Organic Dielectric Layer Research Articles

9‐1: Invited/Distinguished Paper: Flexible TFT Backplane Development for Extremely Small Bending Radius with Organic ILD and Island TFT Structure

A new flexible low‐temperature polycrystalline silicon thin film transistors (LTPS TFTs) backplane on PI substrate with organic dielectric layer and low‐stress structure of island TFT is proposed. This new extremely low stress backplane is able to withstand 20,000 inward bending cycles at a bending radius of 1mm without any noticeable TFT degradation. After 200,000 bending inward cycles at bending radius of 1 mm, the stability test results show that the change of on‐current is 10.07% for hot carrier instability (HCI) test, the change of threshold voltage ‐0.31V for negative bias thermal instability (NBTI) test, the change of threshold voltage ‐0.24V for hysteresis test, and the breakdown voltage of gate insulator 7.73MV/cm, respectively. The AMOLED display panel manufacured with organic ILD does not show any visible degradation of panel image just after 200K rolling at rolling radius of 4mm or 200K folding at folding radius of 1mm.

Wafer-scale organic-on-III-V monolithic heterogeneous integration for active-matrix micro-LED displays.

The organic thin-film transistor is advantageous for monolithic three-dimensional integration attributed to low temperature and facile solution processing. However, the electrical properties of solution deposited organic semiconductor channels are very sensitive to the substrate surface and processing conditions. An organic-last integration technology is developed for wafer-scale heterogeneous integration of a multi-layer organic material stack from solution onto the non-even substrate surface of a III-V micro light emitting diode plane. A via process is proposed to make the via interconnection after fabrication of the organic thin-film transistor. Low-defect uniform organic semiconductor and dielectric layers can then be formed on top to achieve high-quality interfaces. The resulting organic thin-film transistors exhibit superior performance for driving micro light emitting diode displays, in terms of milliampere driving current, and large ON/OFF current ratio approaching 1010 with excellent uniformity and reliability. Active-matrix micro light emitting diode displays are demonstrated with highest brightness of 150,000 nits and highest resolution of 254 pixels-per-inch.

Plasmonics-based gas sensor with photonic spin hall effect in broad terahertz frequency range under variable chemical potential of graphene.

Graphene monolayer of sub-nanometer thickness possesses strong metallic and plasmonic behavior in a broad terahertz (THz) frequency range. This plasmonic effect can be considerably manipulated when graphene layer is subjected to a variable chemical potential (Ef) via chemical doping or electrical gating. The strong adsorption characteristics of graphene layer is another important advantage. In this work, a photonic spin Hall effect (PSHE) based plasmonic sensor consisting of germanium prism, organic dielectric layer, and graphene monolayer is simulated and analyzed in THz range aiming at highly sensitive and reliable gas sensing. Modified Otto configuration and Kubo formulation for graphene at room temperature are considered. The sensor’s performance is examined in terms of figure of merit (FOM). The analysis indicates that under angular interrogation scheme of sensor operation, the FOM improves for smaller chemical potential (moderate doping) and higher THz frequency. Moreover, the influence of temperature on gas sensor’s performance (FOM) is negligible, which suggests that the sensor is capable of providing stable sensing performance against temperature variation. The sensor design is highly flexible in terms of selection of THz frequency as an alternative interrogation scheme (i.e., measuring the variation in spin-dependent shift peak value of PSHE spectrum upon change in gas medium refractive index) can also be implemented. It is found that there is no need to change the moderate doping of graphene monolayer (i.e., Ef remains around its normal value ~ 0.1 eV) as the sensitivity achievable with this alternative method has considerably greater magnitude at smaller THz frequency (e.g., 2 THz). The magnitudes of FOM (with angular interrogation method) and sensitivity (with alternative method) are found to be significantly greater for rarer gaseous media, which might possibly assist in early detection of airborne viruses such as SARS-Cov-2 (while using appropriate specificity method) and to measure the concentration of a particular gas in a given gaseous mixture.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11082-022-03626-7.

Coplanar-Gate Synaptic Transistor Array With Organic Electrolyte Using Lithographic Process

The artificial neural networks based on biomimetic synaptic devices attempts to process and memorize information by simulating a human brain. Electrolyte-gated transistors (EGTs) are promising candidates for artificial synaptic devices, due to their large specific capacitance, low operating voltage, and sensitive interfacial properties. Although large-scale EGTs array based on inorganic electrolytes and small-scale EGTs arrays based on organic electrolytes are reported, large-scale EGTs arrays based on organic electrolytes have not yet been focused. In this work, a large-scale EGTs array (<inline-formula> <tex-math notation="LaTeX">$10\times10$ </tex-math></inline-formula>) based on coplanar-gate structure using lignin as electrolyte is fabricated. The organic dielectric layer fabricated in the last step can be protected from the damage caused by wet etching, thus providing an opportunity for the application of other organic electrolytes in the large-scale EGTs array. Lignin exhibits good electric performance and is environment-friendly due to its renewable property. The synaptic properties of single transistor in the EGTs array, such as excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), and high-pass filtering are realized. The real-time learning and forgetting procedures stimulated by different spike numbers of EGTs array are also demonstrated.

Graphene-Based Plasmonic Sensor at THz Frequency with Photonic Spin Hall Effect Assisted by Magneto-optic Phenomenon.

Graphene monolayer of sub-nanometer thickness shows strong metallic and plasmonic behavior in terahertz (THz) frequency range. This plasmonic effect varies considerably when graphene layer is placed under a magnetic field of appropriate strength. The strong adsorption characteristic of graphene layer is another advantage. In this work, a photonic spin Hall effect (PSHE)-based plasmonic sensor consisting of germanium prism, organic dielectric layer, and graphene monolayer is simulated and analyzed in THz aiming at highly sensitive and reliable sensing under variable magnetic field. Modified Otto configuration and magneto-optic effect in graphene are considered. The sensor’s performance is examined in terms of sensitivity, limit of detection (LOD), and figure of merit (FOM). The analysis indicates that LOD of the order of 10−5 RIU for gas sensing is achievable, which is finer than recently reported gas sensors based on different techniques. Further, the FOM improves when a larger magnitude of magnetic field is applied. The FOM is even greater for rarer gaseous media, which can make the sensor extremely useful in early detection of airborne viruses such as SARS-Cov-2 (while using appropriate specificity method) and to measure the concentration of a particular gas in a given gaseous mixture. The results further indicate that the same sensor design can be used for magnetic field detection while the FOM of magnetic field detection is significantly greater for rarer gaseous medium (e.g., air), which may enable the probe to be used in early detection of radiation leakage in nuclear reactors. For larger magnitudes of magnetic field, the corresponding LOD becomes finer.

Electrohydrodynamic‐Printed Polyvinyl Alcohol‐Based Gate Insulators for Organic Integrated Devices

Organic printable dielectric layers are required for next‐generation electronic circuits, particularly those used as gate insulators (GIs) in organic field‐effect transistors (OFETs). Herein, optimized electrohydrodynamic (EHD) jet printing with an electrostatic force‐assisted mode is suggested for the fabrication of polyvinyl alcohol (PVA)‐based dielectric patterns after careful consideration of the ink system. The PVA molecules maintain good solubility in polar solvents, even when a crosslinking agent is added, which enables a continuous PVA jet stream with a width smaller than the diameter of the nozzle to be stably obtained. Subsequent EHD printing produces uniform PVA patterns with a smooth surface morphology suitable for the crystal growth of overlying organic semiconductors. The addition of a crosslinking agent in PVA results in direct GI patterns that exhibit superior insulating properties with high dielectric strength as compared with PVA without crosslinking agents. The OFETs with PVA‐based GIs prepared with EHD printing show stable operation with low gate leakage currents. In addition, the feasibility of EHD‐printed PVA layers is demonstrated for application as GIs in organic complementary logic gates.

Controlling Nanostructure in Inkjet Printed Organic Transistors for Pressure Sensing Applications.

This work reports the development of a highly sensitive pressure detector prepared by inkjet printing of electroactive organic semiconducting materials. The pressure sensing is achieved by incorporating a quantum tunnelling composite material composed of graphite nanoparticles in a rubber matrix into the multilayer nanostructure of a printed organic thin film transistor. This printed device was able to convert shock wave inputs rapidly and reproducibly into an inherently amplified electronic output signal. Variation of the organic ink material, solvents, and printing speeds were shown to modulate the multilayer nanostructure of the organic semiconducting and dielectric layers, enabling tuneable optimisation of the transistor response. The optimised printed device exhibits rapid switching from a non-conductive to a conductive state upon application of low pressures whilst operating at very low source-drain voltages (0–5 V), a feature that is often required in applications sensitive to stray electromagnetic signals but is not provided by conventional inorganic transistors and switches. The printed sensor also operates without the need for any gate voltage bias, further reducing the electronics required for operation. The printable low-voltage sensing and signalling system offers a route to simple low-cost assemblies for secure detection of stimuli in highly energetic systems including combustible or chemically sensitive materials.

Electrical Properties of Horizontal Array Capacitor Using Composite Organic Dielectric Layer of Impregnated Glass-Fiber Epoxy

We introduce a horizontal array capacitor with nine capacitances in a single body using an organic dielectric layer impregnated with glass fiber as a prepreg sheet. An organic solid horizontal array capacitor with a dielectric of prepreg materials of the epoxy type can implement the nine capacitances in a single body via a unique simple lamination and cutting process. We then investigate the basic electrical properties of a horizontal array capacitor. The organic solid array capacitors with five electrodes and four dielectrics are Cu/PPG layer/Cu/PPG layer/Cu/PPG layer/Cu/PPG layer/Cu with a horizontal array structure. The size of a completed array capacitor is 2.85 × 2.85 mm. The height of the fabricated array capacitor in the vertical direction is 0.5 mm, with nine capacitances possessing a series-type structure. The average capacitance value of C1, C2, C3, and C4 is 1.98 nF, and each tolerance has a value within 1% based on the average value. The temperature change rate in the capacitance maintains a nearly linear characteristic, but the rate of change tends to increase finely from 120°C or more. The capacitance values of C5, C6, and C7 with the parallel circuit were measured according to the voltage. Impedance and ESR (equivalent series resistor) of C1 were measured according to frequency and temperature.

High-efficiency nitrene-based crosslinking agent for robust dielectric layers and high-performance solution-processed organic field-effect transistors

High-performance organic field-effect transistors (OFETs) need not only high-mobility organic semiconductors, but also suitable organic dielectric layers. OFETs with the polystyrene (PS) dielectric layer have shown superior electrical characteristics and operational stability levels due to the hydrophobicity of PS. However, PS is usually compatible with vacuum-evaporated semiconductors because the solution process dissolves the bottom PS layer, making it difficult to obtain a high-quality semiconducting film. In this study, we develop the chemically robust dielectric film ‘FPS’, made up of 5 wt% ethylene glycol bis(4-azido-2,3,5,6-tetrafluoro-benzoate) (sFPA) in a PS film. Addition of UV-responsive sFPA causes the film to become crosslinked after UV irradiation while maintaining its surface properties compared to the PS film. Crosslinking with a very small amount of sFPA is possible due to high crosslinking efficiency of sFPA, not requiring two specific functional groups. In addition, an as-cast FPS film is readily photo-patterned by UV irradiation with a shadow mask. The solution-processed TES-ADT semiconductor film shows a highly crystalline morphology on the FPS layer, and its OFETs show much higher field-effect mobility levels than those with the plain PS layer. These results are attributed to the chemical robustness of the FPS layer, and its favorable environment for growth of TES-ADT crystals. Therefore, we expect the FPS layer to be used as a versatile dielectric layer with various solution-processed semiconductors.

Processing Through Glass Via (TGV) Interposers

For the next generation of 3D-IC applications, glass offers many desirable properties that make it an ideal interposer. In order to realize these applications, it becomes necessary to be able to deposit a Cu layer on both surfaces as well as in the glass via. In this paper we will present results of interposers that have fine line circuitry on both sides of the glass and have through glass vias (TGVs). One major challenge is how to create a TGV using either electrically conductive adhesives (ECAs) or Cu plating methods to achieve a high conductivity and reliable via connection. Most work to date uses ECA materials to fill the TGV, however, many of these ECA materials have limitations in that they cannot withstand temperatures in excess of 400°C, are not hermetic and do not have the electrical properties of bulk Cu. In the first phase of our work, we have evaluated different commercially available ECA materials to fill the glass vias. A major challenge in using these materials is how to effectively fill the vias. Our test vehicle has a TGV with dimensions of 50um diameter and a 300um thickness on 2″ × 2″ glass substrate. A positive pressure screen printing process was used to fill the vias. These ECA materials exhibited a very high viscosity, due to the high Cu metal filler content, which made them very difficult to fill. Different pressures, squeegee speeds, and standoff conditions were evaluated to arrive at an optimized process to effectively fill the glass vias. A tilted X-ray inspection approach was used to characterize the yield for ECA hole fill. A DOE of different sintering conditions was conducted to determine how to maximize the electrically conductivity of the paste. From this work we will show the impact of sintering conditions on electrical conductivity and hermeticity. In parallel, we have developed a process to Cu plate a TGV. In this approach, a dry deposition process was used to produce a conductive seed layer in the via having a diameter of 100um and a total substrate thickness of 300um. We proceeded to use a Cu electroplating process to plate up a conformal Cu layer in the glass via. In this work we have pattern plated circuitry on both surfaces along with the via to produce a glass interposer. In addition, we were able to deposit a very thin layer of Si3N4 between two thick Cu metal layers on the topside of a glass interposer to produce a capacitor. The success of plating a TGV also enables the formation of inductors in a glass substrate. An organic dielectric layer using an Ajinomoto Buildup Film (ABF) was used to create a single buildup layer on the glass interposer. The combination of these individual technologies allows the capability to manufacture a glass interposer for advanced packaging applications. A discussion of the challenges associated in building glass interposers will be presented, which will include examples of recent builds of a double-sided Cu circuitry glass interposer and a diplexer module having Cu MIM (metal-insulator-metal) capacitors and inductors for RF applications.

Flexible terahertz modulators based on graphene FET with organic high-k dielectric layer

Graphene field-effect-transistor (GFET) based terahertz (THz) modulators usually possess an unfulfilling modulation depth (MD) of 15% ∼ 20%. In this work we developed a flexible GFET based THz modulator, where the graphene monolayer is coated with an organic high-K dielectric as the screening layer and an ion-gel layer as the gate. With this exquisite composite modulating structure, the new device possesses a significantly enhanced modulation depth (MD) up to 70% over a broad frequency band, an extremely low insert loss (IL) of 1.3 dB, and unexpected good structural and properties stability. The large intrinsic MD, low IL, as well as its flexibility, render this performance enhanced modulator versatile in fabrication of novel THz devices, such as multi-level modulator, for nonplanar or wearable applications.

Graphene Terahertz Amplitude Modulation Enhanced by Square Ring Resonant Structure

A terahertz amplitude modulator based on graphene on a metallic square ring resonant structure is proposed. By separating the graphene and the metallic structure with a thin organic dielectric layer, both the resonant frequency and the amplitude of the transmission resonant peak are modulated when graphene is electrically tuned by a bias voltage. A maximal amplitude modulation depth of 72% at the frequency of 0.6 THz is achieved for the fabricated Terahertz modulator. An analysis model based on the transmission line theory is built to explore the modulation mechanism. Results of the transmission spectrum and the amplitude modulation indicate a good agreement between the transmission line theoretical predictions and the experimental measurements.

Space Environment Effects on Flexible, Low-Voltage Organic Thin-Film Transistors.

Organic electronic devices fabricated on flexible substrates are promising candidates for applications in environments where flexible, lightweight, and radiation hard materials are required. In this work, device parameters such as threshold voltage, charge mobility, and trap density of 13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene)-based organic thin-film transistors (OTFTs) have been monitored for performing electrical measurements before and after irradiation by high-energy protons. The observed reduction of charge carrier mobility following irradiation can be only partially ascribed to the increased trap density. Indeed, we used other techniques to identify additional effects induced by proton irradiation in such devices. Atomic force microscopy reveals morphological defects occurring in the organic dielectric layer induced by the impinging protons, which, in turn, induce a strain on the TIPS-pentacene crystallites lying above. The effects of this strain are investigated by density functional theory simulations of two model structures, which describe the TIPS-pentacene crystalline films at equilibrium and under strain. The two different density of states distributions in the valence band have been correlated with the photocurrent spectra acquired before and after proton irradiation. We conclude that the degradation of the dielectric layer and the organic semiconductor sensitivity to strain are the two main phenomena responsible for the reduction of OTFT mobility after proton irradiation.

New mechanism of semiconductor polarization at the interface with an organic insulator

A semiconductor—organic-insulator system with spatially distributed charge is created with a uniquely low density of fast surface states (N ss ) at the interface. A system with N ss ≈ 5 × 1010 cm–2 is obtained for the example of n-Ge and the physical characteristics of the interface are measured for this system with liquid and metal field electrodes. For a system with an organic insulator, the range of variation of the surface potential from enrichment of the space-charge region of the semiconductor to the inversion state is first obtained without changing the mechanism of interaction between the adsorbed layer and the semiconductor surface. The effect of enhanced polarization of the space-charge region of the semiconductor occurs due to a change in the spatial structure of mobile charge in the organic dielectric layer. The system developed in the study opens up technological opportunities for the formation of a new generation of electronic devices based on organic film structures and for experimental modeling of the electronic properties of biological membranes.

High Mobility Flexible Amorphous IGZO Thin-Film Transistors with a Low Thermal Budget Ultra-Violet Pulsed Light Process.

Amorphous, sol-gel processed, indium gallium zinc oxide (IGZO) transistors on plastic substrate with a printable gate dielectric and an electron mobility of 4.5 cm2/(V s), as well as a mobility of 7 cm2/(V s) on solid substrate (Si/SiO2) are reported. These performances are obtained using a low temperature pulsed light annealing technique. Ultraviolet (UV) pulsed light system is an innovative technique compared to conventional (furnace or hot-plate) annealing process that we successfully implemented on sol-gel IGZO thin film transistors (TFTs) made on plastic substrate. The photonic annealing treatment has been optimized to obtain IGZO TFTs with significant electrical properties. Organic gate dielectric layers deposited on this pulsed UV light annealed films have also been optimized. This technique is very promising for the development of amorphous IGZO TFTs on plastic substrates.

Highly Bendable In-Ga-ZnO Thin Film Transistors by Using a Thermally Stable Organic Dielectric Layer.

Flexible In-Ga-ZnO (IGZO) thin film transistor (TFT) on a polyimide substrate is produced by employing a thermally stable SA7 organic material as the multi-functional barrier and dielectric layers. The IGZO channel layer was sputtered at Ar:O2 gas flow rate of 100:1 sccm and the fabricated TFT exhibited excellent transistor performances with a mobility of 15.67 cm2/Vs, a threshold voltage of 6.4 V and an on/off current ratio of 4.5 × 105. Further, high mechanical stability was achieved by the use of organic/inorganic stacking of dielectric and channel layers. Thus, the IGZO transistor endured unprecedented bending strain up to 3.33% at a bending radius of 1.5 mm with no significant degradation in transistor performances along with a superior reliability up to 1000 cycles.

Plasma-enhanced chemical vapor-deposited organic dielectric layer for low voltage, flexible organic thin-film transistor

We investigated the electronic properties of pentacene-based organic thin-film transistors (OTFTs) incorporated with an organic dielectric layer (ODL) fabricated using a homemade, cold plasma–enhanced chemical vapor deposition system. Ar-carried trimethylaluminum monomer was introduced into an evaporator mixer controlled at 130°C, and an ODL was deposited on Si- or indium tin oxide (ITO)–coated plastic substrate under N2 plasma. High radio frequency power (100W)-deposited ODL (dielectric constant κ=1.6) supported the fabrication of pentacene-based OTFT on Si substrate at low voltages (<1.5V). The decreased deposition time (10min) reduced the film thickness of the ODL (95nm), thereby increasing its dielectric constant to 3.3. We also deposited ODL on ITO–coated plastic substrate to fabricate a flexible OTFT with a mobility of 2.1cm2/Vs and an on/off current ratio (Ion/Ioff) of 103.

Dielectric interface-dependent spatial charge distribution in ambipolar polymer semiconductors embedded in dual-gate field-effect transistors

The spatial charge distribution in diketopyrrolopyrrole-containing ambipolar polymeric semiconductors embedded in dual-gate field-effect transistors (DGFETs) was investigated. The DGFETs have identical active channel layers but two different channel/gate interfaces, with a CYTOP™ organic dielectric layer for the top-gate and an octadecyltrichlorosilane (ODTS) self-assembled monolayer-treated inorganic SiO2 dielectric for the bottom-gate, respectively. Temperature-dependent transfer measurements of the DGFETs were conducted to examine the charge transport at each interface. By fitting the temperature-dependent measurement results to the modified Vissenberg–Matters model, it can be inferred that the top-channel interfacing with the fluorinated organic dielectric layers has confined charge transport to two-dimensions, whereas the bottom-channel interfacing with the ODTS-treated SiO2 dielectric layers has three-dimensional charge transport.

Memory properties of Au nanoparticles prepared by tuning HAuCl4 concentration using low-temperature hydrothermal reaction

An organic memory device with embedded gold nanoparticles (AuNPs) as charge storage elements and polymethylsilsesquioxane (PMSSQ) as organic dielectric layer was fabricated in this work. The AuNPs were prepared using sacrificial hydrothermal reaction on Al template, and the effect of HAuCl4 on size of AuNPs was studied. Field-emission scanning electron microscopy confirmed the increase of particle size with the increase in HAuCl4 concentration. X-ray diffraction analysis confirmed the formation of AuNPs with face-centered cubic (FCC) structure. The electrical properties of the memory system were investigated using a semiconductor characterization system. Current–voltage analysis confirmed the nonvolatile electrical bistability behavior and charge transport mechanism. Capacitance–voltage measurement indicated a memory behavior with flatband voltage shift because the AuNPs trapped and stored charges in the device. The optimal memory properties of AuNPs embedded in PMSSQ were obtained for FCC-structured AuNPs grown in 0.010M HAuCl4, with a mixture of 82±5nm (area density, 1.3×1012m−2) and 42±7nm (area density, 2.7×1012m−2) grain sizes that produced the lowest switch ON voltage of 2.2V and highest flatband voltage of 2.6V.

Growth of gold nanoparticles using aluminum template via low-temperature hydrothermal method for memory applications

Gold nanoparticles (AuNPs) were grown on a Si(100) substrate using a sacrificial hydrothermal method on Al template prepared by direct current (DC) sputtering, followed by thermal annealing. The effect of annealing temperature on the Al template was investigated. The morphology and phases of the grown AuNPs were observed by field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. FESEM and XRD results revealed the uniform face-centered cubic structure of AuNPs with isolated 83 nm particle size and 1.48 × 1013 m−2 area density; these properties were considered optimum for AuNPs formed on the non-annealed sputtered Al template. The electrical properties of AuNPs embedded in polymethylsilsesquioxane (PMSSQ) as an organic dielectric layer were studied using a semiconductor characterization system. A metal–insulator-semiconductor device with embedded AuNPs was proven to exhibit memory effects. Optimum memory properties of AuNPs embedded in PMSSQ were obtained for AuNPs grown on the non-annealed sputtered Al template with the lowest threshold voltage of 2.4 V in I–V characteristics and 34 electrons stored per AuNP in C–V measurement.