Ayumu Matsumoto

Electropolishing is an essential process for the surface treatment of metallic materials. To determine the appropriate replacement timing of electropolishing solutions for their efficient use and improved productivity, it is important to periodically analyze the amounts of dissolved metals in the solutions. However, these solutions are typically highly corrosive, and on-site analytical techniques that can be easily applied at production sites have not yet been established. In this study, we demonstrated microvolume liquid analysis using low-energy laser-induced breakdown spectroscopy (LIBS) combined with a porous silicon substrate fabricated by metal-assisted etching (metal-assisted chemical etching) and a non-contact gas-blowing pretreatment. In the analysis of electropolishing solutions used for niobium superconducting cavities and stainless steel products, emission lines of niobium and of iron and chromium were successfully detected after blowing the respective microdroplet samples on porous silicon, and linear correlations were observed between the spectral line intensity and the polished amounts. The present results provide a basis for future on-site application of LIBS to highly corrosive electropolishing solutions in the metal finishing industry.

In gallium arsenide (GaAs) semiconductor devices, wafer warpage caused by backside electrode stress complicates wafer handling during subsequent processing and affects device characteristics. Warpage has been primarily attributed to a high-stress nickel–gallium–arsenide (Ni3GaAs) reaction layer that forms at the interface between the electroless Ni–P plating film and the GaAs substrate during annealing at 240 °C for 1 h. In this study, the crystallinity of the Ni3GaAs layer and its lattice-matching state with the GaAs substrate were investigated using transmission electron microscopy. Ni3GaAs grew epitaxially, with its (0-111) plane aligned with the GaAs(001) plane, and four differently oriented microtwin crystals coexisted. Each microtwin crystal exhibited a columnar morphology approximately 10 nm in width and was aligned along the [001] GaAs direction. Ni3GaAs crystals were lattice-matched to the (220) and (2–20) planes perpendicular to the GaAs(001) surface and exhibited 0.8% compression and 3.3% elongation in the Ni3GaAs [01-12] and [2-110] directions, respectively, resulting in an average tensile strain of 1.3%. This strain was identified as the cause of wafer warpage. These findings clarify the microstructure and lattice strain characteristics of the Ni3GaAs alloy layer, contributing to a better understanding of crystal growth at the Ni3GaAs/GaAs interface and informing optimization strategies for GaAs device manufacturing.

This study represents the first use of porous silicon as a substrate in underwater LIBS for detecting dissolved elements.

This is the first report that applied LIBS to the analysis of electropolishing solution. Quantitative analysis of Nb dissolved in the solution was demonstrated by using porous silicon as the sample loading substrate.

Metal-assisted etching (metal-assisted chemical etching) is an efficient method to fabricate porous silicon (Si). When using platinum (Pt) particles as metal catalysts in metal-assisted etching, a composite porous structure of straight macropores formed beneath the Pt particles and a mesoporous layer formed on the entire surface of Si can be fabricated. The formation mechanism of the composite structure is still open to discussion. We previously demonstrated that the ratio of mesoporous layer thickness to macropore depth showed a large value (approximately 1.1) in the case of highly-doped p-Si. In this study, we investigated the composite structure formation by using p-Si substrates with different doping densities and etching solutions with different concentrations of hydrogen peroxide (H₂O₂). There was not significant difference in the structures formed on low- and moderately-doped Si, despite the large difference in doping density. The ratio of mesoporous layer thickness to macropore depth increased within the range approximately from 0.1 to 0.4 with increasing the H₂O₂ concentration in the case of low- and moderately-doped Si, but it did not change in the case of highly-doped Si. We discussed the observation results based on the spatial distribution of hole consumption and the band structures at Pt/Si and Si/electrolyte interfaces.

Strong correlations were found between underwater LIBS signals and bubble collapse time. Signal fluctuation caused by the repeated irradiation at a fixed position was successfully reduced by the normalization with bubble collapse time.

Platinum (Pt) is one of the interesting catalysts in metal-assisted etching (metal-assisted chemical etching) of silicon (Si). The Pt-assisted etching induces not only the dissolution of Si under the Pt catalysts but also the formation of mesoporous layer on the Si surface away from them. In this work, we etched n-Si and p-Si by using patterned Pt films with a diameter of 5 μm and an interval of 50 μm. For both the cases, the Si surface under the Pt catalysts was selectively etched and macropores with a diameter of 5 μm were formed. The macropores formed on n-Si were deeper than those formed on p-Si. The mesoporous layer was observed only around the macropores on n-Si, while it was observed over the entire surface of p-Si. We also measured the open circuit potential of Si in the etching solution. The positive shift of potential of n-Si by the Pt deposition was smaller than that of p-Si except for the initial stage of etching, which can be explained by the polarization characteristics. We discussed the etching behavior of n-Si and p-Si on the basis of the results of structure observation and electrochemical measurements.

For this study, we investigated anodization to improve the adhesion and corrosion resistance of A5052 aluminum alloy. Sulfuric acid anodization was found to have excellent corrosion resistance but poor adhesion. Phosphoric acid anodization showed excellent adhesion but poor corrosion resistance. Two-step anodization with phosphoric acid anodization and sulfuric acid anodization produced both excellent adhesion and corrosion resistance.

A nickel-metal hydride (Ni–MH) prototype battery completely immersed in an aqueous electrolyte solution of KOH under high-pressure was fabricated to examine the effects of high-pressure on the quality of Ni–MH batteries. The small battery cell comprised positive and negative electrode materials, as used in electric vehicles, and an Ag/AgO reference electrode. The electric capacity of the Ni–MH battery was measured at different temperatures and pressures with small currents and charge/discharge voltages of 1.6–1.0 V. High-pressure was found to clearly and effectively enhance the electric capacity of the Ni–MH battery at larger currents. The considerable effect of high-pressure on the Ni–MH battery was elucidated by the change in internal resistance during the charge/discharge cycle life experiment, indicating that the voltage of the positive electrode did not appreciably change under high-pressure compared to that of the negative electrode. Moreover, the use of large currents in rapid charge/discharge cycle tests at high pressures of up to 30 MPa resulted in charge/discharge cycles that were five times faster and a quick recovery of capacity was achieved in the 0.5–2.1 V range.

<p>The LIBS signal of the dry residue from a small amount of liquid sample is significantly enhanced by using a porous silicon substrate produced by gold-nanoparticle-assisted etching.</p>