Infrared Spectroscopy Studies of Electron Induced Reaction Mechanisms in EUV Photoresists




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Extreme ultraviolet (EUV) lithography, with approximately 13.5 nm photons is the new standard of the semiconductor industry. The use of EUV photons allows for further miniaturization of integrated circuits, enabling industry and researchers alike to explore the 1 – 10 nm regime. Despite the desire to begin mass producing devices with EUV tools by 2020, a clear direction for the best EUV capable photoresists is not understood. In this dissertation, a novel class photoresist material is investigated to understand key areas of their reaction mechanisms for next-generation photolithography. These photoresists are composed of a hybrid nanocluster architecture with a small HfOₓ core surrounded by methacrylic acid ligand (HfMAA) and can achieve high sensitivity and etch-resistance due to their small molecular nature, high-absorption metal core, and ease of ligand tunability. However, many aspects about their properties and reactivity are still poorly understood. To investigate the reaction mechanisms, the photoresists were probed with a bream of energetic electrons, corresponding to primary and secondary energies produced during EUV ionizations. Their chemical transformation upon electron irradiation, along with the effects of annealing, were tracked using in situ infrared (IR) spectroscopy. After post-application bake (PAB) to 105 °C, the IR spectra show the formation of new Hf-O-Hf bonds through the consumption of terminal hydroxyl groups. This bond formation negatively affects the intrinsic solubility characteristic of the photoresists. Additionally, a crosslinking pathway is initiated by a decarboxylation mechanism of the methacrylate ligands (MAA) under electron irradiation. To understand further the role of secondary electrons in HfMAA, a ligand exchange procedure was employed to change ~20% of the MAA with 4-hydrobenzoic acid (HBA) and phenyl acetic acid (PAA). In situ IR spectroscopy was used to monitor the amount of alkyl CH produced by both 90 and 20 eV electron irradiations. The addition of the co-ligand enhanced the secondary electron sensitivity by 40% when compared to HfMAA. In addition, using mass spectrometry, two different reaction pathways are observed for each co-ligand due to the benzene ring of each ligand decomposes differently. Finally, a number of fundamental studies were performed to investigate EUV/electron-induced resist chemistry in thin-film model systems. Using methacrylic acid (MAA), isobutyric acid (IBA), and 4-hydrobenzoic acid (HBA) as prototypical probe molecules, we find spectroscopic evidence for a decarboxylation mechanism among each of the grafted carboxylate molecules. Differences in selection rules for EUV absorption vs impact ionization for 90 eV electrons are found to play an important role in the reactivity of ligands with different metal centers. Lastly, ab initio model calculations are compared to experimental data and demonstrate their potential use to screen reactivity of different carboxylate ligands and provide validation of first principles method for predicting reactivity of candidate resist chemistries. Additionally, we successfully grafted trivinyl-, dimethylsilamine on SiO₂ to fundamentally study the effect of electron irradiation of organosilane based molecules. Results show with FTIR spectroscopy we can study reactivity of the silicon-vinyl groups by spin coating a thin siloxane based polymer layer on top of the monolayer. We demonstrated interaction between the two layers can occur with electron irradiation through the formation of Si-C and Si-O bonds.



Fourier transform infrared spectroscopy, Photoresists, Extreme ultraviolet lithography, Irradiation


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