Application of aziridine reactive rinses in a post-development process to reduce photoresist pattern collapse (2012)

One of the problems for lithographic processes at very small feature scales is pattern collapse caused by unbalanced capillary forces experienced by the photoresist patterns during the final deionized water rinse and drying process. The use of surfactants or super critical fluids to reduce collapse problems has been proposed and studied by many research groups. However, the patterns rinsed with low surface tension fluids appear in many cases to shrink or such treatments cause other feature deformations. Super critical fluid processing requires major changes to the ways in which current track systems operate and can result in swelling and deformation of the resist features as well. Instead of utilizing super critical fluids or adding surfactants to the rinse liquid, one general methodology we have pursued for alleviating such pattern collapse problems involves the actual strengthening of the resist feature itself during wet processing in the development and rinse cycle. One method we have investigated extensively is the use of post-development resist surface crosslinking reactions while the resist structures are still in their wet state, a process we term "reactive rinsing". Such reactive rinse processes have shown significant impact on improving resist pattern collapse. However, previous chemistries used for such reactive rinse processes have either: (1) been complex mixtures that potentially have long term stability problems (i.e. thus making their application in a fab environment more difficult) or (2) been specific to a certain resist types in that the chemistries react with only certain resist functional groups that may not be present in all resists of interest (e.g. some chemistries only work with phenolic resins such as those found in DUV or EUV resists). Therefore, the goal of this work has been to investigate other novel reactive rinse chemistries that are both more robust and which can function with different families of resist materials. Poly-functional aziridines offer one potential such family of compounds that could be the basis for a reactive rinse chemistry. Such aziridines are often used industrially in both aqueous and nonaqueous coatings, inks and adhesives to promote both physical and chemical properties and are known to react with carboxylic acid functional groups such as those commonly found in many 193 nm resists. The goal of this specific study was to determine if poly-functional aziridine chemistry could be used as a reactive rinse for preventing photoresist pattern collapse in 193nm type resists. Contact angle studies and x-ray photoelectron spectroscopy (XPS) were used to characterize the surface crosslinking reaction using aziridine treatments of model resists containing carboxylic acid functionality. Pattern collapse test structures were fabricated and analyzed using a PMMA/PMAA copolymer resist commonly employed for e-beam lithography. SEM studies of the resulting patterns confirm that use of multifunctional aziridine crosslinkers during a post-development process can significantly reduce pattern collapse and enable production of higher resolution features.