Influence of Additives on Formation of through-Si Via in Si Substrate Using Metal Assisted Chemical Etching

Abstract

Recently, metal assisted chemical etching (MacEtch) attracts attention to fabricate a nano/microscale structures in Si substrate, such as nanowire and nanohole arrays. In this method, using a noble metal as a catalyst, only Si under the catalyst is selectively etched in the etching solution. Several papers have been already reported on the formation of microscale patterning using MacEtch. However, it is difficult to obtain high aspect vertical holes using MacEtch process because of its instability of etching direction for prolonged process as shown in figure (a). In this study, we present a new approach using surfactant agents as an additive in MacEtch solution to obtain vertical Si holes in Si substrates. Microscale holes were prepared in Si (100) substrates using the MacEtch process with a disc-shape patterned Au catalyst. The patterned Au discs ( 10 μm in diameter), which were deposited on a Ti / Si substrate using a photolithography lift-off process. For the MacEtch process, a mixture of hydrofluoric acid (HF), hydrogen peroxide (H2O2) and deionized water was used as the base etching solution. The PEG was added to the etching solution as an additive. The MacEtch of the Si substrate with patterned Au were performed for 120 min at 40 oC in the etching solution. Figure (a) shows cross-sectional SEM images of typical morphology of etched Si using MacEtch process without PEG in the etching solution. The etching direction of Si were changed during MacEtch as indicated with dashed arrow in Fig. (a), and these phenomena were observed in more than 60 % Si holes etched with MacEtch without PEG. Adding PEG in the etching solution, drastic improvement in verticality of etched Si holes were observed. Almost all holes were vertically etched to the Si substrate in PEG samples, and typical cross-sectional image of the etched holes is shown in Fig. (b). Fig. (c) and (d) shows enlarged image of Au catalyst films after MacEtch process. The catalyst in the bended hole prepared without PEG was deformed and partially detached from Si substrate, shown in Fig. (c). In contrast, in the case of straight holes etched with PEG shown in Fig. (d), the catalyst tightly contacted to the substrate, and showed relatively less defamation. Thus, there is a strong correlation between defamation of catalyst and formation of bended hole. The detailed mechanism of PEG additive in etching solution during Macetch process and the MacEtch results using other surfactants as additives will be presented. Figure 1