Thin Polyimide Research Articles

Terahertz Metamaterials Broadband Perfect Absorber Based on Molybdenum Disulfide

In this letter, a terahertz metamaterials broadband perfect absorber based on molybdenum disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is proposed. The absorber is composed of a single layer of MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and a metal plate separated by thin polyimide. The single-layer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> structure consists of four rectangular strips transformed by fillets. Numerical simulations using the full-vector finite element method show that the new hybrid mode can be excited and the electric fields of the modes can be redistributed due to the introduction of fillet MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> metamaterial structure, which effectively improves the absorption performance of devices. In the range of 1.8 to 2.2 THz, the absorptivity is close to 100%, and the absorption curve is much flatter, which provide a useful reference for terahertz modulation equipment.

A flexible implantable microelectrode array for recording electrocorticography signals from rodents.

Electrocorticography signals, the intracranial recording of electrical signatures of the brain, are recorded by non-penetrating planar electrode arrays placed on the cortical surface. Flexible electrode arrays minimize the tissue damage upon implantation. This work shows the design and development of a 32-channel flexible microelectrode array to record electrocorticography signals from the rat's brain. The array was fabricated on a biocompatible flexible polyimide substrate. A titanium/gold layer was patterned as electrodes, and a thin polyimide layer was used for insulation. The fabricated microelectrode array was mounted on the exposed somatosensory cortex of the right hemisphere of a rat after craniotomy and incision of the dura. The signals were recorded using OpenBCI Cyton Daisy Biosensing Boards. The array faithfully recorded the baseline electrocorticography signals, the induced epileptic activities after applying a convulsant, and the recovered baseline signals after applying an antiepileptic drug. The signals recorded by such fabricated microelectrode array from anesthetized rats demonstrate its potential to monitor electrical signatures corresponding to epilepsy. Finally, the time-frequency analyses highlight the difference in spatiotemporal features of baseline and evoked epileptic discharges.

Development of MEMS Flow and Pressure Sensor Device for Detection of Extravasation at an Early Stage

We developed a flow and pressure measurement device for detection of extravasation by intravenous injection of anticancer drugs at an early stage. Intravenous injection of a drug solution using a syringe pump enables a low‐speed and highly accurate drug‐administration rate. However, because the syringe pump forcibly delivers the drug solution, the drug solution is administrated at a constant administration rate regardless of the leakage of the drug solution to the outside of the blood vessel (extravasation) due to the indwelling needle coming out of the blood vessel. The proposed device is integrated into a flow‐channel structure consisting of a PDMS layer in which a MEMS thermal‐flow sensor formed on thin polyimide film and a pressure sensor are stacked on a plastic plate as a substrate. Therefore, it is possible to measure both the drug‐administration rate and injection pressure. Early detection of extravasation can be achieved by identifying the state of needle‐puncture based on changes in injection pressure. Then, the amount of leaked drug after leak occurs can be accurately measured with a flow sensor. We evaluated the flow and pressure detection function of the proposed device and confirmed that it is possible to detect extravasation early through an experiment involving a piece of raw chicken simulating human skin and flesh. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

VO2 films on flexible thin polyimide films: Fabrication and characterization of electrical and optical properties in insulator-metal transition

We report on the realization of thin polyimide films on which phase transition vanadium dioxide (VO2) films grow. Biased reactive sputtering achieved b-axis-oriented growth of VO2 films on ZnO-buffered polyimide films with a thickness of 8.5 μm. By peeling off the polyimide films from a quartz substrate, stand-alone VO2/ZnO/polyimide layered films that exhibited insulator-metal transition (IMT) with nearly three orders of resistivity change were fabricated. Dependence of IMT on a mechanical curvature was investigated for demonstrating the high flexibility. Temperature-dependent optical transmittance at 1.45 μm showed a high switching ratio for infrared light in VO2/ZnO/polyimide layered films. The proposed structure can be utilized for active metasurfaces that control terahertz waves with quite low reflection loss due to its small thickness.

Tribological investigations of Mo films deposited on thin polyimide substrates

In flexible and/or wearable electronics the thin films deposited on compliant substrates are not only subjected to straining and bending but also to tribological loading conditions. A frequently used method to assess tribological properties of materials systems is the ball-on-disc test. In the current work, Mo films were deposited by high power impulse magnetron sputtering on polyimide substrates to serve as a model system. To establish different tribological contact conditions, a series of different counterpart materials was chosen for the performed ball-on-disc tests. A detailed analysis by 3D confocal laser scanning microscopy and scanning electron microscopy revealed details about the wear mechanisms active during sliding where mainly abrasion of the softer one of the material pair was noticed. The obtained results for friction and wear together with the performed simulations of the induced stresses due to the loading conditions show that ball-on-disc tests can contribute to the understanding of the tribological response of metallic films on compliant substrates which in turn may help to improve the lifetime of components in flexible and/or wearable electronic applications.

Simultaneous Wireless Power Transfer and Data Telemetry Using Dual-Band Smart Contact Lens

An efficient noninvasive system for simultaneous transmission of wireless power and data is vital for continuous human health monitoring. In this article, we present a self-tuned spiral-shaped antenna with dual-band characteristics operating at 920 MHz and 2.4 GHz for efficient wireless power transfer (WPT) and data telemetry, respectively. To avoid vision blockage, the proposed smart contact lens (SCL) dipole antenna comprises two symmetrical spiral arms with inner and outer diameters of 10 and 14 mm, respectively. The SCL antenna was printed on a thin flexible polyimide substrate with a biocompatible polydimethylsiloxane (PDMS) coating. The influence of dispersed dielectric properties of PDMS on SCL antenna performance was investigated. Furthermore, the antenna was combined with a miniaturized rectifier and a micro-LED to demonstrate wireless LED illumination on a saline-filled model head. The power transfer efficiency (PTE) as a function of eye movement in the form of a path gain was also investigated. In addition, the PTE across the distance variations from 10 to 30 mm was computed, and a &#x2212;17.85 dB was achieved at a separation of 12 mm. A specific absorption rate analysis was performed to determine human eye safety under radio frequency exposure from the external transmitter and the results complied with the International Commission on Non-Ionizing Radiation Protection guidelines. Furthermore, a theoretical link power budget analysis was experimentally validated using software-defined radio. Measurements of s-parameters were noted through the use of a saline solution to mimic a realistic human eye and were found to have a reasonable correlation with the simulated results.

Thermal and Mechanical Characterization of 2.5-D and Fan-Out Chip on Substrate Chip-First and Chip-Last Packages

Heterogeneous integration technology makes possible the integration of multiple separately manufactured components into a single higher level assembly with enhanced functionality and improved operating characteristics. Various types of advanced heterogeneous packages are available, including 2.5-D integrated circuit (IC), fan-out chip on substrate (FOCoS) chip-first, and FOCoS chip-last. This study constructs a nonlinear simulation technique (3-D simulation model) to explore the warpage, extreme low-<inline-formula> <tex-math notation="LaTeX">$k$ </tex-math></inline-formula> (ELK) interconnect stress, redistribution layer (RDL) trace stress, and board-level solder joint reliability of all three packages. The validity of the simulation model is confirmed by comparing the numerical results for the in-plane dimensional change of an FOCoS chip-last package with the experimental observations over the temperature range of 30 &#x00B0;C&#x2013;260 &#x00B0;C. The thermal performance (i.e., junction-to-ambient thermal resistance) of the three packages is then examined and compared. Finally, <inline-formula> <tex-math notation="LaTeX">$2^{5}$ </tex-math></inline-formula> factorial designs with the analysis of variance (ANOVA) are conducted to examine the effects of the main structural design parameters of the FOCoS chip-last package on its thermomechanical reliability under typical thermal loading conditions. It is shown that a thinner polyimide (PI) layer is beneficial in improving the package reliability by minimizing the coefficient of temperature expansion (CTE) mismatch between the PI layer and the RDL trace, respectively.

Thermoelectric properties of p-type Bi-=SUB=-0.5-=/SUB=-Sb-=SUB=-1.5-=/SUB=-Te-=SUB=-3-=/SUB=- films on flexible substrate

Bi2Te3-based compounds are excellent candidates for the low-temperature thermoelectric application. In the present work, a technology for fabrication of p-Bi0.5Sb1.5Te3 films with high thermoelectric efficiency on a thin flexible polyimide substrate has been developed. The preparation of films was carried out by a flash evaporation method. A systematic study of the transport properties (Hall coefficient, Seebeck coefficient, electrical conductivity, transverse Nernst coefficient) over the entire temperature range of 80-400 K for p-Bi0.5Sb1.5Te3 films has been performed. The power factor (PF) for the Bi0.5Sb1.5Te3 (doped with 0.5 wt% Te) film reached the value of ~ 30 μW/cm · K2, which is among the highest values of the PF reported in the literature to date for a film on a flexible polyimide (amorphous) substrate. The measured thermal diffusivity along the film allowed us to accurately estimate the figure of merit Z for p-Bi0.5Sb1.5Te3 films considering the anisotropic effect of Bi2Te3-based materials. A significant enhancement of Z up to ~ 3.0·10-3 K has been obtained for these films, which is state-of-the-art even compared to bulk materials. This research can provide insight into the fabrication of p-type branch of the film thermoelectric modules (FTEM), which could be a candidate for application in micro-scale thermoelectric generators. Keywords: p-Bi0.5Sb1.5Te3 film, thin flexible substrate, thermoelectric properties.

Novel, flexible, and transparent thin film polyimide aerogels with enhanced thermal insulation and high service temperature

Due to their high service temperature, excellent thermal insulation, and nanoporous morphology, polyimide (PI) aerogels have the potential capability to be used in the next generation of microelectronic devices and flexible electronics.

A Biaxially Stretchable and Self-Sensing Textile Heater Using Silver Nanowire Composite.

Wearable heaters have garnered significant attention from academia and industry for their great potential in thermotherapy. Silver nanowire (AgNW) is a promising conductive material for flexible and stretchable electrodes. Here, a resistive, biaxially stretchable heater based on AgNW composite is reported for the first time, where a AgNW percolation network is encased in a thin polyimide (PI) film and integrated with a highly stretchable textile. AgNW/PI is patterned with a 2D Kirigami structure, which enables constant resistance under a large tensile strain (up to uniaxial 100% strain and 50% biaxial strain). The heater can achieve a high temperature of ∼140 °C with a low current of 0.125 A, fast heating and cooling rates of ∼16.5 and ∼14.1 °C s-1, respectively, and stable performance over 400 heating cycles. A feedback control system is developed to provide constant heating temperature under a temperature change of the surrounding environment. Demonstrated applications in applying thermotherapy at the curvilinear surface of the knee using the stretchable heater illustrate its promising potential for wearable applications.

Intracortical probe arrays with silicon backbone and microelectrodes on thin polyimide wings enable long-term stable recordings in vivo

Objective. Recording and stimulating neuronal activity across different brain regions requires interfacing at multiple sites using dedicated tools while tissue reactions at the recording sites often prevent their successful long-term application. This implies the technological challenge of developing complex probe geometries while keeping the overall footprint minimal, and of selecting materials compatible with neural tissue. While the potential of soft materials in reducing tissue response is uncontested, the implantation of these materials is often limited to reliably target neuronal structures across large brain volumes. Approach. We report on the development of a new multi-electrode array exploiting the advantages of soft and stiff materials by combining 7-µm-thin polyimide wings carrying platinum electrodes with a silicon backbone enabling a safe probe implantation. The probe fabrication applies microsystems technologies in combination with a temporal wafer fixation method for rear side processing, i.e. grinding and deep reactive ion etching, of slender probe shanks and electrode wings. The wing-type neural probes are chronically implanted into the entorhinal-hippocampal formation in the mouse for in vivo recordings of freely behaving animals. Main results. Probes comprising the novel wing-type electrodes have been realized and characterized in view of their electrical performance and insertion capability. Chronic electrophysiological in vivo recordings of the entorhinal-hippocampal network in the mouse of up to 104 days demonstrated a stable yield of channels containing identifiable multi-unit and single-unit activity outperforming probes with electrodes residing on a Si backbone. Significance. The innovative fabrication process using a process compatible, temporary wafer bonding allowed to realize new Michigan-style probe arrays. The wing-type probe design enables a precise probe insertion into brain tissue and long-term stable recordings of unit activity due to the application of a stable backbone and 7-µm-thin probe wings provoking locally a minimal tissue response and protruding from the glial scare of the backbone.

Scalable Batch Transfer of Individual Silicon Dice for Ultra-Flexible Polyimide-Based Bioelectronic Devices.

Demands on flexible neural interfaces in terms of functionality, spatial resolution and longevity have increased in the past years. These requirements can be met by sophisticated integrated circuits developed in CMOS (complementary metal oxide semiconductor) technology. Embedding such fabricated dice into flexible polymeric substrates greatly enhances the adaption to the mechanical environment in the body. With the process developed here, 100 % of individual dice (n = 34, 390 x 390 μm2) could be transferred simultaneously into polyimide (PI) substrates with simple and exact positioning (0.2° rotational and 5 μm translational error). Levelled layer build-up and standard microfabrication technologies could be used for CMOS-post-processing in order to manufacture metal interconnections between contact pads of 100 μm thin dice and PI insulation as selectively patterned device substrate. The process allows for individual positioning according to desired shape of the final chip-in-foil-system and for upscaling the number of dice to be transferred. Furthermore, final distribution and embedding of dice on the flexible substrate is independent from their distribution on the CMOS fabrication wafer the and does not require additional adhesion promoters. During fabrication the transfer method is insensitive to high temperatures (450 °C in this study) and hence enables a wide range of post-processes. Shear strength between dice and PI substrate was characterized by shear tests and results (58.1 ± 13.7 MPa) are in the range achieved with the adhesive benzocyclobutene (BCB).

Lasergate: A windowless gas target for enhanced laser preheat in magnetized liner inertial fusion

At the Z Facility at Sandia National Laboratories, the magnetized liner inertial fusion (MagLIF) program aims to study the inertial confinement fusion in deuterium-filled gas cells by implementing a three-step process on the fuel: premagnetization, laser preheat, and Z-pinch compression. In the laser preheat stage, the Z-Beamlet laser focuses through a thin polyimide window to enter the gas cell and heat the fusion fuel. However, it is known that the presence of the few μm thick window reduces the amount of laser energy that enters the gas and causes window material to mix into the fuel. These effects are detrimental to achieving fusion; therefore, a windowless target is desired. The Lasergate concept is designed to accomplish this by “cutting” the window and allowing the interior gas pressure to push the window material out of the beam path just before the heating laser arrives. In this work, we present the proof-of-principle experiments to evaluate a laser-cutting approach to Lasergate and explore the subsequent window and gas dynamics. Further, an experimental comparison of gas preheat with and without Lasergate gives clear indications of an energy deposition advantage using the Lasergate concept, as well as other observed and hypothesized benefits. While Lasergate was conceived with MagLIF in mind, the method is applicable to any laser or diagnostic application requiring direct line of sight to the interior of gas cell targets.

Microchip Transfer Process for Implantable Flexible Bioelectronic Devices

Abstract The demands on flexible implants for recording of neural signals and electrical stimulating have increased in recent years with regard to their functionality, miniaturization, and spatial resolution. These requirements can be met best by embedding powerful complementary metal oxide semiconductor (CMOS) microchips into thin biocompatible polymer substrates. So-called chip-in-foil systems thus combine mechanical properties of a polymer substrate and performance of CMOS technology. The development of a process for direct transfer of multiple CMOS microchips (edge length &lt;400 μm) simultaneously into thin polyimide (PI) substrates is subject of this study. It allows the use of standard microelectromechanical systems (MEMS) processes for further levelled superficial layer build-up. This is achieved with the help of a silicon carrier wafer equipped with cavities for precise chip placement and a sacrificial layer to facilitate release of the chip-in-foil systems. In a post-processing step all silicon chips are thinned down to 100 μm. With this process a transfer yield of 100 % (n = 34) was achieved for the silicon chips on a die level. Chip rotational error on substrates was determined to be as low as 0.21° ± 0.10°. Die adhesion was examined by shear tests, resulting in shear strength of 58.1 MPa ± 13.7 MPa, which dropped to 15.2 MPa ± 10.5 MPa after accelerated ageing in 60 °C phosphate buffered saline solution (PBS) for 16 days (equivalent to 78 days at 37 °C). This study demonstrated a reliable microchip transfer process with low positioning error into flexible PI substrates with post-processing thinning of the dies. The use of a carrier silicon wafer allowed precise electrical interconnect fabrication with standard MEMS processing techniques and without handling of thin and fragile chips. These results are a prerequisite to meet needs of reliability and structural biocompatibility in implantable flexible bioelectronic devices.

High-Performance Washable PM2.5 Filter Fabricated with Laser-Induced Graphene.

This study demonstrates a novel application of laser-induced graphene (LIG) as a reusable conductive particulate matter (PM) filter. Four types of LIG-based filters were fabricated based on the laser-induced pyrolysis of thin polyimide (PI) sheets, each pyrolyzed on either a single side or both sides, with or without densification. The LIG filters exhibited a high removal efficiency while maintaining minimal pressure drop compared to a commercial fiberglass filter. The densified LIG (dLIG) filters displayed a higher PM2.5 removal efficiency (>99.86%) than regular LIG filters. The dLIG filters also exhibited excellent durability when tested for washability by ultrasonication in tap water. After being cleaned and left to dry, the structures of the dLIG filters were well-maintained; their filtration efficiencies were also well-maintained (less than a 7% change in PM2.5 removal efficiency), and their resistances only marginally increased (less than a 7% increase after five uses). These results demonstrate the robustness and reusability of the dLIG filters and the accessibility of their cleaning (not requiring aggressive cleaning agents). These promising features will enable the application of LIG in economical, scalable, and high-performance air cleaning.

Thermoelectric Characteristics of Ultrathin Ag₂Se Films with Durability Against Mechanical Stress.

In this study, we investigated thermoelectric materials with durability against mechanical stress using Ag₂Se nanoparticle (NP) thin films and colorless polyimide (CPI) substrates. Ag₂Se NP thin films and CPI substrates were produced by spin-coating, and their thicknesses were 40 nm and 15 μm, respectively. A bendable thermoelectric film with a channel length of 40 μm and a channel area of 1.6 μm² generated a Seebeck voltage of 1.43 mV at a temperature difference of 4.5 K. Owing to the thickness of the extremely thin thermoelectric film and substrate, the mechanical strain was only 0.15% even when the thermoelectric devices were bent with a curvature of 3 mm. Therefore, it was determined that the bendable thermoelectric film was robust against mechanical stress.

Erratum: “Formation of Plastic Creases in Thin Polyimide Films” [ASME J. Appl. Mech., May 2020, Vol. 87; DOI: 10.1115/1.4046002

Abstract In the original publication, the finite element model (Sec 3.2) had a width W of 20 mm while it should have been 25 mm. Based on the corrected analysis, Fig.10 (page 7) and Fig.13 (page 8) should be updated as follows.

Inorganic Perovskite Quantum Dot-Based Strain Sensors for Data Storage and In-Sensor Computing.

Although remarkable improvement has been achieved in stretchable strain sensors, challenges still exist in aspects including intelligent sensing, simultaneous data processing, and scalable fabrication techniques. In this work, a strain-sensitive device is presented by fabricating a CsPbBr3 quantum dots (QDs) floating-gate field-effect transistor (FET) sensing array on thin polyimide (PI) films. The FET exhibits an excellent on/off ratio (>103) and a large memory window (>2 V). With the introduction of CsPbBr3 QDs as the trapping layer, an additional UV response is obtained because of the photogenerated charge carriers that significantly enhance the source-drain current (IDS) of the device. At each electrical state, the IDS varies with the strains and the sensing range is from compressive +12.5% to tensile -10.8%. Excellent data retainability and mechanical durability demonstrate the high quality and reliability of the fabricated sensors. Furthermore, synapse functions including long-term potentiation (LTP), long-term depression (LTD), etc., are emulated at the device level. Linearity factor changes of LTP/LTD in different sensing scenarios demonstrate the reliability of the device and further confirm the different sensing mechanisms with/without UV illumination. Our results exhibit the potential of transistor-based devices for multifunctional intelligent sensing.

Flexible electrochemical uric acid and glucose biosensor

Fully integrated uric acid (UA) and glucose biosensors were fabricated on polydimethylsiloxane/polyimide platform by facile one step laser scribed technique. The laser scribed graphene (LSG) on the thin polyimide film was functionalized using pyrenebutanoic acid, succinimide ester (PBSE) to improve the electrochemical activity of the biosensors. The LSG was further decorated with platinum nanoparticles (PtNPs) to promote the electrocatalytic activity towards the oxidation of UA. Glucose oxidase was immobilized on the PtNPs modified surface for selective detection of glucose. The fabricated biosensors were characterized via scanning electron microscopy (SEM), Energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and electrochemical methods (cyclic voltammetry and amperometry measurements). Outstanding electrocatalytic activities toward oxidation of UA and glucose were demonstrated. A wide detection range of 5 µM to 480 µM UA with a high sensitivity of 156.56 µA/mMcm2 and a calculated detection limit (LOD) of 0.018 μM (S/N = 3) were achieved for the UA biosensor. The glucose biosensor exhibited a detection range of 5 µM to 3200 µM with a sensitivity of 12.64 µA/mMcm2 and an LOD of 2.57 µM (S/N = 3). These integrated biosensors offer great promise for potential applications in wearable UA and glucose sensing due to their good sensitivity, selectivity, and stability properties.

Versatile Solution‐Processed Organic–Inorganic Hybrid Superlattices for Ultraflexible and Transparent High‐Performance Optoelectronic Devices

AbstractCrystalline or amorphous metal oxides are widely used in various optoelectronic devices as key components, such as transparent conductive electrodes, dielectrics or semiconducting active layers for thin‐film transistor (TFT) backplanes in large‐area displays, photovoltaics, and light‐emitting diodes. Although crystalline inorganic materials demonstrate outstanding optoelectronic performance, owing to their wide bandgaps, large conductivities, and high carrier mobilities, their inherent brittleness makes them vulnerable to mechanical stress, thereby limiting the use of metal‐oxide films in emerging flexible electronic applications. In this study, stress‐diffusive organic–inorganic hybrid superlattice nanostructures are developed to overcome the mechanical limitation of crystalline oxides and to provide high mechanical stability to metal‐oxide semiconductors. In particular, hybrid transparent superlattice electrodes based on crystalline indium–tin oxide exhibit high electrical conductivities of up to 555 S cm–1 (resistance variation &lt; 3%) and effectively reduce the mechanical stress on the inorganic layer (up to 10 000 bending cycles with a radius of 1 mm). Furthermore, to ensure the viability of the hybrid superlattice flexible electronics, all solution‐processed superlattice crystalline indium–gallium‐oxide TFTs are implemented on a thin (≈5 µm) polyimide substrate, providing highly robust and excellent electrical performance (average mobility of 7.6 cm2 V–1 s–1).