The process flow for KOH wet-etching silicon

In semiconductor and MEMS manufacturing, etching technology is a core process for achieving pattern transfer and three-dimensional structure formation. Wet chemical etching, particularly anisotropic wet etching, leverages the varying etch rates of corrosive solutions on different crystal orientations of single-crystal silicon, enabling automatic termination of etching along specific crystal planes. This allows the precise creation of structures such as V-shaped grooves, cantilever beams, or cavities. Among the various anisotropic etchants available, KOH solution has emerged as one of the mainstream choices due to its advantages: fast etching speed, high anisotropy ratio, relatively low cost, and well-established processing techniques.

The image above illustrates the typical process flow for KOH (potassium hydroxide) etching of single-crystal silicon, with the following steps:

Process Flow

1. Wafer Cleaning

The process begins with proper substrate preparation. Typically, a P-type single-crystal silicon wafer with a <100> crystal orientation is selected. To ensure the quality of subsequent processes, it's essential to thoroughly remove contaminants such as particles, organic residues, and metal ions from the wafer surface. Standard RCA cleaning methods or a simplified SC1/SC2 chemical cleaning protocol can be employed to achieve a clean, contamination-free silicon wafer surface.

2. Buffer Oxide Layer and Silicon Nitride Deposition

To achieve selective etching, a mask layer resistant to KOH corrosion must be fabricated on the silicon wafer surface.

• First, a thin layer of silicon dioxide (SiO₂) is grown on the surface of the silicon wafer via a thermal oxidation process, serving as a buffer layer to relieve stress between the subsequent silicon nitride and the silicon substrate.
• Then, a layer of silicon nitride (Si₃N₄) is deposited onto the SiO₂ using methods such as chemical vapor deposition (CVD). Si₃N₄ exhibits excellent resistance to corrosion by KOH, effectively protecting the underlying silicon areas from etching.

3. Photolithography

The purpose of this step is to transfer the designed groove or window patterns onto the silicon nitride mask.

• Coating and Baking: Sequentially apply the anti-reflective coating (ARC) and photoresist (PR), followed by soft baking on a hot plate to evaporate the solvent.
• Exposure and Post-Exposure Bake: UV light exposure is performed through a mask, triggering a chemical reaction in the photoresist within the exposed areas. This is followed by a post-exposure bake (PEB) step to stabilize the latent image formed during exposure.
• Developing and Hard Baking: Use a developer solution (such as TMAH) to dissolve and remove the photoresist from exposed (or unexposed, depending on the type of photoresist) areas, thereby revealing the pattern intended for etching. Finally, perform hard baking to enhance the stability of the photoresist during subsequent etching steps.

4. Mask Etching

• Using the pattern formed by photolithography as a protective layer, dry etching of the mask is performed.
• Using reactive ion etching (RIE) technology, the exposed Si₃N₄ and SiO₂ layers are sequentially removed.
• After etching is completed, use a specific stripping solution or plasma-based resist removal process to thoroughly eliminate any remaining photoresist and anti-reflective coating, ultimately yielding a silicon nitride mask with precise patterns.

5. KOH Wet Etching of Silicon

This step is the core of the entire process.

• Immerse the wafer with the prepared mask into the heated KOH solution (typically at a concentration of 20% to 40%, at a temperature of 60 to 90°C).
• By leveraging the anisotropic etching properties of KOH on single-crystal silicon—specifically, its etching rate on the <100> crystal plane is significantly faster than on the <111> plane—etching progresses rapidly along the <100> crystal direction. However, when the etching encounters the <111> crystal plane, it slows down dramatically or even halts altogether, ultimately forming structures such as V-shaped grooves or pyramid-like pits bounded by (111) planes.
• The final etching depth can be achieved by precisely controlling the etching time or by leveraging the self-stop effect of the (111) surface.

6. Strip the Silicon Nitride Mask

After the etching structure is formed, the protective silicon nitride mask layer must be removed.

• Typically, a hot phosphoric acid (H₃PO₄) solution is used for wet removal due to its high selectivity toward Si₃N₄ (relative to SiO₂ and Si).
• Dry etching processes can also be used for patterning and removal.
• After the mask is removed, the final cleaning and drying steps are carried out, completing the entire KOH wet-etching process.

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