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:
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)