TrFE) polymer. The piezoelectric polymer generates a charge on application of a mechanical force which influences the gate bias of the transistor. Gate bias variations, in turn, are directly reflected into drain current variations, that can be further processed by electronic circuits. The measured response to applied forces is linear, as reported in Fig 1a, with a piezoelectric coefficient up to 47 pC/N. The frequency response was tested in the range 30 - 1200 Hz and presented a high-pass behavior (see Fig.1b), according to the adopted common-source bias configuration.The pH sensor was fabricated according to the extended gate structure, as this structure offers many advantages over the conventional ISFET, such as the low cost, simple passivation and package, insensitivity of temperature and light, flexibility of shape of the extended-gate structure, and better long-term stability. The sensitive layer of the extended gate is a nanostructured ZnO film selectively deposited on the Al-gate electrode in a chemical bath at low-temperature. The pH-sensitivity was tested by exposing the extended gate to different pH-solutions and using as a reference electrode an Ag/AgCl electrode. In Fig. 2 the transfer characteristics, measured at low Vds and for different pH-solutions, are reported. In the inset of Fig.2, the threshold voltage shift induced by the pH variation is shown, giving a slope of 59 mV/pH for the pH range 1 – 9, close the ideal Nernstian response. The present results can pave the way to advanced flexible sensing systems, where sensors and local signal conditioning circuits can be integrated on the same flexible substrate.

Flexible sensing systems based on polysilicon thin film transistors technology

Flexible sensors are gaining increasing interest in a number of applications, including biomedical, food control, domotics and robotics, having very light weight, robustness and low cost. In order to improve signal-to-noise ratio, integration of readout electronics is crucial and several technologies are available for the fabrication of thin film transistors (TFTs) based circuits on flexible substrates. Among these technologies, the low temperature polycrystalline silicon (LTPS) is particularly attractive, since LTPS TFTs show excellent electrical characteristics, good stability and offer the possibility to exploit CMOS architectures. The different aspects for the direct fabrication of LTPS TFTs on polymer substrates are reviewed and the specific fabrication process adopted on ultrathin polyimide substrates is described in some detail. Then, as examples of flexible sensing systems, we present both chemical and physical sensors integrated with LTPS TFTs frontend electronics. The present results can pave the way to advanced flexible sensing systems, where sensors and local signal conditioning circuits can be integrated on the same flexible substrate.

Flexible PVDF-TrFE pyroelectric sensor driven by polysilicon thin film transistor fabricated on ultra-thin polyimide substrate

In this work we present a flexible pyroelectric sensor composed by a PVDF-TrFE capacitor realized on ultra-thin polyimide film (5μm thick), integrated with a n-channel low temperature polysilicon thin film transistor also fabricated on ultra-thin polyimide (8μm thick). Exploiting a multi-foil approach, the pyroelectric capacitors and the transistors were attached one over the other reaching a final thickness of about 15μm. The bottom contact of the sensor capacitance was connected to the gate of the transistor by a silver ink, while, for bias and load resistances, we used external elements. The active sensor area was defined by a circular capacitor with a diameter of about 2mm. In order to enhance PVDF-TrFE pyroelectric properties, an external stepwise voltage was applied to the structure up to values of 160V at a temperature of about 80°C. The devices were then tested, at different working frequencies (up to 800Hz) under a specific infrared radiation provided by a He–Ne laser, with a wavelength of 632nm and maximum power of 5mW. An output signal of tens of millivolt was observed at 10Hz, exploiting the pre-amplification of polysilicon thin film transistor.

Flexible sensorial system based on capacitive chemical sensors integrated with readout circuits fully fabricated on ultra thin substrate

In this paper we present the design and fabrication of a fully flexible sensorial system, composed of three different sensor units implemented on an ultrathin polyimide substrate of 8 μm thick. Each unit is composed by a capacitive chemical sensor integrated with readout electronics. The sensors are parallel plate capacitors with the top electrode properly patterned to allow analytes diffusion into the dielectric that acts as chemical interactive material. Three different polymers, poly(tetrafluoroethene) (PTFE), poly(methyl 2-methylpropenoate) (PMMA) and benzocyclobutene (BCB), were used as dielectrics. A ring oscillator circuit, implemented with polysilicon thin film transistors (PS-nTFT), was used to convert the capacitance variations into frequency shifts. The electronic tests show oscillating frequencies of about 211 ± 2 kHz and negligible frequency shifts under different bending radius conditions. Furthermore, system response to some alcohols concentrations (Methanol, ethanol, 1-butanol, and 1-propanol) is reported and data analysis proves that the system is able to discriminate methanol from ethanol.

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.

Mechanical Reliability of a Segmented Silicon Layer on Ultra-Thin Polyimide Substrates

Substrate transfer technology for SOI and non-SOI single-crystalline silicon wafers was demonstrated allowing for high-performance low-power RF applications. 3D deformable electronics could be realized by vertically thinning and laterally partitioning of the silicon substrate on sub-millimeter scale. By varying the partition dimensions and the geometry of connecting bridges, the level of acceptable deformations can be controlled. The targeted applications of this technology are wireless ID tags and sensor networks. The mechanical properties such as crack, interfacial delamination are critical to reach the flexible substrate. In this contribution, results of our work on mechanical reliability issues of poly- and single crystalline silicon on ultra-thin polyimide substrates are presented. To improve reliability, square and hexagonal segmentation with different size is applied to the silicon layer before it is transferred onto an ultra-thin polyimide substrate using wafer-to-wafer substrate transfer technique based on a temporary glass carrier. Generation of cracks within the silicon and dielectric layers is then studied under controlled bending and tensile loads. The formation of cracks is studied experimentally using specially for this purpose designed bending and tensile tools. Ultra-thin interfacial delamination are also focused in this work by experimental and FE simulation method. A new test setup is designed for mixed mode bending testing which has capacity to observation specimen and recording the crack length and crack opening by microscope. The critical energy release depended on mixed angle can be reach by combination experimental data with FEM simulation.