Removers have the task to completely dissolve all resist structures or residues after the respective techonological process step (doting, etching or others) is finished. In the early years of photolithography, mostly strong removers were utilized for this purpose. Environmental thoughts were not given as much concern as today, and solvents like DMF (dimethylformamide), DMSO (dimethyl sulfoxide) or amino alcohols were still in use. Resist manufacturers today make significant efforts to provide environmentally safe removers. Immediately after NMP (N-methylpyrrolidone) was classified as health-hazardous substance in 2011, Allresist replaced this compound by the ecologically harmless NEP (N-ethyl pyrrolidone).
The classical remover is acetone which is, together with isopropanol, used as cleaning agent in probably every lab worldwide. The dissolving power of acetone for non- or only low-baked films (up to 120 °C) is excellent. Care must however be taken with respect to the low boiling point (56 °C) and a flashpoint of only -20 °C. Under unfortunate circumstances, already electrostatic charge may cause an explosion. This problem does not occur with NEP (N-ethyl pyrrolidone) or NMP. Both removers have a boiling point of more than 200 °C. The dissolving power is comparable to the power of acetone and may be further enhanced by heating to max. 80 °C (which is harmless with respect to safety concerns, but at this temperature disturbing vapors begin to develop). In principle also other solvents are suitable as removers: IPA, PGMEA (PMA), methyl ethyl ketone (MEK) or thinners (see Thinners) dissolve residuals of not too hard-baked resists. In most cases however considerably more time will be needed if these solvents are used. Solvent removers are equally suitable for novolac-based resists as well as for all polymer resists (e.g. PMMA).
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.
In difficult cases (e.g. after intensive plasma etching or sputtering), ultra- or megasound applications support the process of resist removal. Sensitive structures on the wafer however have to be protected during this procedure or are destroyed otherwise. Removers tend to become weaker during a prolonged usage since the content of dissolved photoresist increases. Even though the theoretical saturation limit (which is far above a solid content of 50 %) is never reached, used removers will nevertheless successively enrich particles and become opaque so that a reproducible cleaning is no longer possible. The dissolving rate furthermore decreases and the removal process will require more time. Common practice is a cascade cleaning. During this procedure, the remover is used for three different bath steps. Coated wafers are placed in the first bath where resist residuals are almost completely removed. Wafers are then transferred into the second bath, followed by the third bath in which the very last residues are removed before the wafers are rinsed with water. As soon as the pre-determined dissolution capacity is reached, the first bath is discarded and second and third bath move one place up.
According to an estimation of the usability of removers like e.g. AR 300-70 (NEP), approximately 100 4-inch wafers with a film thickness of 2 µm can safely be cleaned with one litre of remover. The function of a resist remover is also fulfilled by strongly oxidizing acids like aqua regia, piranha, sulphuric acid or nitric acid. These acids are used for a final cleaning. But in addition to environmental protection aspects (disposal of used acids), these mixtures often not only attack the resist but also other materials of the wafer surface.