BOE (Buffer Oxide Etchant) is a mixed solution of hydrofluoric acid (HF) and ammonium fluoride (NH₄F). It controls etch rate via a buffer system and is widely used for SiO₂ etching in semiconductor manufacturing. Its selectivity ratio(defined as the etch rate ratio of SiO₂ to Si₃N₄) is significantly affected by temperature, with specific trends as follows:

1. Mechanism of Temperature Effects on BOE Etch Rates

Enhanced Activity of HF

Elevated temperature accelerates molecular motion of HF, increasing its chemical reaction rate with SiO₂. For example, 40% HF exhibits an etch rate of ~833 nm/min for SiO₂ at 21°C, which may rise to over 1200 nm/min when temperature increases to 40°C.

Buffering Capacity of NH₄F

NH₄F releases F⁻ ions that react with HF to form HF₂⁻ (HF + F⁻ ⇌ HF₂⁻), inhibiting excessive HF-induced corrosion of silicon. However, high temperature promotes NH₄F decomposition (NH₄F → NH₃↑ + HF↑), reducing its buffering capacity. This enhances HF activity and leads to a nonlinear increasein etch rate.

2. Temperature Effects on Selectivity Ratio (SiO₂/Si₃N₄)

The selectivity of BOE for SiO₂ vs. Si₃N₄ primarily depends on differences in reaction kineticsbetween the two materials and F⁻:

Etching of SiO₂

SiO₂ reacts with HF to form soluble hexafluorosilicic acid (SiO₂ + 6HF → H₂SiF₆ + 2H₂O). This reaction has a low activation energy(~20–30 kJ/mol), making its etch rate highly sensitive to temperature.

Etching of Si₃N₄

Si₃N₄ requires breaking strong Si-N bonds, resulting in a higher activation energy(~50–60 kJ/mol). Thus, its etch rate is far less temperature-sensitive than SiO₂.

Temperature-Selectivity Relationship

•Low temperature (20–30°C): BOE retains strong buffering capacity, leading to a low SiO₂ etch rate (e.g., 30–80 nm/min at 21°C). Si₃N₄ remains nearly uncorroded, yielding a selectivity ratio of >100:1.

•Medium temperature (40–60°C): NH₄F decomposition intensifies, enhancing HF activity and increasing SiO₂ etch rate (e.g., up to 1500 nm/min at 50°C). Si₃N₄ still etches slowly, reducing selectivity to 50–80:1.

•High temperature (>70°C): Significant NH₄F decomposition releases excess HF, which corrodes silicon. SiO₂ etch rate rises sharply (e.g., >3000 nm/min at 80°C). Si₃N₄ remains resistant due to high bond energy, but selectivity drops further to 20–30:1.

3. Temperature Control in Practical Processes

•Low selectivity requirements: For applications like trimming passivation layers (where rapid SiO₂ removal is prioritized and minor Si₃N₄ loss is acceptable), medium-to-high temperature BOE (50–60°C) is used.

•High selectivity requirements: For precision tasks such as gate oxide etching, temperature must be strictly controlled at 20–30°C to maintain high selectivity (>100:1) and prevent damage to the silicon substrate.

This temperature-dependent selectivity makes BOE a versatile etchant, balancing speed and precision in semiconductor fabrication.