Unconstrained Path to Unconstrained Metal Films Paves the Way for Next Generation Circuits

(top left) An illustration of the HiPIMS process (top right) The energy distribution of tungsten ions arriving at the substrate over time. In the short term, there is a large proportion of high energy ions. (bottom) Stress-free tungsten films created with the selective pulsed polarization technique. (a) Transmission electron microscopy (TEM) image in plan view of the film; (b) a higher resolution image; (c) reconstructions of the area selected in (b) on the basis of inverse Fourier transforms, with two enlarged regions. Credit: Tokyo Metropolitan University

Researchers at Tokyo Metropolitan University have used High Power Pulse Magnetron Scattering (HiPIMS) to create thin films of tungsten with unprecedented levels of film stress. 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 obtained by annealing. Their work promises effective avenues for creating metallic films for the electronics industry.

Modern electronics rely on the complex, nanoscale deposition of thin metallic films on surfaces. It’s easier said than done; Unless done correctly, film stresses resulting from the microscopic internal structure of the film can cause buckling and curvature over time. Getting rid of these stresses usually requires heating or annealing. Unfortunately, many of the best metals for the job, for example tungsten, have high melting points, which means the film must be heated to over 1000 degrees Celsius. Not only does this consume a lot of power, but it greatly limits the substrate materials that can be used. The race is on to create films from high melting point metals without these constraints in the first place.

A team led by Associate Professor Tetsuhide Shimizu of Tokyo Metropolitan University worked with a technique known as High Power Pulse Magnetron Scattering (HiPIMS), a sputtering technique. Sputtering involves applying a high voltage across a metal target and a substrate, creating a plasma of charged gas atoms which bombards the metal target and forms a charged metal vapor; these metal ions fly to the substrate where they form a film. In the case of HiPIMS, the voltage is pulsed in short, powerful bursts. After each pulse, we know that there is a certain separation between the arrival of metal and gas ions at the level of the substrate; a synchronized substrate polarization pulse can help selectively accelerate metal ions, creating denser films. Yet despite many efforts, the issue of residual stress remained.

Unconstrained Path to Unconstrained Metal Films Paves the Way for Next Generation Circuits

Measurements of film voltage and lattice properties for films created without polarization (float), with 50 V DC polarization, with synchronized pulsed substrate polarizations (50 V, 100 V, 200 V) using l argon as the sputtering gas and with 50V synchronized pulsed substrate polarization using krypton as the sputtering gas. FWHM (Full Width at Half Maximum) is a measure of the degree of order of atoms in films (the lower the FWHM, the more ordered); the lattice parameter is the size of the repeating cells of the crystal film, with a limit given by the hypothetical “perfectly relaxed” or unconstrained crystal. Credit: Tokyo Metropolitan University

Now, using argon gas and a tungsten target, the team examined how ions of different energies arrived at the substrate over time in unprecedented detail. Instead of using a bias pulse triggered at the same time as the HiPIMS pulse, they used their knowledge of when different ions arrived and introduced a tiny delay, 60 microseconds, to precisely select the arrival of high energy metal ions. They found that this minimized the amount of gas entering the film and efficiently delivered high levels of kinetic energy. The result was a dense crystalline film with coarse grains and low film stress. By reinforcing the bias, movies have become increasingly stress-free. The efficient supply of energy to the film meant that they had, in fact, achieved an effect similar to annealing while they were depositing the film. By further replacing argon with krypton, the team achieved films with a stress as low as 0.03 GPa, comparable to what can be achieved with post-annealing.

Unconstrained Path to Unconstrained Metal Films Paves the Way for Next Generation Circuits

(a) – (d) show cross sections of films made using different sputtering gases and pulsed bias voltages, imaged by scanning electron microscopy. Notice how the columns (or “grains”) thicken from (a) – (c); columns of similar thickness are visible in (d). Credit: Tokyo Metropolitan University

An efficient path to unconstrained films will have a significant impact on metallization processes and next generation circuit fabrication. The technology can be applied to other metals and promises big gains for the electronics industry.

Electrons in plasma can be used to produce metallic films

More information:
Tetsuhide Shimizu et al, Low-temperature growth of stress-free single-phase α-W films using HiPIMS with synchronized pulsed substrate bias, Journal of Applied Physics (2021). DOI: 10.1063 / 5.0042608

Provided by Tokyo Metropolitan University

Quote: A Stress Free Path to Stress Free Metallic Films Paves the Way for Next Generation Circuits (2021, July 5) retrieved July 5, 2021 from https://phys.org/news/2021-07-stress-free -path- metal-paves-next-gen.html

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