The significant improvements of DC magnetron sputtering plasma source via cryogenic cooling method for spintronics applications

This study investigates the significant enhancement of DC magnetron sputtering plasma sources using a cryogenic cooling system to focus on its impact on thin-film deposition processes. The efficiency of magnetron-sputtering plasma sources directly depends on the in-plane magnetic fields, which are generated by permanent magnets installed inside the plasma source head. However, the permanent magnets are severely deteriorated when the system temperature is increased due to the plasma ignitions. In this case, the cryogenic cooling system effectively overcomes this limitation and it can improve the magnetic flux density and performance stability of the plasma source. Experimental results demonstrate that cryogenic cooling enhances deposition rates and film uniformity for both ferromagnetic (cobalt) and non-magnetic (tungsten) materials. Cobalt (Co) and tungsten (W) targets serve as representative examples, with deposition rates improving by approximately 27 % and 19 %, and uniformities increasing by 18 % and 19 %, respectively, compared to water-cooled systems. Additionally, the cryogenic cooling method allows for the use of thicker ferromagnetic targets that the magnetic target thickness limit is widely extended to 5.0 mm, which can reduce maintenance costs and process interruptions. From the systematic numerical simulations, one can be revealed that a strong magnetic field distortion due to a thin magnetic target is observed and it can occur the degradation of the efficiency of the plasma source. Moreover, the in-plane magnetic fields from permanent magnets are dramatically increased with decreasing the system temperatures. This study can highlight that the cryogenic cooling method of the magnetron sputtering plasma source is a transformative technology to improve the efficiency and reliability of the thin film deposition process for spintronic devices.