The simplest but nevertheless highly effective removers are sodium hydroxide (NaOH) and potassium hydroxide (KOH) solutions. Already a 4 % KOH solution will remove basically all novolac-based photo- and e-beam resists within a few seconds. Exceptions are only resists particularly designed as alkali-stable resists (see Alkali-stabile and solvent-stable negative resist). Increasing the concentration of NaOH or KOH up to 40 % is possible. These strongly alkaline solutions are also suitable for difficult, particularly hard-baked resist films. Alkaline solutions are often not able to remove resist residuals entirely, but will in this case creep under the film. Residuals are then more or less lifted and subsequently completely removed. It should however be taken into account that highly concentrated alkaline solutions may also attack the silicon of the wafer and thus destroy the surface. An alternative to the above-mentioned alkaline solutions is the concentrated developer AR 300-26 which is based on buffered alkaline salts. The undiluted developer will also quickly remove most resist films.
Tetramethyl ammonium hydroxide (TMAH) solutions are also used as remover, with a maximum concentration of 25 %. At this concentration, TMAH is comparable with highly concentrated NaOH- and KOH-solutions. TMAH will also attack silicon and caution is likewise required during usage. The fourfold less concentrated TMAH-based remover AR 300-73 is considerably easier (i.e. safer) to be handled and also more environmentally friendly due to the lower consumption of TMAH. Aqueous-alkaline removers are not suitable for all polymers (PMMA, polystyrene, carbon hydrogens). This feature however can be specifically used in two-layer systems (PMMA/photoresist; carbon hydrogens/photoresist) for a selective development of the upper photoresist layer.