How high is the etch resistance of photoresist 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 removers for persistent resist structures. Already moderate dilution of these acids prevents this reaction with the resist. Non-oxidising acids however (hydrochloric acid, hydrofluoric acid) leave the resist film intact, even if concentrated solutions of these acids are used. Hydrofluoric acid however removes the undamaged resist layer in most cases completely (see Question 19). After a bake of all photoresist films (AR-P 3000 – 5000) at 95 °C for 25 min in a convection oven, 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. Slow stripping of AR-PC 503, 504, X AR-P 3100/10, SX AR-PC 5000/40).
• 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 4340 shows adhesion problems after 10 min (film is floating off).
• Hydrofluoric acid 2 %: Films float off immediately for coatings < 5 µm thickness, thicker layers remain stable for 1 – 3 min, but structures begin to peel off already after 30 s. Exceptions: X AR-P 3100/10, AR-PC 503, 504, X AR-P 3100/10, SX AR-PC 5000/40 ( Question 12: Protective coating).
• Hydrofluoric acid 5 %: Processing of X AR-P 3100/10 is only possible at short etch times (< 5 min) and with protective coating AR-PC 503, 504, SX AR-PC 5000/40.
• Hydrofluoric acid conc. 50 % : Processing is only possible with protective coating SX AR-PC 5000/40 ( Question 19 HF etching).

A postbake of resist films (100 – 130 °C) leads only to a minor improvement with respect to the stability in concentrated sulphuric acid and hydrofluoric acid.
1) Aqua regia: Mixture of hydrochloric acid and nitric acid (3 : 1)
2) Piranha: Mixture of sulphuric acid and hydrogen peroxide (1 : 1)