Acid Mixture Determination in Silicon Etching Bath

The application note describes the determination of both HF and HNO₃ in an etching solution in the fabrication of microelectronic component and also the concentration of H₂SiF₆ that is formed upon etching. Three successive thermometric titrations are used to obtain the concentration of all the three species.

Background:

Etching is an important process in the fabrication of microelectronic components. It involves selective removal of one or more layers of a semiconductor wafer to engrave a certain pattern in it. Wet chemical etching uses chemicals capable of dissolving the wafer material. To obtain a specific pattern, a mask of resistant material is applied on the wafer’s surface. For silicon wafers, the most common chemical etching agent is acid mixture of hydrofluoric (HF) and nitric acid (HNO₃).

This note describes the determination of both HF and HNO₃ in an acid mixture used as etching solution and also the concentration of H₂SiF₆ that is formed upon etching. Three successive thermometric titrations are used to obtain the concentration of all the three species. In the first titration, the amount of HF is determined by complexation with aluminum, sodium and potassium. The second titration is for the total acid content, determined with NaOH and the third step is a back titration to determine the amount of H₂SiF₆.

Reaction:

Titration of HF:

    6 HF + Al³⁺ + 2 K⁺ + Na⁺ → K₂NaAlF₆ + 6 H⁺

Total acid content:

    1) H₂SiF₆ + 2NaOH → Na₂SiF₆ + 2H₂O

    2) Na₂SiF₆ + 4NaOH → Si(OH)₄ + 6NaF

    3) HNO₃ + NaOH → NaNO₃ + H₂O

    4) HF + NaOH → NaF + H₂O

 H2SiF6 content:

    1) Si(OH)₄ + 4HCl + 6NaF → Na₂SiF₆ + 4H₂O + 4NaCl

    2) NaOH + HCl → NaCl + H₂O

Related Resources:

Download brochure 'GTP – Hints & Tips'  to learn more about Hints and Tips for Good Titration Practice