Quantum Yields of Photoacid Generation in 193-nm Chemically Amplified Resists by Fluorescence Imaging Spectroscopy

We have made significant improvements in the development of a fast, convenient fluorescence imaging technique for evaluating the acid generation efficiency of photoacids in chemically amplified photoresists. A pH sensitive fluorescent molecule, coumarin 6 (C6), is doped into a 193-nm commercial resist system for imaging. For the 193-nm resist systems studied here, the Ka of C6 lies within the range of acid concentrations that can be photogenerated, making it a suitable probe of the acid generation efficiency of various photoacid generators (PAGs). Resist formulations, each containing a candidate PAG, are doped with C6. The resist is spin coated onto wafers and patterned with 5-(mu)m features, with each feature at a different exposure dose. Each wafer is then spectroscopically imaged with an epifluorescence microscope. The spectroscopic content of each feature in the dose ramp is determined and the resulting data are analyzed using a two-level optical titration model. The relative quantum yields of acid generation for several PAGs have been measured multiple times over the span of several months and are shown to be reproducible. Furthermore, we have compared the on-wafer fluorescence imaging technique with two nonindicator based (C-parameter and P-parameter) methods. Our results demonstrate an excellent agreement among the three techniques.