Most novolac-based photoresists are characterized by high etch stability in the presence of acids (except in highly concentrated oxidizing acids or in concentrated hydrofluoric acid), while resists are comparably sensitive in alkaline solutions. A developers composed of aqueous-alkaline components with a pH of 12 to 13 will dissolve exposed areas of positive-tone resists within seconds, but unexposed areas are likewise dissolved within 5 – 20 minutes.
It has now been possible to develop a novolac-based negative-tone resist with improved features in this respect. After an additional bake, resists structures are stable in the presence of 1 n NaOH for 4 hours without any measurable removal. A 2:1 dilution of TMAH-remover AR 300-73 was used as developer in these experiments. Surprisingly, the resist is highly sensitive despite the drastic development procedure and can be processed with generally used lithography equipment. Removal is also easily possible.
This negative resist is thus well suited for an application in strongly alkaline galvanic baths and for an etching of aluminium films with strong TMAH developers. Cross-linked resist films are furthermore remarkably solvent-resistant. If structures are baked at only 120°C, these are protected for hours against acetone, IPA, PMA and NEP. At higher bake temperatures, films are stabilized to such an extent that even commonly used removers are ineffective. In these cases, removal of the resist is only possible with plasma etching procedures or using piranha-solution.
The resistance in the presence of solvents can be adjusted by a thorough choice and combination of raw materials (see SX AR-N 4340/7).