Mechanical properties optimization of tungsten nitride thin films grown by reactive sputtering and laser ablation

doi:10.1016/j.vacuum.2010.04.004

Vacuum

Volume 85, Issue 1, 23 July 2010, Pages 69-77

https://doi.org/10.1016/j.vacuum.2010.04.004 Get rights and content

Abstract

Transition metal nitrides coatings are used as protective coatings against wear and corrosion. Their mechanical properties can be tailored by tuning the nitrogen content during film synthesis. The relationship between thin film preparation conditions and mechanical properties for tungsten nitride films is not as well understood as other transition metal nitrides, like titanium nitride. We report the synthesis of tungsten nitride films grown by reactive sputtering and laser ablation in the ambient of N₂ or N₂/Ar mixture at various pressures on stainless steel substrates at 400 C. The composition of the films was determined by XPS. The optimal mechanical properties were found by nanoindentation based on the determination of the proper deposition conditions. As nitrogen pressure was increased during processing, the stoichiometry and hardness changed from W₉N to W₄N and 30.8–38.7 GPa, respectively, for films deposited by reactive sputtering, and from W₆N to W₂N and 19.5–27.7 GPa, respectively, for those deposited by laser ablation.

Introduction

The term of hard coatings is applied to structures which improve wear resistance and extend the lifetime of the structure. Hard coatings are industrially used onto cutting tools, automotive engine parts, turbine blades, structural components, etc [1], [2]. Traditionally, materials utilized as hard coatings had a single composition, crystalline phase, and microstructure. Some of the most common hard coatings used as thin films are diamond-like carbon (DLC), BN, B₄C, SiC, Al₂O₃, Si₃N₄ and WC [1]. A growing interest in the synthesis of transition metal nitrides has risen due to their chemical inertness and high hardness to be used as wear-resistant and hard protective coatings. In particular, TiN thin films and nanocomposites using TiN satisfy these requirements and have been extensively investigated by a variety of deposition techniques in the recent literature [1], [2], [3], [4], [5]. For instance, there is a close relationship between film composition and its mechanical properties for TiN films [3]. However, there are not as many reports as expected on other transition metal nitrides.

Tungsten nitride films have generated considerable interest in the manufacturing of semiconductor devices because they are an excellent barrier for Cu diffusion into Si at high temperatures [6]. The deposition of diffusion barriers motivated the synthesis of WN_x thin films by several techniques like CVD [7], [8], [9], dc reactive sputtering [10], [11], [12], [13], ALD [14], rf reactive sputtering [15], [16], ion beam sputtering [17], reactive laser ablation [18], [19] and cathodic arc [20]. There are reports concerning to the mechanical properties of alloys containing tungsten nitride sputtered coatings, like Cr–W–N [21], Ti–W–N [22] and Si–W–N [23]. WN_x thin films are also a good candidate to be used as a hard coating [9], [13], [16], [19], [20]. The composition, mass density, electrical and optical properties of WN_x thin films on silicon wafers at room temperature grown by reactive laser ablation have been already published by our group but the study of the mechanical properties was missing [18]. This study investigates the best possible mechanical properties as a function of film composition and microstructure for WN_x coatings on stainless steel substrates synthesized by two different deposition techniques: reactive sputtering and laser ablation.

Section snippets

Experiment

As previously mentioned, the WN_x thin films were grown by two different deposition methods: RPLD and dc-sputtering. The experiment was carried out in a laser ablation system described elsewhere [24]. The system consists of three UHV stainless steel chambers: sample introduction, growth and analysis, as shown in Fig. 1. The base pressure of the chambers is approximately 10⁻⁷ Pa. A KrF excimer laser is focused onto a high purity (99.9% at.) tungsten disc of 5 cm in diameter at an angle of 50° off

Results

The WN_x thin films were deposited on stainless steel disks kept at 400 °C on both deposition methods. The nitrogen partial pressure was varied from 0 to 10.0 Pa in the RPLD system and from 0 to 1.6 Pa in the sputtering system. The analysis was in situ performed immediately after each deposition for the films grown by RPLD, while it was done ex situ for those grown by reactive sputtering.

The only elements present in a typical XPS spectrum of a WN_x film grown by RPLD were W, N, and O. Meanwhile,

Discussion

RPLD and reactive sputtering are non-equilibrium thermodynamic processes. The different tungsten ablated species in RPLD collide with the N₂ molecules in the plume expansion process and react with them, after dissociation, to form a WN_x compound as a thin film on the substrate [35]. The species are released from their initial bonds in ground state in the reactive gas and free radicals, only in the ablation plume, and they rearrange themselves somewhere in the network of the growing layers

Conclusions

In summary, RPLD and reactive sputtering complement each other in the comprehension about finding the adequate processing conditions to obtain hard coatings of transition metal nitrides with optimal mechanical properties. These properties for coatings grown by both deposition techniques depend on the nitrogen content in the films, reaching a threshold value to obtain the best properties. The WN_x coatings processed by reactive sputtering are superior to those synthesized by RPLD, although the

Acknowledgements

We gratefully acknowledge the technical assistance of Victor García, Pedro Casillas, Juan Antonio Peralta, Margot Sainz and Carlos González. CONACyT provided financial support.

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The high-power impulse magnetron sputtering (HiPIMS) technology starts a new era in the thin film fabrication, which already attracts researchers and industries' interest on its further application in many fields. For improving the low deposition rate of HiPIMS, the superimposition concept by introducing mid-frequency (MF) pulses during the off-time of HiPIMS pulsing was adopted for the fabrication of transition metal nitride hard coatings. Tungsten nitride coating is characterized by its high hardness, good thermal stability and low electrical resistivity. In this study, tungsten nitride coatings were fabricated using five different N₂ gas flow rate ratios by a superimposed HiPIMS-MF power system. The W target showed a hysteresis-free behavior during the reactive sputtering process of WN_x coating, where x indicated the chemical composition ratio between W and N elements. The peak current and peak power density values of both HiPIMS and MF powers decreased with increasing N₂ flow rate ratio. The nitrogen content of WN_x coating increased with increasing nitrogen flow rate ratio, whereas the tungsten concentration shows a decreasing tendency. The x value and the nitrogen content of WN_x coating increased from 0.05 to 0.60 and from 5.1 to 36.7 at.%, respectively, when the nitrogen flow rate increased from 13 to 37 sccm. The crystalline structure of WN_x coatings changed from a pure BCC-W phase to a mixture of BCC-W and FCC β-W₂N phases when the x value increased from 0.05 to 0.12. A pure FCC β-W₂N phase structure was obtained when the x values of WN_x coating reached 0.36, 0.43 and 0.60. The hardness values of WN_x coatings were ranging from 26.7 to 31.6 GPa. The WN_0.12 coating with a low coefficient of frictions of 0.33, a high hardness of 31.5 GPa and high H/E and H³/E² ratios were achieved due to the solution hardening and smoother microstructure induced by the substrate bias and the higher energy of superimposed HiPIMS-MF system.
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While most of the works devoted to the synthesis of transition metal nitride coating by magnetron sputtering use N₂ as reactive gas, an alternative method is proposed, namely using ammonia (NH₃) as reactive gas. The magnetron sputtering of chromium in Ar/NH₃ atmosphere is studied and compared to Ar/N₂ atmosphere, with in situ (hysteresis curve, target voltage measurement, optical emission spectroscopy, deposition rate) and ex situ characterizations (XPS, XRD, SEM, RBS, UV–Vis, nanoindentation, 4-points probe). It appears that ammonia as a reactive gas influences the optical, electrical and mechanical properties as well as the microstructure of the thin films in a very similar way to HiPIMS with N₂ reactive gas; improved mechanical properties are obtained, attributed to a change of the morphology, from columnar-like to nanocrystalline.
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Mechanical properties optimization of tungsten nitride thin films grown by reactive sputtering and laser ablation

Abstract

Introduction

Section snippets

Experiment

Results

Discussion

Conclusions

Acknowledgements

Surf Coat Technol

Thin Solid Films

Surf Coat Technol

Scr Mater

Thin Solid Films

Appl Surf Sci

Surf Coat Technol

Wear

Appl Surf Sci

Surf Coat Technol

Appl Surf Sci

Thin Solid Films

Surf Coat Technol

Surf Coat Technol

Surf Coat Technol

Surf Coat Technol

Surf Coat Technol

J Alloys Compd