Saccharin-Controlled Electrodeposition of Ultra-Thin Fe–Ni Foils for Low CTE

The fine metal mask (FMM) used in OLED display manufacturing is a thin metallic sheet with precision holes that enable the deposition of diode materials exclusively onto sub-pixel areas. To prevent deformation and misalignment of the holes caused by heat generated during the RGB organic material deposition process, Invar sheets with a coefficient of thermal expansion (CTE) close to zero are employed. Currently, FMMs are fabricated by rolling to about 20 μm, but this top-down process cannot achieve thinner foils. Therefore, to realize next-generation UHD displays requiring FMMs below 10 μm, a bottom-up electrodeposition method is needed. Achieving both dimensional stability and a low CTE comparable to commercial Invar alloys in electrodeposited ultra-thin Invar sheets requires prolonged high-temperature heat treatment above 600 °C using thermomechanical processing. If the electrodeposited Invar alloy can achieve a homogeneous nanocrystalline microstructure in the as-deposited state, the required heat-treatment temperature and duration can be reduced, improving overall process efficiency. In this study, we investigated the effect of controlling saccharin, a key additive in the Fe–Ni electrolyte, on the microstructural evolution of ultra-thin electrodeposited Invar sheets. By regulating surface stress during electrodeposition, Fe and Ni ion diffusion was promoted, and the relationship between microstructural uniformity and CTE was systematically analyzed.