Common Wet Etching Formulations for Semiconductor Materials

A wide variety of materials are used in semiconductor manufacturing, among which silicon and its oxides, as well as silicon nitride, serve as the most fundamental substrate and dielectric materials; research on their etching recipes is the most mature and representative.

1. Etching Systems for Silicon and Silicon Dioxide

1.1 Silica Etching: HF-Based Solutions

• As mentioned earlier, HF is the standard etchant for SiO₂; however, in industrial applications, in addition to BOE, mixtures of HF and ethylene glycol (EG) are also commonly used. A typical formulation consists of 49% HF mixed with EG at a ratio of 4:96, operated at a temperature of 70–80°C. This mixture exhibits virtually no etching of silicon, making it particularly suitable for shallow-trench-isolation (STI) processes where oxide removal is required without damaging the silicon substrate.

1.2 Isotropic Etching of Silicon: HNA System

• The HNA system consists of hydrofluoric acid (HF), nitric acid (HNO₃), and acetic acid (CH₃COOH). Nitric acid acts as an oxidizing agent to oxidize the silicon surface into SiO₂, which is then dissolved by HF; acetic acid serves to dilute the solution and provide buffering, thereby suppressing the ionization of nitric acid and ensuring a stable, controllable etching process. By adjusting the relative proportions of the components, the etching rate can be tuned: in regions with a high HF concentration, the etching rate is limited by the oxidation step and is relatively slow, whereas in regions with a high nitric acid concentration, the rate is limited by the dissolution step, allowing etching rates of 2–5 μm/min.

1.3 Anisotropic Etching of Silicon: Alkaline Solutions

By exploiting the differential etching rates of different crystal planes in single-crystal silicon in alkaline solutions, three-dimensional structures such as V-grooves and cantilever beams can be fabricated, which are widely used in MEMS manufacturing.

• KOH solution exhibits a significantly higher etching rate on the Si(100) surface than on the Si(111) surface, with a selectivity ratio of up to several dozen; however, the risk of metal-ion contamination limits its application in CMOS processes.
• TMAH solution: free of metal ions and highly compatible with CMOS. The addition of surfactants (such as Triton X-100) and isopropyl alcohol (IPA) can enhance surface smoothness and protect sharp corners.

2. Etching of silicon nitride: hot phosphoric acid system

Silicon nitride (Si₃N₄) undergoes hydrolysis in hot phosphoric acid: Si₃N₄ + 6H₂O → 3SiO₂ + 4NH₃, with phosphoric acid acting as a catalyst. A typical process employs 85% concentrated phosphoric acid at a temperature of 150–170°C; the etch rate is highly sensitive to temperature, increasing by approximately 40% for every 10°C rise. To enhance the selectivity ratio with respect to the underlying silicon oxide, silicon can be pre-dissolved in the phosphoric acid, which consumes water and alters the reaction kinetics, thereby increasing the selectivity from the usual 3:1 to greater than 10:1.

3. Introduction to Other Material Systems

Metal etching (e.g., aluminum, titanium, tungsten) typically employs acid-based formulations (such as phosphoric acid–nitric acid–acetic acid), whereas polymer etching generally uses organic solvents or strongly oxidizing acids. The formulation design follows the same principles outlined above: the oxidant provides the driving force for the reaction, the complexing agent prevents the deposition of byproducts, and the diluent controls the etching rate.

Conclusion

Etching recipes for different material systems each follow their own design rationale, yet they all share a common principle: the synergistic coordination of functional components such as oxidizers, etchants, buffers, and diluents. For silicon-based materials, HF-based solutions and alkaline etchants are typical; for silicon nitride, high-temperature catalysis by hot phosphoric acid is relied upon. By mastering the underlying principles of these classic formulations, one can flexibly tailor and optimize them in actual processes to meet specific requirements. This section focuses on the formulations themselves, while the next will turn to the optimization of process parameters and defect control, exploring how to integrate formulations with factors such as temperature, time, flow field, and endpoint detection to establish a comprehensive wet-etching process system.

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