Formation and Dissolution of Mesoporous Layer during Metal-Assisted Etching of Silicon
Authors: Matsumoto Ayumu , Eguchi Makiho , Yae Shinji 
Published: February 2020

Metal-assisted etching of silicon (Si) is one of the methods to fabricate porous Si.1 In this method, Si surface can be selectively etched without electrical bias by immersing metal-modified Si in a hydrofluoric acid (HF) solution containing an oxidizing agent, such as hydrogen peroxide (H2O2)2 or oxygen3. The dissolution of Si proceeds under the metal catalysts due to the injection of positive holes generated by the reduction of oxidizing agent on the metal catalysts. The etching behavior can be controlled by changing experimental conditions.1 In the previous work, the catalytic activity of different metal species was studied.3 Recently, we investigated the mass reduction and the porous structure obtained for different metal particles under a constant condition.4 The depth of pores estimated from the mass reduction was much larger than that observed when gold or platinum (Pt) particles were used. This result could be explained by the dissolution of mesoporous layer formed at the top-surface of Si (Fig. 1a). In this study, we directly observed the dissolution of the top-surface using a Si substrate on which Pt particles are partly deposited.
As a Si substrate, single-crystalline n-type Si wafer (CZ, (100), 0.5–10 Ω cm) was used. A part of the Si substrate was covered by a masking tape. Pt particles were deposited by immersing the partly masked Si substrate in a 1.0 mM K2PtCl4 + 0.15 M HF aq. at 313 K for 90 s. After removing the masking tape, the Pt-deposited Si substrate was etched in a 0.08 M H2O2 + 6.6 M HF aq. at 298 K for 2 h in the dark. Step measurement was performed using a stylus-type surface texture measuring instrument (Tokyo Seimitsu Co., Ltd., Surfcom 570A). The cross-sectional observation was performed using scanning electron microscopes (SEM) (JEOL Ltd., JSM-7001F and JSM-7800F).
Figure 1b shows the result of step measurement. A height difference of 7.5 µm was clearly observed between the deposited area and the un-deposited area. This indicates that general corrosion occurred at the deposited area. Figure 1c shows the cross-sectional SEM image of the boundary between Pt-deposited and un-deposited areas. The mesoporous layer with a thickness of 10 µm was observed at the top-surface of Si at the deposited area, suggesting that the general corrosion occurred due to the dissolution of mesoporous layer. The mesoporous layer was also observed at the un-deposited area and it was thinner than that observed at the deposited area. This could be an evidence that the mesoporous layer was formed due to the diffusion of the excess positive holes and the reaction with HF at the top-surface of Si.5 Considering the mass reduction, depth of macropores, thickness of mesoporous layer, and depth of general corrosion, the porosity of mesoporous layer was estimated to be about 0.5. In addition, we found that the depth of macropores, thickness of mesoporous layer, and depth of general corrosion increased with etching time in the range from 10 min to 2 h. This indicates that the reactions proceed faster in the order of the formation of macropores, formation of mesoporous layer, and general corrosion.
In conclusion, the general corrosion occurs during the metal-assisted etching of Pt-deposited Si. The general corrosion would be explained by the formation and dissolution of mesoporous layer.

Acknowledgements

This work was partly supported by JSPS KAKENHI Grant number JP26289276. We are grateful to Prof. K. Okuda and Prof. M. Nunobiki, University of Hyogo, for their kind support in the step measurement.

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Figure 1