ICMCTF2016 Session TS4-2: Plasma Diagnostics and Modeling

Molecular dynamics (MD) computer simulations are an almost ideal tool for studying interactions between ions and materials at eV and keV energies. In this energy regime, current computer capacity allows simulating all the atoms involved in a single ion deposition and collisional process [1,2] and also makes it feasible to simulate thousands of consecutive impacts. Moreover, modern reactive interatomic potentials can also describe the chemical processes involved during deposition to some extent [3], and are also capable of describing inherently many-body processes such as plastic flow of atoms [4]. Thus MD methods can be used to examine a wide range of complex nonlinear processes involved in physical vapor deposition, ion implantation and plasma surface processing [1-4].

In this talk, I will give a brief background to how MD can be used to simulate ion-surface interactions, and then present examples of some of our work describing nonlinear surface effects. These include description of a nonlinear increase in damage production during Si bombardment of Si surfaces in the energy range 25 – 500 eV [5] and the formation of massive craters due to atom flow during 10-200 keV ion bombardment of surfaces [4]. I will further discuss how scaling down the energy of the impinging ions to the 100 eV range changes the mechanism from actual cratering to “effective crater formation”, i.e. hundreds of small displacements that on average form a crater-like shape. Such displacements can, via a scheme using averaged displacement momenta, be incorporated into a mathematical model that can describe the surface instability leading to formation of surface nanoripples [6]. Finally, I will describe the computational efficiency of MD simulations of irradiation and deposition processes, to discuss the possibility whether they would be fast enough to be incorporated in process simulator softwares.

[1] A. V. Krasheninnikov and K. Nordlund, J. Appl. Phys. (Applied Physics Reviews) 107, 071301 (2010)

[2] K. Nordlund and F. Djurabekova, J. Comput. Electr. 13, 122 (2014)

[3] K. Nordlund, C. Björkas, T. Ahlgren, , A. Lasa, and A. E. Sand, J. Phys. D: Appl. Phys. 47, 224018 (2014)

[4] J. Samela and K. Nordlund, Phys. Rev. Lett. 101, 027601 (2008)

[5] J. Tarus, K. Nordlund, A. Kuronen, and J. Keinonen, Phys. Rev. B 58, 9907 (1998)

[6] S. A. Norris, J. Samela, C. S. Madi, M. P. Brenner, L. Bukonte, M. Backman, F. Djurabekova, K. Nordlund, and M. J. Aziz, Nature communications 2, 276 (2011).

A couple of decades ago we posed this question in Ref. 1 and the answer was “no”. The average energy E_d per deposited atom is a product of the mean ion energy E_i and the ion-to-metal flux ratio J_i /J_Me. We showed that the evolution of film nanostructure follows completely different reaction paths when varying E_ivs.J_i /J_Me. Low gas-ion densities, J_i /J_Me ≲ 1, combined with relatively high ion energies, E_i ≳ 50 eV are suffice,ent to continuously generate linear cascade effects thus enhancing film density . However, this, in turn, leads to significant concentrations of residual defects and high stresses.

By superimposing an external axial magnetic field² on the field of the permanent magnets in a magnetron, we achieve independent control of E_i and J_i/J_Me. The method allows J_i/J_Me ratios incident at the growing film to be varied over extremely wide ranges (up to > 50) at low ion energies (e.g., below bulk lattice-atom displacement energies). Using this technique, we have realized reduced-temperature growth of high-quality epitaxial layers and independent control of texture and density in TM nitrides..

It was realized early on in the HIPIMS literature³ that there exist a time separation between the Ar and metal-ion dominated fluxes at the substrate which opens the possibility for selection one of the components for ion-assisted by using a pulsed bias voltage with suitable synchronization. We recently investigated this alternative route for ion-assisted growth via the use of bias synchronized to the metal-rich portion of the plasma pulse. Stresses can be dramatically reduced, or even eliminated, since metal (as opposed to inert-gas) ions are components of the film.⁴ We use metastable NaCl-structure Ti_1-xAl_xN,⁵ known to be very sensitive to ion-irradiation-induced phase separation, as a model system deposited in a hybrid high-power pulsed and dc magnetron co-sputtering configuration to show that the average metal-ion momentum per deposited atom, rather than the average metal-ion energy controls film phase composition and stress evolution.

¹ I. Petrov, F. Adibi, J. E. Greene, L. Hultman and J.‑E. Sundgren, APL.63 36 (1993).

²I. Petrov, F. Adibi, J. E. Greene, W. D. Sproul and W.‑D. Münz, JVST. A,103283 (1992).

³K. Macák, V. Kouznetsov, J. Schneider, U. Helmersson, and I. Petrov, JVSTA 18 (2000) 1533-1537

⁴G. Greczynski, J. Lu, I. Petrov, J.E. Greene, S. Bolz, W. Kölker, Ch. Schiffers, O. Lemmer and L. Hultman, JVST. A 32 (2014) 041515.

⁵G. Greczynski, J. Lu, M. Johansson, J. Jensen, I. Petrov, J.E. Greene, and L. Hultman, SCT 206 (2012) 4202.

⁶ G. Greczynski, et al, TSF 556 (2014) 87.

Ions have been long recognized to be important to the microstructure evolution of thin films deposited by physical vapor deposition (PVD). Early discoveries go back to the 1930s, followed by ion plating in the 1960s, cathodic arc deposition in the 1970s and 80s, pulsed laser deposition since the 1970s, plasma immersion ion implantation and deposition in the 1980s and 90s, i-PVD in the 1990s, and high power impulse magnetron sputtering in the 2000s. The underlying idea for each of those deposition techniques is actually the same: making use of ions from plasma by utilizing their kinetic and potential energies to obtain desired film properties. Not surprisingly, the role of ions was studied and the findings were cast in simplifying schemes such as structure zone diagrams. Classifying parameters were defined, most prominently the average energy deposited per atom. Indeed, it would be very helpful if a universal PVD parameter could be defined applicable to all PVD processes. However, research dating back to the 1990s has shown that the average energy per deposited atom is a useful but not a universal parameter. For one, the averaging process eliminates the effect of the shape of the distribution functions: it matters for the microstructure and resulting properties how the energy is distributed. Therefore, in this review, besides looking at the historical development of the field, an overview is given what energy distribution functions are produced in each of the plasma generation schemes, and how ion energy is controlled. There are also sets of material-specific energy parameters like the surface bonding energy and the activation energy for surface atom diffusion. The overview suggests that a “universal PVD parameter” does not exist.

This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

High power magnetron sputtering (HiPIMS) discharges generate ions with high kinetic energies in comparison to conventional dc magnetron sputtering. The peculiar shape of the ion energy distribution function (IEDF) is correlated to the formation of localized ionization zones (IZ) in the racetrack of a HIPIMS discharge, so called spokes. By using ion energy mass spectrometry, probe experiments and plasma spectroscopy the connection between IZ and IEDFs is evaluated with high

In the first part of this study, the total energy flux transferred to the substrate was varied in a broad range by modifying systematically the working conditions. A titanium target was sputtered in an Ar/O₂ reactive atmosphere (P= 0.6 Pa) either in DC Magnetron Sputtering (DCMS) or in the High Power Impulse Magnetron Sputtering (HiPIMS) mode of operation. Unbalanced (UB) and Balanced (B) magnetron cathodes were utilized and the time-averaged power delivered to the plasma was set to either 400 or 800W. The total energy flux was measured in situ thanks to a heat flux sensor located at the substrate position. The normalized energy flux (φ_norm) was calculated by taking into account the number of Ti atoms deposited per unit time and the discharge regimes were compared accordingly. Regardless of the sputtering method, the phase constitution evolves from phase pure anatase to rutile rich anatase/rutile mixtures as φ_norm increases. φ_norm is the highest as the HiPIMS discharge is ignited with an UB magnetron. Surprisingly the energy flux related to the B-DCMS discharge at 800W is higher than the one measured for the UB-DCMS, at the same power. The enhanced magnetic field confinement for the balanced cathode intensifies the target ion bombardment, which, in turn, promotes the heating of the target surface. Consequently the contribution of the IR photon flux emitted by the hot surface to the total energy flux is significantly augmented. In fine, we show that the phase constitution of TiO₂ films depends on the total energy flux supplied during growth.

In the second part, the phase formation in zirconia (ZrO₂) thin films synthesized by DC-MS has been investigated. A 2-inch Zr target is sputtered in an Ar/O₂ ambient, in the poisoned mode or in the transition zone with the help of voltage feedback control loop. The films grown in the poisoned regime at 200 mA, 1.3 mTorr always exhibits the low-temperature monoclinic phase. While working in the transition zone, the phase constitution is dramatically modified and the XRD spectra only exhibit peaks from the high-temperature cubic crystals. Elemental analysis shows that films grown in the transition zone are oxygen deficient. DFT-based calculations data remarkably agree with these experimental observations and thus highlight that the formation of the cubic phase is solely due to the incorporation of O vacancies.

However, it should be noticed that a slight modification in these optimized working conditions would alter the phase constitution, i.e. the monoclinic phase appears if i) the film thickness is increased, ii) the energy of the depositing species is increased (by lowering the deposition pressure).