Evolution of 248 nm Positive Resists

The acid-catalyzed deprotection has become the exclusive foundation for all the modern positive resists. The first generation system was built on lipophilic 

From the time of the inception, chemical amplified positive resists suffered from formation of a surface-insoluble layer, which typically showed up as a skin or T-top profile (Fig. 49). The problem has been ascribed to contamination by 

In addition to the air filtration, several techniques to stabilize chemical amplification resists toward airborne contamination have been proposed  

Application of a protective overcoat to seal off airborne contaminants was also a popular approach initially. Although many polymers, lipophilic and hydrophilic, have been evaluated as a topcoat, water-soluble poly[ (meth)acryhc acid] is most commonly employed, which can be cast from a water solution without interfacial mixing with the resist layer and can be removed during aqueous base development. However, it has been reported that a poly(acrylic acid) overcoat allows diffusion of water, which reportedly contaminates a chemical amplification resist [211]. Poly(cx-methylstyrene) has been recommended as a good barrier against both airborne base and water [211]. 

Chemical amplification resists require PEB in general to accelerate acid-catalyzed reactions and thus the acid generated by irradiation could be deactivated (by airborne contamination) upon standing before PEB. Then, generation of acid when it is needed (at the time of PEB) could eliminate/minimize the 

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