A low-temperature Cu–Cu bonding technique using a thin metal intermediate layer deposited by atomic layer deposition (ALD) was developed. A thin Pt intermediate layer was selectively deposited on the Cu surface at the angstrom level by ALD without any mask under low vacuum conditions (24 Pa). To suppress the deterioration of bonding reliability caused by impurities at the bonding interface, quasi-direct bonding was realized by using a thin Pt intermediate layer. The Cu–Cu quasi-direct bonding with a thin Pt layer provided a bonding strength of 9.5 MPa, which was five times higher than that without the intermediate layer (1.9 MPa). These results will contribute to the development of low-temperature Cu–Cu bonding for three-dimensional integrated circuit chips.

A materials-informatics technique for designing strong flat interfaces has been developed by use of advanced molecular simulations that can calculate the delamination energy as the adhesion strength. In this study, this technique is applied to the design of metals with strong adhesion to organic materials such as polyimide and DNA. At the first stage, the interatomic spacings were selected as the important, dominant metal parameters from four metal parameters (the short-distance and long-distance interatomic spacings, electronegativity, and surface energy density) by using sensitivity analysis based on the design-of-experiments method with the delamination-energy data calculated from advanced molecular simulations. At the second stage, the adhesion strength (delamination energy) was expressed as a function of the important metal parameters (i.e., the short-distance and long-distance interatomic spacings) by using a response-surface method (Kriging method). At the third stage, by solving the maximum-value problem of the function, it was found that the metal that has the same short-distance and long-distance interatomic spacings as those of organic materials has the strongest adhesion to the organic materials. Finally, it was found that the metal (Ni-12%Mn) whose lattice matches with polyimide has the strongest adhesion to polyimide and that the Zr/Dy/Y multilayer whose lattice matches with B-DNA's lattice has the strongest adhesion to B-DNA.

To investigate the influence of the difference in module structures on the degradation of crystalline silicon solar cells, two different photovoltaic modules were fabricated, and a high-temperature and high-humidity test was carried out. The degradation modes of these modules were compared to each other using electrical characteristics and electroluminescence images. Degradation of outputs occurred in both modules, and different degradation modes were confirmed by electroluminescence images. The difference in degradation modes between these modular structures could be due to the difference in concentration distribution attributable to the moisture and acetic acid generated from the encapsulant inside the module.

For realizing stable and high-speed communication systems, the spectral of signals need to be purified, thus, an asymmetric third-order bandpass filter, which composed by open-circuited stubs loaded dissimilar stepped impedance resonators, is proposed to realize 20 dB stop-band attenuation up to 20.6f₀ (17.85 GHz). Open-circuited stubs are loaded to resonators for scattering harmonics and suppressing the unexpected harmonics. Asymmetric coupling structure has been applied to suppress the harmonics and the inevitable fly-over coupling between resonators. Stepped impedance resonators are designed with extremely low impedance ratio (R_z) for enhancing the open-circuited stubs and asymmetric coupling structures' suppression on harmonics and reducing the circuit size. Resonance frequencies, external quality factor (Q_e) and coupling coefficient (K) between dissimilar resonators have been synthesized and derived by admittance analysis. The open-circuited stub and K's influence on fundamental resonance, and Q_e have been derived by equations and investigated by simulations respectively. Both the calculations and simulations show that the open-circuited stub and asymmetric coupling structure influence fundamental resonance little while the harmonics significantly.

This paper reports a method for determining the gate bias voltage condition during transient temperature measurement using a SiC–MOSFET body diode. The Vg–Vsd characteristic of the device was investigated. There exists a plateau where Vsd does not change with changes in Vg. In this plateau, transient temperature measurement using a body diode is possible under the gate bias condition. This method was applied to three devices with different characteristics. The results confirmed that transient temperature measurement is possible by selecting appropriate Vg values according to the relevant device characteristics. The transient temperature of each module in which three devices (Device A, Device B1, and Device B2) are mounted in the same structure package are measured. The three transient temperature graphs agree well, and the three structure function graphs are also in good agreement. Thus, the proposed method allows the rapid determination of parameters of a transient thermal analysis for an unknown device.

The polymorphic transformation that occurs in the Cu₆Sn₅ intermetallic compound (IMC) at 186°C has the potential to generate stresses that could lead to cracking of that phase in soldered joints during the multiple reflow cycles of a typical printed board assembly process and the thermal cycles to which electronic assemblies are exposed during service. In this paper the authors report on the effect of variations in the cooling stage of a reflow soldering thermal profile on the incidence and extent of cracking in the Cu₆Sn₅ at the interface between solder alloys and copper substrates. The solder alloy/substrate combinations studied were Sn-3.0Ag-0.5Cu/Cu and Sn-0.7Cu-0.05Ni-1.5Bi/Cu. The cooling conditions were (i) the direct-cooling of a conventional reflow profile, and (ii) an alternative reflow profile with one of three extended isothermal holding periods of 30, 60, and 180 seconds at 140°C during the cooling stage. It was found that the alternative reflow profiles reduced cracking in the interfacial Cu₆Sn₅ IMC layer and this resulted in improved resistance of the reflowed solder ball to failure in high speed impact shear when the distribution of stress tends to favor crack propagation though the interfacial IMC rather than through the bulk solder.

LiTaO₃ (LT)/quartz or LiNbO₃ (LN)/quartz bonded surface acoustic wave (SAW) substrates with an amorphous intermediate layer have been proposed for high-frequency communication. Requirements for 5G mobile communication are high-performance SAW substrates with a large SAW velocity, a small temperature coefficient of frequency, and a large electromechanical coupling factor. Reduction of the residual stress of the amorphous intermediate layer is expected to improve the bonding strength and SAW characteristics of the bonded substrate. In this report, a method of low-residual-stress amorphous film deposition for LT/quartz or LN/quartz bonding was studied. The residual stresses of amorphous SiO₂ and Al₂O₃ films deposited by ion beam sputtering, electron cyclotron resonance sputtering, and atomic layer deposition (ALD) were evaluated. The LT substrate with an amorphous Al₂O₃ film deposited by ALD (ALD-Al₂O₃) had the minimum warpage of 0.152 µm and residual stress of 127.3 MPa. The ALD-Al₂O₃ film formed with near-identical thicknesses on both sides of the LT substrate simultaneously, which is likely the source of the low residual stress of the ALD-Al₂O₃ film. A maximum LN/quartz bonding strength of 3.7 MPa was achieved with the ALD-Al₂O₃ film. These results indicate that ALD-Al₂O₃ films are promising materials for LT/quartz or LN/quartz SAW substrates in 5G devices.

We made the large experimental tool where Photodesmear could process an actual size of the print circuit board by static irradiation. And we proved that handling of such panel size was technically possible. The connection reliability of the contact via was evaluated after electric copper plating by quick via peel examination. We made a cross-section sample of the contact via holes and the smear removal properties of wet desmear processing and the Photodesmear processing were compared with the residual smear and the oxidation thin layer by observation of the connection interface. The interfacial surface state after desmear processing was analyzed in an X-ray photoelectron spectrum analyzer. We produced the test vehicle using Photodesmear technology and sputtering seed technology. We compared it with the same pattern sample produced by a conventional process. In this paper, we report the results of the highly accelerated temperature and humidity stress test.

Herein we report on a copper (Cu) sinter die bonding paste that sinters under pressureless bonding conditions for power devices operating at high temperatures. The shear strengths of pressureless sintered Cu on four different metallization layers (Ni, Cu, Ag, and Au) were studied by experiment. The sintering behavior of Cu nanoparticles and diffusion coefficients at interfaces between a bulk Cu layer and the metallization layers was also evaluated by molecular dynamics (MD) simulation. After aging (573 K, 8 h), the shear strength of the Cu sintered to the Ni and Cu layers increased, whereas that of Cu sintered to the Ag and Au layers decreased. The interdiffusion at the interfaces between the sintered Cu layer and the Ag or Au layer increased the interfacial porosity of sintered Cu, which decreased the shear strengths in the sintered Cu/Ag and Cu/Au systems. In contrast, the interfacial porosity between the sintered Cu and the Ni or Cu layer hardly changed after aging. MD simulations revealed that Kirkendall voids were promoted by higher interdiffusion coefficients and a higher ratio of intrinsic diffusion coefficients between a bulk Cu layer and an Ag or Au layer. This consequently increased the porosity of Cu sintered near the interfaces.

Eleventh-order Chebyshev function types of 2.6-GHz-band low-pass filter (LPF) and 5.4-GHz-band high-pass filter (HPF) are configured using both chip components and patterned lumped elements. Compact matching circuits for an LPF-HPF diplexer are also proposed. They are able to absorb the first elements of the LPF and the HPF. The fabricated LPF-HPF diplexer has realized high isolation characteristics of more than 45 dB, reflection characteristics of more than 15 dB, and low transmission loss of less than 1.74 dB in each passband. Finally, three types of the bandpass filter-bandpass filter (BPF-BPF) diplexers with two BPFs are composed with the proposed LPF-HPF diplexer, conventional matching circuits and a commercial diplexer. When compared using a circuit simulator, the BPF-BPF diplexer composed of the proposed LPF-HPF diplexer and two BPFs has higher isolation characteristics between two channels than other circuits.

This paper discusses a transient thermal testing approach for Gallium Nitride High Electron Mobility Transistor (GaN HEMT) with p-type gate. Conventional transient thermal characterization methods for Si (Silicon) based device have difficulties in characterizing GaN HEMTs. The non-ideal characteristics of such power devices make thermal characterization difficult. The proposed transient thermal characterization method for GaN HEMTs uses the knee voltage of PN junction in gate and source of GaN HEMT as a junction temperature sensor. The proposed method enables the acquisition of the time response of temperature with sufficient accuracy. The proposed method is validated on a commercially-available GaN HEMT device.

Maintaining the water content in the human body is necessary to prevent heatstroke on construction sites in the hot sun. As such, determining levels of perspiration for the entire body is important. This study focused on helmets (or hard hats), which are mandatory on construction sites, to develop a wearable helmet device that can measure the amount of perspiration while completing various activities. The results of our experiments indicate the feasibility of measuring whole-body perspiration levels using this device. In addition, we show that the estimated perspiration rate can potentially be used as an index for the earlier identification of heatstroke occurrence compared with the traditional indicator, i.e., a rise in core body temperature.

This paper describes sintering properties and bonding abilities of mechanical pressure-assisted sintered copper (pressure-sintered Cu) in N₂ atmosphere for power devices with large chips. The pressure-sintered Cu showed equal or improved sintering and bonding properties compared with pressureless-sintered Cu and pressure-sintered silver (Ag). The bonding abilities of pressure-sintered Cu on two different Ag plating (Ti/Ni/Ag, Ti/Ni/Au/Ag) were also investigated. The Cu pressure-sintered at 2 MPa and 300°C for 5 min was successfully bonded to 10 × 10 mm² chips with Ti/Ni/Ag. Thus, pressure-sintered Cu has a potential as a reliable die-bonding material for power devices with large chips.

The construction industry is known for a high number of casualties resulting from heat strokes. Heatstroke risk management at construction sites is therefore a crucial issue and needs to be addresses. One method to prevent heatstroke is to maintain the amount of water in the human body. To achieve this, it is essential to monitor the amount of perspiration of construction workers during working hours. In this study, we shifted focus toward helmets that are worn by workers at construction sites. The helmet that we developed can measure the amount of perspiration and we use the data to estimate whole-body perspiration.

The electrochemical migration resistance was improved by plating Ni-P on the surface of L/S = 2/2 μm Cu wiring. Ni-P plating film became more uniform by increasing dissolved oxygen in the Pd catalyst solution. This is because oxidation of the Pd-substituted film increased a catalytic activity of Pd.

We have been developing a three-dimensionally (3D) structured complementary metal-oxide semiconductor (CMOS) image sensor (CIS), which has individual signal processing circuits in each pixel under the photoelectronic conversion area for high-performance and multi-functional operation. In this paper, we report on our experimental 3D integrated circuits developed using multi-stack technology, which enables us to fabricate 3D-CISs with small pixels. The results showed the fundamental operation of the prototype circuit, which indicates the feasibility of highly integrated 3D-CIS of More-than-Moore type applications.

We investigated an effect of a solution-processed electron injection layer (EIL) on an electroluminescent (EL) performance of organic light-emitting diode with a liquid emitting layer (Liquid OLED). A pyrene-based liquid organic semiconductor, 1-pyrenebutyric acid 2-ethylhexyl ester (PLQ), was used as a greenish-blue emitter. Zinc oxide nanoparticles (ZnO NPs) were deposited on an indium tin oxide (ITO) cathode by a spin-coating method. We fabricated two kinds of liquid OLEDs with ITO anode/PLQ/ITO cathode and ITO anode/PLQ/ZnO NPs (EIL)/ITO cathode. Current density and luminance were increased significantly by inserting the ZnO NPs between the ITO cathode and PLQ. We expect that the solution-processed ZnO NPs can be useful in developing microfluidic OLED displays.

We designed hardware-based neural networks (HNNs) that resemble neural functions of the lamprey using electronic circuits. The lamprey has a relatively simple nervous system among vertebrates. In addition, the central pattern generator of the lamprey is better understood than that of other vertebrates. Hence, we designed the HNNs based on the known structure of the lamprey nervous system. The HNNs can generate burst firing that imitates two functions of lamprey. In one function, the left and right segments alternately generate oscillatory burst firing. In the other function, burst firing propagates from head to tail of the lamprey. Therefore, we achieved biomimetic HNNs that generate oscillatory activity from the lamprey nervous system.

We studied a method of quad flat package (QFP)/quartz adhesive bonding. This bonding technology is required for developing physical security of edge artificial intelligence (AI) devices using a nanoartifact metrics (NAM) chip. Relationships between contact angle and bonding strength of QFP/adhesive or quartz/adhesive were investigated under several surface treatment methods including vacuum ultraviolet (VUV) treatments, and plasma treatments. VUV irradiation in the presence of nitrogen gas (VUV/N₂) treatment reduced the contact angle of the QFP and quartz surface from 99.2° to 5.0°, which improved QFP/adhesive bonding strength 5.7 times. Furthermore, the bonding strength of QFP/adhesive/quartz bonding with the VUV/N₂ treatment shows a shear strength of 14.2 MPa. It is 2.4 times higher value than that without the treatment. These results are a promising step for highly reliable QFP/quartz bonding.

In this study, a 160°C low-temperature bonding method was developed for fine pitch chip-stacking application by Solid-Liquid Inter-diffusion (SLID) bonding technology. The chip-to-wafer test was conducted by using Cu/Sn-Ag and Cu/Ni/In as the micro pillar bump structure for top chip and bottom wafer, respectively. Indium, with a low melting point of 157°C, was chosen to realize the SLID bonding mechanism in this study. A thin indium layer with a thickness of 1 μm was plated on nickel to induce low temperature bonding with tin. These 30 μm-pitch interconnects bonded at low temperature were well-bonded and exhibited excellent electrical continuity through 3,264 I/Os. Furthermore, the bonded samples were tested under reliability assessments to verify the thermal stability.

The electrical resistance of interconnection fabricated by electroless Ni(P) and Au plating was measured by four-point probe measurement technique. Prior to the bonding, the average electrical resistance of electroplated copper pillars was 0.21 mΩ. After bonding with electroless Ni(P) plating, the average electrical resistance of joints was 8.4 mΩ and even decreased to 4.8 mΩ with 400°C post-aging process. As for bonding with electroless Au plating, two different kinds of surface finish processes were applied. The averaged electrical resistances of electroless nickel immersion gold (ENIG) joints and direct immersion gold (DIG) joints were 3.00 mΩ and 1.76 mΩ, respectively. In comparison with conventional SnAgCu solder joints (760 μm) (7.1 mΩ,), joints bonding with electroless plating has smaller electrical resistance.