Stoppers in the classical sense are mainly used for electron beam lithography with PMMA resists. These substances have the function to interrupt the development process after a development of exposed PMMA films and to cleanly rinse off the developer solvent which now contains residual PMMA. If no stopper is used, polymer residues from the contaminated developer may be deposited during the bake step on the surface. These almost invisible veil-like deposits can only be removed with difficulty after the bake (in this case, O 2 -plasma recommended), but will impair e.g. a subsequent metallization step.
Even for other solvent-developable polymers (e.g. SX AR-PC 5000/40 or polystyrene), the use of stoppers is recommended for the same reason.
A stopper in the non-classical sense is DI-water for photoresists (novolac-based). The development process is here also abruptly stopped and the novolac-based developer is rinsed off. In contrast to solvent-developable polymers however which are first soaked in the (solvent-) developer, covered with a solvent “coat” and then finally removed, cresole-based novolacs dissolve immediately. Under alkaline conditions, cresols are converted into cresolates (anions) which represent a polyelectrolyte (novolac consists of approx. 10 – 20 cresoles which all may be converted into cresolate). This polyelectrolyte is well soluble in alkaline developers.
Overview Process Chemicals- Stopper