Discussion
Hydrofluoric acid is preferred in wet etching. Which of the following is/are correct for Hydrofluoric acid (HF) ?
(1) HF does not attack SiO2
(2) HF attacks SiO2
(3) HF affects silicon substrate underneath
(4) HF does not affect silicon substrate underneath
Answer: C
The correct statements are 2 and 4.
Hydrofluoric acid etches silicon dioxide readily, through the reaction SiO2 + 6HF → H2SiF6 + 2H2O, so statement 2 is correct. It hardly attacks the silicon underneath, which makes it highly selective: an oxide layer can be removed without damaging the substrate, so statement 4 is correct.
Statement 1 contradicts statement 2, and statement 3 contradicts statement 4, so every option containing 1 or 3 is wrong. This selectivity is exactly why HF (often buffered with ammonium fluoride) is preferred for oxide wet etching.
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