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.
KEYWORDS: Deep ultraviolet, Hard x-rays, Photoemission spectroscopy, Photoresist materials, Extreme ultraviolet lithography, Oxidation, Chemical species, Extreme ultraviolet, Absorption, 3D modeling
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)12O14(OH)6]X2 (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)12O14(OH)6](OH)2, either fresh directly after spin-coated vs. DUV-exposed materials under either ambient condition or under a dry N2 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 N2 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.
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