Fabrication and adhesion strength of Cu/Ni-Cr/polyimide films for flexible printed circuits

The adhesion strength of a Cu/Ni-Cr/polyimide flexible copper clad laminate (FCCL), was evaluated according to the thickness of the Ni-Cr (Ni:Cr = 95:5 ratio) seed layer using the 90° peel test. The changes in the morphology, chemical bonding and adhesion properties were characterized by SEM, AFM and XPS. The peel strength of the FCCL increased with increasing thickness of the Ni-Cr seed layer, due to the increase in the ion bombardment caused by the higher power used in the Ni-Cr sputtering process. This increase in the FCCL peel strength was attributed to the lower proportion of C-N bonds and higher proportion of C-O bonds in the polyimide surface. The adhesion strength between the metal and polyimide was mostly attributed to the chemical interaction between the metal layer and the functional groups of the polyimide.     

Fabrication and characterization of CdTe nano pattern on flexible substrates by nano imprinting and electrodeposition

Nano imprinting technology and the electrodeposition method were applied to make CdTe nano patterns on flexible substrates. An ammonia based aqueous solution was prepared at pH 10.7 and indium tin oxide (ITO)/polyethylene naphthalate (PEN) film with template was used as the working electrode. ITO thin film which was coated on PEN film had good electrical conductivity and optical transmittance. The template was manufactured by nano imprinting technology on ITO/PEN film. It was made from benzyl methacrylate and had nano rod arrays. It was used as the working electrode and for making CdTe nano pattern. CdTe nano pattern were analyzed by X-ray diffractometer, dual beam (DB)-focused ion beam (FIB)-scanning electron microscopy (SEM), Raman spectroscopy, and ultraviolet (UV)-visible (VIS) spectroscopy. The structure and optical properties of CdTe nano pattern on flexible substrates was analyzed. The crystalline size of nano pattern had 8.26 nm. The Te particles that were precipitated on CdTe surface seems to be exist. The absence of annealing process influenced to have low absorption coefficient and narrow band gap compared to bulk CdTe. However, nano pattern increased reflectance.     

A novel fabrication process of MEMS devices on polyimide flexible substrates

Micro-electro-mechanic-system (MEMS) devices on flexible substrate are important for non-planar and non-rigid surface applications. In this paper, a novel and cost-effective fabrication process for an 8 × 8 MEMS temperature sensor array with a lateral dimension of 2.5 mm × 5.5 mm on a polyimide flexible substrate is developed. A 40 μm thick polyimide substrate is formed on a rigid silicon wafer using as a mechanical carrier throughout the fabrication by four successive spin coating liquid polyimide. The arrayed temperature sensing elements made of 1200 Å sputtered platinum thin film on polyimide substrate show excellent linearity with a temperature coefficient of resistance of 0.0028/°C. The purposed sensor obtains a high sensitivity of 0.781 Ω/°C at 8 mA at constant drive current. Because of the low heat capacity and excellent thermal isolation, the temperature sensing element shows excellent high sensitivity and a fast thermal response. The finished devices are flexible enough to be folded and twisted achieving any desired shape and form. Employing spin-coated liquid polyimide substrate instead of solid polyimide sheet minimizes the thermal cycling as well as improves the production yield. This fabrication technique first introduces the spin-coated PDMS (Polydimethylsiloxane) interlayer between the silicon carrier and the polyimide substrate and makes the polyimide-based devices separate much easier and greatly simplifies the fabrication process with a high production yield. A non-successive two-stage cure procedure for the polyimide precursor is developed to meet low-temperature requirement of the PDMS interlayer. The fabrication procedure developed in this research is compatible with conventional MEMS technology through an optimized integration process. The novel flexible MEMS technology can benefit the development of other new flexible polyimide-based devices.     

Fabrication and characterization of bilayer metal wire-grid polarizer using nanoimprint lithography on flexible plastic substrate

In this work, we demonstrate the fabrication of bilayer metal wire-grid polarizers and the characterization of their performance. The polarizers with 200 nm period were fabricated on flexible plastic substrates by nanoimprint lithography (NIL), followed by aluminum deposition. Transmission efficiency over 0.51 and extinction ratio higher than 950 can be achieved in the visible range when the aluminum thickness of the polarizer is 100 nm. The fabrication process only involves direct imprinting on flexible plastic substrates and aluminum deposition, without any resist spin-coating, lift-off, and etching processes, which is much simpler, less costly, and applicable to large volume production.     

Study of bending reliability and electrical properties of platinum lines on flexible polyimide substrates

We have experimentally studied the variation in electrical resistance of flexible platinum lines patterned on polyimide foil when they are subjected to circular bending constraints. The lines were patterned by means of standard photolithography and sputtering deposition. Two different photolithography masks were used for comparative evaluation: an un-expensive transparency mask and a standard chromium mask. Measurements of the temperature coefficient of resistance (TCR) and time stability of the resistance have been acquired for lines bent down to 1.25 mm radius of curvature on a customized bending setup, showing good reliability results. The robustness of the lines has been also assessed by registering their change in resistance while bending at different radii of curvature. The lines showed reliability issues for radii of curvature below 1.25 mm, presenting a resistance variation of 19% for transparency mask-fabricated lines and 9% for chromium mask-fabricated lines. The worse reliability performances of transparency mask lines, compared to the chromium mask ones, was found to be due to their imperfect edges, which promoted the formation and propagation of cracks during bending. The results of the experiments in this work permitted to compare the performances of flexible conductive lines with different geometry and fabricated with two different masks, establishing quantitative and qualitative bending limits for their appropriate operation in flexible electronics systems.     

与半导体IC相融合的互连板的电镀技术动向

概述了IC芯片的铜镶嵌构造和高端电子安装领域中与半导体IC相融合的互连板的电镀技术动向。     

Light extraction of flexible OLEDs based on transparent polyimide substrates with 3-D photonic structure

We report a flexible organic light-emitting diode (OLED) based on transparent polyimide (PI) substrate with 3-D photonic structure, which shows a maximum gain factor of ∼1.7 for current efficiency at large viewing angle. The PI substrate is a replicate from glass carrier with hexagonal closely-packed convex-truncated-cone array. Green OLEDs are fabricated on the planar surface of the PI substrate before being mechanically de-bonded from the glass template. The proposed OLEDs exhibit excellent angular optical properties including stable CIE coordinates with Δx = −0.006 and Δy = 0.002 as the viewing angle varies from 0° to 50°. Surface scattering effect of the 3-D photonic structure eliminates the periodic distortion phenomenon in electroluminescence spectrum of flexible OLEDs.     

Enhanced outcoupling in flexible organic light-emitting diodes on scattering polyimide substrates

We demonstrate an upscalable approach to increase outcoupling in organic light-emitting diodes (OLEDs) fabricated on flexible substrates. The outcoupling enhancement is enabled by introducing a thin film of microporous polyimide on the backside of silver nanowire (AgNW) electrodes embedded in neat colorless polyimide. This porous polyimide film, prepared by immersion precipitation, utilizes a large index contrast between the polyimide host and randomly distributed air voids, resulting in broadband haze (>75%). In addition, the composite polyimide/AgNW scattering substrate inherits the high thermal (>360 °C), chemical, and mechanical stability of polyimides. The outcoupling efficiency of the composite scattering substrate is studied via optical characterization of the composite substrate and electron microscopy of the scattering film. The flexible scattering substrates compared to glass/indium tin oxide (ITO) allows for a 74% enhancement in external quantum efficiency (EQE) for a phosphorescent green OLED, and 68% EQE enhancement for a phosphorescent white OLED. The outcoupling enhancement remains unharmed after 5000 bending cycles at a 2 mm bending radius. Moreover, the color uniformity over viewing angles is improved, an important feature for lighting applications.     

Large scale,flexible organic transistor arrays and circuits based on polyimide materials

Polyimide (PI) materials are lightweight, flexible, resistant strongly to heat and chemicals, and have been widely used in electronics industry such as working as electronic packaging materials in large-scale integrated circuits. In this letter, PI materials, for the first time, are introduced into organic field-effect transistors (OFETs) and circuits as insulator layers in order to be compatible with the photolithography process. Moreover, a novel method is developed to make the PI films strong enough to endure the critical processes of photolithography (e.g., the influence of developer on polyimide layer). Based on the intact PI insulator and the modified photolithographic technique, large scale, flexible transistor arrays and circuits were fabricated with high resolution and high performance (mobility up to 0.55 cm² V⁻¹ s⁻¹ for bottom-contact bottom-gate OFETs). It provides a new way for the fabrication of large-area organic devices and circuits beyond solution printed techniques, especially for the application of organic semiconductors with poor solubility, e.g., pentacene.     

聚酰亚胺无胶型挠性覆铜板的卷曲及其模型

用直接涂覆聚酰胺酸于铜箔的方法制作了聚酰亚胺无胶型挠性覆铜板,对无胶型挠性覆铜板发生卷曲的原因进行了分析推导,并建立了模型,用建立的卷曲模型公式成功地评估了挠性板的铜箔基材与覆于铜箔上的聚酰亚胺树脂的线性热膨胀系数之间的差值。文章还就聚酰亚胺的化学结构、聚酰亚胺薄膜的厚度及聚酰胺酸酰亚胺化工艺对卷曲程度的影响进行了初步分析和探讨。     

Flexible QLED and OPV based on transparent polyimide substrate with rigid alicyclic asymmetric isomer

A series of isomeric alicyclic-functionalized polyimide with chemical imidization and thermal imidization (CPI-x and TPI-x) were prepared from a rigid alicyclic-functionalized isomerism diamine, 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindane (DAPI). The influences of incorporation of the isomeric rigid alicyclic structure onto the backbone of the polymers were systematically investigated in terms of optical, thermal, mechanical, dielectric and surface properties, respectively. Due to the moderate chemical imidization condition, CPI-x series retain higher glass transition temperature (Tg) in the range of 329–429 °C than the Tg of TPI-x (from 321.9 °C to 370.7 °C). Therefore, the CPI-1 film based 5-DAPI was chosen as the flexible substrate. MoO₃ was chosen as the interface layer to improve the compatibility between PI substrate and the metal electrode. Then a ultra-thin layer of MoO₃ (3 nm)/Au(2 nm)/Ag(4 nm) was utilized to be the transparent electrode. After annealing at 220 °C for 0.5 h, zinc oxide (ZnO) was deposited onto the electrode to maintain the superior electron mobility and improve the transparency of the electrode. Consequently, the flexible quantum dots light emitting diode (QLED) obtained a high luminance of 5230 (cd/m²) and EQE of 5.2%, meanwhile, a device performance of 4.36% was achieved in organic photovoltaic (OPV) devices.     

Impact of adhesive rheology on stress-distortion of bonded plastic substrates for flexible electronics applications

For fabrication of flexible electronics using standard microelectronics toolsets, a temporary bond-debond method has been developed that requires minimization of the distortion of bonded flexible substrate and bow of bonded system (flexible substrate-adhesive-carrier) during processing. To elucidate the critical parameters of the adhesive used in the bonding that control the stress (bow) and distortion, adhesives with different viscoelastic properties are examined systematically. By blending a high modulus adhesive into a low modulus adhesive, the storage modulus, loss modulus and loss factor of the adhesive can be tuned by orders of magnitude. Detailed examination of the impact of these three rheological parameters on the stress and distortion of bonded system reveals that the relative viscoelastic flow properties of the bonding adhesive to that of the bonded flexible substrate are directly correlated to bow and distortion. When the loss factor of the adhesive is less than that for the plastic substrate, precise registration of layers during photolithography is observed. These results provide insight into the rheological parameters critical to the adhesive formulations for the temporary bond-debond method in the fabrication of flexible electronics.     

Enhanced printed temperature sensors on flexible substrate

In this paper we present the development of enhanced printed temperature sensors on large area flexible substrates. The process flow is a fully screen printed technology that uses exclusively solution-processed materials. These Screen printed temperature sensors are based on resistive pastes integrated in a Wheatstone bridge circuit. Substrate is a commercial Poly Ethylene Naphtalate (PEN) with a thickness of 125 µm. Functional temperature sensors are demonstrated and characterized with good electrical properties, showing a good sensitivity of 0.06 V/°C at V_in=4.8 V. This sensitivity is enhanced by the annealing and the O₂ plasma treatment. Based on this temperature sensor, we have developed a demonstrator for human body temperature detection.     

Fabrication techniques and applications of flexible graphene-based electronic devices

In recent years, flexible electronic devices have become a hot topic of scientific research. These flexible devices are the basis of flexible circuits, flexible batteries, flexible displays and electronic skins. Graphene-based materials are very promising for flexible electronic devices, due to their high mobility, high elasticity, a tunable band gap, quantum electronic transport and high mechanical strength. In this article, we review the recent progress of the fabrication process and the applications of graphene-based electronic devices, including thermal acoustic devices, thermal rectifiers, graphene-based nanogenerators, pressure sensors and graphene-based light-emitting diodes. In summary, although there are still a lot of challenges needing to be solved, graphene-based materials are very promising for various flexible device applications in the future.     

Roller nanoimprint lithography for flexible electronic devices of a sub-micron scale

This paper proposes a new concept of a RNIL (roller nanoimprint lithography) system. The system does not require the roll stamp that is necessary in the conventional RNIL system, and it easily transfers patterns from a hard stamp to a flexible substrate. Generally, hard stamps such as Si wafers are of a circular shape. While imprinting with a hard stamp using the RNIL system, the pressing force of the press roller in the system varies as the length of the contact line between the circular-type hard stamp and the roller changes. In this study, the contact force profile is presented and is then implemented. Micro- and nano-scale patterns are transferred from Si stamps onto thin and flexible PC (polycarbonate) substrates. Then performance of the system is the evaluated by SEM images.     

Micro-scale twistable organic field effect transistors and complementary inverters fabricated by orthogonal photolithography on flexible polyimide substrate

We fabricated micro-scale organic field effect transistors (OFETs) and complementary inverters on a twistable polyimide (PI) substrate by applying orthogonal photolithography. By applying a highly fluorinated photoresist and development solvent, it becomes possible to create organic electronic devices with a micro-scale channel length without damaging the underlying polymer films. The 3 μm-channel twistable pentacene OFET devices and complementary inverters created using p-type pentacene and n-type copper hexadecafluorophthalocyanine exhibited stable electrical characteristics from flat to twist configurations (angle of up to ∼50°). The realization of twistable micro-scale OFETs and inverter devices on a PI substrate may enable the production of functioning organic devices in practical, flexible configurations.     

柔性OLED显示及封装技术的研究

有机发光二极管(OLED)是最具发展潜力的显示技术之一,而器件的使用寿命短的缺点影响了OLED的发展.有机电致发光器件在柔性衬底上的制备是下一代显示技术发展的重要方向.介绍了柔性OLED及封装技术和金属薄片、聚合物基片及超薄玻璃—聚合物系统的特点,OLED器件的有效封装可以延长器件的寿命.     

Silated acidic copolymers for nanoimprint lithography on flexible plastic substrates

A new silated acidic polymer was developed as the resist for nanoimprint lithography on flexible substrates. This polymer was synthesized from methylmethacrylate, n-butylacrylate, methacrylic acid and 3-[tris(trimethylsiloxy)silyl]propyl methacrylate by free radical copolymerization with an azobisisobutyronitrile (AIBN) initiator at 90 °C. The resist has excellent reactive ion etching (RIE) resistability, a lower T_g (43 °C) compared to poly(methyl methacrylate) (PMMA) and good flowability. It is suitable to use on flexible plastic substrates. The resist can be easily removed by an aqueous base solution at the final stripping step, instead of using an organic solvent or RIE. A 100 nm/50 nm (line/space) feature pattern was obtained on a flexible polyethylene terephthalate (PET)/ITO substrate.     

Fabrication and characterization of 4H-SiC planar MESFETs

The planar 4H-SiC MESFETs were fabricated by employing an ion-implantation process instead of a recess gate etching process, which is commonly adapted in compound semiconductor MESFETs, to eliminate potential damage to the gate region during etching process. Excellent ohmic and Schottky contact properties were achieved by using the modified RCA cleaning of 4H-SiC surface and the sacrificial thermal oxide layer. The fabricated MESFETs was also free from drain current instability, which the most of SiC MESFETs have been reported to suffer for the charge trapping. The drain current recovery characteristics were also improved by passivating the surface with a thermal oxide layer and eliminating the charge trapping at the surface. The performance of fabricated MESFETs was characterized by analyzing the small-signal equivalent circuit parameters extracted from the measured parameters.     

Influence of flexible substrates on inverted organic solar cells using sputtered ZnO as cathode interfacial layer

Zinc oxide (ZnO) has recently shown to be of considerable interest for the development of interfacial buffer layers in inverted organic solar cells (OSCs). High quality ZnO thin films can indeed be prepared on large-area ITO-coated flexible substrates, using low temperature deposition techniques such as sputtering, a compatible technique with roll-to-roll process. However, further studies are still needed for a better understanding of the influence of the flexible substrate properties on the photovoltaic performances of those devices. In this work, ZnO films have been sputtered on ITO-coated flexible (PEN) substrates and annealed at different temperatures. The role of the surface morphology and the crystalline quality of ZnO films has been investigated. In the window of flexible compatible process, we found that moderate annealing temperatures of ZnO (?180 °C) lead to improved structural properties and performances. Interestingly, we achieve optimal performances for an annealing temperature of 160 °C, resulting in power conversion efficiency (PCE) equivalent to the highest performances usually achieved on rigid cells.