Catalytic Activity of Ru for Metal-Assisted Etching of Si
Authors: Sadakane Daisuke , Yamakawa Kano , Fukumuro Naoki , Yae Shinji 
Published: February 2020

Metal-assisted chemical etching of Silicon (Si) in hydrofluoric acid (HF) solution is an electroless method to produce porous Si (1, 2). Such etching generally uses not only metal-modified Si but also an oxidizing agent as hydrogen peroxide, metal ions and oxygen. We reported that etching rate of platinum (Pt), rhodium (Rh), gold (Au) and silver (Ag)-particle-modified Si depended on catalytic activity of metal for oxygen reduction (3), and palladium (Pd) exhibits high catalytic activity for the etching of Si without an oxidizing agent even under dissolved oxygen-free and dark conditions (4). This etching reaction using Pd is explained by electron injection into the conduction band of Si due to Pd assisted anodic oxidization of Si (Fig. 1). In this study, we have investigated the catalytic activity of ruthenium (Ru), which is in the same platinum group as Pd, for etching of Si in a HF solution.
Single-crystalline n-Si wafers (CZ, (100), ca. 10 Ω cm) were used as substrates. Si wafers were immersed in 1 mM (M = mol dm-3) ruthenium chloride (RuCl3) solution including 0.15 M HF at 313 K for electroless displacement metal deposition. Metal deposited Si wafers were immersed in a 7.3 M HF solution at 298 K under dark conditions for 24 h. Argon or oxygen gas bubbling was applied to the HF solution before and during etching to remove or saturate the dissolved oxygen in the solution, respectively. The etching rate of Si was measured by a gravimetric procedure.
No metal deposition occurred on mirror-polished Si wafers even by immersing in the deposition solution for even long time of 24 h. On the other hand, on Si wafers roughed with sand paper (#1200), Ru particles were deposited by immersing in the solution for 5 min. The etching rates of such Ru-particle-deposited Si wafers in 7.3 M HF solution were 1.1×10-5 and 2.0×10-4 g cm-2 h-1 under oxygen-free and -saturated conditions, respectively. The etch rate in the oxygen-free case is much higher than 1.2×10-6 g cm-2 h-1 of Pt-particle-modified case and one order lower than 3.4×10-4 g cm-2 h-1 the of Pd case. Figure 2 shows the SEM images of Ru-particle-deposited Si wafers before and after etching with the oxygen free HF solution. Ru particles were localized on scratches before etching (fig. 2a). After etching, a porous layer covered whole surface of Si (Fig. 2b and c). Pores, which have Ru particle at the bottom, were formed. The depth of pores, ca. 0.2 mm, is much shorter than 1 mm of depth calculated using the gravimetric procedure. These results indicate that Ru has much lower activity for electroless displacement deposition than that of other noble metals and high catalytic activity for metal assisted etching of Si similar to Pd.
ACKNOWLEDGEMENTS
The present work was partly supported by JSPS KAKENHI (26289276).
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Figure 1