Real-Time Compositional Monitoring of Sputtered Li₃PO₄ Electrolyte Films via In Situ Optical Emission Spectroscopy

Lithium phosphate (Li₃PO₄) is a key solid electrolyte for all-solid-state thin-film batteries. Although sputtering enables high-throughput film deposition, precise compositional control is essential because ionic conductivity is highly sensitive to stoichiometry. In this study, optical emission spectroscopy was employed for real-time compositional monitoring of Li₃PO₄ films during sputtering. Emission spectra acquired under various powers (20-40 W) and chamber pressures (1-5 Pa) were analyzed, and the intensities of Li I, P I, and O I lines were correlated with the atomic ratios determined by inductively coupled plasma mass spectrometry and field-emission electron probe microanalysis. Strong correlations were observed between P I 253.5 nm and O I 777.3 nm (R² = 0.977, 1-5 Pa), Li I 670.8 nm and P I 253.5 nm (R² = 0.837, 1-5 Pa), and Li I 391.5 nm and O I 777.3 nm (R² = 0.940, 3-5 Pa), demonstrating that these emission lines are suitable for in situ compositional analysis. The emission lines composing these pairs exhibited identical input power dependence, providing a useful criterion for selecting appropriate diagnostic lines. This study represents the first demonstration of real-time compositional monitoring for Li- and P-containing films, offering a promising approach for process control during sputtering.