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, novolac resists are easily removed, while PMMA resists begin to swell and are then removed slowly. Already moderate dilution of oxidising acids with di-water however prevents this attack of acidic solutions. In contrast, non-oxidising acids (hydrochloric acid, hydrofluoric acid) leave resist films intact, even if concentrated solutions of these acids are used.
After a tempering of e-beam resist films (AR-P 600(0), 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 %: dissolution of films after 15 s, except for 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 %: coatings < 2 µm film thickness float off immediately
With respect to the stability in concentrated sulphuric acid and hydrofluoric acid, a postbake of resists films (PMMA 190 °C, novolac 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)