Tetsuhide Shimizu¹, Caroline Hain^2,3, Yuji Oshida^1,2, Eva Vogt², Thomas Nelis^2,3, Johann Michler^2
1Tokyo Metropolitan University, Tokyo, Japan
²EMPA - Swiss Federal Laboratories for Materials Science and Technology, Thun, Switzerland
³University of Applied Sciences, Biel/Bienne, Switzerland
Nitride thin films are indispensable materials across diverse industrial sectors, including hard coatings, semiconductors, and optical devices, typically fabricated by reactive sputtering with nitrogen (N_₂). Film stoichiometry and crystallinity are strongly governed by the N_₂ partial pressure, but less attention has been given to the actual incorporation efficiency of nitrogen into the growing film. In particular, dissociation of N_₂ molecules into atomic nitrogen within the plasma is expected to critically influence surface reaction kinetics. This study investigates the role of highly ionized metal ions of high-power impulse magnetron sputtering (HiPIMS) in the discharge with activated nitrogen species during nitride film growth, with a focus on AlN deposition by microwave (MW)-assisted reactive HiPIMS. In this approach, AlN thin films were synthesized at very low N_₂ flow rates within the metallic regime, where enhanced deposition rates and improved process stability are advantageous for industrial application. To analyze discharge characteristics during the reactive mode transition, energy- and time-resolved mass spectrometry was performed using a time-of-flight mass spectrometer (E-ToFMS), enabling detailed analysis of ion dynamics under varying reactive gas conditions. The results demonstrate that highly crystallized, (0002)-oriented AlN films can be deposited at very low N₂ flow rates when MW plasma assistance is applied, whereas conventional HiPIMS under the same conditions yielded metallic Al films. Mass spectrometry revealed that even at reduced N_₂ flows, high fluxes of atomic and molecular nitrogen ions were present, particularly during the pulse-off time, highlighting their decisive role in sustaining film-forming reactions. These findings clarify the mechanism of AlN growth under low N₂ pressures and emphasize the importance of dissociated nitrogen species to improve the incorporation efficiency of nitrogen during reactive sputtering. The insights gained not only improve process control for AlN but also provide broader implications for the synthesis of other transition metal nitrides by HiPIMS in industrially relevant conditions.