10. How high is the etch resistance of e-beam resists in the presence of strong acids?

Concentrated oxidising acids (sulphuric acid, nitric acid, aqua regia 1), piranha 2)) attack resist films already at room temperatures and are often used as remover for persistent resist structures.

In the presence of these agents, novolak resists are easily removed, while PMMA resists begin to swell and are then removed slowly. Already moderate dilution of these oxidising acids however prevent this reaction. In contrast, non-oxidising acids (hydrochloric acid, hydrofluoric acid) leave resist films intact, even if concentrated solutions of these acids are used.

After a bake of e-beam resist films (AR-P 6000, AR-P/N 7000) at 95 °C for 25 min, the following parameters were determined:

• Sulphuric acid 50 %: no attack after 2 hours (room temperature and heated to 60° C)
• Sulphuric acid 96 %: films peel off after 15 s, only PMMA protective coatings are heavily attacked and gradually removed
• Hydrochloric acid 20 %: no attack after 2 hours (room temperature and heated to 60° C)
• Hydrochloric acid conc. 37 %: no reaction with films observed, only AR-N 7500 shows adhesion problems after 10 min (film is floating off)
• Hydrofluoric acid 2 %: Films float off immediately for coatings < 2 μm film thickness

With respect to the stability in concentrated sulphuric acid and hydrofluoric acid, a postbake of resists films (PMMA 190 °C, novolak 150 °C) only leads to a minor improvement.

1) Aqua regia: Mixture of hydrochloric acid and nitric acid (3:1)

2) Piranha: Mixture of sulphuric acid and hydrogen peroxide (1:1)