*3.6. Nano-Indentation*

Figure 7a,b show that the hardness and Young's modulus increase as the thickness of the CoFeW film increases. Mostly, the nanoindentation hardness is determined from the loading and unloading curve by the Pharr–Oliver method [37], which indicates the mixed hardness of the silicon substrate and the CoFeW film. Since the thickness of the CoFeW film is too thin, it can be reasonably concluded that there must be a substrate effect in the nanoindentation measurement. In the nanoindentation measurement, the corresponding hardness and Young's modulus values of the substrate are 4.1 and 133.3 Gpa, when the Si(100) substrate is measured. As the thickness increased from 10 nm to 50 nm under as-deposited condition, the hardness and Young's modulus of the CoFeW films increased from 12.9 to 13.4 GPa and 146.8 to 187.2 GPa, respectively. At annealed 250 ◦C, as the thickness increases from 10 nm to 50 nm, the hardness and Young's modulus of the CoFeW films increased from 10.9 to 13.5 GPa and 169.4 to 188.5 GPa, respectively. At annealed 350 ◦C, as the thickness increases from 10 to 50 nm, the hardness and Young's modulus of the CoFeW films increased from 12.0 to 13.4 GPa and 163.5 to 186.4 GPa, respectively. When the thickness is from 10 to 50 nm, the hardness and Young's modulus of CoFeW films display a tendency to saturate. According to the result, the Young's modulus of adding the W effect to CoFe films in thicker films is larger than CoFe film. The thinner CoFeW film has a more significant impact on the substrate. It shows a similar behavior as diamond films on crystalline silicon [38].

**Figure 7.** Nano-indentation of CoFeW films. (**a**) Hardness and (**b**) Young's modulus.

#### **4. Conclusions**

This study investigated the structure and surface property of CoFeW thin films. The CoFeW magnetic thin films were deposited on a Si(100) substrate by sputtering, while the Scherrer equation was estimated to calculate the grain size of CoFe(110). The results

indicate that the grain size increased along with the thickness of the CoFeW films, and the grain size was affected by the temperature of post-annealing. When the post-annealing temperature increased, the grain size also increased, suggesting that the post-annealing process provided energy to CoFeW thin films and caused the grain size to grow. The contact angles of all CoFeW samples were less than 90◦, indicating that CoFeW thin films show changes in the direction of higher hydrophilicity. The contact angle decreased along with the increase of post-annealing temperature, while the grain size affected the contact angle of CoFeW films. The contact angle of CoFeW thin films reduced when the grain size of CoFe(110) increased. Moreover, the formation of the oxide layers on the thin films' surface resulted in a decrease in the contact angle and an increase in the surface energy. The results suggest that the surface energy of the annealed CoFeW films was higher than the as-deposited CoFeW samples. When the post-annealing temperature increased, the surface energy increased, thus making the adhesion stronger. In the future, we hope to combine the layered MTJs with free and pinning layers. Due to the scattering of electrons at the grain boundaries or the presence of impurities, the sheet resistance decreases as the film thickness and annealing temperature increase. In addition, as the thickness increases, the hardness and Young's modulus of the CoFeW film shows a tendency to saturate.

**Author Contributions:** Conceptualization, W.-J.L., Y.-H.C., S.-H.L. and Y.-T.C.; methodology, Y.-T.C. and T.-Y.J.; validation, formal analysis, Y.-T.C.; investigation, Y.-T.C. and W.-J.L.; resources, C.-L.F., K.-W.L. and T.-H.W.; writing—original draft preparation, Y.-T.C.; writing—review and editing, Y.-T.C. and W.-J.L.; supervision, Y.-T.C. and P.-C.C.; project administration, Y.-T.C. and T.-H.W.; funding acquisition, W.-J.L. and Y.-H.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Ministry of Science and Technology (grant No. MOST108- 2221-E-224-015-MY3 and MOST105-2112-M-224-001) and the National Yunlin University of Science and Technology (grant No. 110T06).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on reasonable request from the corresponding author.

**Acknowledgments:** The author thanks Sin-Liang Ou for his help with the SEM experiment and images.

**Conflicts of Interest:** The authors declare that there is no conflict of interest regarding the publication of this paper. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
