Absorption and exposure kinetics of photoresists at EUV
The experimental measurement of the time-dependent absorption of photoresists at extreme ultraviolet wavelength is of great interest for the modeling of the lithographic process. So far, several technical challenges have made the accurate determination of the linear absorption coefficient and the Dill parameters nontrivial. In this work, we use a dedicated equipment and synchrotron light source to experimentally measure the transmittance of thin layers of photoresists on transparent silicon nitride membranes, and their thickness was measured with the spectroscopic ellipsometry. The absorption of negative tone photo-condensed metal oxide photoresists based on Sn cage structures, and of Zr and Hf oxoclusters was measured and compared to the estimated values. It was found that tin based materials absorb considerably more light than conventional chemically amplified resists based on organic polymer. Hafnium-based materials have about twice absorption, while zirconium based are basically comparable to organic resists. Furthermore, the exposure kinetics of several chemically amplified resists with varying photo-acid concentration and backbone polymer was studied. The rate of bleaching, described by the Dill parameter C, was measured and conclusions are drawn based on the specific resist formulation.