Mean Free Path of Electrons in Organic Photoresists for Extreme Ultraviolet Lithography in the Kinetic Energy Range 20-450 eV.

The blur caused by the nonzero mean free path of electrons in photoresists exposed by extreme ultraviolet lithography has detrimental consequences on patterning resolution, but its effect is difficult to quantify experimentally. So far, most mean free path calculations use the dielectric formalism, which is an approximation valid in the optical limit and fails at low kinetic energy. In this work, we used a modified substrate-overlayer technique that exploited the attenuation of the Si 2p core level originating specifically from the native silicon dioxide to evaluate the attenuation of electrons traveling through 2 and 4 nm of photoresist overlayers to provide a close estimation of the inelastic mean free path relevant for photoresist lithography patterning and for electron microscopy. The photoemission spectra were collected in the proximity of the Si 2p edge (binding energy ∼101 eV) using synchrotron light of energy ℏω ranging between 120 and 550 eV. The photoresist films were prototypical chemically amplified resists based on organic copolymer of poly hydroxystyrene and tertbutyl methacrylate with and without triphenyl sulfonium perfluoro-1-butanesufonate photoacid generator and trioctylamine quencher. The inelastic mean free path of electrons, in the range that is relevant for photoresist exposure in extreme ultraviolet lithography (20-92 eV), was found to be between 1 and 2 nm. At higher kinetic energy, the mean free path increased, consistently with the well-known behavior. The presence of the photoacid generator and quencher did not change the mean free path, within experimental error. Our results are discussed and compared with the existing literature on organic molecules measured via dielectric formalism and electron transmission experiments.

Multilayer beamsplitter polarizers for key UV-FUV spectral lines of solar polarimetry.

Multilayer beamsplitter polarizers have been developed for improved solar polarimetry at key spectral lines. The advantage of beamsplitter polarizers is that a single device separates s from p polarization; this helps minimize attenuation and enables a more compact and lighter polarimeter, which is important for space instruments. Polarizers based on Al/AlF3 multilayers were prepared for both C IV (155 nm) and Mg II (280 nm) lines, and based on Al/MgF2 multilayers for H Lyman α line (121.6 nm). Polarizers were designed to mainly reflect (transmit) s (p) polarization. Beamsplitter performance and throughput are shown to compare advantageously with polarizers in the literature. Beamsplitter polarizers kept a valuable performance after several years of ageing.

Fluorine-Rich Zinc Oxoclusters as Extreme Ultraviolet Photoresists: Chemical Reactions and Lithography Performance.

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 (D 50) 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.

Ultrafast Dynamics of Plasmon-Mediated Charge Transfer in Ag@CeO2 Studied by Free Electron Laser Time-Resolved X-ray Absorption Spectroscopy.

Expanding the activity of wide bandgap semiconductors from the UV into the visible range has become a central goal for their application in green solar photocatalysis. The hybrid plasmonic/semiconductor system, based on silver nanoparticles (Ag NPs) embedded in a film of CeO2, is an example of a functional material developed with this aim. In this work, we take advantage of the chemical sensitivity of free electron laser (FEL) time-resolved soft X-ray absorption spectroscopy (TRXAS) to investigate the electron transfer process from the Ag NPs to the CeO2 film generated by the NPs plasmonic resonance photoexcitation. Ultrafast changes (<200 fs) of the Ce N4,5 absorption edge allowed us to conclude that the excited Ag NPs transfer electrons to the Ce atoms of the CeO2 film through a highly efficient electron-based mechanism. These results demonstrate the potential of FEL-based TRXAS measurements for the characterization of energy transfer in novel hybrid plasmonic/semiconductor materials.

Kossel Effect in Periodic Multilayers.

The Kossel effect is the diffraction by a periodically structured medium, of the characteristic X-ray radiation emitted by the atoms of the medium. We show that multilayers designed for X-ray optics applications are convenient periodic systems to use in order to produce the Kossel effect, modulating the intensity emitted by the sample in a narrow angular range defined by the Bragg angle. We also show that excitation can be done by using photons (X-rays), electrons or protons (or charged particles), under near normal or grazing incident geometries, which makes the method relatively easy to implement. The main constraint comes from the angular resolution necessary for the detection of the emitted radiation. This leads to small solid angles of detection and long acquisition times to collect data with sufficient statistical significance. Provided this difficulty is overcome, the comparison or fit of the experimental Kossel curves, i.e., the angular distributions of the intensity of an emitted radiation of one of the element of the periodic stack, with the simulated curves enables getting information on the depth distribution of the elements throughout the multilayer. Thus the same kind of information obtained from the more widespread method of X-ray standing wave induced fluorescence used to characterize stacks of nanometer period, can be obtained using the Kossel effect.

Phase measurement of soft x-ray multilayer mirrors.

We propose a new model enabling the extraction of the phase of a multilayer mirror from photocurrent measurements in the soft x rays. In this range, the effects of the mean free path of the electrons inside the stack can no longer be neglected, which prevents the phase reconstruction by conventional photocurrent measurements. The new model takes into account this phenomenon and thus extends up to the x rays the applicability range of the technique. This approach has been validated through a numerical and experimental study of chromium/scandium multilayers used near 360 eV. To our knowledge, this work constitutes the first measurement of the phase of a multilayer mirror in the soft x-ray range.

Transmittance and optical constants of Ca films in the 4-1000 eV spectral range.

The low expected absorption of Ca in the extreme ultraviolet (EUV) makes it an attractive material for multilayers and filters because most materials in nature strongly absorb the EUV. Few optical constant data had been reported for Ca. In this research, Ca films of various thicknesses were deposited on grid-supported C films and their transmittance measured in situ from the visible to the soft x-rays. The measurement range contains M2,3 and L2,3 absorption edges. Transmittance measurements were used to obtain the Ca extinction coefficient k. A minimum k of 0.017 was obtained at ∼23 eV, which makes Ca a promising low-absorption material for EUV coatings. A second spectral range of interest for its low absorption is below the Ca L3 edge at ∼343 eV. Measured k data and extrapolations were used to calculate the refractive index n using Kramers-Krönig relations. This is the first self-consistent data set on Ca covering a wide spectral range including the EUV.

Pushing the high-energy limit of plasmonics.

The localized surface plasmon resonance of metal nanoparticles allows confining the eletromagnetic field in nanosized volumes, creating high-field "hot spots", most useful for enhanced nonlinear optical spectroscopies. The commonly employed metals, Au and Ag, yield plasmon resonances only spanning the visible/near-infrared range. Stretching upward, the useful energy range of plasmonics requires exploiting different materials. Deep-ultraviolet plasmon resonances happen to be achievable with one of the cheapest and most abundant materials available: aluminum indeed holds the promise of a broadly tunable plasmonic response, theoretically extending far into the deep-ultraviolet. Complex nanofabrication and the unavoidable Al oxidation have so far prevented the achievement of this ultimate high-energy response. A nanofabrication technique producing purely metallic Al nanoparticles has at last allowed to overcome these limits, pushing the plasmon resonance to 6.8 eV photon energy (≈180 nm) and thus significantly broadening the spectral range of plasmonics' numerous applications.

Deep ultraviolet plasmon resonance in aluminum nanoparticle arrays.

Small aluminum nanoparticles have the potential to exhibit localized surface plasmon resonances in the deep ultraviolet region of the electromagnetic spectrum, however technical and scientific challenges make it difficult to attain this limit. We report the fabrication of arrays of Al/Al2O3 core/shell nanoparticles with a metallic-core diameter between 12 and 25 nm that display sharp plasmonic resonances at very high energies, up to 5.8 eV (down to λ = 215 nm). The arrays were fabricated by means of a straightforward self-organization approach. The experimental spectra were compared with theoretical calculations that allow the correlation of each feature to the corresponding plasmon modes.

Solidlike and liquidlike behavior in monolayers and multilayers of metal-bearing amphiphiles.

Atomic force microscopy (AFM) of cadmium stearate (CdSt) and cobalt stearate (CoSt) Langmuir-Blodgett films show differences in their in-plane morphologies. CdSt films, with a huge number of in-plane "pinhole" defects, follow self-affine behavior, whereas CoSt films, which are almost void of such in-plane defects, show deviation from self-affinity especially at small length scales, suggesting liquidlike behavior, imparting flexibility to the system, in plane. Phase images of CoSt obtained from tapping mode AFM show gentle undulations or hemispherelike features in contrast to its smooth topography, unlike the CdSt system where both height and phase images show self-affine domains. Near edge x-ray absorption fine structure spectroscopy indicates no preferred in-plane orientation of the head group in CoSt films. The undulating features in CoSt is explained by invoking a radially symmetric orientational distribution in the tilt of adjacent hydrocarbon tails, causing a small in-plane density variation which shows up in the phase image. These orientational disorders in adjacent tails probably allow "filling up" of in-plane defects thereby giving rise to its excellent in-plane coverage and hence a "liquidlike" behavior in CoSt. Brewster angle microscopy shows that parent Langmuir monolayers of stearic acid in the presence of Cd and Co ions in the aqueous subphase behave as two-dimensional "solids" and "liquids," respectively, suggesting the phenomena to be inherent in the amphiphiles and probably independent of their organization as monolayers and multilayers.

Transmittance and optical constants of erbium films in the 3.25-1580 eV spectral range.

The optical constants of erbium (Er) films were obtained in the 3.25-1580 eV range from transmittance measurements performed at room temperature. Thin films of Er were deposited by evaporation in ultra high vacuum conditions and their transmittance was measured in situ. Substrates consisted of a thin C film supported on a grid. Transmittance measurements were used to obtain the extinction coefficient k of the Er films. The refractive index n of Er was calculated using the Kramers-Krönig analysis. k data were extrapolated both on the high- and low-energy parts of the spectrum by using experimental data and calculated k values available in the literature. Er, similar to other lanthanides, has a low-absorption band below the O(2,3) edge onset; the smallest absorption was measured at ~22.5 eV. Therefore, Er is a promising material for filters and multilayer coatings in the energy range below the O(2,3) edge, in which materials typically have an absorption stronger than at other energies. Good consistency of the data resulted from the application of f and inertial sum rules.

UPS, XPS, and NEXAFS study of self-assembly of standing 1,4-benzenedimethanethiol SAMs on gold.

We report a study of the self-assembly of 1,4-benzenedimethanethiol monolayers on gold formed in n-hexane solution held at 60 °C for 30 min and in dark conditions. The valence band characteristics, the thickness of the layer, and the orientation of the molecules were analyzed at a synchrotron using high resolution photoelectron spectroscopy and near edge X-ray adsorption spectroscopy. These measurements unambiguously attest the formation of a single layer with molecules arranged in the upright position and presenting a free -SH group at the outer interface. Near edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis is oriented at 24° with respect to the surface normal. In addition, valence band features could be successfully associated to specific molecular orbital contributions thanks to the comparison with theoretically calculated density of states projected on the different molecular units.

Chemistry at air/water interface versus reaction in a flask: tuning molecular conformation in thin films.

Atomic force microscopy and X-ray reflectivity studies of cobalt stearate Langmuir-Blodgett (LB) films (CoStp) deposited from a preformed bulk sample on quartz substrates showed formation of a Volmer-Weber type monolayer but no multilayers as compared to the excellent multilayers of cobalt stearate films (CoStn) deposited at the air/water interface by the usual LB technique, in spite of both showing bidentate bridging type coordination of cobalt ions with the carboxylate group. The difference is attributed to existence of different headgroup conformers, observed from Fourier transform infrared (FTIR) studies. The CoStp films had a higher energy 'boat' conformation with linear O-Co-O linkage, whereas CoStn formed a low energy conformer with a bent O-Co-O configuration (bond angle of 105 degrees). Present results support the necessity of bidentate bridging coordination in multilayer deposition, but rejects its sufficiency by bringing out the crucial role played by air/water interface. Differences in surface pressure-molecular area isotherms and hydrocarbon tail-tail interactions (evident from FTIR spectra) of the films support the above statement. Methyl-methyl interactions observed in CoStn samples suggest hierarchy of supramolecular chemistry at the air/water interface in tuning the C-O-Co bond angle essential to satisfy the wetting condition with the substrate and subsequently form LB multilayers.

Optical constants of Yb films in the 23-1700 eV range.

The optical constants of Yb films have been determined in the 23-1700 eV spectral range from transmittance measurements performed in situ on Yb films deposited by evaporation in ultrahigh vacuum conditions. Yb films were deposited over grids coated with a thin carbon film. Transmittance measurements were used to obtain the extinction coefficient of Yb films at each individual photon energy investigated. The energy range investigated encompasses Yb edges from M(4,5) to O(2,3). The current results, along with data in the literature, show that Yb has an interesting low-absorption band in the approximately 12-24 eV range, which may be useful for the development of transmittance filters and multilayer coatings. The current data along with literature data and extrapolations were used to obtain n, the real part of the complex refractive index, using a Kramers-Krönig analysis. The application of the sum rules showed a good consistency of the results.

Determination of optical constants of scandium films in the 20-1000 eV range.

The transmittance of thin films of Sc deposited by evaporation in ultrahigh vacuum conditions has been investigated in the 20-1000 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.

3-methylthiophene self-assembled monolayers on planar and nanoparticle Au surfaces.

In this article the adsorption of 3-methylthiophene on planar and nanoparticle Au surfaces is investigated. The resulting systems are compared with a benchmark system based on 1-decanethiol. The characterization data collected evidence the formation of a packed 3-methylthiophene SAM on the planar surface. In particular, spectroscopic investigations suggest that 3-methylthiophene aromatic system is not adsorbed on the surface through the pi-electron system but rather through the S atom alone. On the other hand, the behavior of 3-methylthiophene on nanoparticle surfaces is notably different from that of the alkanethiol. Only a limited fraction of the surface of Au nanoparticles results to be actually coated after purification; this notwithstanding, the nanoparticle growth seems to be strongly influenced by the presence of such a labile encapsulating agent.