Tokyo, Japan – Researchers at Tokyo Metropolitan University have used high power pulse magnetron scattering (HiPIMS) to create thin layers of tungsten with unprecedented low voltage. By optimizing the timing of a “substrate bias pulse” with microsecond precision, they minimized impurities and defects to form crystalline films with stresses as low as 0.03 GPa, similar to those achieved by annealing. Their work promises efficient ways of making metal films for the electronics industry.

Modern electronics are based on the complex, nano-scale deposition of thin metal films on surfaces. Easier said than done; If not done correctly, “film stress”, which results from the microscopic internal structure of the film, can, over time, cause it to kink and bend. To get rid of this tension, heating or “glowing” is usually required. Unfortunately, many of the best metals for this job, such as tungsten, have high melting points, which means the foil must be heated to over 1000 degrees Celsius. This is not only energy-intensive, but also severely restricts which substrate materials can be used. The race to produce films from metals with a high melting point without these stresses is on.

A team led by Associate Professor Tetsuhide Shimizu of Tokyo Metropolitan University has worked with a technique known as high power pulse magnetron scattering (HiPIMS) Sputtering Technology. In sputtering, a high voltage is applied to a metallic “target” and a substrate, creating a plasma of charged gas atoms that bombard the metallic target and form a charged metal vapor; these metal ions fly towards the substrate, where they form a film. With HiPIMS, the voltage is pulsed in short, powerful bursts. It is known that after each pulse there is a certain separation between the arrival of metal and gas ions at the substrate; A synchronized “substrate bias” pulse can help to selectively accelerate the metal ions and create denser films. But despite many efforts, the issue of internal stress remained.

Now the team used argon gas and a tungsten target to investigate how ions with different energies reached the substrate over time in unprecedented detail. Instead of using a bias pulse that was triggered simultaneously with the HiPIMS pulse, they used their knowledge of when different ions were arriving and introduced a tiny delay of 60 microseconds to precisely select for the arrival of high-energy metal ions. They found that this minimizes the amount of gas that gets into the film and efficiently delivers a great deal of kinetic energy. The result was a dense crystalline film with large grains and low film stress. By increasing the bias, the films became more and more stress-free. The efficient transfer of energy to the film actually achieved an effect similar to that achieved with tempering while they put the film off. By further exchanging argon for krypton, the team realized films with a voltage of just 0.03 GPa, comparable to what can be produced after annealing.

An efficient route to stress-free films will have a significant impact on metallization processes and the manufacture of next generation circuits. The technology can be transferred to other metals and promises great advantages for the electronics industry.

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This work was supported by the Fund for the Promotion of Joint International Research (No. 17KK0136) of the Japan Society for the Promotion of Science (JSPS), the Swedish Research Council (No. VR 2018-04139) and the Swedish Government Strategic. supports Research Area in Materials Science on Functional Materials at Linköping University (faculty grant SFO-Mat-LiU No. 2009-00971).

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