Surface Investigation of Ni 81 Fe 19 Thin Film: Using ARXPS for Thickness Estimation of Oxidation Layers

: This work demonstrates the dependence between magnetic properties and the thickness of NiFe thin ﬁlms. More importantly, a quantitative study of the surface composition of NiFe thin ﬁlm exposed to atmospheric conditions has been carried out employing angle-resolved X-ray photoelectron spectroscopy (ARXPS). In this study, we fabricated Ni 81 Fe 19 (NiFe) thin ﬁlms on Si (100) substrate using electron beam evaporation and investigated their surface morphologies, magnetic properties, and the thickness of the surface oxide layer. The coexistence of metallic and oxidized species on the surface are suggested by the depth proﬁle of ARXPS spectra. The thickness of the oxidized species, including NiO, Ni(OH) 2 , Fe 2 O 3 , and Fe 3 O 4 , are also estimated based on the ARXPS results. This work provides an effective approach to clarify the surface composition, as well as the thickness of the oxide layer of the thin ﬁlms. Fe 2 O 3 2 p 3/2 (710.7 eV), and Fe 3 O 4 2 p 3/2 (709.3 eV) [27–29], results of are shown in Figure 8. Figure 9a shows the ﬁtted O 1s spectra to further conﬁrm the existence of iron oxides. Table 3 shows the relevant parameters including I s (Fe), I o (Fe 2 O 3 ), I o (Fe 3 O 4 ), and the intensity ratios between iron and its oxides. For the same reason mentioned above, I s (Fe) is increased with increasing α , as higher take-off angle results in deeper detection depth.


Introduction
Microwave magnetic devices, such as circulators, filters, and phase shifters are indispensable components in satellite and mobile communications systems [1][2][3]. Recently, the rapid development in the electronic communication industry has proposed higher requirements for microwave devices including higher operating frequency, lower loss, and higher integration level. As magnetoelectronic devices are the core components of microwave devices [4][5][6], there is an urgent demand to improve their magnetic properties at the high-frequency range. However, most microwave/radio-frequency magnetic components are discrete devices based on bulk materials, and directly reducing the size of high integration often leads to degradation in performance. The progress of chip-type technology is still behind for the perfect integration of magnetic components with existing semiconductor devices. Given the above context, ferromagnetic/anti-ferromagnetic (FM/AF) thin films with a multilayer structure are considered as ideal candidates. The higher saturation magnetization (M s ), permeability (µ), and self-biased ferromagnetic resonance (FMR) frequency of the magnetic thin film make them suitable for high-frequency applications as nanostructured magnetic media and magnetic sensors [7][8][9][10][11][12][13]. Meanwhile, the multilayer structure has been widely used in giant magnetoresistance spin valves based on the exchange bias phenomenon.
The operating frequency, FMR linewidth and the resonance field of periodically arranged FM/AF multilayer structure can easily be affected by the change in thickness of each single layer, as well as the surface morphology. Especially, when the thickness of individual layers falls below the critical thickness, the states and properties of surface and interface dominate the magnetic properties. In the multilayer structure of thin films, the thickness of each single layer can reach down to the level of several nanometers. However, such high accuracy is often susceptible to environmental oxidation, which leads to the change in the effective thickness, as well as the change in composition.
To study the composition of surface and interface as well as the thickness distribution in multilayer structures, atomic force microscopy (AFM) and angle resolved X-ray photoelectron spectroscopy (ARXPS) are commonly applied. Among the previous studies, Yu G.H. et al. [14] studied the relationship between the oxidation states of Ni in NiO x and the exchange bias field H ex of the NiFe film by ARXPS, but the critical thickness of the pinning of NiFe film was not studied. S.S. Sakhonenkov et al. [15] presented an in-depth study of Mo/Si multilayer systems using ARXPS. They reported that a MoSi 2 interlayer with a thickness of 0.19 ± 0.05 nm was identified on the Si-on-Mo interface. J. Zemek et al. [16] investigated the surface and in-depth distribution of sp 2 and sp 3 coordinated carbon atoms in modified diamond-like carbon films. A. Sanchez-Martinez et al. [17] discovered that small amounts of oxidized gallium and metallic arsenic are located at the HfO 2 /InGaAs interface. Meanwhile, they studied the structure of TiN/HfO 2 nanofilms grown on InGaAs substrates by ARXPS. The above evidences have shown ARXPS as a useful tool in characterizing the surface/interface of the thin films. In this study, AFM and ARXPS are used to characterize the surface oxidation layers of the NiFe thin film, which is known for its high permeability while having a relatively high M s . More importantly, the thickness of each oxidation layer is estimated based on the ARXPS results. The static magnetic properties of the NiFe thin films were also investigated.

Experimental Details
In this experiment, Ni 81 Fe 19 thin films were deposited on 5 mm × 5 mm Si (100) substrates by electron beam evaporation (EB-500) in vacuum (≤5.0 × 10 −5 Pa) at 25 • C. The thicknesses of the Ni 81 Fe 19 films are 90, 100, 110, and 120 nm. The growth rate of the film is 0.03 nm/s. The Si (100) substrates were preliminarily cleaned in a sequential bath of acetone, alcohol, and deionized water and dried with ionized dry N 2 flux. Once prepared, the samples were stored at room temperature to be oxidized under ambient environment.
The surface morphologies of the NiFe thin films were investigated using an atomic force microscope (AFM, Bruker MultiMode8) with a scanned area of 2 µm × 2 µm. Static magnetic properties of NiFe films were measured using a vibrating sample magnetometer (VSM, Lake Shore 8604) at room temperature. The samples were analyzed using ARXPS (ULVAC-PHI5000 Veraprobe III) with an Al Kα emission source (1486.6 eV). The XPS measurements were conducted at a base pressure of 8 × 10 −10 Pa. The pass energy of the spectrometer was set to 69 eV. The energy was calibrated by setting the adventitious C 1s binding energy to 284.8 eV. The chemical depth profile was acquired at six angles (α) of 20 • , 35 • , 45 • , 60 • , 75 • , and 90 • . Figure 1 illustrates the AFM surface morphology and roughness of the NiFe thin films with a NiFe thicknesses of 100 nm. The surface roughness of the films with different NiFe thicknesses from 90 to 120 nm was 1.31 nm, 1.27 nm, 1.45nm, and 1.67 nm, respectively. The minimum roughness of 1.27 nm is seen for the sample with a thickness of 100 nm, whose average grain size also appears to be the smallest among the samples (see Figure 3a). The experimental results are in good correspondence with a previous report [18], which shows the positive correlation between the average grain size and surface roughness.  Figure 2a,b shows the in-plane and out-of-plane magnetic hysteresis loops of NiFe thin films with the thicknesses from 90 to 120 nm. The samples with thickness of 90-110 nm show narrow M-H loops, while the 120-nm-thick sample shows a much wider curve, suggesting a higher coercivity (Hc) [19]. Figure 3 shows the static magnetic properties of the NiFe thin films, including the saturation magnetization (4πMs) and Hc. According to Figure 3a, as the thickness of NiFe thin film increases, the 4πMs is increased. Concomitantly, in-plane (Hc//) and out-of-plane (Hc⊥) coercivity are displayed in Figure 3b. Both Hc// and Hc⊥ exhibit a similar trend, which shows a slight reduction upon the increase of thickness from 90 to 110 nm, followed by a dramatic increase as the thickness is further increased to 120 nm. As the Hc is dependent on the quality and defects of the film [20], the dramatic increase of Hc observed in Figure 3b can be attributed to the high roughness of the 120 nm thick sample.    Figure 2a,b shows the in-plane and out-of-plane magnetic hysteresis loops of NiFe thin films with the thicknesses from 90 to 120 nm. The samples with thickness of 90-110 nm show narrow M-H loops, while the 120-nm-thick sample shows a much wider curve, suggesting a higher coercivity (H c ) [19]. Figure 3 shows the static magnetic properties of the NiFe thin films, including the saturation magnetization (4πM s ) and H c . According to Figure 3a, as the thickness of NiFe thin film increases, the 4πM s is increased. Concomitantly, in-plane (H c// ) and out-of-plane (H c⊥ ) coercivity are displayed in Figure 3b. Both H c// and H c⊥ exhibit a similar trend, which shows a slight reduction upon the increase of thickness from 90 to 110 nm, followed by a dramatic increase as the thickness is further increased to 120 nm. As the H c is dependent on the quality and defects of the film [20], the dramatic increase of H c observed in Figure 3b can be attributed to the high roughness of the 120 nm thick sample. Figure 4 shows the ARXPS spectra of Ni in the NiFe thin film with a thickness of 100 nm from various take-off angles. For clear identification of each peak, as well as to obtain the peak intensity, fitted results for each spectrum are obtained (examples of α = 20 • and 90 • are shown in Figure 5). The relationship between the detection depth (d), photoelectron take-off grazing angle, and the mean free path of inelastic scattering (λ), is as follows [21].  Figure 2a,b shows the in-plane and out-of-plane magnetic hysteresis loops of NiFe thin films with the thicknesses from 90 to 120 nm. The samples with thickness of 90-110 nm show narrow M-H loops, while the 120-nm-thick sample shows a much wider curve, suggesting a higher coercivity (Hc) [19]. Figure 3 shows the static magnetic properties of the NiFe thin films, including the saturation magnetization (4πMs) and Hc. According to Figure 3a, as the thickness of NiFe thin film increases, the 4πMs is increased. Concomitantly, in-plane (Hc//) and out-of-plane (Hc⊥) coercivity are displayed in Figure 3b. Both Hc/ and Hc⊥ exhibit a similar trend, which shows a slight reduction upon the increase of thickness from 90 to 110 nm, followed by a dramatic increase as the thickness is further increased to 120 nm. As the Hc is dependent on the quality and defects of the film [20], the dramatic increase of Hc observed in Figure 3b can be attributed to the high roughness of the 120 nm thick sample.      Figure 4 shows the ARXPS spectra of Ni in the NiFe thin film with a thickness of 100 nm from various take-off angles. For clear identification of each peak, as well as to obtain the peak intensity, fitted results for each spectrum are obtained (examples of α = 20° and 90° are shown in Figure 5). The relationship between the detection depth (d), photoelectron take-off grazing angle, and the mean free path of inelastic scattering (λ), is as follows [21].  As can be seen in Figure 5, there are several noticeable peaks representing the signals from Ni 2p3/2 (852.3 eV), NiO 2p3/2 (853.3 eV), Ni(OH)2 2p3/2 (856.6 eV), as well as their satellite peaks [22][23][24]. The coexistence of metallic and oxide components is further confirmed by the spectra from O 1s photoemission, which is shown in Figure 6a. The thickness of the oxide film (do) can be calculated according to the ARXPS results through the following equations [25]:

Oxidation Thickness of NiFe Films
where α is different take-off angles: 20°, 35°, 45°, 60°, 75°, and 90°, and R is the ratio of photoelectron peak intensity from the oxides (Io) to that from nickel (Is): In Equation (2), R∞ can be calculated as follows: where the values of M are the molecular weight of nickel oxides (Mo) and nickel (Ms), values of ρ are the density of nickel oxides (ρo) and nickel (ρs), and values of λ are the mean free path of inelastic scattering for nickel oxides (λo) and nickel (λs). Because the photoelectron kinetic energy of nickel and its oxides are almost the same, thus λo and λs are approximately equal [21]. Table 1 shows the values of density, molecular weight for Ni, NiO, and Ni(OH)2, as well as the calculated R∞ for both nickel oxides.   Table 2 shows the relative peak intensities of nickel (Is) and nickel oxides (Io) at different take-off angles based on the fitting results shown in Figure 5, as well as the calculated ratios between them (R). As α is increased from 20 to 90°, Is(Ni) is seen to increase from ~20 to ~43%, while both Io(NiO) and Io(Ni(OH)2) are reduced from ~46 to ~32% and from ~33 to ~26%, respectively. As the oxidation reactions take place at the top surface of the metal substrate, with a limited traveling distance of photoemission signal, it is natural that the detection depth becomes deeper as the take-off angle becomes closer to 90°. Therefore, one can observe the obvious enhancement in Is(Ni), and slight reduction in Io(NiO) and Io(Ni(OH)2).    As can be seen in Figure 5, there are several noticeable peaks representing the signals from Ni 2p3/2 (852.3 eV), NiO 2p3/2 (853.3 eV), Ni(OH) 2 2p3/2 (856.6 eV), as well as their satellite peaks [22][23][24]. The coexistence of metallic and oxide components is further confirmed by the spectra from O 1s photoemission, which is shown in Figure 6a. The thickness of the oxide film (d o ) can be calculated according to the ARXPS results through the following equations [25]:
As with nickel oxides, the composition and thickness of iron oxides can also be determined. Figure 7 shows the Fe 2p photoemission spectra of partially oxidized NiFe thin film. The signals from each take-off angle can be fitted with Fe 2p3/2 (706.8 eV), Fe2O3 2p3/2 (710.7 eV), and Fe3O4 2p3/2 (709.3 eV) [27][28][29], the results of which are shown in Figure 8. Figure 9a shows the fitted O 1s spectra to further confirm the existence of iron oxides. Table 3 shows the relevant parameters including Is(Fe), Io(Fe2O3), Io(Fe3O4), and the intensity ratios between iron and its oxides. For the same reason mentioned above, Is(Fe) is increased with increasing α, as higher take-off angle results in deeper detection depth. In Equation (2), R ∞ can be calculated as follows: where the values of M are the molecular weight of nickel oxides (M o ) and nickel (M s ), values of ρ are the density of nickel oxides (ρ o ) and nickel (ρ s ), and values of λ are the mean free path of inelastic scattering for nickel oxides (λ o ) and nickel (λ s ). Because the photoelectron kinetic energy of nickel and its oxides are almost the same, thus λ o and λ s are approximately equal [21]. Table 1 shows the values of density, molecular weight for Ni, NiO, and Ni(OH) 2 , as well as the calculated R ∞ for both nickel oxides.  Table 2 shows the relative peak intensities of nickel (I s ) and nickel oxides (I o ) at different take-off angles based on the fitting results shown in Figure 5, as well as the calculated ratios between them (R). As α is increased from 20 to 90 • , I s (Ni) is seen to increase from~20 to~43%, while both I o (NiO) and I o (Ni(OH) 2 ) are reduced from~46 tõ 32% and from~33 to~26%, respectively. As the oxidation reactions take place at the top surface of the metal substrate, with a limited traveling distance of photoemission signal, it is natural that the detection depth becomes deeper as the take-off angle becomes closer to 90 • . Therefore, one can observe the obvious enhancement in I s (Ni), and slight reduction in I o (NiO) and I o (Ni(OH) 2 ). According to Equation (2), to calculate the thickness of the oxidation layer, one can plot the results of ln(1 + R/R ∞ ) against 1/sinα, the slope of which indicates the thickness divided by the mean free path of inelastic scattering (d o /λ). Figure 6b shows the results of ln(1 + R/R ∞ ) against 1/sinα for Ni(OH) 2 and NiO. Given that λ = 6 Å [26], the thickness of NiO in NiFe thin film is determined to be 0.2 nm, the thickness of Ni(OH) 2 is determined to be 0.3 nm.
As with nickel oxides, the composition and thickness of iron oxides can also be determined. Figure 7 shows the Fe 2p photoemission spectra of partially oxidized NiFe thin film. The signals from each take-off angle can be fitted with Fe 2p3/2 (706.8 eV), Fe 2 O 3 2p3/2 (710.7 eV), and Fe 3 O 4 2p3/2 (709.3 eV) [27][28][29], the results of which are shown in Figure 8. Figure 9a shows the fitted O 1s spectra to further confirm the existence of iron oxides. Table 3 shows the relevant parameters including I s (Fe), I o (Fe 2 O 3 ), I o (Fe 3 O 4 ), and the intensity ratios between iron and its oxides. For the same reason mentioned above, I s (Fe) is increased with increasing α, as higher take-off angle results in deeper detection depth. According to Equation (2), to calculate the thickness of the oxidation plot the results of ln(1 + R/R∞) against 1/sinα, the slope of which indicate divided by the mean free path of inelastic scattering (do/λ). Figure 6b show ln(1 + R/R∞) against 1/sinα for Ni(OH)2 and NiO. Given that λ = 6 Å [26], t NiO in NiFe thin film is determined to be 0.2 nm, the thickness of Ni(OH)2 to be 0.3 nm.
As with nickel oxides, the composition and thickness of iron oxides termined. Figure 7 shows the Fe 2p photoemission spectra of partially oxid film. The signals from each take-off angle can be fitted with Fe 2p3/2 (70 2p3/2 (710.7 eV), and Fe3O4 2p3/2 (709.3 eV) [27][28][29], the results of which Figure 8. Figure 9a shows the fitted O 1s spectra to further confirm the ex oxides. Table 3 shows the relevant parameters including Is(Fe), Io(Fe2O3), Io( intensity ratios between iron and its oxides. For the same reason mentione is increased with increasing α, as higher take-off angle results in deeper de     Table 4 shows the values of density, molecular weight for Fe, Fe3O4, and Fe2O3, as well as the calculated R∞ for both iron oxides. The values in Tables 3 and 4 are integrated into Equations (2)-(4), and the results of ln(1 + R/R∞) against 1/sinα for Fe 2p photoemission in the NiFe thin film are shown in Figure 9b. As λ = 7.5 Å [26], the thickness of Fe2O3 layer determined is 0.9 nm and that of Fe3O4 is 0.7 nm. It means that oxidation in the natural environment occurs only in the extreme depth of the surface. To summarize, a schematic illustration is shown in Figure 10 to better demonstrate the multilayer structure of the oxidized NiFe.    Table 4 shows the values of density, molecular weight for Fe, Fe3O4, and Fe2O3, as well as the calculated R∞ for both iron oxides. The values in Tables 3 and 4 are integrated into Equations (2)-(4), and the results of ln(1 + R/R∞) against 1/sinα for Fe 2p photoemission in the NiFe thin film are shown in Figure 9b. As λ = 7.5 Å [26], the thickness of Fe2O3 layer determined is 0.9 nm and that of Fe3O4 is 0.7 nm. It means that oxidation in the natural environment occurs only in the extreme depth of the surface. To summarize, a schematic illustration is shown in Figure 10 to better demonstrate the multilayer structure of the oxidized NiFe.    Table 4 shows the values of density, molecular weight for Fe, Fe 3 O 4 , and Fe 2 O 3 , as well as the calculated R ∞ for both iron oxides. The values in Tables 3 and 4 are integrated into Equations (2)-(4), and the results of ln(1 + R/R ∞ ) against 1/sinα for Fe 2p photoemission in the NiFe thin film are shown in Figure 9b. As λ = 7.5 Å [26], the thickness of Fe 2 O 3 layer determined is 0.9 nm and that of Fe 3 O 4 is 0.7 nm. It means that oxidation in the natural environment occurs only in the extreme depth of the surface. To summarize, a schematic illustration is shown in Figure 10 to better demonstrate the multilayer structure of the oxidized NiFe.

Conclusions
In summary, NiFe thin films with thickness from 90 to 120 nm were fabricated by electron beam evaporation. The sample with a thickness of 100 nm shows the lowest surface roughness and the smallest grain size. The saturation magnetization of the films is increased from 9891 to 10,300 Gs as the thickness of film increases. The optimum magnetic properties (4πMs = 9930 Gs, Hc = 1.37 Oe) are obtained when the thickness of film is 100 nm. In addition, the coexistence of NiFe and its oxides on the top surface is observed along the probing depth of ARXPS. By fitting the ARXPS spectra and obtaining the relative intensity of each peak, the thickness of NiO, Ni(OH)2, Fe2O3, and Fe3O4, which are 0.3 nm, 0.2 nm, 0.9 nm, and 0.7 nm, respectively, are determined. This work demonstrates the beneficial application of ARXPS in characterizing the oxide layers including their compositions and thicknesses as a quick and damage-free approach, especially for future investigations with even lower film thickness.

Conclusions
In summary, NiFe thin films with thickness from 90 to 120 nm were fabricated by electron beam evaporation. The sample with a thickness of 100 nm shows the lowest surface roughness and the smallest grain size. The saturation magnetization of the films is increased from 9891 to 10,300 Gs as the thickness of film increases. The optimum magnetic properties (4πM s = 9930 Gs, H c = 1.37 Oe) are obtained when the thickness of film is 100 nm. In addition, the coexistence of NiFe and its oxides on the top surface is observed along the probing depth of ARXPS. By fitting the ARXPS spectra and obtaining the relative intensity of each peak, the thickness of NiO, Ni(OH) 2 , Fe 2 O 3 , and Fe 3 O 4 , which are 0.3 nm, 0.2 nm, 0.9 nm, and 0.7 nm, respectively, are determined. This work demonstrates the beneficial application of ARXPS in characterizing the oxide layers including their compositions and thicknesses as a quick and damage-free approach, especially for future investigations with even lower film thickness.