RBS Depth Profiling Analysis of (Ti, Al)N/MoN and CrN/MoN Multilayers
© The Author(s). 2017
Received: 27 November 2016
Accepted: 14 February 2017
Published: 1 March 2017
(Ti, Al)N/MoN and CrN/MoN multilayered films were synthesized on Si (100) surface by multi-cathodic arc ion plating system with various bilayer periods. The elemental composition and depth profiling of the films were investigated by Rutherford backscattering spectroscopy (RBS) using 2.42 and 1.52 MeV Li2+ ion beams and different incident angles (0°, 15°, 37°, and 53°). The microstructures of (Ti, Al)N/MoN multilayered films were evaluated by X-ray diffraction. The multilayer periods and thickness of the multilayered films were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) and then compared with RBS results.
KeywordsRBS MoN Multilayer Microstructure Depth profiling
Multilayered, multicomponent, and nanostructured films are widely used in modern material engineering for their great contributions to improving protective properties of versatile industrial products involving hardness, wear, and corrosion resistance, oxidation resistance at high temperature [1–5]. Generally, a multilayered film containing two alternating sublayers has a significant parameter of modulation period defined as the thickness of a bilayer at nanoscale. Among the multilayer-film family, hard nitride-based coatings as one of the most prospective functional materials are so attractive that have been exploring by means of the optimized preparation processes and novel analytical techniques [6–9]. It has already been proved that the multilayered coatings have better properties compared with the monolayers [10–12] because the combination of two kinds of coatings can provide superior performance for the cutting tools and the thickness of the sublayer inlayed in the multilayered structure plays a significant role for vigorous properties of nano-composite coatings [13, 14]. It is necessary to adopt appropriate methods to fabricate and probe new multilayered structure films for further industrial application.
MoN films are remarkable for the self-lubrication over a wide temperature range, which leads to a low-coefficient friction and low wear rate [15–17]. The excellent tribological signatures are introduced by the formation of lubricious oxides, such as MoO3 demonstrated by Koshy . CrN coatings can exhibit the extraordinary oxidation, wear, and corrosion resistance [19, 20] while has rather high-friction coefficient (0.4–0.8 in air) [21–23]. Fabrication of CrN/Mo2N multilayered structure is an effective route to decrease the friction coefficient of bearing coatings like CrN from 0.6–0.8 to 0.3–0.4 at room temperature. TiAlN coatings are usually used as the cutting tools, wear protections, and contact materials in the microelectronics due to its high hardness, chemical inertness, and thermodynamic stability [24–28]. Addition of Mo to TiAlN forming TiAlN/MoN multilayers or nanocomposites can diminish the friction coefficient ranging from 0.8–0.9 to 0.3–0.4 at higher temperatures [29, 30]. The super hardness in nano-composite thin films is obtained when the small crystallites are detached by a discrepant boundary with the high cohesive strength (Patscheider et al.) . Briefly, it is extremely important and useful to analyze the multilayered structure consisting of MoN, CrN as well as other functional interlayers in ion plating applications.
Rutherford backscattering spectrometry (RBS) based on elastic collision has been utilized as a conventional tool for analysis of the thin film or the solid compound comprised intrinsic elements with enormous mass difference [32–34]. It is determinate to act as one of the most efficient non-destructive techniques for the depth profiling among nanoscale characterizations for the thin film, which can evaluate the relative atomic concentration as a function of depth at a unit of the areal density. It can also give the evidence on the atomic diffusion located at the interface of two individual layers as depth profiling in the film resulted from annealing. Usually, cross-section scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are employed to probe the microstructure feature of the multilayered films. However, the destructiveness and complication of accurate sampling detected by these methods have to take into consideration. By contrast, RBS is certainly a better alternative for the composition and depth profiling due to its significant advantage and quantification for both the thin films and bulk materials [35, 36].
To some extent, the systemic resolution of RBS measurement depends on the incident ion species, initial energy, and energy resolution of detector besides vacuum degree for beam transportation. Frequently, selecting proton as incident ion has better sensitivity for light element detection than 4He2+ or other heavy ions ascribed to its great penetrability and smaller straggling, but it is not sensitive for ultrathin film as the multilayered films consisting of number of sublayers at a thickness of several nanometers. In order to obtain more reasonable detection results, a heavier incoming MeV ion is probable to contribute much better mass resolution and depth resolution instead of energy resolution.
In this paper, we have used 2.42 and 1.52 MeV 7Li2+ ion beams as projectile and different incident angles (0°, 15°, 37°, and 53°) to study the structure of (Ti, Al)N/MoN and CrN/MoN multilayered films. X-ray diffraction (XRD) was employed to probe the microstructure of (Ti, Al)N/MoN. The element composition of CrN/MoN multilayered films was measured by Sirion FEG SEM with EDAX genesis 7000 EDS. X-ray photoelectron spectra (XPS, XSAM800 KRATOS) collected by Thermo Scientific Escalab 250 Xi spectrometer were used to confirm the elemental composition of (Ti, Al)N/MoN. At the same time, SEM and cross-sectional high resolution TEM (HRTEM) were also used to measure their modulation periods comparing with the results of RBS.
The (Ti, Al)N/MoN multilayered films were deposited on the polished Si(100) with Ti0.7Al0.3 and Mo targets by the cathodic arc plasma deposition system whose configuration was detailed in our previous work . During the deposition process, the nitrogen gas was fed into the chamber and the deposition pressure kept at 2.5 Pa while bias voltage of the substrate was −300 V. The (Ti, Al)N/MoN films with different modulation periods were fabricated by varying the substrate rotation speed (SRS) from 1 to 3 rpm. Similarly, the CrN/MoN multilayered films were deposited on the Si (100) substrates using Cr and Mo metal targets with a purity of more than 99.95%. Prior to deposition, the substrates were cleaned by a standard technique using ultrasonic degreasing and exposed to bombardment of Cr+ ions at −800 V for 10 min so as to remove the surface contaminants and reduce the roughness. After feeding reactive nitrogen gas to deposition process continuously, the vacuum and negative bias were about 2.0 Pa and 200 V, respectively. To achieve the multilayered films with various modulation periods, SRS was also varied from 2 to 6 rpm.
Results and Discussion
This subtle phase angle shift may be ascribed to the reduction of lattice parameter which is probable to be explained by smaller interstitial Al3+ ions replacing Ti3+ ions. It is concluded that the bilayer in films has no influence on the phase formation at the different SRSs but can interact on grain sizes of crystals in the sublayers.
where, Z1, Z2 is atomic number of the incident ion and the target atom, M1, M2 is their relative atomic mass, and E and θ are corresponding to incident ion energy and backscattering angle, respectively . Decreasing the initial energy E 0, the backscattering cross section is increased that can dedicate better mass differences to target atoms in the sample. In Fig. 5c, when E 0 is reduced from 2.42 to 1.52 MeV, the backscattering yields of all the elements have increased to five times higher than that of 2.42 MeV. A straightforward variety is that visible signal peaks from neighbor-surface sublayers reduce, such as from 7 to 5 peaks for Mo, revealing a shallower detected depth in the same condition. Comparatively, the fitting data give a thickness of 1.9 × 1017 ~ 2.8 × 1017 atoms/cm2 for monolayer MoN and 1.15 × 1017 ~ 1.8 × 1017 atoms/cm2 for monolayer (Ti, Al)N, which is corresponding to an average thickness of 47.5 and 34.5 nm, respectively. When the angle of incidence changes from 0° to 53° (Fig. 5d), the effective thickness of the outmost monolayer film is increased intensely while the sublayers located in deeper positions have worse energy resolution, especially for larger tilt angle 37° and 53°. It is concluded that incident ion beam with low energy can lead to a relative shallower detected depth but can be beneficial to detect ultrathin film beneath 10 nm.
where, Y is yield of backscattered ions, σ R is Rutherford backscattering cross section, Q is total number of incident ion charges, ρ is bulk density of target sample, and d 0/cos α is the detected depth when the incident angel is α. The fitting data from SIMNRA give the areal density ρ ⋅ d 0/cos α at atoms/cm2 (unit) . The thickness (nm) is proportional to the areal density and molecular mass of compound. However, during quantitative RBS measurements via fitting data of CrN/MoN, the areal density and molecular mass of homogenous compound instead of hybridized structures consisting of single phase compound and amorphous phase mixtures were taken into inconsideration that can lead to calculated value is larger than the actual value.
We have analyzed (Ti, Al)N/MoN and CrN/MoN multilayered films on Si substrate by using 1.52 and 2.42 MeV Li2+ ion beams of RBS. The results demonstrated that the 1.52 MeV ion beam is superior in depth resolution, whereas the 2.42 MeV ion beam is advantageous for deeper path detection. It is seen that SIMNRA simulation data agree well with the SEM results of (Ti, Al)N/MoN films at 2 rpm. The scattering cross-section peaks were broadened gradually with increases in the angle of incidence (α) of ion beam which implies the improvement in the detected resolution at a certain depth. The monolayers of CrN and MoN are 8.5 and 10 nm, respectively, when the film has smallest bilayer period 18.5 nm which provided that a relative good depth resolution about 10 nm. Finally, RBS depth profiling proved to be a useful structural tool to evaluate the multilayer structure and chemical composition.
This work was supported by National Natural Science Foundation of China under grants 11375133 and 11405133, Suzhou Scientific Development Project ZXG201448, and Wuhan Science and Technology Bureau under 2016030409020219.
DF, CL, and BH conceived and designed the study. ZW, NL, and WZ performed the experiments. BH wrote the paper. ZW, ND, and KKK reviewed and edited the manuscript. All authors read and approved the manuscript.
The authors declare that they have no competing interests.
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