Annealing Effect on the Characteristics of Co40Fe40W10B10 Thin Films on Si(100) Substrate

This research explores the behavior of Co40Fe40W10B10 when it is sputtered onto Si(100) substrates with a thickness (tf) ranging from 10 nm to 100 nm, and then altered by an annealing process at temperatures of 200 °C, 250 °C, 300 °C, and 350 °C, respectively. The crystal structure and grain size of Co40Fe40W10B10 films with different thicknesses and annealing temperatures are observed and estimated by an X-ray diffractometer pattern (XRD) and full-width at half maximum (FWHM). The XRD of annealing Co40Fe40W10B10 films at 200 °C exhibited an amorphous status due to insufficient heating drive force. Moreover, the thicknesses and annealing temperatures of body-centered cubic (BCC) CoFe (110) peaks were detected when annealing at 250 °C with thicknesses ranging from 80 nm to 100 nm, annealing at 300 °C with thicknesses ranging from 50 nm to 100 nm, and annealing at 350 °C with thicknesses ranging from 10 nm to 100 nm. The FWHM of CoFe (110) decreased and the grain size increased when the thickness and annealing temperature increased. The CoFe (110) peak revealed magnetocrystalline anisotropy, which was related to strong low-frequency alternative-current magnetic susceptibility (χac) and induced an increasing trend in saturation magnetization (Ms) as the thickness and annealing temperature increased. The contact angles of all Co40Fe40W10B10 films were less than 90°, indicating the hydrophilic nature of Co40Fe40W10B10 films. Furthermore, the surface energy of Co40Fe40W10B10 presented an increased trend as the thickness and annealing temperature increased. According to the results, the optimal conditions are a thickness of 100 nm and an annealing temperature of 350 °C, owing to high χac, large Ms, and strong adhesion; this indicates that annealing Co40Fe40W10B10 at 350 °C and with a thickness of 100 nm exhibits good thermal stability and can become a free or pinned layer in a magnetic tunneling junction (MTJ) application.


Introduction
Ever since the conception of nanocrystalline materials by Rozlin in 2012, the CoFe alloy found in magnetic materials has presented excellent soft magnetic properties, categorized by high saturation magnetization (Ms) and a high Curie temperature (T C ) [1]. This topic has continued to attract the attention of scholars researching CoFe material, which can be applied to magnetic equipment for sensor, actuator, and read-write recorder applications [2][3][4][5][6]. The CoFe matrix with magnetic properties combines with boron (B) to form CoFeB material, which is a soft magnetic material that is applied widely to spin electronic devices. The thickness of the CoFeB thin film is thin enough that it shows perpendicular magnetic anisotropy (PMA) and is applied to magnetoresistance random access memory (MRAM). CoFeB can be a free layer or pinned layer for increasing ferromagnetic (FM)/antiferromagnetic (AFM) exchange-biasing anisotropy in magnetic tunnel junction (MTJ), creating large tunnel magnetoresistance (TMR) [7][8][9][10][11][12]. Among all metals, tungsten (W) has a high melting point, high tensile strength, and thermal conductivity, as well as a low thermal expansion coefficient. The addition of refractory elements such as W to the CoFeB system is worthwhile to study their specific properties. In 2012, Pai et al. examined the effect of W as a seed layer on MTJ and observed different crystal structures formed by single-layer W films due to a different thickness and a spin Hall angle [13]. An experiment in 2015 found that the W layer influenced the PMA effect at different annealing temperatures [14]. The PMA has high stable efficiency at high temperatures with the CoFeB alloy presenting the major effect of the PMA simultaneously, indicating that the W layer probably has the factor of heat resistance and is realized at a higher annealing temperature in order to obtain a strong PMA feature. In 2016, researchers studied the electrodeposition efficiency of CoFeW films in a citrate solution by changing the chemical composition ratio of the films with different W content in the electrolytes. The morphology, microstructure, and magnetic properties of the films were analyzed. A CoFeW alloy with low coercivity (H C ) and high saturation magnetization (M S ) was obtained [15]. Findings show that the coating of the CoFeW alloy with a high PH value produces better magnetic properties. However, the coating of the CoFeW alloy with a low PH value causes a lack of surface tension and the phenomenon of the grain size to decrease. Adding W into the CoFe alloy can increase its hardness, durability, corrosion resistance, and heat resistance [16]. However, excessive B content leads to a decrease in saturation magnetization at high temperatures [17]. Higher saturation magnetization has the advantage of heating stability. Due to the above reasons, magnetic components are usually operated in a higher temperature environment than room temperature (RT). CoFeWB film is usually inserted into MTJ as a free layer, pinned layer, or combined with other layers in a multilayer structure. It can be widely used in magnetic and semiconductor applications. Adhesion is an important factor in CoFeWB film. The experiment investigates Co 40 Fe 40 W 10 B 10 films deposited on Si (100) substrates when an annealing process occurs at 200 • C, 250 • C, 300 • C, and 350 • C, respectively. This experiment elected to add B and W into CoFe material and investigated their specific properties, including structure, adhesion, and magnetic characteristics, after annealing treatments.

Materials and Methods
Co 40 Fe 40 W 10 B 10 with a thickness of 10-100 nm was sputtered onto Si(100) substrate at room temperature (RT) by a magnetron DC sputtering direct method of 50 W power and under the following four conditions: (a) annealed at a treatment temperature (T A ) of 200 • C for 1 h, (b) annealed at 250 • C for 1 h, (c) annealed at 300 • C for 1 h, and (d) annealed at 350 • C for 1 h. The chamber base pressure was 8.5 × 10 −7 Torr, and the Ar working pressure was 3 × 10 −3 Torr. The pressure in the ex-situ annealed condition was 3 × 10 −3 Torr with a selected Ar gas. The target alloy composition of CoFeWB was Co (40%), Fe (40%), W (10%), and B (10%). The structure of the CoFeWB thin films was detected by grazing incidence X-ray diffraction (XRD) patterns obtained with CuKα1 (PAN analytical X'pert PRO MRD, Malvern Panalytical Ltd, Cambridge, UK) and a low angle diffraction incidence of roughly 2 degrees. The in-plane low-frequency alternate-current magnetic susceptibility (χ ac ) and hysteresis loop of the Co 40 Fe 40 W 20 were studied using an χ ac analyzer (XacQuan, MagQu Co. Ltd. New Tapei City, Taiwan) and an alternating gradient magnetometer (AGM, PMC, Westerville, OH, USA). Moreover, in χ ac measurement, the χ ac analyzer was used to calibrate the standard sample under the action of an external magnetic field. Then, the sample was inserted into the χ ac analyzer. The driving frequency was between 10 and 25,000 Hz. χ ac was measured using magnetization. All test samples had an equivalent shape and size to eliminate demagnetization. The χ ac valve acted as an arbitrary unit (a.u.) because the AC result corresponded to the reference standard sample and could have been a comparison value. The connection between magnetic susceptibility and frequency was measured by an χ ac analyzer. The best resonance frequency (f res ) was measured by an χ ac analyzer and represented the frequency of the maximum χ ac . Before measurement, the contact angle was properly air cleaned on the surface. The contact angle of the CoFeWB film was measured with deionized (DI) water and glycerol. The contact angle was measured when the samples were taken out of the chamber. The surface energy was obtained by measuring the contact angle and employing specific calculations [18][19][20].  Figure 1a of annealed 200 • C result displays an amorphous status in all films owing to insufficient heating drive force. Furthermore, the thicknesses and annealing temperatures of body-centered cubic (BCC) CoFe (110) peaks were detected at around a diffracted angle of 2θ = 44.7 • when annealing at 250 • C with thicknesses from 80 nm to 100 nm, annealing at 300 • C with thicknesses from 50 nm to 100 nm, annealing at 350 • C with thicknesses from 10 nm to 100 nm, and annealing at 400 • C with thickness of 150 nm. It is generally observed in CoFeWB thin films that the intensity of CoFe(110) peaks increases with greater thickness and increased annealing temperature. When the annealing temperature increases above 350 • C, it is found that the thickness of the initial crystallization of CoFe (110) becomes thinner.

X-ray Diffraction
Materials 2021, 14, x 3 of 11 magnetic field. Then, the sample was inserted into the χac analyzer. The driving frequency was between 10 and 25,000 Hz. χac was measured using magnetization. All test samples had an equivalent shape and size to eliminate demagnetization. The χac valve acted as an arbitrary unit (a.u.) because the AC result corresponded to the reference standard sample and could have been a comparison value. The connection between magnetic susceptibility and frequency was measured by an χac analyzer. The best resonance frequency (fres) was measured by an χac analyzer and represented the frequency of the maximum χac. Before measurement, the contact angle was properly air cleaned on the surface. The contact angle of the CoFeWB film was measured with deionized (DI) water and glycerol. The contact angle was measured when the samples were taken out of the chamber. The surface energy was obtained by measuring the contact angle and employing specific calculations [18][19][20].

X-ray Diffraction
Figures 1a-e show Co40Fe40W10B10 thin films when analyzed for the crystal structure by XRD at the annealed temperatures of 200 °C, 250 °C, 300 °C, 350 °C, and 400 °C, respectively. Figure 1a of annealed 200 °C result displays an amorphous status in all films owing to insufficient heating drive force. Furthermore, the thicknesses and annealing temperatures of body-centered cubic (BCC) CoFe (110) peaks were detected at around a diffracted angle of 2θ = 44.7° when annealing at 250 °C with thicknesses from 80 nm to 100 nm, annealing at 300 °C with thicknesses from 50 nm to 100 nm, annealing at 350 °C with thicknesses from 10 nm to 100 nm, and annealing at 400 °C with thickness of 150 nm. It is generally observed in CoFeWB thin films that the intensity of CoFe(110) peaks increases with greater thickness and increased annealing temperature. When the annealing temperature increases above 350 °C, it is found that the thickness of the initial crystallization of CoFe (110) becomes thinner.  Figure 2a shows the corresponding full width at half maximum (FWHM, B) of the CoFe (110) peak obtained under four conditions. The result reveals that FWHM decreases with increased thicknesses and post-annealing temperature, using the FWHM determined by XRD, while the grain size of CoFe (110) is calculated using the Scherrer formula [21,22]. By using the Scherrer formula (1), and considering the CoFe (110) peak, this study calculated the average crystallite size for the CoFeWB thin films under the studied conditions.

Full-Width at Half Maximum (FWHM) and Grain Size Distribution
The Scherrer formula is In the formula, k (0.89) is Scherrer's constant; λ is the X-ray wavelength of the Cu Kα1 line; B is the FWHM diffraction CoFe (110) peak; and θ is the half-angle of the diffraction peak. Figure 2b shows the average grain sizes, which were estimated using the FWHM of the CoFe (110) peak under four annealed conditions. The results indicate that the grain sizes depend on the thickness and annealed temperature, and that the crystallization of the films rose with the thickness and annealed temperature, indicating annealing treatment supports the heating drive force to grain growth [23][24][25]. To investigate the thickness and annealing temperature effect, the performance of the grains at a thicker and higher temperature, 150 nm and 400 °C, was also studied. In Figure 2a, there is little difference in the FWHM and grain size when annealing at 350 °C with 100 nm and at 400 °C with 150 nm. The grain size of 400 °C is slightly larger than 350 °C. Therefore, the grain size tends to be saturated when annealing above 350 °C.  Figure 2a shows the corresponding full width at half maximum (FWHM, B) of the CoFe (110) peak obtained under four conditions. The result reveals that FWHM decreases with increased thicknesses and post-annealing temperature, using the FWHM determined by XRD, while the grain size of CoFe (110) is calculated using the Scherrer formula [21,22]. By using the Scherrer formula (1), and considering the CoFe (110) peak, this study calculated the average crystallite size for the CoFeWB thin films under the studied conditions.  Figure 2a shows the corresponding full width at half maximum (FWHM, B) of th CoFe (110) peak obtained under four conditions. The result reveals that FWHM decreas with increased thicknesses and post-annealing temperature, using the FWHM dete mined by XRD, while the grain size of CoFe (110) is calculated using the Scherrer formu [21,22]. By using the Scherrer formula (1), and considering the CoFe (110) peak, this stud calculated the average crystallite size for the CoFeWB thin films under the studied co ditions.

Full-Width at Half Maximum (FWHM) and Grain Size Distribution
The Scherrer formula is In the formula, k (0.89) is Scherrer's constant; λ is the X-ray wavelength of the C Kα1 line; B is the FWHM diffraction CoFe (110) peak; and θ is the half-angle of the d fraction peak. Figure 2b shows the average grain sizes, which were estimated using th FWHM of the CoFe (110) peak under four annealed conditions. The results indicate th the grain sizes depend on the thickness and annealed temperature, and that the crysta lization of the films rose with the thickness and annealed temperature, indicating a nealing treatment supports the heating drive force to grain growth [23][24][25]. To investiga the thickness and annealing temperature effect, the performance of the grains at a thick and higher temperature, 150 nm and 400 °C, was also studied. In Figure 2a, there is litt difference in the FWHM and grain size when annealing at 350 °C with 100 nm and at 4 °C with 150 nm. The grain size of 400 °C is slightly larger than 350 °C. Therefore, th grain size tends to be saturated when annealing above 350 °C.  The Scherrer formula is In the formula, k (0.89) is Scherrer's constant; λ is the X-ray wavelength of the Cu Kα1 line; B is the FWHM diffraction CoFe (110) peak; and θ is the half-angle of the diffraction peak. Figure 2b shows the average grain sizes, which were estimated using the FWHM of the CoFe (110) peak under four annealed conditions. The results indicate that the grain sizes depend on the thickness and annealed temperature, and that the crystallization of the films rose with the thickness and annealed temperature, indicating annealing treatment supports the heating drive force to grain growth [23][24][25]. To investigate the thickness and annealing temperature effect, the performance of the grains at a thicker and higher temperature, 150 nm and 400 • C, was also studied. In Figure 2a, there is little difference in the FWHM and grain size when annealing at 350 • C with 100 nm and at 400 • C with 150 nm. The grain size of 400 • C is slightly larger than 350 • C. Therefore, the grain size tends to be saturated when annealing above 350 • C. It was also found that the Ms of the CoFeWB thin films increased by raising the annealing temperature. Apparently, the M S, when annealed at 350 • C, is much larger than other conditions because of magnetocrystalline anisotropy [26,27]. CoFeWB films show in-plane magnetization because the CoFeWB film is too thick and, when deposited on a Si substrate, perpendicular magnetic anisotropy (PMA) originates from the Fe-O bond and the in-plane demagnetization field is too big, owing to a thick CoFeWB [28,29]. In this study, the effect of greater CoFeWB thickness is demonstrated more than the weaker Fe-O bonding effect. The M S value of Co 40 Fe 40 W 10 B 10 thin films is increased to 350 • C, which shows that the thermal stability of Co 40 Fe 40 W 10 B 10 thin films is better than that of other research findings [30].

Magnetic Analysis
Figures 3a-d display the magnetic hysteresis loops of Co40Fe40W10B10 thin films u der four annealed conditions, with thicknesses ranging from 10 to 100 nm. The extern magnetic field of 200 Oe in the plane is enough to observe the saturation magnetic sp state. The figure shows low coercivity (HC), which indicates that Co40Fe40W10B10 films a soft magnetic. The saturation magnetization (MS) of Co40Fe40W10B10 thin films under fo post-annealing conditions illustrates the magnetic properties of Co40Fe40W10B10 thin film that were measured by AGM, as shown in Figure 4. The results show that Co40Fe40W10B10 films, Ms increases with the increase of thickness, indicating the effect thickness on Ms. It was also found that the Ms of the CoFeWB thin films increased b raising the annealing temperature. Apparently, the MS, when annealed at 350 °C, is mu larger than other conditions because of magnetocrystalline anisotropy [26,27]. CoFeW films show in-plane magnetization because the CoFeWB film is too thick and, when d posited on a Si substrate, perpendicular magnetic anisotropy (PMA) originates from th Fe-O bond and the in-plane demagnetization field is too big, owing to a thick CoFeW [28,29]. In this study, the effect of greater CoFeWB thickness is demonstrated more tha the weaker Fe-O bonding effect. The MS value of Co40Fe40W10B10 thin films is increased 350 °C, which shows that the thermal stability of Co40Fe40W10B10 thin films is better tha that of other research findings [30].    Figures 5a-d show the low-frequency alternating-current magnetic susc (χac) result of CoFeWB films with thicknesses ranging from 10 to 100 nm un conditions. The low frequencies were measured in the range of 50-25,000 Hz. Th display that the χac values decrease with an increasing frequency (Hz) under conditions. The corresponding maximum χac values of various CoFeWB thickn der four conditions are shown in Figure 6. It was found that the maximum χac v 0.24 when the thickness was 100 nm at 200 °C ; the maximum χac value was 0.26 w thickness was 100 nm at 250 °C; the maximum χac value was 0.36 when the thick 100 nm at 300 °C; and the maximum χac value was 1.17 when the thickness was 1 350 °C. These results clearly display an increased χac owing to the thickness e magnetocrystalline anisotropy in the CoFeWB films. Table 1 shows the optim nance frequency (ƒres) of CoFeWB. The maximum χac demonstrates that the spin ity is highest at the optimal resonant frequency [31]. The χac peak indicates the change-coupling interaction and dipole moment of the domain under frequenc tionally, the ƒres value is below 1000 Hz, which makes CoFeWB films for applic soft magnetism devices. It was found that the ƒres values of all CoFeWB thicknes in the range of 50-1000 Hz, suggesting the maximum χac had the strongest spin ity at this frequency [32].    Figure 6. It was found that the maximum χ ac value was 0.24 when the thickness was 100 nm at 200 • C; the maximum χ ac value was 0.26 when the thickness was 100 nm at 250 • C; the maximum χ ac value was 0.36 when the thickness was 100 nm at 300 • C; and the maximum χ ac value was 1.17 when the thickness was 100 nm at 350 • C. These results clearly display an increased χ ac owing to the thickness effect and magnetocrystalline anisotropy in the CoFeWB films. Table 1 shows the optimal resonance frequency (ƒ res ) of CoFeWB. The maximum χ ac demonstrates that the spin sensitivity is highest at the optimal resonant frequency [31]. The χ ac peak indicates the spin exchange-coupling interaction and dipole moment of the domain under frequency. Additionally, the ƒ res value is below 1000 Hz, which makes CoFeWB films for applications in soft magnetism devices. It was found that the ƒ res values of all CoFeWB thicknesses were in the range of 50-1000 Hz, suggesting the maximum χ ac had the strongest spin sensitivity at this frequency [32].  Figure 6. It was found that the maximum χac val 0.24 when the thickness was 100 nm at 200 °C ; the maximum χac value was 0.26 wh thickness was 100 nm at 250 °C; the maximum χac value was 0.36 when the thickne 100 nm at 300 °C; and the maximum χac value was 1.17 when the thickness was 100 350 °C. These results clearly display an increased χac owing to the thickness effe magnetocrystalline anisotropy in the CoFeWB films. Table 1 shows the optima nance frequency (ƒres) of CoFeWB. The maximum χac demonstrates that the spin se ity is highest at the optimal resonant frequency [31]. The χac peak indicates the s change-coupling interaction and dipole moment of the domain under frequency. tionally, the ƒres value is below 1000 Hz, which makes CoFeWB films for applicat soft magnetism devices. It was found that the ƒres values of all CoFeWB thicknesse in the range of 50-1000 Hz, suggesting the maximum χac had the strongest spin se ity at this frequency [32].

Contact Angle and Surface Energy
The contact angles were measured using DI water and glycerol. The data are presented in Table 2. The contact angles of all Co 40 Fe 40 W 10 B 10 thin films were less than 90 • , representing that Co 40 Fe 40 W 10 B 10 film exhibits good hydrophilicity and wettability. The surface energy of thin films is an important parameter because it relates to the adhesion of thin films. When CoFeWB thin film is used as a seed, buffer, free, or pinned layer, the strong adhesion of the thin films is essential. The data of contact angles are used to calculate the surface energy via Young's equation [19,20]: Here, σsg is the surface free energy of the solid; σsl denotes the liquid-solid interface tension; σlg is the surface tension of the liquid, and θ is the contact angle. Figure 7 displays the surface energy of Co 40 Fe 40 W 10 B 10 thin films. These data of surface energy are shown in Table 2. It can be observed that the surface energy of high postannealing temperature was larger than low annealing temperature. As the post-annealing temperature increased, the surface energy clearly increased. As a consequence, the surface energy of annealed 200 • C CoFeWB thin films was 28.15 mJ/mm 2 at 50 nm, which was the highest value. When the post-annealing temperature was 250 • C, the highest surface energy of 60 nm was 31.01 mJ/mm 2 . When the post-annealing temperature achieved 300 • C, it was 50.56 mJ/mm 2 at 100 nm. When the post-annealing temperature achieved 350 • C, it was 54.95 mJ/mm 2 at 100 nm. When the surface energy is high, the liquid absorption capacity of the surface is correspondingly high. High surface energy corresponds to strong adhesion [33]. Surface energy is the key factor affecting the adhesion of the film. Because CoFeWB is compatible with spin-value MTJ and applied to MRAM application, it can also be used as a free layer and in combination with other layers. One can observe the Co 40 Fe 40 W 10 B 10 properties and compare them with other specific Co 40 Fe 40 V 10 B 10 film magnetic properties and surface energies, as mentioned in Table 3 [34]. The χ ac and surface energy of 100 nm Co 40 Fe 40 W 10 B 10, annealed at 350 • C, is larger than that of the Co 40 Fe 40 V 10 B 10 film, indicating that Co 40 Fe 40 W 10 B 10 thin film is more suitable as a free or pinned layer in MTJ and can be applied in MRAM applications.
Here, σsg is the surface free energy of the solid; σsl denotes the liquid-solid interface tension; σlg is the surface tension of the liquid, and θ is the contact angle. Figure 7 displays the surface energy of Co40Fe40W10B10 thin films. These data of surface energy are shown in Table 2. It can be observed that the surface energy of high post-annealing temperature was larger than low annealing temperature. As the post-annealing temperature increased, the surface energy clearly increased. As a consequence, the surface energy of annealed 200 °C CoFeWB thin films was 28.15 mJ/mm 2 at 50 nm, which was the highest value. When the post-annealing temperature was 250 °C, the highest surface energy of 60 nm was 31.01 mJ/mm 2 . When the post-annealing temperature achieved 300 °C, it was 50.56 mJ/mm 2 at 100 nm. When the post-annealing temperature achieved 350 °C, it was 54.95 mJ/mm 2 at 100 nm. When the surface energy is high, the liquid absorption capacity of the surface is correspondingly high. High surface energy corresponds to strong adhesion [33]. Surface energy is the key factor affecting the adhesion of the film. Because CoFeWB is compatible with spin-value MTJ and applied to MRAM application, it can also be used as a free layer and in combination with other layers. One can observe the Co40Fe40W10B10 properties and compare them with other specific Co40Fe40V10B10 film magnetic properties and surface energies, as mentioned in Table 3 [34]. The χac and surface energy of 100 nm Co40Fe40W10B10, annealed at 350 °C, is larger than that of the Co40Fe40V10B10 film, indicating that Co40Fe40W10B10 thin film is more suitable as a free or pinned layer in MTJ and can be applied in MRAM applications.

Conclusions
XRD patterns revealed that CoFeWB thin films are composed of an amorphous status when annealed at a temperature of 200 • C. BCC CoFe (110) peaks were detected when annealing at 250 • C with thicknesses ranging from 80 nm to 100 nm, annealing at 300 • C with thicknesses ranging from 50 nm to 100 nm, and annealing at 350 • C with thicknesses ranging from 10 nm to 100 nm. The intensity and grain size of CoFe (110) peaks generally increased with increasing film thickness and annealing temperatures, indicating a development in crystallographic texture. CoFeWB films presented soft magnetism owing to low H c and in-plane magnetization because thicker CoFeWB thicknesses showed a large in-plane demagnetization field. M s and χ ac also increased as the thickness and annealed temperature increased. The χ ac and M S of films annealed at 350 • C are much larger than in other conditions, owing to the thickness effect and magnetocrystalline anisotropy in the CoFeWB films. Alloying additions of W improved the thermal stability of CoFeB films. The maximum Ms and χ ac values were achieved for CoFeWB films with a thickness of 100 nm after annealing at 350 • C. The f res values of all films were less than 1000 Hz. The contact angles of CoFeWB were smaller than 90 • , indicating that the films were hydrophilic and had a good wetting effect. The surface energy of a high post-annealing temperature was larger than that of a low annealing temperature. As the post-annealing temperature increased, the surface energy clearly increased. Based on the magnetic properties and surface energy, the optimal condition of CoFeWB film is a thickness of 100 nm with an annealed temperature of 350 • C, which is suitable as a free or pinned layer in MTJ for magnetic component applications.