Removers have the task to completely dissolve all resist structures or residues after the respective technological 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).
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.
Furthermore the eco-friendly removers AR 300-76 and AR 600-71 are available.
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.