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Article

Microstructure and Stress of Ni/C Multilayer Films Prepared by Reactive Sputtering

by
Jichang Peng
* and
Zhen Ouyang
Department of Physics, Jinggangshan University, Ji’an 343009, China
*
Author to whom correspondence should be addressed.
Coatings 2022, 12(11), 1718; https://doi.org/10.3390/coatings12111718
Submission received: 15 September 2022 / Revised: 3 November 2022 / Accepted: 4 November 2022 / Published: 10 November 2022
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Abstract

:
Magnetron-sputtered Ni/C multilayers with a periodic thickness below 4 nm are difficult to produce, and reactive sputtering with nitrogen is a feasible method. The effects of nitrogen on the reflectivities of Ni/C multilayers were investigated. Pure argon and three mixing ratios of 4%, 8%, and 15% nitrogen-argon gas mixture were used as the working gas. For all Ni/C multilayer samples, each contains 40 bilayers. The nominal structure has a periodic thickness of 3.8 nm, with a ratio of the thickness of the Ni layer to the periodic thickness of Г = 0.39. The results of grazing incidence X-ray reflectivity (GIXRR) measurements indicate that reactively-sputtered Ni/C multilayers have a lower interface width and higher specular reflectance. It was shown in transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) measurements that a periodic layered structure can still be clearly observed in Ni/C multilayers with pure argon, but with quite rough interfaces between the adjacent layers. For Ni/C multilayers with the mixing ratio of 4% nitrogen-argon gas mixture, it is found that the interfaces between Ni and C layers become smoother and sharper. Additionally, nitrogen incorporation can reduce the mobility of Ni atoms, which decreases the threshold thickness that Ni layers would become continuous. This may be also a reason which accounts for the better interface quality of reactively-sputtered Ni/C multilayers. Meanwhile, Ni/C multilayers deposited with a nitrogen-argon gas mixture have lower stress due to the reduction in Ni adatom mobility.

1. Introduction

Multilayer optical elements attract extensive interest due to their unusual optical properties applied in extreme ultraviolet (EUV) and X-ray wavelength regions, such as EUV lithography, synchrotron radiation, plasma diagnostics, and astronomy [1,2,3]. In the ‘carbon-window’ region [4,5], Ni/C multilayers have great potential as normal incidence reflective optics compared to other material combinations because of a large electron density contrast. In the X-ray wavelength region, laterally-graded Ni/C multilayers can be served as monochromators for Cu Kα radiation in “Göbel Mirrors”, which have unique properties of a tiny divergence for the reflected beam and an excellent inhibition for Cu Kβ radiation [6,7,8]. In addition, depth-graded Ni/C multilayers can be applied in grazing incidence telescopes for astronomical observations up to 100 k eV [9,10].
Due to their unique optical properties, Ni/C multilayers have been widely studied over the past several decades. However, the reflectivity performances of Ni/C multilayers are strongly affected by the large interface width [11], especially for samples with a small periodic thickness working in the X-ray wavelength region. For instance, Borchers et al., have found that the X-ray reflectivity dropped greatly for the magnetron sputtered Ni/C multilayers having a periodic thickness under 4 nm [12,13]. The drop in reflectivity can be attributed to the critical thickness below which Ni crystalline grains are formed and Ni layers would not become continuous any more. Other deposition techniques such as pulsed laser deposition or ion beam sputter deposition [14,15] can reduce the critical thickness because the deposited particles which have higher kinetic energy are used [16]. Nevertheless, Ni/C multilayers having periodic thickness under 3 nm have still been difficult to produce up to now [8,17]. Consequently, the applications of Ni/C multilayers would be restricted to a degree.
Furthermore, film stress is another issue that must be taken into consideration, especially for sputtered thin films. The presence of stress would influence the adhesion between the film and the substrate and also result in the deformation of the substrate. In certain applications, such as EUV projection lithography and diffraction-limited X-ray imaging [18], the substrate deformation caused by stress would affect the surface figure precision of optical elements and then deteriorate the imaging quality.
The chemical methods to change the properties of matter are widely used in various fields [19,20,21,22,23,24,25,26]. In the process of growing the thin films, reactive sputtering is often applied to improve the properties of thin films [27,28]. Reactive sputtering with nitrogen is a deposition technique which could effectively reduce roughness and stress in thin films. In some cases, nitrogen ions may take up interstitial sites in the unit cell of sputtered species and cause a distortion in the unit cell, which would result in a nanocrystalline or amorphous structure of the deposited film [29]. For sputtered metal layers, a nanocrystalline or amorphous phase could result in smoother interfaces in multilayers in virtue of smaller crystallite grains. The reduction in interface roughness and stress has been observed for certain multilayers, such as W/B4C [30]. On the other hand, due to the nitridation and, thereby, passivation of the layers, the interdiffusion and compound formation at the interfaces are reduced, which has been observed for several multilayer systems including Cr/Sc [31], La/B [32,33], La/B4C [34], and Pd/Y [35]. Moreover, the incorporation of nitrogen may also enhance the thermal stability of multilayers such as Co/C [36], Cr/Sc [37], and La/B4C [34].
In this paper, we report on research into the microstructure and stress of Ni/C multilayers deposited by reactive direct-current (DC) magnetron sputtering employing a nitrogen-argon gas mixture. The effects of the incorporation of nitrogen on the interfacial microstructures of Ni/C multilayer films were investigated by characterization techniques including grazing incidence X-ray reflectivity (GIXRR) high-resolution cross-sectional transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The stress of Ni/C multilayer films was determined by a wafer curvature measurement. Compared to Ni/C multilayers prepared by pure argon sputtering, reactively-sputtered Ni/C multilayers have better interface quality and lower stress.

2. Experimental

The studied Ni/C multilayer films were fabricated on super-smooth silicon wafers with a size of 20 mm × 20 mm employing DC magnetron sputtering (LeyFond Vacuum, Beijing, China) at room temperature. The background pressure before deposition was 2.5 × 10−4 Pa. For non-reactive sputtering, high purity argon was applied as the working gas, and the pressure of argon was kept at 1.5 mTorr; for reactive sputtering with nitrogen, a mixture of argon and nitrogen was applied as the working gas, and the total pressure of argon and nitrogen was also kept at 1.5 mTorr. Three different mixing ratios, i.e., the ratios of the partial pressure of N2 to the total pressure in the bottle, were used: 4%, 8%, and 15% nitrogen. The powers which were applied to Ni and C targets were kept constant at 20 W and 100 W, respectively. For all Ni/C multilayer samples, each of them includes 40 bilayers. The bottom layer is Ni and the top layer is C. The nominal structure has a periodic thickness of 3.8 nm, and a ratio of the thickness of the Ni layer to the periodic thickness is Г = 0.39.
The layer structures of Ni/C multilayers were first characterized by GIXRR measurements (Bede Scientific Instruments Ltd., Durham, UK) based on Bede D1 X-ray diffractometer applying a Cu Kα source (λ = 0.154 nm). High resolution transmission electron microscopy, and selected area electron diffraction, provided by Materials Analysis Technology Inc., were employed to investigate the layer morphology and crystalline structure. For the transmission electron microscopy (TEM) (FEL Company, Hillsboro, OR, USA) samples, first, the wedge polishing technique was employed to mill the samples, which made the thickness of the sample below 10 µm. Then, the samples were thinned to electron transparency by ion-milling, which used 3 kV Ar+ ions and a Gatan Model 691 Precision Ion Polishing System. Finally, the cross-sectional samples were measured using a FEI Tecnai G2 F20 instrument (FEL Company, Hillsboro, OR, USA) which was operating at 200 kV. In order to deeply understand nitrogen depth distribution after nitridation of the Ni/C multilayers structure, we employed the energy dispersive X-ray spectroscopy (EDS) technique performed with the same FEI Tecnai G2 F20 instrument used for TEM. With regard to the stress of Ni/C multilayers, selected Ni/C multilayers were deposited onto 30 mm quartz glass wafers of nominal 1 mm thickness, and the wafer curvature was measured before and after film deposition by using the Fizeau interferometer. The total film thickness, which was determined by GIXRR, was used to compute film stress, following the standard formalism which is based on the Stoney equation [38].

3. Results

3.1. Grazing Incidence X-ray Reflectivity

Shown in Figure 1 are the GIXRR measurement results for all samples. The drift of the Bragg peaks’ position indicates that the period thickness is changed from 3.7 nm to 3.87 nm for different Ni/C multilayers. The major difficulty with depositing multilayers with precisely the same period and thickness ratios is that the deposition rate of C increases notably with an increasing proportion of N2, whereas the deposition rate of Ni is varied only mildly. A similar case was observed in the previous research of W/B4C multilayers [30]. For the Ni/C multilayers deposited with pure Ar, only two Bragg peaks can be observed, indicating that there is a larger interface width for the sample. It can be seen that the widths of diffraction peaks of the sample are relatively wider, suggesting that there is a fluctuation of the thickness of each bilayer in the non-reactively-sputtered Ni/C multilayers, which in turn results in the decreasing of intensities and the increasing of widths in the diffraction peaks, while, for the other three Ni/C multilayers fabricated with nitrogen-argon gas mixture, the fourth order peak arises, and the first to second Bragg peaks’ reflectivity was also enhanced as compared with the non-reactively-sputtered sample. The increasing of the number of Bragg peaks demonstrates that there is a smaller interface width for the reactively-sputtered samples, which is similar with the previous research of Co/C multilayer films by incorporation of nitrogen [39,40]. Meanwhile, the widths of diffraction peaks of the reactively-sputtered samples are relatively narrower, indicating that reactive sputtering with nitrogen reduces the fluctuation of thickness of each bilayer. For the Ni/C multilayers deposited with pure Ar and Ar + 4% N2, the first Bragg peaks’ reflectivities are 30.4% and 59.1%, respectively. From these results, it is observed that the reactively-sputtered Ni/C multilayers have lower interface width and higher specular reflectance and also have uniform thickness of each bilayer. Additionally, it can be deduced that the microstructures of Ni/C multilayers have only tiny changes as compared with the Ni/C multilayers deposited with Ar + 4% N2 when further increasing the mixing ratio to 8% and 15%. Hence, our relevant investigation mainly focuses on the Ni/C multilayers fabricated with Ar + 4% N2.

3.2. High Resolution Transmission Electron Microscopy

High resolution TEM images and SAED (FEL Company, Hillsboro, OR, USA) patterns of Ni/C multilayers deposited with pure Ar and Ar + 4% N2 are shown in Figure 2, where the Ni layers are dark and the C layers are bright. For the Ni/C multilayers deposited with pure Ar, although a periodic layered structure still remains, the interfaces between the adjacent layers are quite rough, as indicated in Figure 2a. There are some grains with ellipsoidal shapes inside the Ni layers, and the orientations of these grains are of random distribution. The sizes of these Ni grains are approximately 2–3 nm along the multilayer growth direction, which is generally smaller than the lateral sizes of these grains. Furthermore, it can be seen that these grains are separated from each other or merely partially in contact. Hence, it can be concluded that there are some gaps or interstices between these Ni grains, which are stuffed with the amorphous carbon, as displayed in Figure 2a with an arrow. The SAED pattern (inset) of this sample is shown in Figure 2a. One diffraction ring was evidently observed, which corresponds with the Ni(111) plane’s diffraction with an Ni fcc structure. The observed phase is consistent with the reported XRD results [41,42]. No apparent intensity modulations are observed in this obscure diffraction ring, indicating that these Ni crystalline grains display no preferential orientation inside the Ni layers. From these results shown in Figure 2a, it can be deduced that the rough interfaces of Ni/C multilayers fabricated with pure Ar are mainly attributed to these polycrystalline Ni grains of random distribution in the Ni layers. This phenomenon was also observed in the investigations of magnetron-sputtered Ni/C graded multilayers performed by Borchers et al. [12].
For the sample deposited with Ar + 4% N2 (dNi = 1.53 nm, dC = 2.22 nm), the interfaces between Ni and C layers are quite smooth and sharp, which is consistent with the GIXRR measurement results, as displayed in Figure 2b. There are some visible grains inside the Ni layers, and the orientations of these grains are randomly distributed, suggesting that the Ni layers are polycrystalline. Meantime, compared with the sample fabricated with pure Ar, the sizes of these Ni grains along the multilayer growth direction become smaller due to the incorporation of nitrogen. Shown in Figure 2b is the SAED pattern (inset) of this specimen, where one visible diffraction ring was observed corresponding to fcc Ni(111) planes. Since no apparent intensity modulations are observed in this obscure diffraction ring, these Ni crystalline grains have no preferred orientation inside the Ni layers, which is in agreement with the TEM image.

3.3. EDS Compositional and Chemical Analysis

In order to investigate the effect of nitridation on the change in chemical composition of the layers, the Ni/C multilayers deposited with pure Ar and Ar + 4% N2 were analyzed using energy dispersive X-ray spectroscopy (EDS). The energy dispersive X-ray spectroscopy (EDS) technique was operated with the same FEI Tecnai G2 F20 instrument employed for TEM. EDS is a valuable tool for analyzing element composition. This technique is much more sensitive to heavy atoms such as Ni and W compared with light atoms such as C and N. Thus, the absolute concentration of the chemical element can only be determined qualitatively in our experiment. Even so, EDS can ascertain the relative variation in the concentration of the same chemical element, which is sufficient for the aims of our research. A strong focusing electron beam (1–2 nm size) was projected normally onto the cross section of the multilayer and moved in the direction which is perpendicular to interfaces. The characteristic spectral line of different chemical elements was obtained, which allowed the composition distribution of chemical elements in the depth of a multilayer structure to be determined.
For the Ni/C multilayers deposited with pure Ar and Ar + 4% N2, the periodic thicknesses are 3.7 nm and 3.75 nm, and the total film thicknesses are 148 nm and 150 nm, respectively. For the Ni element, the characteristic spectral lines are Ni Lα, Ni Kα, and Ni Kβ, and the corresponding energies are 0.849 k eV, 7.478 k eV, and 8.265 k eV, respectively; for the N element, the characteristic spectral line is N Kα and the corresponding energy is 0.392 keV; for the C element, the characteristic spectral line is C Kα, and the corresponding energy is 0.282 keV. The atomic concentrations determined by EDS as a function of depth for Ni/C multilayers deposited with pure Ar and Ar + 4% N2 are shown in Figure 3. For the sample deposited with pure Ar, the concentration curves of Ni and C have the periodic oscillation with depth, suggesting that a periodic layered structure still remains. Nevertheless, the amplitude of the Ni and C concentration oscillation is smaller, suggesting that the interfaces are not abrupt due to the mixing and diffusion between the adjacent layers of Ni and C. For the sample deposited with Ar + 4% N2, the concentration curves of Ni and C also have the periodic oscillation with depth, but with the larger amplitude of the concentration oscillation, which indicates that the interfaces between Ni and C layers are quite smooth and sharp. Thus, the addition of 4% nitrogen to the working gas could improve the interface quality of Ni/C multilayers, which is consistent with the results determined by GIXRR and TEM for the same sample. Furthermore, as shown in Figure 3b, the nitrogen concentration has the same variation tendency as the carbon concentration but contrary to the nickel concentration; that is to say, the concentration of nitrogen is maximal inside carbon layers rather than in nickel layers.

3.4. The Stress of Ni/C Multilayers

From the above-mentioned results, it can be seen that N2 does not react with the Ni atoms, but the Ni layers become continuous after incorporation of nitrogen. Since the film stress is closely related to the film microstructure, stress measurements can be used for studies of the film microstructure. In order to search for the reason why the Ni layers become continuous after incorporation of nitrogen, the stress of Ni/C multilayers deposited with pure Ar or a nitrogen-argon gas mixture was determined by a wafer curvature measurement. The stresses as a function of N2 Gas Fraction for Ni/C multilayers deposited with pure Ar and a nitrogen-argon gas mixture are shown in Figure 4. For the sample fabricated with pure Ar, the stress was measured to be approximately −438.3 MPa (compressive); for reactively-sputtered Ni/C multilayers, there is a marked reduction in total film stress. Hence, compared to Ni/C multilayers prepared by pure argon sputtering, reactively-sputtered Ni/C multilayers have better interface quality and lower stress.

4. Discussion

The above-mentioned experimental results indicate that the reactively-sputtered Ni/C multilayers have better interface quality and lower stress. As displayed in Figure 2a,b, the sizes of Ni crystalline grains of the sample fabricated with pure Ar are greater than those of the sample fabricated with Ar + 4% N2. Judging from the SAED patterns (insets), the diffraction ring of fcc Ni(111) of the sample fabricated with pure Ar is lighter than that of the sample fabricated with Ar + 4% N2. Therefore, it can be deduced that the incorporation of nitrogen can weaken the crystallization of Ni. According to the investigations of Abermann et al. [43,44], the introduction of impurity gas (e.g., O2, N2, H2O) into the vacuum system during deposition could affect the adatom mobility, which, in turn, could influence the film microstructure and stress. For magnetron-sputtered polycrystalline film, when the adatom mobility is reduced, the film microstructure evolves from a zone T-type microstructure comprising tightly-packed columnar grains to a zone 1-type microstructure characterized by porous columnar grains, accompanied by a stress reversal from compressive to tensile stress [45,46]. For Ni/C multilayers prepared by pure argon sputtering, when Ni atoms are grown on an amorphous C layer, Ni atoms have high adatom mobility and appear in island growth mode induced by the interfacial nonwetting [12]. Hence, the incorporation of nitrogen could reduce the Ni adatom mobility, which results in the decrease in the threshold thickness at which the Ni film becomes continuous. That is to say, when the thickness of the Ni layer is below the thickness of 2.0 nm, the Ni layer is still continuous. On the other hand, according to the investigations of Bellotti et al. [39], reactive sputtering with nitrogen can produce a smoother C single-layer film, compared with pure argon sputtering. Thus, reactively-sputtered Ni/C multilayers have better interface quality as compared with Ni/C multilayers prepared by pure argon sputtering. Meanwhile, due to the reduction in Ni adatom mobility induced by the incorporation of nitrogen, there is a stress reversal from compressive to tensile stress in Ni layers, while, as for C layers, according to the investigations of Hellgren et al. [47], the stress as a function of nitrogen concentration stays fairly constant at a low temperature. It indicates that the reduction in Ni adatom mobility can reduce the stress of Ni/C multilayer films at least to a certain degree.

5. Conclusions

The effects of the incorporation of nitrogen on the interfacial microstructures of Ni/C multilayers were studied by characterization techniques. The results of GIXRR measurements indicate that reactively-sputtered Ni/C multilayers have lower interface width and higher specular reflectance. When further increasing the mixing ratio to 8% and 15%, the microstructures of Ni/C multilayers have only tiny changes. It was shown in TEM and EDS measurements that a periodic layered structure can still be clearly observed in Ni/C multilayers with pure argon, but with quite rough interfaces between the adjacent layers. For Ni/C multilayers with the mixing ratio of 4% nitrogen-argon gas mixture, it is found that the interfaces between Ni and C layers become smoother and sharper. Additionally, nitrogen incorporation can reduce the mobility of Ni atoms, which decreases the threshold thickness at which Ni layers would become continuous. This may be also a reason which accounts for the better interface quality of reactively-sputtered Ni/C multilayers. Meanwhile, Ni/C multilayers deposited with a nitrogen-argon gas mixture have lower stress due to the reduction of Ni adatom mobility.

Author Contributions

Study design, J.P.; data collection, J.P.; writing, J.P.; data analysis, Z.O. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 11865008) and the Science and Technology Research Project of Jiangxi Provincial Department of Education (No. GJJ211028).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. Measured GIXRR curves for the Ni/C multilayers fabricated with pure Ar and nitrogen-argon gas mixture.
Figure 1. Measured GIXRR curves for the Ni/C multilayers fabricated with pure Ar and nitrogen-argon gas mixture.
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Figure 2. Cross-sectional HRTEM images and SAED patterns (insets) of Ni/C multilayers deposited with (a) pure Ar and (b) a mixture Ar + 4% N2 as the working gas.
Figure 2. Cross-sectional HRTEM images and SAED patterns (insets) of Ni/C multilayers deposited with (a) pure Ar and (b) a mixture Ar + 4% N2 as the working gas.
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Figure 3. The atomic concentrations determined by EDS as a function of depth for Ni/C multilayers deposited with (a) pure Ar and (b) a mixture Ar + 4% N2 as the working gas.
Figure 3. The atomic concentrations determined by EDS as a function of depth for Ni/C multilayers deposited with (a) pure Ar and (b) a mixture Ar + 4% N2 as the working gas.
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Figure 4. The stresses as a function of N2 Gas Fraction for Ni/C multilayers deposited with pure Ar and a nitrogen-argon gas mixture.
Figure 4. The stresses as a function of N2 Gas Fraction for Ni/C multilayers deposited with pure Ar and a nitrogen-argon gas mixture.
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Peng, J.; Ouyang, Z. Microstructure and Stress of Ni/C Multilayer Films Prepared by Reactive Sputtering. Coatings 2022, 12, 1718. https://doi.org/10.3390/coatings12111718

AMA Style

Peng J, Ouyang Z. Microstructure and Stress of Ni/C Multilayer Films Prepared by Reactive Sputtering. Coatings. 2022; 12(11):1718. https://doi.org/10.3390/coatings12111718

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Peng, Jichang, and Zhen Ouyang. 2022. "Microstructure and Stress of Ni/C Multilayer Films Prepared by Reactive Sputtering" Coatings 12, no. 11: 1718. https://doi.org/10.3390/coatings12111718

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