(Invited) Selective Wet Etching of Semiconductor Materials by Novel Catalyst-assisted Modes
Authors: Arima Kenta , Li Junhuan , Murase Shiika , Inagaki Kouji 
Published: December 2024

Metal-assisted chemical etching (MaCE) is an emerging technology in the formation of semiconductor surfaces with various structures and functions [1, 2]. In general, precious metals such as Pt and Ag are used as catalysts in this etching mode. By controlling solution conditions, a semiconductor surface in contact with a catalyst can be etched selectively. Because this etching utilizes chemical reactions, it does not leave mechanical damages on a processed surface. So far, our group reported the application of this etching mode for local patterning and surface flattening of Ge and SiC surfaces [3, 4]. In this paper, we present two recent topics on this mode.
One is the development of “chemical knife” to separate neighboring terraces on a Si surface. We aim at forming ultrathin Si ribbons from a silicon-on-insulator (SOI) layer possessing a vicinal surface by total wet chemical processing. To test this concept, we used a p-type Si(111) sample of which surface is misoriented in the <11-2> direction at an angle of 0.2°. After wet cleaning, it was immersed in water with an ultralow level of dissolved O2 molecules less than 2 ppb, which is referred to as the first LOW (Low dissolved-Oxygen Water) treatment. The first LOW treatment formed a Si(111) surface composed of flat terraces and biatomic steps. Then it was immersed in LOW containing Ag+ ions at a concentration of 5 ppm, which is referred to as the second LOW treatment. We have found that Ag atoms were selectively reduced at the edges of atomic steps on Si(111) to form Ag nanowires, as reported by another group [5]. Finally, the Si sample with Ag nanowires was immersed into a mixture of HF and H2O2. We found that the self-assembled Ag nanowires after the second LOW treatment were replaced by almost continuous nano trenches [6]. This is probably because the Ag nanowires acted as a catalyst to enhance chemical etching of the Si surface underneath. By applying this sequence for a thin SOI layer, we expect to form Si ribbons of which width and thickness are both controlled in a self-assembled manner.
The other topic is graphene-based catalysts to enhance etching of a Ge surface just in water. A problem with conventional MaCE is the use of noble metals as catalysts. In most applications, residual metals on the semiconductor surface have to be removed after MaCE. For example, aqua regia is effective for dissolving Pt. However, such a strong oxidative solution causes severe damage to the surfaces of some semiconductor materials such as Ge. To solve this issue, graphene can be used as a substitute for noble metals. Another group has reported graphene-assisted chemical etching of Si with chemical-vapor-deposition-grown graphene as a catalyst [7]. Recently, we reduced a commercial graphene oxide (GO) solution by several synthesis methods to obtain a black suspension including reduced GO (rGO) sheets. Then we found that enhanced etching occurred on a Ge surface covered by the chemically modified rGO sheets in O2-dissolved water [8, 9]. This is probably because the rGO sheets catalyze Ge oxidation underneath the sheets. We propose that this reaction is triggered by the adsorption of O2 molecules in water at local defects in the sheets. As investigated in our surface science studies by scanning probe techniques [10] and first-principles simulations [11], both a finite-sized and chemically modified graphene exhibit electronic structures different from those of a simple and large graphene sheet. This indicates that the control of local defects and their density in an rGO sheet is a key in realizing rGO-assisted chemical etching with a higher performance. Finally, we show a result combining this metal-free etching mode with a lithography process to fabricate a trench pattern on Ge.
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