Airborne molecular base

The results also showed that the amount of absorbed NMP, or for that matter any airborne molecular base, depends on the solubility parameter of the polymer, on the nature of the contaminant, and on the Tg of the polymer. This was a remarkable result because it suggested that the environmental sensitivity of a resist polymer could be controlled by merely changing the Tg or the solubility parameter of the resist polymer. A verification of this result was obtained when the environmental stability of two polymers synthesized from two different isomers of BOCST monomers—the 3-BOCST and the 5-BOCST isomers—were found to have their environmental stability very well correlated with their Tg. The polymer with the significantly lower 7g—the 3- BOCST isomer—showed the least sensitivity to exposure to low levels of NMP. 

Figure 7.13 PEB delay stability of 160-nm lines printed on Sumitomo s PARlOl resist that is based on poly(2-methyladamantyl methacrylate-co-gamma butyrol lactone). Airborne molecular base concentration NH3 0.64 ppb NMP 0.08 ppb. Processing conditions PAB PEB 130°C/60 s. Resist thickness 4384 A. Developer CD26 (2.38% TMAH). Dose 15mJ/cm. NA 0.6, partial coherence 0.7.

Next, 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... 

Thiuram Sulfides. These compounds, (8) and (9), are an important class of accelerator. Thiurams are produced by the oxidation of sodium dithiocarbamates. The di- and polysulfides can donate one or more atoms of sulfur from their molecular stmcture for vulcanization. The use of these compounds at relatively high levels with litde or no elemental sulfur provides articles with improved heat resistance. The short-chain (methyl and ethyl) thiurams and dithiocarbamates ate priced 2/kg. Producers have introduced ultra-accelerators based on longer-chain and branched-chain amines that are less volatile and less toxic. This development is also motivated by a desire to rninirnize airborne nitrosamines. 

Figure 9 Atmospheric transmission in the submillimetre and far-IR from (top) a very good high-altitude ground-based site (Mauna Kea at 4.2 km) and from (bottom) an airborne observatory (e.g. KAO at 12 km). The blocked regions are mostly caused by molecular absorption. Reproduced with permission from Phillips TG and Keene J (1992) Submillimetre astronomy. Proceedings of IEEE 80 1662.

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