Magnetron deposition of chromium nitride coatings using a hot chromium target: Influence of magnetron power on the deposition rate and elemental composition
Introduction
Nowadays, the deposition of modifying films and coatings on the surface of various materials using magnetron sputtering systems is widely used in science and industries. The demand and promising nature of this method is due to the good quality of the coatings obtained, as well as a wide variety of control parameters providing some flexibility in regulating various structural and functional properties.
The reactive magnetron deposition of thin films and coatings has been intensively investigated since 1980. Sputtering of metal targets in the presence of a reactive gas facilitates the formation of complex films such as nitrides, oxides, carbides, or their combinations [1], [2], [3], [4], [5], [6], [7], [8]. This method makes it possible to obtain sufficiently dense coatings with high hardness and good adhesion. Here, sputtering of the target surface in a reactive gas atmosphere is the main mechanism for the formation of fluxes of deposited particles. It can take place in three modes: in a metallic mode, in a transition and in a chemical compound one, depending on the concentration of a reactive gas in the vacuum chamber [2], [3], [4], [7], [8]. In the metallic mode, predominantly sputtering of metal atoms by ions of the working gas (usually argon) takes place. In the transition mode, the surface layers of the target contain a certain amount of chemical compounds of target atoms and reactive gas, the concentration of which changes in an uncontrolled manner. In the chemical compound mode, the surface layers of the target almost completely react with the reactive gas (the so-called target poisoning occurs), so the chemical compound is sputtered. All three modes are in use. Each of them has its own advantages and disadvantages, depending on a specific task and requirements for functional properties of the requested coatings. As a rule, the highest deposition rates are characteristic of the metallic mode. However, a problem often arises here in connection with the formation of the stoichiometric elemental composition, which is critical for some types of functional coatings. In any case, since the rate of the coating formation is limited by the intensity of target sputtering, the formation rate in deposition modes on stationary substrates cannot be raised above 1–5 nm/s [3], [5], [6], [9]. If it were possible to develop a way to dramatically increase the productivity of technologies based on reactive magnetron sputtering, then their attractiveness would increase significantly, especially for industrial production.
The reliable evidence has recently been found for the fact that the rate of magnetron deposition of coatings can be increased significantly, i.e. by about an order of magnitude, due to evaporation or sublimation effects on the target surface in addition to the actual sputtering. For the moment, this possibility has been studied quite deeply in relation to fabricating films and coatings from simple substances (some metals, silicon, etc.) [10], [11], [12], [13], [14], [15]. For the deposition of films made of metals with oxygen, nitrogen, and other reactive gases, a similar task associated with an increase in the deposition performance due to evaporation or sublimation of a target substance has not yet found a systematic solution. However, this matter is of great interest because these films and coatings are very widely used in various industries because of their characteristics. For example, coatings made of metal nitrides (ZrN, TiN, HfN, CrN, etc.) have a high melting point, corrosion resistance in various environments, increased hardness, mechanical and thermal stability, etc. [16], [17], [18], [19], [20], [21], [22], [23], [24]. Research results have already been published in the scientific literature, indicating a noticeable effect of heating a titanium target on the stabilization of the sputtering process and an increase in the deposition rate of titanium nitride coatings (for example, [25], [26], [27], [28]). Magnetron deposition of nitrides of other metals during sputtering of hot targets has not yet been given enough attention. In this work, we consider the formation of coatings based on chromium nitride. These coatings are often applied as a protective layer on various products due to their strength, wear, and corrosion resistance [29], [30], [31], [32], [33], [34], [35]. Since there is a problem associated with ensuring a dramatic increase in the rate of magnetron deposition of coatings made from chemical compounds, chromium is still of interest because it has a high sublimation rate. Consequently, performing high-rate deposition of compounds with chromium can be facilitated by using the target even in a solid state.
Ref. [36] displays that the process of deposition of coatings based on chromium and nitrogen compounds during the magnetron sputtering of a strongly heated chromium target can be stably implemented in the metallic mode. A separate inlet of argon as a working gas and nitrogen as a reactive gas into the vacuum chamber helps to prevent the poisoning of the target by the atoms of the reactive gas. Using an assisting radio-frequency source of inductively-coupled plasma (RF-ICP source) makes it possible to atomize molecular nitrogen injected into the vacuum chamber and thereby enhance its chemisorption on the surface of the coating being formed. The nitrogen concentration in the vacuum chamber has a significant effect on the elemental composition of the formed chromium nitride (CrNx) coatings. In Ref. [36] all experiments were carried out at a target power density of 15.7 W/cm2, whereby the target was relatively strong heated, but the sublimation rate remained low. However, it turned out that even at this power density, the deposition rate of the CrNx coating was about 50% higher when sputtering a hot target compared to a cooled one. In Ref. [36], the values for the gas flow rate of nitrogen into the chamber and the power of the RF-ICP source were also determined, which made it possible to achieve the elemental composition of the deposited coatings close to stoichiometric chromium nitride at a magnetron power density of 15.7 W/cm2. However, if the magnetron power increases, the temperature increases too, and consequently the rate of sublimation on the surface of the Cr target increases. As a result, the flux density of atomic Cr particles deposited on the substrate surface will increase significantly, and the deposition rate of the CrNx coating also increases. In terms of practical application, it is important to find out to what extent the deposition rate of such coatings can be increased in the respective power range of modern magnetrons. At the same time, it is important to understand how the elemental composition of the resulting coatings changes with an increase in power and how it can be controlled. It should be noted that in usual practice the processing of products is carried out in batches, so here we consider the case of the formation of coatings on the surface of substrates moving in the space of a vacuum chamber. Therefore, the main objectives of the study, the findings of which are presented in this article, are the following. Firstly, to find out the contribution of target sublimation to enhancing the flux density of deposited chromium atoms, as well as to obtain data on the rate of formation of coatings from chromium and nitrogen compounds with increasing magnetron power. Secondly, to investigate the kinetics of particles arriving at the surface of the growing coatings in an argon and nitrogen atmosphere, depending on the position of the substrate in the space of the vacuum chamber, as well as to analyze the elemental composition of coatings formed at different magnetron power with a hot Cr target.
Section snippets
Coating deposition
To ensure the stable and high-rate formation of the coatings based on CrNx, the following requirements are to comply.
- 1)
A magnetron with a hot and sublimating Cr target is used to create a high-density flux of metal atoms deposited. Due to this factor, an increase in the deposition rate of the coating is supposed.
- 2)
The substrate with the growing coating is immersed in an atmosphere of argon and nitrogen. To minimize the poisoning of the chromium target by compounds with nitrogen, a separate inlet of
Deposition rate of the coatings
Since the creation of an erosion flux of atoms on the surface of a Cr target occurs in a metallic mode, the rate of formation of a coating based on CrNx compounds is controlled by the flux density of atomic chromium Fdep,Cr arriving at the substrate. The sublimation of a hot Cr target, the intensity of which increases nonlinearly with an increase in magnetron power [12], [14], should increase the Fdep,Cr value significantly. This raises the question of the magnetron power at which its
Conclusions
The possibilities of magnetron sputtering systems with hot targets for increasing the deposition rate of coatings based on chromium and nitrogen compounds have been investigated. The elemental and phase composition features of coatings formed in the mode of planetary rotation of the substrates, depending on the power density of the magnetron, have been studied. The main conclusions are as follows.
- 1.
Due to the effect of sublimation of a hot chromium target in addition to its sputtering, the
Funding
The study was supported by the Russian Foundation for Basic Research (Project number 20-38-90134).
CRediT authorship contribution statement
V.A. Grudinin: Investigation, Visualization. G.A. Bleykher: Methodology, Investigation, Writing – original draft, Writing – review & editing. D.V. Sidelev: Conceptualization, Methodology. Yu.N. Yuriev: Investigation. A.D. Lomygin: Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by TPU Development Program.
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