The absorption of extreme ultraviolet (EUV) radiation by a photoresist strongly depends on its atomic composition. Consequently, elements with a high EUV absorption cross section can assist in meeting the demand for higher photon absorbance by the photoresist to improve the sensitivity and reduce the photon shot noise induced roughness. In this work, we enhanced the EUV absorption of the methacrylic acid ligands of Zn oxoclusters by introducing fluorine atoms. We evaluated the lithography performance of this fluorine-rich material as a negative tone EUV photoresist along with extensive spectroscopic and microscopic studies, providing deep insights into the underlying mechanism. UV–vis spectroscopy studies demonstrate that the presence of fluorine in the oxocluster enhances its stability in the thin films to the ambient atmosphere. However, the EUV photoresist sensitivity (D₅₀) of the fluorine-rich oxocluster is decreased compared to its previously studied methacrylic acid analogue. Scanning transmission X-ray microscopy and in situ X-ray photoelectron spectroscopy in combination with FTIR and UV–vis spectroscopy were used to gain insights into the chemical changes in the material responsible for the solubility switch. The results support decarboxylation of the ligands and subsequent radical-induced polymerization reactions in the thin film upon EUV irradiation. The rupture of carbon–fluorine bonds via dissociative electron attachment offers a parallel way of generating radicals. The mechanistic insights obtained here will be applicable to other hybrid materials and potentially pave the way for the development of EUV materials with better performance.

We developed a quantitative approach for the determination of molecular arrangement and electronic structure in anisotropic organic ultrathin films based on the measurement of polarized reflectivity at the carbon K-edge. The reflectivity spectra were fitted to a parameterized model calculation. The method was applied to a self-assembled monolayer of 1,4-benzenedimethanethiol on gold. To simulate reflectivity, the organic anisotropic film was described by a dielectric tensor, obtained by ab initio calculations for the single molecule and suitable rotations to describe the molecular organization in film domains. Film structure was obtained though the best fit of the simulation to the experiment. Results were consistent with a monolayer-thick film composed of domains of molecules with in-plane isotropic distribution of orientations. In each domain, molecules adopted a standing configuration, with a tilt of 28° relative to the substrate normal. Information on the modification of the molecular electronic states due to chemical bonding was derived. DOI: http://dx.doi.org/10.1103/PhysRevB.89.045401 ©2014 American Physical Society

Organometallic tin-oxo-hydroxo cage compounds offer a promising photoresist platform for extreme ultraviolet photolithography (EUVL). Their reactivity is dominated by the facile breaking of the tin-carbon bonds upon photon or electron irradiation. As the cage is dicationic, it exists as a complex with anions for charge compensation. In the present work, we explore the n-butyltin-oxo cage with two tetrakis(pentafluorophenyl)borate counteranions (TinPFPB). In contrast to the small counterions that are typically used, the bulky PFPB anion absorbs a substantial fraction (∼30%) of the impinging EUV radiation (13.5 nm, 92 eV), and it has its own reactivity upon photoionization. When thin films of the complex are irradiated with EUV radiation at low doses, a positive-tone development is possible, which is rather unique as all other known tin-oxo cage resists show a negative tone (cross-linking) behavior. We propose that the initial positive tone behavior is a result of the chemical modification of the Sn cluster by fragments of the borate anions. For comparison, we include the tetrakis(p-tolyl)borate anion (TB) in the study, which has similar bulkiness, and its complex with the n-butyltin-oxo cage (TinTB) shows the usual negative tone EUV resist behavior. This negative-tone behavior for our control experiment rules out a hypothesis based purely on the steric hindrance of the anion as the cause of the different EUV reactivity.

The unoccupied electronic structure of thick films of tetraphenylporphyrin and tetrakis(pentafluorophenyl)porphyrin Cu(II) complexes (hereafter, CuTPP and CuTPP(F)) deposited on Au(111) has been studied by combining the outcomes of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy with those of spin-unrestricted time-dependent density functional (TD-DFT) calculations carried out either within the scalar relativistic zeroth order regular approximation (ZORA) framework (C, N and F K-edges) or by using the Tamm–Dancoff approximation coupled to ZORA and including spin–orbit effects (Cu L2,3-edges). Similarly to the modelling of NEXAFS outcomes pertaining to other Cu(II) complexes, the agreement between theory and experiment is more than satisfactory, thus confirming the open-shell TD-DFT to be a useful tool to look into NEXAFS results pertinent to Cu(II) compounds. The combined effect of metalation and phenyl (Ph) fluorine decoration is found to favour an extensive mixing between Phσ* and pristine porphyrin macrocyle (pmc) pmcπ* virtual levels. The lowest lying excitation in the C and N K-edge spectra of both CuTPP and CuTPP(F) is associated with a ligand-to-metal-charge-transfer transition, unambiguously revealed in the CuTPPN K-edge spectral pattern. Moreover, the comparison with literature data pertaining to the modelling of the Cu(II)L2,3 features in the phthalocyanine–Cu(II) (CuPc) complex provided further insights into how metal-to-ligand-charge-transfer transitions associated with excitations from 2pCu(II) AOs to low-lying, ligand-based π* MOs may contribute to the Cu(II) L2,3-edge intensity and thus weaken its believed relationship with the Cu(II)–ligand symmetry-restricted covalency. Despite the coordinative pocket of CuTPP/CuTPP(F) mirroring CuPc, the ligand-field strength exerted by the phthalocyanine ligand on the Cu(II) centre is experimentally found and theoretically confirmed to be slightly stronger than that experienced by Cu in CuTPP and CuTPP(F). On the whole, the obtained results complement those published in the near past by the same group on the occupied and empty states of the H2TPP and H2TPP(F) free ligands as well as on the occupied states of both CuTPP and CuTPP(F), thus providing the final piece to get a thorough description of electronic perturbations associated with the metalation and the Ph halogen decoration of H2TPP.

The transmittance of thin films of Sc deposited by evaporation in ultrahigh vacuum conditions has been investigated in the $20–1000\mathrm{eV}$ spectral range. Transmittance measurements were performed in situ on Sc layers that were deposited over grids coated with a C support film. Transmittance measurements were used to obtain the extinction coefficient of Sc films at each individual photon energy investigated. These data, along with the data available in the literature for the rest of the spectrum, were used to obtain the refractive index of Sc by means of the Kramers–Krönig analysis. Sum-rule tests indicated an acceptable consistency of the data.