Why Does Alkaline Corrosion of Silicon Exhibit Anisotropy?

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Corrosion involves an oxidation-reduction reaction. The reaction between silicon and hydroxide ions illustrates this: silicon is oxidized to silicate ions, and hydrogen from water is reduced to hydrogen gas, which evolves.

Si+2OH^–+2H₂O→SiO₂(OH)₂^2-+2H₂↑

However, since silicon can react with hydroxide ions, why does anisotropy still occur? In fact, people have been studying the alkaline corrosion of silicon for a long time, which can be divided into two theoretical directions: one is the binding energy of atoms on each crystal plane of silicon crystal, and the other is the number and back bond structure of dangling bonds on the surface of silicon wafer. In silicon crystals, there are many different crystal planes. Taking the typical three crystal planes as an example for analysis. The distribution of dangling bonds on silicon’s low-index surfaces is illustrated below for (100), (110), and (111) planes. Surface atoms on (111) and (110) planes each possess one dangling bond, whereas those on (100) planes each exhibit two.

Fig. 1 Crystal structure and different crystal planes of single crystal silicon

When the {100} surface reacts, the first step is for the two dangling bonds of silicon atoms to bind two OH- ions and inject two electrons into the conduction band, corresponding to the diffusion of light oxygen ions onto the wafer surface;

Then, due to the presence of bonding OH groups on the silicon surface, the back bond strength of the silicon surface atoms decreases. The Si-O bond binding energy is 193 Kcal/mol, while the Si-Si bond binding energy is only 78 Kcal/mol. Therefore, the reaction proceeds further. Cleavage of the Si–Si bond in the Si(OH)2 group occurs when its electrons are thermally excited to the conduction band (as shown in the band diagram). This generates a positively charged silicon hydroxide complex, which subsequently reacts with hydroxide ions to form orthosilicic acid – completing the reactant-to-product conversion;

After that, due to the instability of ortho silicic acid, ortho silicic acid decomposes into meta silicate ions when the solution pH is greater than 12, while releasing hydrogen ions and hydroxide ions to form water, accompanied by the desorption of the products.

Si(OH)₄→SiO₂(OH)₂^2-+2H⁺

2H⁺+2OH^–→2H₂O

Later, silicon is oxidized, which means there are excess electrons in the conduction band. The excess electrons in the conduction band are transferred to the water molecules on the silicon surface, producing OH and H. H combines with each other to form hydrogen molecules.
4H₂O+4e^–→4H₂O^–

4H₂O^–→4OH^–+4H→4OH^–+2H₂↑

When the (111) surface reacts, it is similar to the (100) surface, but the initial reaction only binds to one OH-, followed by the breaking of three back bonds on the silicon surface; This reaction differs from the (100) surface in that the (111) surface requires the transfer of 3 electrons to the conduction band and the binding of 3 OH- ions. Once Si (OH) 4 is formed, the reaction is similar to that of the (100) surface. Moreover, due to the three back bonds of silicon atoms on the (111) surface, the electrons on the back bonds correspond to lower energy levels, resulting in a much slower corrosion rate on the (111) surface compared to the (100) surface. When the (110) surface reacts, although each surface silicon atom has a dangling bond, the back bond is relatively complex, with one back bond binding to the internal atom and the other two binding to adjacent surface atoms. Therefore, although the initial reaction rate is similar to that of the (111) surface, the density of initial Si-OH bonds is more similar to that of the (100) surface. The generally observed higher corrosion rate on the (110) surface may be due to two reasons: one is the higher energy level of the (110) surface back bond surface states; Another reason may be that the (110) surface corresponds to the direction of the tunnel, and H2O is easily penetrated, so the corrosion rate of the (110) surface is relatively high.

The difference in corrosion rates between different crystal planes causes anisotropy. As for which one has a faster corrosion rate between the (100) and (110) planes, it is sometimes affected by the corrosive solution and wafer type, and requires verification by process.

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