Polyimide Thin Films Research Articles

FBG based shape sensing of a silicone octopus tentacle model for soft robotics

The shape sensing of a soft silicone octopus tentacle model using FBG sensors is presented in this paper. A polyimide thin film layer integrates FBG sensors is embedded into a soft silicone model as the strain limited layer for pneumatic actuation. The relation between the Bragg wavelength shift (BWS) value and the bending curvature of the soft silicone model is measured through the FBG calibration process. The curvature values of the soft silicone model at four bending shapes are calculated based on the BWS values and the interpolation algorithm. The reconstruction of the 3D shapes of the soft silicone model at different bending status are realized using the curve fitting function, and the correctness is verified. The results show that the error between the calculated and actual values of the bending curvature is no more than 2.1%, the proposed FBG-based sensing approach is effective for the measurement of the bending shape of the soft silicone model, it has application prospect in the field of soft robotics.

Gas Separation Properties of Polyimide Thin Films on Ceramic Supports for High Temperature Applications.

Novel selective ceramic-supported thin polyimide films produced in a single dip coating step are proposed for membrane applications at elevated temperatures. Layers of the polyimides P84®, Matrimid 5218®, and 6FDA-6FpDA were successfully deposited onto porous alumina supports. In order to tackle the poor compatibility between ceramic support and polymer, and to get defect-free thin films, the effect of the viscosity of the polymer solution was studied, giving the entanglement concentration (C*) for each polymer. The C* values were 3.09 wt. % for the 6FDA-6FpDA, 3.52 wt. % for Matrimid®, and 4.30 wt. % for P84®. A minimum polymer solution concentration necessary for defect-free film formation was found for each polymer, with the inverse order to the intrinsic viscosities (P84® ≥ Matrimid® >> 6FDA-6FpDA). The effect of the temperature on the permeance of prepared membranes was studied for H2, CH4, N2, O2, and CO2. As expected, activation energy of permeance for hydrogen was higher than for CO2, resulting in H2/CO2 selectivity increase with temperature. More densely packed polymers lead to materials that are more selective at elevated temperatures.

Synthesis and characterization of novel sulfonated polyimide with varying chemical structure for fuel cell applications

Sodium 2-(2, 3-bis (4-aminophenyl) propyl) benzenesulphonate (APBS), a novel sulfonated diamine (NSDA) was synthesized and characterized by 13C NMR. NSDA along with 4,4′-oxydianiline (ODA) and 4,4′-oxydiphthalic anhydride (OPDA) were used for the synthesis of novel sulfonated polyimide (NSPI) thin film through the random copolymerization process via two-step classical method: (1) synthesis of sulfonated poly(amic acid) (SPAA), the precursor; (2) thermal imidization of SPAA film. Novel sulfonated polyimide (NSPI) with a pendant sulfonic acid group was fabricated by the use of this novel NSDA. The imidization of novel NSPI membrane was confirmed by FTIR spectroscopy. Thermal stability, water uptake, ion exchange capacity, hydrolytic and oxidative stabilities were also studied. Relatively high proton conductivity even at 110 °C, makes this SPI membrane suitable to be used as a polymer electrolyte membrane for fuel cell.

High-sensitivity optical fiber relative humidity probe with temperature calibration ability.

A high-sensitivity optical fiber relative humidity (RH) sensing probe with the ability of temperature calibration is proposed and experimentally demonstrated. It consists of a simple Fabry-Perot interferometer constructed by coating a layer of thin polyimide (PI) film on the end face of single-mode fiber and an upstream fiber Bragg grating (FBG). PI is one of the organic polymer humidity-sensitive materials with good comprehensive properties. The cascaded FBG is used for temperature calibration and elimination of the temperature cross-sensitivity in the process of measuring RH. Experimental results show that this sensing probe can realize simultaneous measurement of temperature and RH. The RH response sensitivity reaches up to 986.25pm/%RH. This sensing probe with the advantages of simple structure, compact size, high sensitivity, easy packaging, and dual-parameter measurement has an extensive application prospect.

Optical fiber shape sensing of polyimide skin for a flexible morphing wing.

This paper presents the 3D shape sensing of polyimide thin film skin for a flexible morphing wing using fiber Bragg grating (FBG) sensors. The calibration curves of the FBG sensors are measured experimentally to ensure relative accurate conversion between Bragg wavelength shift (BWS) and bending curvature of the polyimide skin. The reflection spectra of the FBG sensors are measured at different airfoil profiles, and the variation tendency of the BWS values with the airfoil profiles are analyzed. The bending curvatures of the polyimide thin film skin at different airfoil profiles are calculated using the measured BWS values of the FBG sensors and the linear interpolation algorithm. The 3D shapes of the polyimide skin at different airfoil profiles are reconstructed based on the measured bending curvatures and the interpolation and curve fitting functions. The 3D precise visual measurements are conducted using a digital photogrammetry system, and then the correctness of the shape reconstruction results are verified. The results prove that the maximum error between the 3D visual and FBG measurements is less than 5%. The FBG sensing method is effective for the shape sensing of polyimide skin for flexible morphing wing.

Vapor Deposition Polymerization and Molecular Orientation of Polyimide Thin Films

Polyimide thin films were prepared by vapor deposition polymerization. Co-deposition of pyromellitic dianhydride and 1,12-diaminododecane spontaneously yielded a thin film of polyamic acid, which was imidized by annealing in nitrogen atmosphere at 200°C. It was observed that the polyimide thin film has preferential orientation to the (001) crystal plane with the molecular chain standing on the substrate. The orientation was enhanced by matching the ratio of monomer supply for vapor deposition. The molecular orientation was also influenced by modifying the substrate surface with self-assembled monolayers (SAMs). A SAM having anhydrous carboxy terminal was effective in enhancing the molecular orientation.

High-Frequency Magnetic Thin-Film Inductor Integrated on Flexible Organic Substrates

This paper presents integrated inductors with Co-Zr-Ta-B (CZTB) thin films on flexible organic substrates for MHz–GHz applications. This paper demonstrated the fabrication and characterization of magnetic thin-film inductors on flexible organic substrates, such as polyimide and Ajinomoto build-up film thin films. These films have thicknesses of only tens of micrometer and thus are very flexible compared to the same inductor structures fabricated on rigid substrates, such as quartz. The magnetic films on flexible substrates were observed to maintain similar magnetic properties compared with those on Si substrate, but with greater coercivity likely due to the increased roughness of the substrate surface. The fabricated thin-film inductors on organic substrates with inductance $L$ , measured by RF characteristics, have shown significantly increased inductance compared to air-core inductors (without magnetic core materials). This paper also demonstrated a high frequency response, clearly over GHz, consistent with the CZTB type of materials. The frequency response, as well as inductance and Q values, remains similar even after many iterations of bending to a large degree, indicating the robustness of the inductor on flexible organic substrates, as well as that of the magnetic thin films.

3D shape sensing of flexible morphing wing using fiber Bragg grating sensing method

The 3D shape sensing of flexible morphing wing using fiber Bragg grating (FBG) sensors is presented in this paper. A morphing wing prototype with polyimide thin film skin is constructed, and forty-eight FBG sensors are glued on the skin surface. The calibration experiments of the FBG sensors are conducted to ensure relative accurate conversion between Bragg wavelength shift (BWS) and bending curvature of the polyimide skin. The relation of the BWS values with the airfoil profiles are analyzed. The bending curvatures of the polyimide skin are calculated. The 3D shapes of the polyimide skin at different airfoil profiles are reconstructed. The 3D precise visual measurements are conducted using a digital photogrammetry system, and then the correctness of the shape reconstruction results are verified. The results prove that the maximum error between the 3D visual and FBG measurements is less than 5%, the FBG sensing method is effective for the shape sensing of flexible morphing wing with polyimide skin.

Direct-write graphene resistors on aromatic polyimide for transparent heating glass

This study reports a scalable process for the direct-write graphene resistors via a CO2 laser to irradiate a spin-coated aromatic polyimide (PI) thin film on top of the glass substrate. With the help of an XY stage, the resistor patterns can be controlled and designed as transparent heaters. Material characterizations by using Raman, SEM, TEM, XRD and AFM have been performed to verify the functional structures. The laser power density has been optimized as 7.27 kw/cm2 with the scanning speed of 1.6mm/s to achieve the best resistivity value of ca. 2.833×10−4Ω·m for the graphene structures. The effects on the scanning speed and the pulse width to the resulting graphene resistor patterns such as line widths have also been investigated for optimized direct-write process. When the scanning speed and the pulse width were set to 1.6mm/s and 357μs respectively, the mean line width was about 120μm. Experimentally, such laser irradiated graphene was demonstrated to be used as electric transparent heating glass, and different line width and gap have great influence on the heating temperature, heat distribution and transmittance. When the line width and gap distance were 120μm and 2mm on a 4×4cm2 cover glass respectively, the heater can produce a temperature of 92°C on the glass substrate in one minute.

High-Quality Factor Superconducting Flexible Resonators Embedded in Thin-Film Polyimide HD-4110

We present the design, fabrication procedure, and measurement results of Nb superconducting microstrip transmission line resonators fabricated using thin-film polyimide HD-4110, including both nonembedded and embedded versions. These resonators were used to characterize the microwave dielectric loss tangent of 20 $\mu$ m thick polyimide HD-4110 at deep cryogenic temperatures. We observed high-quality factors ( Q ) up to 21 040 at 1.2 K in the frequency range of 2–21 GHz for nonembedded resonators, indicating that the dielectric loss tangent can be less than 5 $\times$ 10−5. Embedded resonators with an additional 20 $\mu$ m thick encapsulation layer also exhibited Q values as high as $\sim 19$ 200. Dielectric and conductor (quasiparticle) loss have been compared between the two types of resonators. This study provides information applicable to the design of future high-density, flexible, multilayer superconducting cables, which are of great interest for potential applications in cryogenic electronics systems, including quantum computers.

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

Abstract 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.

Determination of the glass transition temperature in thin polymeric films used for microelectronic packaging by temperature-dependent spectroscopic ellipsometry

We characterized the glass transition temperature Tg of thin polyimide films by temperature-dependent spectroscopic ellipsometry and compared the results to DSC measurements of the bulk polymer. The effect of the curing temperature on Tg and the thermal expansion α(T) was analyzed. An improved ellipsometric data evaluation was used to get most precise and reliable Tg data. Tg increased with increasing curing temperature, while the bulk Tg was considerably lower than the thin film Tg. Both observations are attributed to the temperature sensitive release of the imidization by-product 2-hydroxyethyl methacrylate (HEMA) and crosslinker components as well as decomposition products from the material. Variation in the curing temperatures of 230–380 °C led to an increase in the Tg of 34 °C.

Fabrication and evaluation of dispersed-Ag nanoparticles-in-polyimide thin films

A thin-film common-mode filter (TF-CMF) for cell phones in the UHF band was fabricated and evaluated. The TF-CMF consisted of multiple metal–insulator–metal (MIM) capacitors and inductors. The sizes of the 0.70–1.0 GHz band-type and 1.8–2.0 GHz band-type TF-CMFs are 1,140 × 1,260 × 10.5 µm3, and 1,060 × 1,060 × 10.5 µm3, respectively. The footprint in both types of TF-CMFs is over 1 mm2. In order to miniaturize the TF-CMF, we proposed to change a polyimide-only to a polyimide with dispersed Ag nanoparticles with high permittivity in the insulator layer for the MIM capacitor of the TF-CMF. A polyimide () thin film with dispersed high-density Ag nanoparticles has a relative permittivity of about 8, which is twice as high as that of the polyimide-only thin film. If the capacitance and distance between electrodes are the same, then the capacitor footprint may be halved.

Investigation on Passive Booster for Improving Magnetic Coupling of Metal Mounted Proximity Range HF RFIDs

In this paper, the design methodology, fabrication, and measurement of a passive circuit for increasing the read range of the near-field-communications (NFCs) gadget in handheld devices to read miniature metal mounted radio-frequency identification (RFID) are discussed. Since, the impedances at source and load are unknown, the conventional methods are not helpful for designing a proper communication. The proposed circuit is fabricated on thin flexible polyimide film hence; it can be simply attached to the back of handheld devices with arbitrary shape. Having only one discrete component in the circuit is another step toward low-cost devices. The magnetic coupling between the small metal mounted high-frequency (HF) RFID and the NFC coil are improved by adding two coils connected in parallel to each other resonating at 13.56 MHz. One coil is designed to be coupled to the NFC coil and the other one to the HF RFID. The operability of the system is investigated not only by standard measurement equipment like vector network analyzer but also utilizing different smartphones. The agreement between the simulation and practical results shows that the booster improves the read range significantly.

Proton-Conductivity Enhancement in Polymer Thin Films.

Highly proton conductive polymers have long attracted the attention of researchers for use in energy conversion, sensors, catalysts, and other applications. From the viewpoint of the scientific history of the creation of highly proton conductive polymers, one fundamental approach is based on the strategy of phase-segregated structures with strong acid groups. This Feature Article presents a new approach to enhancing the proton conductivity of the polymer thin films using an interface that can modify the degrees of freedom for a polymer structure through interaction between the substrate surface and polymers. I introduce suppressed proton conductivity into Nafion thin films and then specifically examine the enhancement in proton conductivity by the molecular orientation of the polymers. As the last topic, a highly proton conductive organized polyimide thin film is demonstrated using the lyotropic liquid-crystal property. Both molecular ordering and the in-plane oriented structure can enhance proton conductivity. Moreover, the optical domain and degree of molecular ordering derived from the molecular weight can contribute strongly to the proton transport property.

Single laser based pump-probe technique to study plasma shielding during nanosecond laser ablation of copper thin films

A plasma shielding phenomenon and its influence on micromachining is studied experimentally and theoretically for laser wavelengths of 355 nm, 532 nm and 1064 nm. A time resolved pump-probe technique is proposed and demonstrated by splitting a single nanosecond Nd3+:YAG laser into an ablation laser (pump laser) and a probe laser to understand the influence of plasma shielding on laser ablation of copper (Cu) clad on polyimide thin films. The proposed nanosecond pump-probe technique allows simultaneous measurement of the absorption characteristics of plasma produced during Cu film ablation by the pump laser. Experimental measurements of the probe intensity distinctly show that the absorption by the ablated plume increases with increase in the pump intensity, as a result of plasma shielding. Theoretical estimation of the intensity of the transmitted pump beam based on the thermo-temporal modeling is in qualitative agreement with the pump-probe based experimental measurements. The theoretical estimate of the depth attained for a single pulse with high pump intensity value on a Cu thin film is limited by the plasma shielding of the incident laser beam, similar to that observed experimentally. Further, the depth of micro-channels produced shows a similar trend for all three wavelengths, however, the channel depth achieved is lesser at the wavelength of 1064 nm.

Infrared photo-resistors based on recrystallized amorphous germanium films on flexible substrates

The fabrication of germanium-based near infrared photo-resistors on a flexible substrate is reported. The devices were fabricated using plasma enhanced chemical vapor deposition of amorphous germanium on an insulating stack layer of silicon nitride and silicon oxide on a flexible polyimide thin film. The deposited films were annealed using a pulsed laser at different pulse rates and laser energy densities to investigate the effects of germanium recrystallization on the characteristics of the photo-resistors. Electrical and opto-electrical responses of the films have been measured and reported. Surface characterization via scanning electron and atomic force microscopy was used to evaluate the variations in surface roughness of the annealed material versus the amorphous germanium layers. Power spectral density analysis was performed to investigate the effect of laser annealing on the crystalline structure of the germanium samples. Providing a low-temperature processing flow, this work lays the foundation for the development of infrared sensors above flexible substrates with potential applications in wearable electronics among others.

Effects of Imidization Process on the Electric Breakdown Strength of Polyimide Thin Film

The electric breakdown voltage (Vbd) is a fundamental performance index for insulation thin film in MEMS, and plays a dominant role for the operation reliability of this kind of devices. In this work, positive polyimide (PI) photoresist was used to prepare thin films by spin-coating and thermal imidization. Electric breakdown experiments were carried out under DC voltage mode. The effect of imidization temperature on Vbd was also studied by setting three different imidization temperature profiles. XRD was applied to test the composition and microstructure of the PI thin films. Results show that the imidization degree of the PI thin film will be increased with the raising of the imidization temperature from 280 to 350°C. This means that a more completed imidization process can be achieved in the PI thin film by using higher imidization temperature. The highest Vbd of about 2740V and an average Vbd of 343kV/mm can be achieved on a 8.5μm thick PI thin film imidized at 350°C.

Design of Flexible Hybrid Nanocomposite Structure Based on Frequency Selective Surface for Wideband Radar Cross Section Reduction

This paper presents the design, synthesis, and testing of a novel flexible hybrid nanocomposite (FHN) structure for the wideband radar cross section (RCS) reduction of targets. The proposed FHN structure consists of a frequency selective surface (FSS) pattern printed on a thin flexible polyimide film backed by aluminum foil, which is then integrated with a customized flexible nanocomposite sheet. The nanocomposite sheet consists of graphite fillers combined with zinc oxide powders, which are embedded into the thermoplastic polyurethane matrix. The fabricated nanocomposite sheet is first characterized for the effective permittivity and the dielectric loss tangent using the free-space measurement technique, and the measured electromagnetic properties are then used for the design of an FHN structure. The proposed FHN structure having overall thickness of 1.335 mm shows minimum reflection coefficient (RC) of −28 dB at 10.2 GHz, along with −10 dB RC bandwidth in the entire $X$ -band. Finally, the fabricated FHN structure is applied on a cylindrical target, which provides minimum 10 dB RCS reduction under various polarizations. The proposed work provides an effective way to combine the strength of FSS with that of a customized nanocomposite sheet in order to design an efficient FHN structure for various strategic applications.

Microwave Loss Measurements of Copper-Clad Superconducting Niobium Microstrip Resonators on Flexible Polyimide Substrates

In this work, we investigated how different under- and capping layers on patterned Nb films impacted the RF losses of flexible thin-film superconducting microstrip transmission line resonators measured in a frequency range from 2 to 20 GHz. We studied how different thicknesses of Ti(10 and 50 nm) under-layer, used for adhesion, impacts conductor losses. We also studied Cu(20, 50, 100, and 200 nm) capping layers and how they affect conductor loss. These studies were carried out on 20- $\mu$ m-thick spin-on polyimide (PI-2611) thin films and characterized at various cryogenic temperatures between 1.2 and 4.2 K. The results indicate normal-superconductor (Ti/Nb) and superconductor-normal (Nb/Cu) bilayer structures have increased surface resistance, which leads to an increase in microwave loss when compared to Nb-only signal traces. We quantified this additional loss by extracting resonator quality factors for weakly coupled resonators with various conductor stack-ups. Our experimental results can help inform decisions regarding material stack-ups when designing multiconductor multilayer superconducting flexible cables intended for use with ultralow-temperature electronic systems.