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New Deposition Method Creates Resilient Thin Films
March 8, 2019

Today, researchers are continuing to explore new approaches to create resilient thin films, a coating as hard as a diamond and a fraction the thickness of a human hair, that could be used in a variety of electronic applications from cutting tools to the aerospace industry.

According to information provided by the American Vacuum Society (AVS) Science and Technology of Materials, Interfaces, and Processing, Grzegorz (Greg) Greczynski, an associate professor and principal research engineer in the Thin Film Physics (TUNNF) division of the Department of Phuysics, Chemistry and Biology at Linköping University, presented a new deposition method that improves the thermal and chemical stability of transition-metal nitride (TMN) thin-film layers during the 65th American Vacuum Society (AVS) International Symposium and Exhibition.

Researchers have alloyed TMNs with aluminum to enhance the stress and high-temperature oxidation resistance that make these films ideal for wear protection in high-temperature machining. Following conventional practices, the aluminum often precipitates as a wurtzite-aluminum nitride complex (w-AlN), that lacks the hardness characteristics necessary for mechanical applications.

During his presentation, “Selectable Phase Formation in Al-based Transition Metal Nitride Films by Controlling Al+ Subplantation depth during HIPIMS Deposition,” Dr. Greczynski discussed a new deposition technique that his team, in cooperation with professor Jochen Schneider’s group at RWTH Aachen in Germany, developed that uses two simultaneous plasma sources. One (dc-magnetron sputtering [DCMS]) produces a continuous flux of vanadium atoms to the metal substrate, while the second (high-power impulse magnetron sputtering [HIPIMS]) delivers periodic fluxes of ionized aluminum.

The nitrogen in the carrier gas, consisting of a mixture of nitrogen and argon combines with the aluminum ions and vanadium neutrals forming the thin film on the metal substrate. The key to this deposition method is the application of the substrate bias, which is synchronized to the aluminum ion flux.

“With our approach, the vanadium and aluminum are not present in the same volume at the same time,” Dr. Greczynski explained. “We intentionally separate vanadium from aluminum to overcome the limitations (i.e., w-AlN precipitation) of conventional processing.”

Dr. Greczynski and his team used Monte Carlo type simulations (TRIDYN) to better understand the interaction of the Al ions with the VAlN thin-film surface, in particular to calculate the depth that the aluminum ions penetrate into the thin film. By comparing the model results to experimental results, they honed the deposition process.

The research team controlled the depth of aluminum penetration in the film by simply adjusting the amplitude of the synchronous bias voltage pulse. By increasing their incident energy, the aluminum ions were driven deeper into the film, below the high-mobility surface layer, while creating supersaturated VAlN alloy films.

Aluminum ionization remains the limiting step in this new method. At best, the HIPIMS method only ionizes about 70 percent of the aluminum flux. The remaining 30 percent arrives to the metal substrate as neutrals that do not separate from the vanadium in the film.

Despite the incomplete ionization of aluminum ions, this new deposition method overcomes the limitations of conventional processing and enables unprecedented control over the phase formation and properties of the thin films. According to Dr. Greczynski, this deposition method could open the door to new opportunities for scientific endeavors.

“The fact that we can control the crystal structure of these metastable films in such a simple way is exciting,” noted Dr. Greczynski, “It is nice to see that we change the settings and obtain an immediate effect that makes sense.”

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