The amount of absorbed light in thin photoresist films is a key parameter in photolithographic processing, but its experimental measurement is not straightforward. The optical absorption of metal oxide-based thin photoresist films for extreme ultraviolet (EUV) lithography was measured using an established methodology based on synchrotron light. Three types of materials were investigated: tin cage molecules, zirconium oxoclusters, and hafnium oxoclusters. The tin-containing compound was demonstrated to have optical absorption up to three times higher than conventional organic-based photoresists have. The absorptivity of the zirconium oxocluster was comparable to that of organic polymer-based photoresists, owing to the low absorption cross section of zirconium at EUV. The hafnium-containing resist shows about twice as high absorptivity as an organic photoresist, owing to the significantly higher absorbance of hafnium. From the chemical composition and crystal structure, the density of the spin-coated films was determined. Using the density of the films and the tabulated data for atomic cross section at EUV, the expected absorptivity of these resists was calculated and discussed in comparison to the experimental results. The agreement between measured and expected absorption was fairly good with some substantial discrepancies due to differences in the actual film density or to thickness inhomogeneity due to the spin coating. The developed method here enables the accurate measurement of the EUV absorption of the photoresists and can contribute to the further development of EUV resists and more accurate lithographic modeling.
We report on the extreme ultraviolet (EUV) patterning performance of tin-oxo cages. These cage molecules were already known to function as a negative tone photoresist for EUV radiation, but in this work, we significantly optimized their performance. Our results show that sensitivity and resolution are only meaningful photoresist parameters if the process conditions are optimized. We focus on contrast curves of the materials using large area EUV exposures and patterning of the cages using EUV interference lithography. It is shown that baking steps, such as postexposure baking, can significantly affect both the sensitivity and contrast in the open-frame experiments as well as the patterning experiments. A layer thickness increase reduced the necessary dose to induce a solubility change but decreased the patterning quality. The patterning experiments were affected by minor changes in processing conditions such as an increased rinsing time. In addition, we show that the anions of the cage can influence the sensitivity and quality of the patterning, probably through their effect on physical properties of the materials.
Molecular inorganic materials are currently considered photoresists for extreme ultraviolet lithography (EUVL). Their high EUV absorption cross section and small building block size potentially allow high sensitivity and resolution as well as low line-edge roughness. The photochemical reaction mechanisms that allow these kinds of materials to function as photoresists, however, are still poorly understood. We discuss photochemical reactions upon deep UV (DUV) irradiation of a model negative-tone EUV photoresist material, namely the well-defined molecular tin-oxo cage compound [(SnBu)12O14(OH)6](OH)2, which is spin-coated to thin layers of 20 nm. The core electronic structures (Sn 3d, O 1s, and C 1s) of unexposed and DUV exposed films were then investigated using synchrotron radiation-based hard x-ray photoelectron spectroscopy. Different chemical oxidation states and concentrations of atoms and atom types in the unexposed and exposed films were found. We observed that the exposure in a nitrogen atmosphere prevented the oxidation but still led to carbon loss, albeit with a smaller conversion. Finally, a mechanistic hypothesis for the basic DUV photoreactions in molecular tin-oxo cages is proposed.
Several metal-containing molecular inorganic materials are currently considered as photoresists for extreme ultraviolet lithography (EUVL). This is primarily due to their high EUV absorption cross section and small building block size, properties which potentially allow both high sensitivity and resolution as well as low line-edge roughness. The photochemical reaction mechanisms that allow these kinds of materials to function as photoresists, however, are still poorly understood. As a step in this direction, we here discuss photochemical reactions upon deep UV (DUV) irradiation of a model negative-tone EUV photoresist material, namely the well-defined molecular tin-oxo cage compound [(SnR)<sub>12</sub>O<sub>14</sub>(OH)<sub>6</sub>]X<sub>2</sub> (R = organic group; X = anion) which is spin coated to thin layers of 20 nm. The core electronic structure (Sn 3d, O 1s and C 1s) of fresh and DUV exposed films were then investigated using synchrotron radiationbased hard X-ray photoelectron spectroscopy (HAXPES). This method provides information about the structure and chemical state of the respective atoms in the material. We performed a comparative HAXPES study of the composition of the tin-oxo cage compound [(SnR)<sub>12</sub>O<sub>14</sub>(OH)<sub>6</sub>](OH)<sub>2</sub>, either fresh directly after spin-coated vs. DUV-exposed materials under either ambient condition or under a dry N<sub>2</sub> atmosphere. Different chemical oxidation states and concentrations of atoms and atom types in the fresh and exposed films were found. We further found that the chemistry resulting from exposure in air and N<sub>2</sub> is strikingly different, clearly illustrating the influence of film-gas interactions on the (photo)chemical processes that eventually determine the photoresist. Finally, a mechanistic hypothesis for the basic DUV photoreactions in molecular tin-oxo cages is proposed.
We report on the extreme ultraviolet (EUV) patterning performance of tin-oxo cages: molecular building blocks that are known to turn insoluble upon EUV exposure, thus having the properties of a negative tone photoresist. In this work, we focus on contrast curves of the materials using open-frame EUV exposures and their patterning capabilities using EUV interference lithography. It is shown that baking steps, such as post-exposure baking (PEB) can significantly affect both the sensitivity and contrast in the open-frame experiments as well as the patterning experiments. In addition, we show that the exchange of the anions of the cage can make a difference in terms of their physical properties. Our results demonstrate the significance of process optimization while evaluating the resist performance of novel molecular materials.
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.