Regulating the coefficient of thermal expansion in electrodeposited Invar alloy films for fine metal masks via vacuum and hydrogen annealing

Electrodeposited Invar alloy film is highly desirable for manufacturing fine metal masks (FMMs) used in Organic Light-Emitting Diodes (OLEDs). However, its development is limited by the high coefficient of thermal expansion (CTE). In this study, we successfully obtained a coarse-grained electrodeposited Invar alloy film with near-zero thermal expansion and inclusions smaller than 300 nm, achieved through abnormal grain growth (AGG) induced by second-phase particles. Our experiments confirmed that these second-phase particles are composed of (Fe, Ni)_1-xS. During AGG, the aggregation and coarsening of these second-phase particles lead to changes in local strain energy, and the combined effects of surface and strain energies results in the rapid preferential growth of (110)-oriented grains. The matrix grains are engulfed by large-sized grains and transformed into sub-grains, which merge with the abnormally growing grains through grain rotation. An inverse relationship was found between CTE and grain size. Following vacuum annealing at 800 °C for 2 h and hydrogen annealing for 1 h, the average grain size increased to approximately 400 µm, while the CTE was reduced to 0.3 × 10^–6/ °C. This study provides a new method for obtaining electrodeposited Invar alloy films that meet commercial applications criteria for and are suitable for FMM fabrication for high-definition OLEDs.