Consequences of Acid Diffusion

While the increased sensitivity of chemical amplification resists is tied to the ability and the ease of the generated acid to diffuse through the free volume of the resist polymer resin, aided hy PEB, to catalyze chemical transformations of the resin that alter their dissolution properties in the developer solution, these same characteristics of chemical amplification can also negatively impact the patterning action of the resist. 

if neutralization of the acid occurs, the dissolution rate of the exposed part of the resist decreases. If the resist is exposed to airborne molecular bases (such as ammonia and N-methyl pyrrillodone) after exposure and prior to PEB (normally called PEB delay), a thin soluble inhibition layer forms on the top surface of the resist, which results in the formation of a characteristic T-top on development (see Fig. 11.26). Acid evaporation during PEB can also lead to T-top formation. Early commercial chemical amplification resists had 

Dammel, Practical photoresist processing, SPIE Short Course No. SC616 (2005). 

PEB delay stability of a few minutes this value is now on the order of hours in current commercial chemical amplification resists. 

The acid can also diffuse from the exposed area to the interface between the resist in the unexposed area and the substrate and interact with basic moieties in the substrate or BARCs, especially inorganic BARCs such as SiON and TiN, leading to the creation of footing. 

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