Polyimide Thin Films Research Articles

Manufacture and Evaluation of Light Emitting Diode Package Substrate Using Flexible Printed Circuit Board.

Unlike other light sources such as fluorescent lamps and incandescent bulbs, light-emitting diodes (LED) convert 70-80% of energy into heat. If the heat produced an LED chip is not effectively released, its luminous efficiency and lifespan are reduced. Therefore, as a method effectively release heat, an LED PKG substrate containing a heat-releasing material with excellent thermal conductance was fabricated, and its thermal resistance and luminous efficiency were analyzed. In this experiment, a thin polyimide film with excellent ductility was used to fabricate the LED PKG substrate. A 35-μm-thick Cu foil with excellent thermal conductance was subjected to high temperature and pressure and attached to both sides of the polyimide film. By electroplating Ag or Au, which has excellent thermal conductance, for us as the electrode and heat-releasing material, LED PKG substrate was fabricated with a thickness of approximately 170 μm. (-40 °C --> RT --> 120 °C). The results revealed that the LED PKG substrate having a Ag electrode with excellent thermal conductance had an excellent thermal resistance of approximately 4.2 °C/W (Au electrode: 5.6 °C/W). The luminous flux after 100 cycles in the thermal shock test was reduced by approximately 0.09% (Au electrode: 2.77%), indicating that the LED PKG substrate had excellent thermal resistance without any mechanical and material defects in a rapid-temperature-changing environment. The advantages and excellent thermal resistance can be exploited in cellular phones and LCD panels, and heat-releasing problems in thin panels be solved.

Transparent Polyimide Film with Improved Water and Oxygen Barrier Property by In‐Situ Exfoliating Graphite

A facile process is demonstrated to fabricate flexible and transparent polyimide (PI) thin films with improved moisture and oxygen barrier property by in situ solution‐blending PI monomers with ultrasonicated graphite sheets in the solvent during imidization process. When the concentration of graphite in solvent is less than 0.3 wt%, graphite can be exfoliated to a thickness of less than 800 nm and homogeneously distributed in the precursor of PI solution. The resultant PI composite films containing exfoliated graphite (xG) show excellent transparency, enhanced mechanical strength, improved thermal stability and sufficient electrical insulation. Most importantly, the barrier property of PI/xG composite film is significantly improved by the distributed xG sheets in PI matrix. With the addition of 0.06 wt% of xG in PI matrix, the PI/xG‐0.06 film show an oxygen‐transmission‐rate (OTR) of 152 cc/m2‐day compared to 301 cc/m2‐day for pure PI. The reduction degree in water‐vapor‐transmission‐rate (WVTR) by xG is higher than that in OTR. The WVTR of PI/xG‐0.06 is 28 g/m2‐day, which is only 17% of the value of pure PI. Those PI/xG thin films show the potential as substrate materials for modern flexible electronic devices.

Influence of Molecular Weight on Molecular Ordering and Proton Transport in Organized Sulfonated Polyimide Thin Films

The effect of molecular ordering and ordered domain size on molecular weight dependent proton conductivity in well-organized sulfonated polyimide (SPI) thin films has been investigated using both low molecular weight (LMw, Mw = 1.3 × 104) and high molecular weight samples (HMw, Mw = 2.6 × 105). Analyses of the films by grazing incidence small-angle X-ray scattering (GISAXS) revealed that the HMw SPI film exhibits the higher degree of inherent liquid crystalline-like lamellar ordering, and that it is consistent with a preferential in-plane main chain packing. Such molecular ordering and the in-plane oriented structure can have considerable influence on the proton transport characteristics in thin films. The larger ordered domains in the HMw SPI thin film with a thickness of approximately 500 nm shows the significant proton conductivity enhancement to a value of 2.6 × 10–1 S/cm (at 25 °C and 95% relative humidity), which is more than an order of magnitude higher value than that of the smaller ordered domain...

Improving retention properties by thermal imidization for polyimide-based nonvolatile resistive random access memories

In this study, a polyimide (PI) thin film was synthesized as a resistive layer for creating resistive random access memory (ReRAM). The switch between high- and low-resistance states is caused by the formation and dissociation of dipole direction and Schottky barrier. The impact of imidization on memory properties was evaluated in detail by clarifying the transmission mechanism, and reliability properties including retention and endurance were improved using thermal imidization. In addition, the proposed PI-based ReRAM demonstrated superior performance levels compared with those of electrochemical-metallization-based and valence-change-based ReRAMs, including higher RON/ROFF ratio (>107) and lower operation energy (<0.16MV/cm).

Insulation lifetime improvement of polyimide thin film neural implants

Objective. This work deals with studying and improving the insulation lifetime of polyimide-insulated thin film neural implants, or related polyimide-based medical implants. Approach. The evolution of the leak impedance of insulated conductors was investigated in saline water at 40 °C. The fabrication process as commonly found in literature for polyimide/platinum/polyimide microelectrode arrays was compared with three possible improvements: one based on lowering the curing temperature of the lower layer, one based on chemical activation and one based on an additional plasma activation step. Main results. The lower curing temperature process was found to yield a 7.5-fold improved lifetime compared with the state of the art process. Also, the leak impedances found after soak testing are an order of magnitude higher compared to the standard process. Significance. By improving the lifetime and insulation impedance of polyimide insulation with one order of magnitude, this work increases the applicability of polyimide in chronic thin film neural implants considerably.

The Microsieve Electrode (μSE) As a Tool for Neuromodulation in Peripheral Nerves: A Design, Modeling and Fabrication study

n BACKGROUND: An electrode array that operates within a cross-section of nerve is an attractive yet unfledged idea. Limitations in selectively recruiting individual fascicles restrict the promise and clinical viability of existing devices. Microsieve electrodes (mSE), a type of regenerative neural interface, have shown promise, providing a robust interface integrated into a transected nerve. We report a hybrid-field model demonstrating graded selective recruitment of fascicles, and a scalable microfabrication process based on current semiconductor technologies. n METHODS: A hybrid-field model was used to calculate the selectivity and recruitment of 16 electrode sites on 14 mSE designs varied by transparency (available regeneration area ranging from 20% to 80%) and hole size (30 mm and 100mm). A physical prototype was created using photolithography, physical vapor deposition and anodic metal dissolution. n RESULTS: All designs demonstrate graded recruitment by adjusting current. The 30 mm hole size favored lower stimulation currents and lessened variance in fascicle selection, however, inhibited the effects of transparency on recruitment. Contrary, the 100 mm version increased stimulation current requirements and increased variance, but enabled transparency effects on recruitment. Microfabrication was made feasible using polyimide thin-film, and platinum metal patterning of the electrodes on glass slides. The process avoided cytotoxic materials and processes, allowed multi-unit scalability, and arbitrary geometries. n CONCLUSIONS: These preliminary results provide a framework for neuromodulation using graded and selective recruitment of nerve fascicles using the mSE. Clinically, mSE could be used for rehabilitation of motor and sensory function, spinal cord injuries, chronic pain and as an enhancement to selective rhizotomies.

Flexible Cu(In,Ga)Se2 solar cells fabricated using a polyimide-coated soda-lime glass substrate

Flexible solar cells with a Cu(In,Ga)Se2 (CIGS) absorber layer were fabricated on a polyimide thin film using a lift-off process. Polyimide-coated soda-lime glass (SLG) was used as a substrate for fabricating CIGS solar cells before the lift-off process conducted to make the cells flexible. A conversion efficiency of 13.4% was achieved by low temperature deposition; this value is comparable to that obtained by direct deposition on a rigid glass substrate even without an external Na source. The final conversion efficiency after the lift-off process was 12.7% with some area correction due to the partial peeling-off between CIGS and Mo. Open-circuit voltage and fill factor did not change before and after the lift-off process, suggesting that the lift-off process did not give any physical damage.

Changes in wetting and contact charge transfer by femtosecond laser-ablation of polyimide

In this study it is demonstrated that the triboelectric charging of polyimide thin films is significantly reduced by using a femtosecond laser to nanostructure its. It is found that the contact charge transfer between laser-ablated Kapton and aluminum is almost negligible, and even much lower than the significant current occurring when non-treated Kapton touches the metal. Scanning electron microscopy demonstrates that laser ablation produces a hierarchical micro and nanostructure, and it is found that the structural anisotropy leads to spatially varying contact angles of water droplets residing on the surface. Raman spectra suggest that the centers of the laser-ablated tracks are carbonized; therefore, the loss of insulation can be responsible for the reduction of charge transfer.

Deposition of ODA-PMDA types of polyimide thin film inside a microscopic-scale space using supercritical carbon dioxide

Aromatic polyimide (API) thin films were deposited onto silicon wafers with microscopic-scale trenches (width: 5μm, depth: 30μm) using supercritical carbon dioxide at deposition temperatures that ranged from 150 to 230°C and at a constant deposition time of 90min. Moreover, the relationship between deposition time and film thickness was also investigated for durations that ranged from 30 to 180min at a deposition temperature of 200°C. We used 4,4′-diaminodiphenyl ether and pyromellitic dianhydride of Kapton-type polyimide API monomers. Although the morphologies of the API films deposited on the outside surfaces of the trenches depended on the distance from the supply outlets of the monomers, the API thin films were deposited onto the surfaces of the walls of the trenches with an approximately equal thickness from the outer surface position to the bottom position regardless of the distance from the supply outlets. However, the film thickness on the walls of the trenches depended on the distance from the supply outlets, because the trenches near the supply outlets were likely to be blocked by the masses of APIs during deposition. The film deposited at 230°C was much smoother than that deposited at either 150 or 200°C, which could have been caused by the differences in imidization rates and the amounts of PAA entering into the trenches based on the differences in the deposition temperatures although the film thickness showed no significant temperature dependency in the experimental conditions. The film thickness increased with increases in deposition time on the walls of the trenches in positions where no masses of APIs were found on the outer surface.

Significant enhancement in dielectric constant of polyimide thin films by doping zirconia nanocrystals

Polyimide thin film has a low dielectric constant of 3.2. In this paper, we fabricated polyimide/ZrO2-nanocrystals hybrid thin films with a high dielectric constant of 6.1. The polyimide/ZrO2-nanocrystals hybrid thin films were formed by a spin-coating and a sintering process. The organic/inorganic hybrid thin films were characterized by X-ray powder diffraction, transmission electron microscopy, scanning electron microscopy, and LCR meter. Our experimental results confirmed that ZrO2 nanocrystals were well dispersed and embedded in the hybrid thin films. Compared with pristine polyimide thin film, the dielectric constant of polyimide/ZrO2-nanocrystal hybrid thin film significantly increased due to the incorporation of inorganic component.

Spectroscopic investigation of different concentrations of the vapour deposited copper phthalocyanine as a “guest” in polyimide matrix

Nanocomposite layers 250nm copper phthalocyanine/polyimide prepared by simultaneous vapour deposition of three different sources were studied. Different concentrations of copper phthalocyanine as a “guest” in polyimide matrix as a function of conditions of the preparation have been determined by FTIR (Fourier Transform Infrared) and UV–VIS (Ultraviolet–Visible) spectroscopies. The aim was to estimate the possibility of the spectroscopic methods for quantitative determination of the “guest” and compare with the quality of the polyimide thin films in relation to the “guest” concentration. The band at 1334cm−1 has been used for quantitative estimation of “guest” in polyimide matrix. The concentrations of the copper phthalocyanine less than 20% require curve fitting techniques with Fourier self deconvolution. The relationship between “guest” concentrations and degree of imidization, as well as the electronic UV–VIS spectra are discussed in relation to the composition, imidization degree and the two crystallographic modification of the embedded chromophore.

Microfabrication of Si3N4-polyimide membrane for thermo-pneumatic actuator

Purpose – The purpose of this paper is to discuss the fabrication technology and test of thermo-pneumatic actuator utilizing Si3N4-polyimide thin film membrane. Thin film polyimide membrane capped with Si3N4 thin layer is used as actuator membrane which is able to deform through thermal forces inside an isolated chamber. The fabricated membrane will be suitable for thermo-pneumatic-based membrane actuation for lab-on-chip application. Design/methodology/approach – The actuator device consisting of a micro-heater, a Si-based micro-chamber and a heat-sensitive square-shaped membrane is fabricated using surface and bulk-micromachining process, with an additional adhesive bonding process. The polyimide membrane is capped with a thin silicon nitride layer that is fabricated by using etch stop technique and spin coating. Findings – The deformation property of the membrane depend on the volumetric expansion of air particles in the heat chamber as a result of temperature increase generated from the micro-heater inside the chamber. Preliminary testing showed that the fabricated micro-heater has the capability to generate heat in the chamber with a temperature increase of 18.8 °C/min. Analysis on membrane deflection against temperature increase showed that heat-sensitive thin polyimide membrane can perform the deflection up to 65 μm for a temperature increase of 57°C. Originality/value – The dual layer polyimide capped with Si3N4 was used as the membrane material. The nitride layer allowed the polyimide membrane for working at extreme heat condition. The process technique is simple implementing standard micro-electro-mechanical systems process.

J2210403 フレキシブルサーミスタ用(AlTi)N薄膜/ポリイミドフィルム界面の密着強度評価 : (AlTi)N薄膜/ポリイミドフィルム界面密着機構の検討

Wurtzite (AlTi)N with high crystallinity has the NTC (negative temperature coefficient) thermistor property, which is applicable to an ultra thin temperature sensor on a thin polyimide film. The flexible sensor actually requires the mechanical reliability for operation, thus, the mechanical property, especially the toughness of the interface between an (AlTi)N thin film and a polyimide film was quantitatively evaluated by sub-micron scale fracture test. In addition, the effect of the fabrication process on the toughness was investigated with different types of specimens in order to elucidate the mechanism of the interface adhesion.

Temperature dependence and the dispersion of nonlinear optical properties of chromophore-containing polyimide thin films

Detailed investigations of the quadratic nonlinear response of a series of new polyimides with covalently attached chromophore DR13 are performed by the Maker fringes method in the range of fundamental wavelength from 850 to 1450 nm. Polymer films with thickness of 100–400 nm were spin-coated on glass substrates and corona poled. For these materials, the maximum values of the second harmonic generation coefficients d33 are 80–120 pm/V. A red shift of the nonlinear response dispersion with respect to the linear absorption spectrum was observed for the DR13 chromophore. The temperature dependences of linear absorption and nonlinear coefficients d33 for studied structures are observed. It was found that the temperature changes of the absorption spectra lead to appreciable contribution to the value of the nonlinear coefficient d33. The demonstrated high temperature stability (up to 120 °C) of chromophore-containing polyimide thin films makes it possible to eliminate the degradation of their nonlinear optical properties in the future applications of such structures.

A thin-film microprocessor with inkjet print-programmable memory.

The Internet of Things is driving extensive efforts to develop intelligent everyday objects. This requires seamless integration of relatively simple electronics, for example through ‘stick-on' electronics labels. We believe the future evolution of this technology will be governed by Wright's Law, which was first proposed in 1936 and states that the cost of a product decreases with cumulative production. This implies that a generic electronic device that can be tailored for application-specific requirements during downstream integration would be a cornerstone in the development of the Internet of Things. We present an 8-bit thin-film microprocessor with a write-once, read-many (WORM) instruction generator that can be programmed after manufacture via inkjet printing. The processor combines organic p-type and soluble oxide n-type thin-film transistors in a new flavor of the familiar complementary transistor technology with the potential to be manufactured on a very thin polyimide film, enabling low-cost flexible electronics. It operates at 6.5 V and reaches clock frequencies up to 2.1 kHz. An instruction set of 16 code lines, each line providing a 9 bit instruction, is defined by means of inkjet printing of conductive silver inks.

Clues to the electrical switching mechanism of carbazole-containing polyimide thin films.

The mechanism behind electrical memory behavior of carbazole-containing polyimides (PIs) in nanoscale thin films was investigated. For this investigation, a series of poly(3,3'-dihydroxy-4,4'-biphenylene-co-3,3'-bis(N-ethylenyloxycarbazole)-4,4'-biphenylene hexafluoro-isopropylidenedi-phthalimide)s (6F-HAB-HABCZn PIs) with various compositions was synthesized as a model carbazole-containing polymer system. The thermal properties, band gaps, and molecular orbital levels of the PIs were determined. Furthermore, the chemical compositions, as well as the nanoscale thin film morphologies and electron densities, were analyzed, providing detailed information on the population and positional distribution of carbazole moieties in thin films of the PIs. PI Devices were fabricated with aluminum electrodes and tested electrically. The PI thin film layers in the devices exhibited electrically permanent memory behavior, which was driven by trap-limited space-charge limited conduction and ohmic conduction. The permanent memory characteristics were found to be attributed to the incorporated carbazole moieties rather than from the other chemical components. Furthermore, the memory characteristics depended significantly on the population and positional distribution of carbazole moieties in the PI layer, as well as the film thickness. Considering that the backbone is not conjugated, the present results collectively indicate that the electrical switching behavior of the PI films is driven by the carbazole moieties acting as charge traps and a hopping process using the carbazole charge-trap sites as stepping-stones.

Surface Morphology of Polyimide Thin Film Dip-Coated on Polyester Filament for Dielectric Layer in Fibrous Organic Field Effect Transistor

Abstract The idea of wearable electronics automatically leads to the concept of integrating electronic functions on textile substrates. Since this substrate type implies certain challenges in comparison with their rigid electronic companions, it is of utmost importance to investigate the application of materials for generating the electronic functions on the textile substrate. Only when interaction of materials and textile substrate is fully understood, the electronic function can be generated on the textile without changing the textile’s properties, being flexible or stretchable. This research deals with the optimization of the dielectric layer in a fibrous organic field effect transistor (OFET). A transistor can act as an electrical switch in a circuit. In this work, the polyimide layer was dip-coated on a copper-coated polyester filament. After thoroughly investigating the process conditions, best results with minimal thickness and roughness at full insulation could be achieved at a dip-coating speed of 50 mm/min. The polyimide solution was optimal at 15w% and the choice for the solvent NMP was made. In this paper, details on the pre-treatment methods, choice of solvent and dip-coating speed and their effect on layer morphology and thickness, electrical properties and roughness are reported. Results show that the use of polyimide as a dielectric layer in the architecture of a fibrous OFET is promising. Further research deals with the application of the semiconductor layer within the mentioned architecture, to finally build an OFET on a filament for application in smart textiles.

Fabrication and characterization of a high-resolution neural probe for stereoelectroencephalography and single neuron recording.

This paper reports on the design, fabrication, and characterization of neural probes for stereoelectroencephalography (SEEG). The probe specifically targets focal epilepsy as key application. However, probes of this type can also be used for the diagnosis and treatment of other neural dysfunctions such as Parkinson's disease or tremor, typically requiring deep brain probes. The probe fabrication, of which most processes are parallel batch processes, relies on a novel fabrication concept for rolling and gluing thin film polyimide sheets with integrated electrodes into permanent cylindrical shapes with diameters down to 800 μm. The SEEG probes, comprise several macro-electrodes designed to record local field potentials, and micro-electrodes positioned in-between, dedicated to monitoring single unit activity, with a total channel count of 32, despite the small diameter. While platinum micro-electrodes with a diameter of 35 μm have impedances of about 255 kΩ at 1 kHz, impedance values down to about 1.5 kΩ have been measured for the macro-electrodes. The devices have shown good compatibility with magnetic resonance imaging in a 9.4 T magnet, enabling the precise post-operative probe localization within the brain.

Deposition of aromatic polyimide thin films in supercritical carbon dioxide

The deposition of aromatic polyimide (API) thin films was carried out in supercritical carbon dioxide (scCO2) with 20mol% of N,N-dimethylformamide (DMF) as a cosolvent, and using 4,4′-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) as monomers. A new apparatus based on a flow method that consisted of a cold-wall type reactor and two supply lines for each of the API monomers was used. The effect on the morphology of the films was investigated at feeding monomer concentrations and deposition temperatures ranging from 5×10−4 to 5×10−6 on a mole fraction basis and from 423 to 523K, respectively. The results showed that increasing the monomer concentration and decreasing the deposition temperature increased the thickness of the films, and a smooth surface of the film was obtained at 423K. Additionally, FT-IR study and thermogravimetric (TG) analysis of the film deposited at monomer mole fractions of 5×10−5 and at a deposition temperature of 473K were also carried out. The FT-IR spectrum of the deposited film in scCO2 with DMF represented imide structures although there was a small peak of amide carbonyl stretching that originated from the polyamic acid and DMF. The TG curve indicated the temperature of 5% weight loss was more than 800K under an air atmosphere after complete imidization.

A thin film polyimide mesh microelectrode for chronic epidural electrocorticography recording with enhanced contactability

Objective. Epidural electrocorticography (ECoG) activity may be more reliable and stable than single-unit-activity or local field potential. Invasive brain computer interface (BCI) devices are limited by mechanical mismatching and cellular reactive responses due to differences in the elastic modulus and the motion of stiff electrodes. We propose a mesh-shaped electrode to enhance the contactability between surface of dura and electrode. Approach. We designed a polyimide (PI) electrode with a mesh pattern for more conformal contact with a curved surface. We compared the contact capability of mesh PI electrodes with conventionally used sheet PI electrode. The electrical properties of the mesh PI electrode were evaluated for four weeks. We recorded the epidural ECoG (eECoG) activity on the surface of rhesus monkey brains while they performed a saccadic task for four months. Main results. The mesh PI electrode showed good contact with the agarose brain surface, as evaluated by visual inspection and signal measurement. It was about 87% accurate in predicting the direction of saccade eye movement. Significance. Our results indicate that the mesh PI electrode was flexible and good contact on the curved surface and can record eECoG activity maintaining close contact to dura, which was proved by in vivo and in vitro test.