Hiroyuki Kuwae

Suppression of external quantum efficiency roll-off of nanopatterned organic-light emitting diodes at high current densities

We developed organic light-emitting diodes (OLEDs) with nanopatterned current flow regions using electron-beam lithography with the aim of suppressing singlet–polaron annihilation (SPA). Nanopatterns composed of lines and circles were used in the current flow regions of nano-line and nano-dot OLEDs, respectively. Excitons partially escape from the current flow regions where SPA takes place. As such, current densities where external quantum efficiencies were half of their initial values (J0) increased as line width and circle diameter were decreased to close to the exciton diffusion length. Circles were more efficient at enhancing exciton escape and increasing J0 than lines. The J0 increase in the nano-dot OLEDs containing nanopatterned circles with a diameter of 50 nm was approximately 41-fold that of a conventional OLED with a current flow region of 4 mm2. The dependence of J0 on the size and shape of the nanopatterns was well explained by an SPA model that considered exciton diffusion. Nanopatterning of...

Joule heat-induced breakdown of organic thin-film devices under pulse operation

We investigated the influence of the substrates thermal conductivities (k) and the widths of the electrical pulses (τpulse) on the maximum current densities (Jmax) in organic thin-film devices. We also estimated the temperature rise (ΔT) inside devices under the pulse operation using numerical calculations to interpret the observed differences in Jmax. For a long τpulse of 5 μs, Jmax is higher for devices with high-k sapphire substrates (around 1.2 kA/cm2) than devices with low-k plastic substrates (around 0.4 kA/cm2). This is because high-k sapphire substrates can work as heat sinks to relax ΔT for such a long τpulse. Operation of devices with high-k sapphire substrates for a short τpulse of 70 ns resulted in further relaxation of ΔT, leading to an increase of Jmax to around 5 kA/cm2. Interestingly, for such a short τpulse, devices with high-k sapphire and low-k plastic substrates showed similar Jmax and ΔT values, the reason for which may be that it is difficult for the generated Joule heat to travel to the substrate across a low-k organic layer within this short time.We investigated the influence of the substrates thermal conductivities (k) and the widths of the electrical pulses (τpulse) on the maximum current densities (Jmax) in organic thin-film devices. We also estimated the temperature rise (ΔT) inside devices under the pulse operation using numerical calculations to interpret the observed differences in Jmax. For a long τpulse of 5 μs, Jmax is higher for devices with high-k sapphire substrates (around 1.2 kA/cm2) than devices with low-k plastic substrates (around 0.4 kA/cm2). This is because high-k sapphire substrates can work as heat sinks to relax ΔT for such a long τpulse. Operation of devices with high-k sapphire substrates for a short τpulse of 70 ns resulted in further relaxation of ΔT, leading to an increase of Jmax to around 5 kA/cm2. Interestingly, for such a short τpulse, devices with high-k sapphire and low-k plastic substrates showed similar Jmax and ΔT values, the reason for which may be that it is difficult for the generated Joule heat to travel t...

A study on low temperature SAM modified POM direct bonding affected by VUV/O 3 irradiation

A direct bonding of polyoxymethelene (POM) was feasible at 100 °C by using self-assembled monolayers (SAM) as surface modification method. (3-Aminopropyl)triethoxysilane (APTES) and (3-Glycidyloxypropyl)trimethoxysilane (GOPTS) were applied in our work. Surface modification carried out with different VUV/O3 irradiation conditions showed different bonding strength. In addition, the bonding condition with highest strength had an average strength of 0.37 MPa. This technology was expected to be used in packaging for micro/nano electromechanical systems (MEMS/NMES), such as bio-/medical devices.

Low temperature direct bonding of single crystal quartz substrates for high performance optical low pass filter using amorphous SiO2 intermediate layers

We proposed a HF-assisted single crystal quartz direct bonding method at low temperature using amorphous SiO2 layer for high performance optical low pass filter (OLPF) to improve heat resistance compared with conventional OLPFs using UV-curing adhesive. Amorphous SiO2 was deposited by ion beam sputtering on backside of both infrared reflection and anti-reflection coated substrates. By the etching rate evaluation, amorphous SiO2 deposition is considered to provide high active surface useful for bonding. The HF bonded sample with amorphous SiO2 layer achieved 0.8 MPa in tensile test and 3.3 MPa in shear test, and also nearly 100 % light transmittance was performed, which is as the same level as conventional UV-curing adhesive one. Therefore, the proposed single crystal quartz direct bonding with amorphous SiO2 layers is considered to be a promising technique to realize high performance OLPFs.

ST-quartz/LiTaO3 direct bonding using SiO2 amorphous layers with VUV/O3 pre-treatment for a novel 5G surface acoustic wave device

This paper describes a novel ST-cut quartz (ST-quartz)/LiTaO3 (LT) direct bonding for surface acoustic wave (SAW) devices of next 5G mobile communication. The ST-quartz and LT were bonded to fabricate temperature compensated piezoelectric substrates using amorphous SiO2 (α-SiO2) intermediate layers. The α-SiO2 thin layer was prepared on each substrate by ion beam sputtering (IBS) to realize highly active bonding interfaces and treated by vacuum ultraviolet irradiation in the presence of oxygen gas (VUV/O3). Then they were bonded under pressure of 5 MPa at 200 °C for 15 min in 100 kPa vacuum atmosphere. The tensile strength of 2.9 MPa was achieved in α-SiO2 substrate which is six times stronger than other samples; without intermediate layers or VUV/O3 pre-treatment. In addition, VUV/O3 bonding was compared with Mega-sonic bonding. VUV/O3 treated sample with AIB method slightly increase the bonding strength and achieved the same level of Mega-sonic bonding sample with AIB. Hence, it is indicated that AIB method could prepare the considerably activated surface even using low vacuum condition and affect effectively to hetero-monocrystalline bonding. This result suggested the proposed ST-quartz/LT direct bonding is a promising technique for future 5G SAW devices.

Fabrication of microchannel-TEM grid for in situ liquid observation of interfacial chemical reaction

We developed a novel microchannel-TEM grid that enables observation of interfacial biochemical reactions. A Y-shaped microchannel, which was sandwiched between two alumina membranes, realizes observation of interfacial reactions by lamellar flow. Reliable sealing of the microchannel was achieved by highly stable Al2O3/SU-8 bonding of 2.90 MPa at 180 °C. TEM images indicate the TEM grid has sufficient electron transparency and it was successfully fabricated without microchannel distortion. These results suggest that proposed TEM grid will become a powerful tool for revealing mechanisms of chemical reactions.

Highly flexible transparent electrodes based on mesh-patterned rigid indium tin oxide

We developed highly bendable transparent indium tin oxide (ITO) electrodes with a mesh pattern for use in flexible electronic devices. The mesh patterns lowered tensile stress and hindered propagation of cracks. Simulations using the finite element method confirmed that the mesh patterns decreased tensile stress by over 10% because of the escaped strain to the flexible film when the electrodes were bent. The proposed patterned ITO electrodes were simply fabricated by photolithography and wet etching. The resistance increase ratio of a mesh-patterned ITO electrode after bending 1000 times was at least two orders of magnitude lower than that of a planar ITO electrode. In addition, crack propagation was stopped by the mesh pattern of the patterned ITO electrode. A mesh-patterned ITO electrode was used in a liquid-based organic light-emitting diode (OLED). The OLED displayed the same current density-voltage-luminance (J-V-L) curves before and after bending 100 times. These results indicate that the developed mesh-patterned ITO electrodes are attractive for use in flexible electronic devices.

Compensation of Surface Roughness Using an Au Intermediate Layer in a Cu Direct Bonding Process

Copper-copper (Cu-Cu) direct bonding assisted by direct immersion gold (DIG) was demonstrated. Cu-Cu direct bonding is a critical technology for inductively coupled memory interconnections. To solve the problems of conventional methods of Cu-Cu direct bonding, a plating process using DIG to form an intermediate layer was selected. The concept of the developed bonding process is to use deformation of DIG to compensate for the surface roughness of the Cu substrates during application of pressure and annealing. Using this method, precise surface flattening of Cu substrates is not necessary. Bonding can be achieved even in an air atmosphere. A sample bonded at a temperature of 350°C failed within the chip in a shear test. It was found that bonding can be achieved when the gold (Au) thickness is greater than the half of the surface roughness of Cu at the bonding temperature. Transmission electron microscopy-energy-dispersive x-ray spectroscopy revealed that Au diffused into Cu during bonding. The diffusion constant of Au into Cu was investigated through a numerical calculation. The obtained results showed good agreement with the literature values.

Highly bendable transparent electrode using mesh patterned indium tin oxide for flexible electronic devices

We developed a highly bendable transparent indium tin oxide (ITO) electrode with mesh pattern for flexible electronic devices. Mesh pattern reduces an effect of tensile stress and propagation of cracks when the electrode is bent. The proposed ITO electrode was fabricated on a polyethylene terephthalate by means of photolithography and wet etching. The bendability was investigated through cyclic bending test. Resistance increase rate of the mesh patterned ITO electrode after 1000 times bending was approximately 9.18 × 102 times as low as that on a plane ITO electrode. In addition, the distinct cracks were not observed on the mesh patterned ITO electrode after cyclic bending. Mesh patterned ITO electrode was applied to a liquid-based organic light-emitting diode (OLED). Even the use of the mesh patterned ITO after 100 times bending, electroluminescence emission was confirmed without obvious damages. These results indicate that the mesh patterned ITO electrode gives an impact in flexible electronic devices.

Fabrication of self-standing curved film with pillar arrays by large area spherical soft-UV imprint lithography

We proposed a novel fabrication method of a pillar patterned self-standing curved film by spherical soft-UV imprint lithography. Soft-UV imprint lithography was performed using a patterned polydimethylsiloxane (PDMS) mold and a convex mold in order to pattern pillar onto large curved film. A mother patterned mold of spherical soft UV imprint lithography was prepared by unique 3D printer method. The curved film with the pillar pattern was successfully fabricated with a curvature radius of 40 mm. The proposed method is useful for 3D-structured fabrication of the optical devices.