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.