Practical Electron Microscopy and Database

In IC devices, fluorine corrosion can be induced by F contamination as some F-based chemical gases and materials such as CF₄, CHF₃, SF₆, HF, BOE (NH₄F + HF) etc are used in microelectronics wafer fab. On the other hand, wafer shipment package foam materials may also cause the F contamination and corrosion.

For instance, it is well-known that fluorine will often be detected near the surface of bondpads. This is because CF₄ gas is used during bondpad opening fab process. Fluorine slightly reacts with Al and forms [AlF_x]^(x-3)- (e.g. [AlF₆]^3-) or compounds Al_xF_yO_z on the surface of bondpads. Unfortunately, these compounds cannot be cleaned away fully by EKC & DI water cleaning processes. Therefore, it is normal that some percentage of fluorine is detected on bondpads.

Table 2436a. Fluorine contamination in IC devices.

Figure 2436a shows an example of C, O, and Si contaminated bond pads. Area S6 had an abnormal film on the native Al oxide, while area S5 presented nonhomogeneous, loose and empty Si_xAl_yC_zO_m materials in the hemispherical defect.  These pad defects existed only at wafer edge because backside grinding was the root cause of the contamination.

The TEM image in Figure 2436b (a) shows that the high resistance, undulate nickel silicide film that was essentially caused by fluoride contamination. This contamination originated from a chemical dry-cleaning process for silicon (Si) substrate prior to Ni sputtering. The undulate film consisted of thicker NiSi areas (e.g. arrowed by 1) and thinner NiSi₂ areas (e.g. arrowed by 2). In Figure 2436b (b), the formation process of the Ni silicide film in different phases was:
         i) Fluoride contaminants initially exist in some areas of the Si surface (indicated by the red lines).
         ii) Fluoride contaminants impede direct Ni diffusion into Si substrate so that the Ni atoms migrated in the horizontal and then downward directions during the initial silicidation annealing, while direct Ni diffusion into Si substrate occurrs in the clean areas (indicated by the blue arrows).
         iii) Vacancies (indicated by the white spots) are formed due to excess Ni diffusion above the contaminants.
         iv) During the second (higher temperature) annealing, the vacancies are filled up by excess Si atoms so that the thin Ni₂Si areas are transformed into thinner NiSi₂, while the thick Ni₂Si areas are transformed into thicker NiSi.

Figure 2436b. Undulate NiSi + NiSi₂ film formed due to fluoride contamination: (a) TEM image of nickel silicide film (Adapted from [5]). White arrow "1" points to thicker NiSi, while arrow "2" points to thinner NiSi₂. (b) Schematic illustration of the formation process of the structure in (a). The blue box represents the area of the as-deposited Ni film. The area marked by the dotted green curve represents the final nickel silicide as shown in (a). The white spots indicate vacancies that are formed due to Ni out-diffusion during the initial annealing process, while the blue arrows indicate the direction of the Ni out-diffusion. The red lines at the initial interface between the as-deposited Ni and Si substrate represent fluoride (F) contaminants. The green arrows indicate the Si diffusion into the vacancy areas during the second annealing process.

Note that the fluorine contamination on Si wafers can be investigated with XPS analysis.

[1] Y. N. Hua, S. Redkar, C. K. Lau, “A Study on Non- Stick Aluminium Bondpads due to Fluorine Contamination using SEM, EDX, TEM, IC, Auger, XPS and TOF-SIMS Techniques,” Proceedings from the 28th International Symposium for Testing and Failure Analysis, Phoenix, Arizona, November, 2002, pp. 495-504.
[2] Paul Yu, Jamie Su, Qiang Gao, Ming Li, Chorng Niou, Study of Aluminum Pad Contamination Sources during Wafer Fabrication, Shipping, Storage and Assembly, International Symposium on High Density packaging and Microsystem Integration, 2007. HDP '07.
[3] R. Yang, N. Su, P. Bonfanti, J. Nie, and J. Ning, “Advanced in situ pre-Ni silicide (Siconi) cleaning at 65 nm to resolve defects in NiSix modules,” J. Vac. Sci. Technol. B, vol. 28, no. 1, pp. 56–61, Jan. 2010.
[4] T. Yamaguchi, K. Kashihara, S. Kudo, T. Tsutsumi, T. Okudaira, K. Maekawa, Y. Hirose, K. Asai, and M. Yoneda, “Characterizations of NiSi2-whisker defects in n-channel metal-oxide-semiconductor fieldeffect transistors with <110> Channel on Si(100),” Japan. J. Appl. Phys., vol. 49, no. 12, pp. 126–503, Dec. 2010.
[5] Takuya Futase, and Hisanori Tanimoto, Fluoride Contamination Induced NiSi2 Film Formation in a Gate NiSi Line, IEEE Transactions on Semiconductor Manufacturing, 26(3), (2013) 355.