Showing papers by "Ulf Helmersson published in 2021"

Experimental verification of deposition rate increase, with maintained high ionized flux fraction, by shortening the HiPIMS pulse

Abstract: High power impulse magnetron sputtering (HiPIMS) is an ionized physical vapor deposition technique, providing a high flux of metal ions to the substrate. However, one of the disadvantages for industrial use of this technique is a reduced deposition rate compared to direct current magnetron sputtering (dcMS) at equal average power. This is mainly due to a high target back-attraction probability of the metal ions with typical values in the range 70%–90% during the pulse. In order to reduce this effect, we focused on the contribution of ion fluxes available immediately after each HiPIMS pulse; a time also known as afterglow. Without a negative potential on the target at this stage of the HiPIMS process, the back-attracting electric field disappears allowing remaining ions to escape the magnetic trap and travel toward the substrate. To quantify the proposed mechanism, we studied the effect of HiPIMS pulse duration on the outward flux of film-forming species in titanium discharges, which are known to exhibit more than 50% reduction in deposition rate compared to dcMS. By shortening the HiPIMS pulse length, it was found that the contribution to the outward flux of film-forming species from the afterglow increases significantly. For example, HiPIMS discharges at a constant peak current density of about 1.10 A cm−2 showed a 45% increase of the deposition rate, by shortening the pulse duration from 200 to 50 μs. Ionized flux fraction measurements, using a gridless quartz crystal micro-balance-based ion meter, showed that this increase of the deposition rate could be achieved without compromising the ionized flux fraction, which remained approximately constant. The key to the achieved optimization of HiPIMS discharges lies in maintaining a high peak discharge current also for short pulse lengths to ensure sufficient ionization of the sputtered species.

Bipolar HiPIMS: The role of capacitive coupling in achieving ion bombardment during growth of dielectric thin films

Abstract: Bipolar high-power impulse magnetron sputtering (HiPIMS) is used to achieve ion acceleration for ion bombardment of dielectric thin films. This is realized by increasing the plasma potential (Up), during the interval in-between the HiPIMS-pulses, using a positive reversed voltage (Urev). As long as the film surface potential (Us) is maintained low, close to ground potential, this increase in Up results in ion-acceleration as ions approach the film surface. The effect of Urev on the ion bombardment is demonstrated by the growth of dielectric (Al,Cr)2O3 films on two sets of substrates, Si (001) and sapphire (0001) utilizing a Urev ranging from 0 to 300 V. A clear ion bombardment effect is detected in films grown on the conductive Si substrate, while no, or a very small, effect is observed in films grown on the dielectric sapphire substrate. This is ascribed to the changes in Us when the substrate is subjected to the bombardment of positive ions. For a film surface that has a high capacitance to ground, Us remains close to ground potential for an extended time in-between the HiPIMS pulses, while if the capacitance is low, Us quickly attains floating potential (Ufloat) close to Up. The simulated temporal evolutions of Us for the films by using capacitors show that for a 1 μm thick (Al,Cr)2O3 film on a conductive substrate, Us is maintained close to ground potential during the entire 20 μs that Urev is applied after the HiPIMS pulse. On the other hand, when a capacitance corresponding to the 0.5 mm thick sapphire substrate is used, Us rapidly attains a potential close to Urev.

Copper thin films deposited using different ion acceleration strategies in HiPIMS

Abstract: The growth of Cu thin films by low-energy ion-bombardment using bipolar and conventional HiPIMS pulse configurations to the target in combination with different biasing methods of the substrate were investigated. For bipolar HiPIMS with a substrate at floating potential, XRD measurements indicate minimal ion acceleration and change in the crystal growth when increasing the substrate holder potential to the same level as the applied positive voltage. In contrast, using bipolar HiPIMS with a substrate at ground potential results in a similar ion current profile as in conventional HiPIMS with a synchronized pulsed bias with the same delay and timing as the positive pulse. Furthermore, the trend in crystal growth is the same such that a significant increase in the (200) intensity is observed within an ion acceleration window, 125 – 175 V. Using conventional HiPIMS with a continuous DC bias also results in Cu films exhibiting significant (200) peaks, but the ion acceleration window is shifted to 175 – 225 V. The observed differences in the film growth could be explained not only by the energy of the ions but also by the type of ions (working gas vs metal ions) that are accelerated during either the positive pulse or substrate biasing.

Low temperature growth of stress-free single phase α-W films using HiPIMS with synchronized pulsed substrate bias

Abstract: Efficient metal-ion-irradiation during film growth with the concurrent reduction of gas-ion-irradiation is realized for high power impulse magnetron sputtering by the use of a synchronized, but delayed, pulsed substrate bias. In this way, the growth of stress-free, single phase α-W thin films is demonstrated without additional substrate heating or post-annealing. By synchronizing the pulsed substrate bias to the metal-ion rich portion of the discharge, tungsten films with a ⟨110⟩ oriented crystal texture are obtained as compared to the ⟨111⟩ orientation obtained using a continuous substrate bias. At the same time, a reduction of Ar incorporation in the films are observed, resulting in the decrease of compressive film stress from σ = 1.80–1.43 GPa when switching from continuous to synchronized bias. This trend is further enhanced by the increase of the synchronized bias voltage, whereby a much lower compressive stress σ = 0.71 GPa is obtained at Us = 200 V. In addition, switching the inert gas from Ar to Kr has led to fully relaxed, low tensile stress (0.03 GPa) tungsten films with no measurable concentration of trapped gas atoms. Room-temperature electrical resistivity is correlated with the microstructural properties, showing lower resistivities for higher Us and having the lowest resistivity (14.2 μΩ cm) for the Kr sputtered tungsten films. These results illustrate the clear benefit of utilizing selective metal-ion-irradiation during film growth as an effective pathway to minimize the compressive stress induced by high-energetic gas ions/neutrals during low temperature growth of high melting temperature materials.

Magnetically collected platinum/nickel alloy nanoparticles – insight into low noble metal content catalysts for hydrogen evolution reaction

Abstract: Magnetically collected platinum/nickel alloy nanoparticles – insight into low noble metal content catalysts for hydrogen evolution reaction