Erdong Chen¹, Caroline Hain², Daniel Lundin³, Stephanos Konstantinidis⁴, Tetsuhide Shimizu^1
1Tokyo Metropolitan University, Tokyo, Japan
²EMPA - Swiss Federal Laboratories for Materials Science and Technology, Thun, Switzerland
³Linköping University, Linköping, Sweden
⁴University of Mons, Mons, Belgium
Vanadium dioxide (VO₂) is a thermochromic material that undergoes a metal-insulator transition (MIT), leading to significant changes in its electrical properties. This transition can be exploited e.g. for use in memristors within neuromorphic perception computing systems. However, achieving high crystalline quality VO₂ films via reactive sputtering is challenging due to the limited controllability over oxygen gas flow. Compared with traditional sputtering technology, reactive high power impulse magnetron sputtering (R-HiPIMS) offers an alternative route to process control by regulating the incident ion fluxes that govern film growth at the substrate. This is achieved via peak current regulated pulse frequency feedback control, rather than by direct adjustment of oxygen gas flow.
Hysteresis measurements were performed to assess process stability by monitoring peak current as a function of oxygen gas flow. But an abrupt decrease in peak current was observed with increasing O₂ flow in traditional single-pulse HiPIMS. This behavior hindered process controllability and indicated the formation of vanadium pentoxide (V₂ O₅) on the target surface.
To address these challenges, very short multi-pulse sequences were applied in the R-HiPIMS process. By comparing the single-pulse and multi-pulse HiPIMS modes, the latter approach eliminated the abrupt drop in peak current evolution. Moreover, QCM (quartz crystal microbalance) revealed that multi-pulse HiPIMS provided high deposition rate at higher oxygen flow. The pulse dynamics were systematically investigated using energy‑resolved ion mass spectrometry to further confirm the stability of the multi‑pulse HiPIMS mode and its improved controllability for VO₂ film deposition. The successful application of multi‑pulse HiPIMS for reactive sputtering process control may open new avenues for other metal‑oxide and metal‑nitride systems and could have significant impact on related industrial applications.