Understanding Reproducibility of Sputter Deposited Metastable Ferroelectric Wurtzite Al0.6Sc0.4N Films using In‐situ Optical Emission Spectrometry

High-Sc Al_1–xSc_xN thin films are of tremendous interest because of their attractive piezoelectric and ferroelectric properties, but overall film quality and reproducibility are widely reported to suffer as x increases. In this study, structural and electrical properties of metastable Al_0.6Sc_0.4N films are connected with plasma changes during film growth, identified via glow discharge optical emission spectroscopy (GD-OES), and linked to the target mode changes. This in situ GD-OES technique uses changes in the N₂(I) intensity, correlated with DC bias hysteresis behavior of a Al_0.6Sc_0.4 target in metallic and poisoned modes, to identify films that subsequently exhibit unacceptable structural and electrical performance. Two representative samples deposited under identical conditions but possessing distinct properties related to phases present in the films are focused on. Films sputtered under a poisoned target mode produce pure wurtzite ferroelectric Al_0.6Sc_0.4N with a reversible 80 μC cm⁻¹ polarization and 3.1 MV cm⁻¹ coercive field. When identical chamber settings are used but the process starts in metallic mode, a mixed wurtzite/rocksalt film is deposited which exhibits nanometer-scale changes to the film microstructure and a nonferroelectric response. These results illustrate the utility of optical emission spectroscopy for tracking target mode fluctuations when fabricating metastable materials such as high-Sc Al_1–xSc_xN films.