Tokyo, Japan - HiPIMS (high power impulse magnetron scanning) has been used by researchers at Tokyo Metropolitan University to produce thin films of tungsten that have unprecedented levels of film stress. They optimized the timing of a substrate bias pulse with microsecond precision to minimize impurities and defect to create crystalline films with stresses as low 0.03 GPa. This is similar to those obtained through annealing. Their research promises to provide efficient methods for creating metallic films for electronics industry.Modern electronics rely on the precise, nanoscale deposition thin metallic films onto surfaces. This can be difficult because "film stresses" can result from the film's microscopic internal structure. Over time, this can lead to buckling or curving. Heating or "annealing" is usually required to eliminate these stresses. Unfortunately, not all the best metals are suitable for this job. The melting points of tungsten are high, so the film must be heated to more than 1000 degrees Celsius. This is not only energy-intensive, but also severely restricts the options for substrate materials. It is now possible to make films from high melting metals without these stresses.Associate Professor Tetsuhide Shimizu from Tokyo Metropolitan University has been leading a team that is using a technique called high power impulse magnetron scanning (HiPIMS), which is a sputtering technique. Sputtering is the process of applying high voltage to a metal "target" and a sub-stratum. This creates a plasma consisting of charged gas atoms that bombard the target and form a metal vapor. These metal ions then fly towards the substrate and form a film. HiPIMS uses short bursts of high voltage. It is clear that there is some separation of the metal and gas ions arriving at the substrate after each pulse. A synchronized "substrate biased" pulse can be used to selectively accelerate these metal ions, creating denser films. However, residual stress remains despite all efforts.The team used argon gas as well as a target made of tungsten to examine how different ions arrived at the substrate over time. They used their knowledge of the time different ions arrive to select high-energy metal ions. Instead of setting off a bias pulse at the same moment as the HiPIMS pulse they created a small delay of 60 microseconds. This reduced the amount of gas that was left in the film, and delivered high levels of energy. This resulted in a dense, crystallized film with large grains and low film stresses. The films became less stressful by strengthening the bias. They were able to deliver energy efficiently to the film, which was similar to annealing. The team also used krypton to replace argon, resulting in films that had a stress of 0.03 GPa. This is comparable to post-annealing.A stress-free pathway to metallization will make a huge impact on the manufacturing of next-generation circuitry and metallization processes. This technology could be used to make other metals, and it promises huge gains for electronics industry.###This work was supported in part by the Fund for the Promotion of Joint International Research of the Japan Society for the Promotion of Science, (No.17KK0136), and the Swedish Research Council (No. VR 2018-04139, and the Swedish Government Strategic Research Area for Materials Science on Functional Materials (Faculty Grant SF-Mat-LiU Number. 2009-00971.
