Effect of Polarity of Surfactant on Formation of through-Silicon Via Using Metal-Assisted Chemical Etching
Authors: Hanatani Syunsuke , Yorioka Takuya , Shimizu Tomohiro , Ito Takeshi , Shingubara Shoso 
Published: December 2020

In this study, we demonstrated formation of high aspect vertical holes in Si substrate using wet chemical approach, MacEtch process, for the application of through silicon via (TSV) holes. It is important that the addition of the surfactants in the etching solution to obtain the well- defined straight holes in the Si substrate using MacEtch, regardless the polarity of the surfactants. MacEtch is expected to be used for formation of TSV instead of Deep reactive ion etching (Deep-RIE) because of lower costs than Deep-RIE [1,2,3]. However, it is difficult to obtain high aspect vertical holes using MacEtch process due to its instability of etching direction for prolonged process as shown in Fig. 1(a). Addition of surfactant is usefulness to improve the morphology of etched holes.
Micro scale holes were prepared in N-type Si (100) substrate using MacEtch process. The disc shape Au deposited by sputtering were used as catalyst. Before deposition of the Au catalyst, a Ti interlayer with a thickness of 10 nm was deposited directly on the Si substrate with patterned resist. The patterned Au discs (10 μm in diameter), which were deposited on a Ti / Si substrate using a photolithography lift-off process. The thicknesses of deposited Au were 10 nm for Ti/Si substrate. The base pressure of the deposition chamber was maintained at 10-5 Pa, and purity of Au and Ti target used for the deposition were 99.99 %, respectively. For the MacEtch process, a mixture of 1.0 M hydrofluoric acid (HF), 1.3 M hydrogen peroxide (H2O2) and deionized water was used as the base etching solution. Benzalkonium chloride (BKC), and polyethylene glycol (PEG), which were cationic and non-polar surfactant, respectively, were used as the additive for the etching solutions. The MacEtch of the Si substrate with patterned Au were performed for 120 min at 40 oC in the etching solution.
Fig.1 shows cross sectional SEM images of typical shape of etched Si holes using MacEtch. The direction of etched Si hole was changed during MacEtch without surfactants, as shown Fig.1 (a), and these changes of direction occurred more than 60% Si holes. By contrast, we observed drastic improvement on direction of etched holes in almost all holes in addition of PEG, as indicated Fig.1 (b). Similarly, adding BKC in MacEtch process was also observed improvement. The catalyst at the bottom of substrate without surfactant was deformed and partially detached from the Si substrate, as shown Fig.1 (d). However, with PEG and BKC, the catalyst contacted to the bottom and was suppressed deformation, as indicated Fig.1 (e), (f). In addition, BKC suppressed at 25 times low concentration compared with PEG. This phenomenon was is considered due to influence of polarity of hydrogen part or structure of surfactants. Particularly, the shape of catalyst film with BKC kept flat during MacEtch process compared with PEG. Therefore, defamation of catalyst has a strong correlation with formation of bended holes. Moreover, the suppression effect of catalyst is stronger in cation, BKC, than in non-ion, PEG.
References
[1] L. W. Schaper, S. L. Burkett, S. Spiesshoefer, G. V. Vangara, Z. Rahman, and S. Polamreddy, IEEE Trans. Adv. Packing. 28, 356 (2005).
[2] T. Shimizu, R. Niwa, T. Ito, and S. Shingubara, Jpn. J. Appl. Phys., 58, SDDF06 (2019).
[3] Y. Asano, K. Matsuo, H. Ito, K. Higuchi, K. Shimokawa, and T. Sato, In Electron. Components Technol. Conf., 2015, p. 853.

Figure 1