Role of ionization fraction on the surface roughness, density, and interface mixing of the films deposited by thermal evaporation, dc magnetron sputtering, and HiPIMS: An atomistic simulation

Abstract

We explore the effect of ionization fraction on the epitaxial growth of Cu film on Cu (111) substrate at room temperature. We compare thermal evaporation, dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS). Three deposition conditions i.e. fully neutral, 50 % ionized and 100 % ionized flux were considered as thermal evaporation, dcMS and HiPIMS, respectively, for ∼20000 adatoms. It is shown that higher ionization fraction of the deposition flux leads to smoother surfaces by two major mechanisms i.e. decreasing clustering in the vapor phase and bi-collision of high energy ions at the film surface. The bi-collision event consists of local amorphization which fills the gaps between islands followed by crystallization due to secondary collisions. We found bi-collision events to be very important to prevent island growth to become dominant and increase the surface roughness. Regardless of the deposition method, epitaxial Cu thin films suffer from stacking fault areas (twin boundaries) in agreement with recent experimental results. In addition, HiPIMS deposition presents considerable interface mixing while it is negligible in thermal evaporation and dcMS deposition, those present less adhesion accordingly.

On the role of ion potential energy in low energy HiPIMS deposition: An atomistic simulation

“…Regarding HiPIMS deposition, MD simulations have indicated that both ionization fraction and substrate bias change the microstructure and film-substrate intermixing [10,14]. Hence, the variation of the film texture by bias voltage that has been observed experimentally [19,20,13] can be attributed to the change of wetting due to interface mixing.…”

“…Due to the high electron density the ionization mean free path of the sputtered species becomes shorter and the ionization probability increases and a significant fraction of the ions reaching the substrate are ions of the film-forming species. As a result HiPIMS deposition presents smoother [8,9,10], denser [11,10], and less defective [12,10] coatings, compared to dcMS deposition. Further, by applying a substrate bias, the bombarding energy of the ions of the film forming material can be tuned in order to achieve desired film properties such as the film texture and grain size [13,14].…”

“…Molecular dynamics (MD) simulations have been shown to be a promising tool in terms of revealing atomistic mechanisms that contribute to various PVD techniques including dcMS and HiPIMS [10,14]. It is worth mentioning that MD simulations of sputter deposition are divided into two categories: (i) modeling the vicinity of the substrate, the so called deposition side, and (ii) modeling the energetic working gas ions bombarding a surface or the target side.…”

“…An interesting possibility is modeling the ion potential as an extra short-ranged repulsive interatomic potential [31,10,14]. The term extra means that we already included an interatomic potential due to E c that might accurately describe forces around the equilibrium interatomic distances but not short-ranged ones [31].…”

“…More recently, we utilized a combination of the ZBL potential and the EAM for a simulation of deposition with partially and fully ionized flux of sputtered species [10]. It has been shown experimentally that surface roughness decreases as higher ionization fraction is utilized [8,9].…”