Absorption coefficient and exposure kinetics of photoresists at EUV

Abstract

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 photocondensed 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 photoacid 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.

Extreme ultraviolet patterning of tin-oxo cages

“…The calculated linear absorption coefficient (α) for TinS is 13.3 μm −1 assuming a density of 1.9 g∕cm 3 taken from the crystal structure, 12 and using atomic EUV absorption coefficients reported in the literature. 13 In 2017, the absorption coefficients α for some of the tin cage materials were measured 14 and found to be in good agreement with the predicted ones.…”

“…The relatively high packing density of Sn atoms provides a highly absorbing material, 14 since Sn is one of the elements most strongly absorbing at 13.5 nm. 13 Optimization of the processing conditions provided a better sensitivity than was reported previously.…”

Photochemical conversion of tin-oxo cage compounds studied using hard x-ray photoelectron spectroscopy

“…15. The molecule contains Sn atoms with high EUV photon absorption, 10,16,17 and it has been demonstrated to work as an EUV photoresist. 6,8 Despite the relatively simple structure of this molecular system, few spectroscopic methods are suitable to study the details of the photochemical reactions occurring in thin films, both due to limited sensitivity and limited chemical contrast.…”

Photoreactions of Tin Oxo Cages, Model EUV Photoresists

“…These systems combine several advantages: the demonstrated EUV patterning underscores their relevance for the EUVL field [7,9], the absorption cross sections are 2 -3 times higher than those of organic polymers [5], and the materials are of a molecular nature, consisting of small (~1 nm) and fully defined building blocks. The goal of our research in this field is to boost the fundamental level of understanding of the photoresist performance in relation to the chemical processes that take place upon activation of these materials by high energy sources such as UV and EUV photons as well as electrons.…”

“…These materials have a number of inherent limitations related to diffusion of the catalyst and to the stochastic distributions of photoacid generators (PAG) and quenchers. Probably even more importantly, the absorption cross-sections of these mostly organic materials at the EUV wavelength of 13.5 nm are only low, [4,5] and their resistance to etch processes is limited. These disadvantages are aggravated by the need to reduce the film thickness in order to maintain a reasonable aspect ratio for the small features that need to be transferred.…”