A Novel Photometric Method for the Determination of Photoacid Generation Efficiencies Using Benzothiazole and Xanthene Dyes as Acid Sensors

We report here on a new photometric method for acid quantification in nonaqueous media which has been developed to facilitate quantification for photoacid generators (PAGs) employed in resist compositions for microlithography, but may find application in other fields. The method employs laser excitation of the PAG and is based on the acid-induced bathochromic shift in the absorption spectra of a selected organic dye such as rhodamine B ( RB ), fluorescein ( Fl ), coumarin 6 ( C6 ), and 2-[4-(3,4-dimethoxyphenyl)-1,3-butadienyl]benzothiazole ( DB ). The differences in signal intensity of the protonated form of the sensor are used to evaluate the differences in acid generation efficiencies of the PAGs by relative actinometry. To test this methodology, we have determined the photoacid generation efficiencies of 2-methyl- ( 1 ), 2-(2‘-furyl ethylidene)- ( 2 ), and 2-(4‘-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine ( 3 ) in acetonitrile by laser flash photolysis, using direct 308 nm laser excitation for 2 and 3 and 355 nm excitation and various phenothiazine derivatives as sensitizers for 1 . The values determined this way for 2 and 3 using the four different dyes coincide fairly well, and the absolute values are in good agreement with those determined independently with an established technique earlier. Under sensitized conditions, the efficiency of acid generation seems to equal the combined quenching efficiencies of the S 1 and T 1 state of the sensitizers. Experiments in thin polymer films indicate that the same technique could be used to perform in situ acid analysis on coated quartz wafers. This approach has a set of advantages over existing methods and should be applicable to virtually every photoacid generating system by simply selecting the sensor that meets the specific requirements of the PAG under investigation.