Abstract
The quest for lowering energy consumption during thin film growth, as by magnetron sputtering, becomes of particular importance in view of sustainable development goals. A recently proposed ...solution combining high power impulse and direct current magnetron sputtering (HiPIMS/DCMS) relies on the use of heavy metal-ion irradiation, instead of conventionally employed resistive heating, to provide sufficient adatom mobility, in order to obtain high-quality dense films. The major fraction of process energy is used at the sputtering sources rather than for heating the entire vacuum vessel. The present study aims to investigate the W
+
densification effects as a function of increasing Al content in (Ti
1-
y
Al
y
)
1-
x
W
x
N films covering the entire range up to the practical solubility limits (
y
~ 0.67). Layers with high Al content are attractive to industrial applications as the high temperature oxidation resistance increases with increasing Al concentration. The challenge is, however, to avoid precipitation of the hexagonal wurtzite AlN phase, which is softer. We report here that (Ti
1-
y
Al
y
)
1-
x
W
x
N layers with
y
= 0.66 and
x
= 0.05 grown by a combination of W-HiPIMS and TiAl-DCMS with the substrate bias
V
s
synchronized to the W
+
-rich fluxes (to provide mobility in the absence of substrate heating) possess single-phase NaCl-structure, as confirmed by XRD and SAED patterns. The evidence provided by XTEM images and the residual oxygen content obtained from ERDA analyses reveals that the alloy films are dense without discernable porosity. The nanoindentation hardness is comparable to that of TiAlN films grown at 400–500 °C, while the residual stresses are very low. We established that the adatom mobility due to the heavy ion W
+
irradiation (in place of resistive heating) enables the growth of high-quality coatings at substrate temperatures not exceeding 130 °C provided that the W
+
momentum transfer per deposited metal atom is sufficiently high. The benefit of this novel film growth approach is not only the reduction of the process energy consumption by 83%, but also the possibility to coat temperature-sensitive substrates.
Metastable Ti1-xAlxN (0.4≤x≤0.76) films are grown using a hybrid approach in which high-power pulsed magnetron sputtering (HIPIMS) is combined with dc magnetron sputtering (DCMS). Elemental Al and Ti ...metal targets are co-sputtered with one operated in HIPIMS mode and the other target in DCMS; the positions of the targets are then switched for the next set of experiments. In both cases, the AlN concentration in the co-sputtered films, deposited at Ts=500°C with R=1.5–5.3Å/s, is controlled by adjusting the average DCMS target power. Resulting films are analyzed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, elastic recoil detection analysis, and nanoindentation. Mass spectroscopy is used to determine ion energy distribution functions at the substrate. The distinctly different flux distributions obtained from targets driven in HIPIMS vs. DCMS modes allow the effects of Aln+ and Tin+ (n=1, 2) ion irradiation on film growth kinetics, and resulting properties, to be investigated separately. Bombardment with Aln+ ions (primarily Al+ in the Al-HIPIMS/Ti-DCMS configuration) during film growth leads to NaCl-structure Ti1-xAlxN (0.53≤x≤0.60) films which exhibit high hardness (>30GPa) with low stress (0.2–0.7GPa tensile). In contrast, films with corresponding AlN concentrations grown under Tin+ metal ion irradiation (with a significant Ti2+ component) in the Ti-HIPIMS/Al-DCMS mode have much lower hardness, 18–19GPa, and high compressive stress ranging up to 2.7GPa. The surprisingly large variation in mechanical properties results from the fact that the kinetic AlN solubility limit xmax in Ti1-xAlxN depends strongly on, in addition to Ts and R, the target power configuration during growth and hence the composition of the ion flux. AlN with xmax~64mol% can be accommodated in the NaCl structure under Aln+ ion flux, compared with ~40mol% for growth with Tin+ flux. The strong asymmetry in film growth reaction paths is due primarily to the fact that the doubly-ionized metal ion flux is approximately two orders of magnitude higher from the Ti target, than from Al, powered with HIPIMS. This asymmetry becomes decisive upon application of a moderate substrate bias voltage, −60V, applied synchronously with HIPIMS pulses, during growth.
► Ti1-xAlxN alloys with high hardness and low residual stress are demonstrated. ► Hybrid HIPIMS–DCMS approach of opposing metal targets is used. ► Film growth pathways depend upon which target is powered by HIPIMS. ► Al-HIPIMS/Ti-DCMS alloys have a much higher solid-solubility limit, xmax=0.64.
In view of the increasing demand for achieving sustainable development, the quest for lowering energy consumption during thin film growth by magnetron sputtering becomes of particular importance. In ...addition, there is a demand for low-temperature growth of dense, hard coatings for protecting temperature-sensitive substrates. Here, we explore a method, in which thermally-driven adatom mobility, necessary to obtain high-quality fully-dense films, is replaced with that supplied by effective low-energy recoil creation resulting from high-mass metal ion irradiation of the growing film surface. This approach allows the growth of dense and hard films with no external heating at substrate temperatures Ts not exceeding 130 °C in a hybrid high-power impulse and dc magnetron co-sputtering (HiPIMS/DCMS) setup involving a high mass (m > 180 amu) HiPIMS target and metal-ion-synchronized bias pulses. We specifically investigate the effect of the metal ion mass on the extent of densification, phase content, nanostructure, and mechanical properties of metastable cubic Ti0.50Al0.50N based thin films, which present outstanding challenges for phase stability control. Ti0.50Al0.50N based thin films are irradiated by group VIB transition metal (TM) target ions generated by Me-HiPIMS discharge, in which Me = Cr (mCr = 52.0 amu), Mo (mMo = 96.0 amu), and W (mW = 183.8 amu). Three series of (Ti1-yAly)1-xMexN films are grown with x = Me/(Me+Al+Ti) varied intentionally by adjusting the DCMS powers, while y = Al/(Al+Ti) also varies as a result of Me+ ion irradiation. Results reveal a strong dependence of film properties on the mass of the HiPIMS-generated metal ions. All layers deposited with Cr+ irradiation exhibit porous nanostructure, high oxygen content, and poor mechanical properties. In contrast, (Ti1-yAly)1-xWxN films are fully-dense even with the lowest W concentration, x = 0.09, show no evidence of hexagonal AlN precipitation, and exhibit state-of the-art mechanical properties typical of Ti0.50Al0.50N grown at 500 °C. The process energy consumption is lowered by 64% with no negative impact on the coating quality. TRIM simulations provide an insight into the densification mechanisms.
•(Ti1-yAly)1-xMexN (Me = Cr, Mo, W) thin films are grown by Me-HiPIMS/TiAl-DCMS.•The effect of the metal ion mass on the Ti0.50Al0.50N densification is studied.•No external heating is applied during coating and substrate temperature is lower than 130 °C.•Dense films are achieved with W ions resulting in hardness of 32 GPa.•Process energy consumption can be decreased by 64%.
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•(Ti1-yAly)1-xWxN coatings are grown by W-HiPIMS/TiAl-DCMS under industrial conditions.•Conventional resistive heating can be replaced by high-mass metal ion irradiation.•The ...densification of the coatings controlled by the W ion energy and W ion dose per deposited metal atom.•The energy consumption during the pre-heating and coating stage is lowered by 70 % and 62 %, respectively.
Decreasing the growth temperature to lower energy consumption and enable deposition on temperature-sensitive substrates during thin film growth by magnetron sputtering is crucial for sustainable development. High-mass metal ion irradiation of the growing film surface with ion energy controlled by metal-ion-synchronized biasing, allows to replace conventionally-used resistive heating, as was recently demonstrated in experiments involving a hybrid high-power impulse and dc magnetron co-sputtering (HiPIMS/DCMS) setup and stationary substrates. Here, we report the extension of the method to industrial scale conditions. As a model-case towards understanding the role of one-fold substrate rotation on Ti0.50Al0.50N film growth employing W+ irradiation, we investigate the effect of two parameters: W ion energy (controlled in the range 45 ≤ EW+ ≤ 630 eV by the amplitude of synchronized substrate bias voltage) and W ion dose per deposited metal atom (determined by the target power). We show that the efficient densification of coatings grown without external heating can be achieved by minimizing the thickness of DCMS-deposited Ti0.50Al0.50N layer that is exposed to an W+ ion flux, or by increasing EW+ at a given Ti0.50Al0.50N thickness.
Reactive sputtering processes are quite complex processes and therefore difficult to understand in detail. However, a number of attempts to clearify the behaviour of reactive sputtering of oxides and ...nitrides have been made. Several process modelling results for such processes have been published that reasonable well mirrors the actual experimental findings. All of these models indicate that the processes normally exhibit hysteresis effects and that the oxides/nitrides will saturate at the stoichiometric compound values. We therefore call these processes saturated reactive sputtering processes. Carrying out reactive sputtering in a hydrocarbon gas like CH4 instead of in oxygen or nitrogen cannot be described with the previously suggested models for oxide or nitride formations. Decomposition of the CH4 molecule in the plasma may result both in carbide formation with the target metal as well as plasma deposited carbon. Depending on the supply of the CH4 the deposited film composition may vary from 0 to 100% of carbon. In the extreme case of very high supply of CH4 a pure carbon film will be deposited. We expect that similar behaviour will be found when carrying out reactive sputtering in other solid material containing gases like e.g. silane or diborane. We have chosen to call such processes non-saturated reactive sputtering processes. In order to understand the behaviour of non-saturated reactive sputtering processes we have developed a new model that enables the user to find the response to individual processing parameters and thus obtain a tool for process optimization. In order to limit the number of parameters our model is outlined for reactive sputtering of Ti in a mixture of argon and CH4. In this article we report that the simulation results reasonable well correlate with our experimental findings.
•Categorizing reactive sputtering processes into saturated and non-saturated processes•Modelling of non-saturated reactive sputtering processes, such as reactive sputtering of Ti in C2H2•Emphasizing and explaining the fundamental differences between the types of reactive processes
The microstructure and composition of CrNx (0 andlt;= x andlt;= 1) films grown by reactive high power pulsed magnetron sputtering (HIPIMS or HPPMS) have been studied as a function of the process ...parameters: N-2-to-Ar discharge gas ratio, (f(N2/Ar)), negative substrate bias (V-s), pulsing frequency, and energy per pulse. The film stoichiometry is found to be determined by the composition of the material flux incident upon the substrate during the active phase of the discharge with no nitrogen uptake between the high power pulses. Scanning electron microscopy investigations reveal that for 0andlt;f(N2/Ar)andlt;0.15 and 150 V bias, a columnar film growth is suppressed in favor of nano-sized grain structure. The phenomenon is ascribed to the high flux of doubly charged Cr ions and appears to be a unique feature of HIPIMS. The microstructure of column-less films for 100 V andlt;= V-s andlt;= 150 V is dominated by the CrN and hexagonal beta-Cr2N phases and shows a high sensitivity to V-s. As the amplitude of V, decreases to 40 V and self-biased condition, the film morphology evolves to a dense columnar structure. This is accompanied by an increase in the average surface roughness from 0.25 nm to 2.4 nm. CrNx samples grown at f(N2/Ar)andgt;= 0.3 are columnar and show high compressive stress levels ranging from -7.1 GPa at f(N2/Ar)=0.3 to -9.6 GPa at f(N2/Ar)=1. The power-normalized deposition rate decreases with increasing pulse energy, independent of f(N2/Ar). This effect is found to be closely related to the increased ion content in the plasma as determined by optical emission spectroscopy. The HIPIMS deposition rate normalized to DC rate decreases linearly with increasing relative ion content in the plasma, independent of f(N2/Ar) and pulsing frequency, in agreement with the so-called target-pathways model. Increasing frequency leads to a finer grain structure and a partial suppression of the columnar growth, which is attributed to the corresponding increase of the time-averaged mean energy of film-forming ions arriving at the substrate.
High power pulsed magnetron sputtering has been used to grow thin chromium layers on substrates facing and orthogonal to the target. It is demonstrated that at low peak target current density,
j
T
<
...0.6
A/cm
2 corresponding to a low ion-to-neutral flux ratio, films grown on substrates facing the target exhibit in-plane alignment. This is due to the rectangular shape of the target that yields an asymmetry in the off-normal flux of sputtered species. With increasing
j
T
the biaxial alignment degrades, as the major portion of the incoming flux (ions) can be effectively steered by the electric field of the substrate to remove asymmetry imposed by geometrical restrictions. Eventually, at
j
T
=
1.7
A/cm
2 a fiber texture is obtained. For films grown on substrates orthogonal to the target, the large column tilt characteristic for growth at low
j
T
, decreases with increasing ion content in the flux and almost disappears at the highest value of
j
T
. The latter indicates that material flux to the substrate is highly ionized so that deposition takes place along substrate normal despite the high nominal inclination angle. Thus, in the limit of high
j
T
the artifacts of conventional physical vapor deposition, resulting from the line-of-sight deposition, are effectively eliminated and the film growth proceeds more or less unaffected by the substrate orientation. Samples mounted orthogonally thus possess a similar texture, morphology, and topography as those facing the target.
Hybrid high power impulse/direct current magnetron sputtering (HiPIMS/DCMS) film growth technique with metal-ion-synchronized substrate bias allows for significant energy savings as compared to ...conventional PVD methods. For carefully selected type of metal ion irradiation, taking into account ion mass, ionization potential, and reactivity towards working gas, fully dense and hard films can be obtained with no intentional substrate heating. The thermally-driven adatom mobility, which is an essential densification mechanism in conventional film growth that takes place at elevated temperatures, is replaced with that supplied by effective low-energy recoil creation. In this contribution we explore effects of the high-mass W+ irradiation, which has proven to be the most efficient in densifying Ti0.50Al0.50N layers, serving here as a model system, grown with no substrate heating. We study the effects of two essential parameters: W+ energy EW+ and W concentration x, on film porosity, phase content, nanostructure, and mechanical properties. EW+ varies from ~90 to ~630 eV (controlled by substrate bias voltage amplitude Vs) and x from 0.02 to 0.12 (controlled by the HiPIMS pulse length), while the HiPIMS peak target current is kept constant. Results reveal that a strong coupling exists between the W+ incident energy and the minimum W concentration required to grow dense layers.
•The effect of W+ irradiation on densification of TiAlN films is studied.•Films are grown by W-HiPIMS/TiAl-DCMS with W+-synchronized bias.•No external heating is applied and substrate temperature is lower than 130 °C.•The effect of W+ energy EW+ and W concentration x on film properties is studied.•Fully-dense films can be obtained with low EW+ without high residual stress.
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