Low-energy inert-gas ion irradiation of the film surface during refractory transition-metal (TM) nitride growth by conventional DC magnetron sputtering has been used widely to overcome the characteristically underdense microstructures with rough surfaces of layers deposited at low temperatures (Ts/Tm < 0.30, in which Ts is the film growth temperature and Tm is the melting point in K)i.
It has been recently demonstrated that high-power pulsed magnetron sputtering (HIPIMS) provides an alternative route for ion-assisted TM nitride film growth by using substrate bias synchronized to the metal-rich portion of the plasma pulse. It is possible to dramatically reduce or even eliminate stresses since metal (as opposed to inert-gas) ions are components of the film.ii, iii
In this project, a hybrid HIPIMS/DCMS two-target co-sputtering configuration is used, in which one target (either Si or Ti ) is powered by HIPIMS while the other is powered by DCMS, for growth of Ti1-xSixN films with compositions 0 ≤ x ≤ 0.26. Markedly different film growth pathways are obtained based on which target is powered by HIPIMS with, in both cases, a substrate bias applied in synchronous with the HIPIMS pulse. The observed divergence in phase content, film nanostructure and mechanical properties between layers grown in Ti-HIPIMS/Si-DCMS and Si-HIPIMS/Ti-DCMS configuration is because of distinctly different metal-ion irradiation conditions, Ti+/Ti2+ vs. Si+/Si2+, during film growth, as established by the ion mass spectrometry analyses carried out at the substrate position with a Hiden Analytical EQP 1000 instrument (Figure 1(a)-(b)).
Figure 1. Ion energy distribution functions measured at the substrate position for (a) singly-charged Ti+ and Si+ ions, and (b) doubly-charged Ti2+ and Si2+ ions during Ti-HIPIMS and Si-HIPIMS pulses; (c) plan-view STEM micrograph, and (d) plan-view EDX/STEM elemental maps of a Ti0.74Si0.26N Ti-HIPIMS/Si-DCMS film, showing spatial distributions, acquired from the area outlined in panel (c); (e) nanoindentation hardnesses H(x) of Ti-HIPIMS/Si-DCMS and Si-HIPIMS/Ti-DCMS Ti1-xSixN films grown on Si(001) substrates at Ts = 500 °C.
Better mass match between incident Ti+ ions and the average film atomic mass, higher metal-ion/metal-atom ratios and a high fraction of doubly-ionized species leads to an average momentum transfer per deposited atom (pd) ~20 times higher for Ti-HIPIMS/Si-DCMS than during Si-HIPIMS/Ti-DCMS. This results in increased adatom mean free paths, leading to the segregation of smaller Si atoms to column boundaries and the formation of a nanocomposite structure comprising of TiN-rich nanocolumns encapsulated in SiNx tissue phases (cf. plan-view STEM micrograph in Figure 1(c), and EDX/STEM elemental maps in Figure 1(d)). Ti-HIPIMS/Si-DCMS Ti1-xSixN films are superhard over a composition range that is significantly wider than reported previously, 0.04 ≤ x ≤ 0.26, with a maximum hardness, H = 45 GPa, for layers with x = 0.13 (Figure 1(e)). However, residual stresses are also high with an average value of 7±1 GPa.
In sharp contrast, during Si-HIPIMS/Ti-DCMS Ti1-xSixN film growth, the flux of doubly-ionized metal ions is lower which, along with the lower mass of Si, low metal-ion/metal-atom flux ratio during HIPIMS pulses, and poorer mass match between incident Si+ ions and average film atomic mass results in comparatively low values. This leads to trapping of Si in the metastable Ti1-xSixN NaCl structure in order to form solid solutions over the highest compositional range yet reported, 0 ≤ x ≤ 0.24.
Project summary by:
Thin Film Physics Division
Department of Physics (IFM)
SE-581 83 Linköping
G. Greczynski et al., (2015) “Control of Ti1-xSixN nanostructure via tunable metal-ion momentum transfer during HIPIMS/DCMS co-deposition”, Surface and Coatings Technology 280, 174-184
i I. Petrov, P.B. Barna, L. Hultman, J.E. Greene J. Vac. Sci. Technol. A 21 (2003) 117
ii G. Greczynski, J. Lu, I. Petrov, J.E. Greene, S. Bolz, W. Kölker, Ch. Schiffers, O. Lemmer and L. Hultman, J. Vac. Sci. Technol. A 32 (2014) 041515.
iii G. Greczynski, J. Lu, J. Jensen, I. Petrov, J.E. Greene, S. Bolz, W. Kölker, Ch. Schiffers, O. Lemmer and L. Hultman, J. Vac. Sci. Technol. A 30 (2012) 061504.
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
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