Flux and fluence effects on the vacuum-UV photodesorption and photoprocessing of CO2 ices.

CO2 is a major component of the icy mantles surrounding dust grains in planet and star formation regions. Understanding its photodesorption is crucial for explaining gas phase abundances in the coldest environments of the interstellar medium irradiated by vacuum-UV (VUV) photons. Photodesorption yields determined experimentally from CO2 samples grown at low temperatures (T = 15 K) have been found to be very sensitive to experimental methods and conditions. Several mechanisms have been suggested for explaining the desorption of CO2, O2 and CO from CO2 ices. In the present study, the cross-sections characterizing the dynamics of photodesorption as a function of photon fluence (determined from released molecules in the gas phase) and of ice composition modification (determined in situ in the solid phase) are compared for the first time for different photon flux conditions (from 7.3 × 1012 photon per s cm-2 to 2.2× 1014 photon per s cm-2) using monochromatic synchrotron radiation in the VUV range (on the DESIRS beamline at SOLEIL). This approach reveals that CO and O2 desorptions are decorrelated from that of CO2. CO and O2 photodesorption yields depend on photon flux conditions and can be linked to surface chemistry. In contrast, the photodesorption yield of CO2 is independent of the photon flux conditions and can be linked to bulk ice chemical modification, consistently with indirect desorption induced by an electronic transition (DIET) process.

Low-energy electron interaction with 2-(trifluoromethyl)acrylic acid, a potential component for EUVL resist material.

Motivated by the current introduction of extreme ultraviolet lithography (EUVL) into chip manufacturing processes, and thus the transition to electron-induced chemistry within the respective resist materials, we have studied low energy electron-induced fragmentation of 2-(trifluoromethyl)acrylic acid (TFMAA). This compound is chosen as a potential resist component, whereby fluorination enhances the EUV adsorption and may at the same time promote electron-induced dissociation. Dissociative ionization and dissociative electron attachment are studied, and to aid the interpretation of the observed fragmentation channels, the respective threshold values are calculated at the DFT and coupled cluster level of theory. Not surprisingly, we find significantly more extensive fragmentation in DI than in DEA and in fact, the only significant DEA fragmentation channel is the cleavage of HF from the parent molecule upon electron attachment. Rearrangement and new bond formation are substantial in DI and are, in fact, similar to DEA, mainly associated with HF formation. The observed fragmentation reactions are discussed in relation to the underlying reactions and potential implications for the suitability of TFMAA as a component of EUVL resist materials.

Cryogenic Chemistry and Quantitative Non-Thermal Desorption from Pure Methanol Ices: High-Energy Electron versus X-Ray Induced Processes.

X-Ray irradiation of interstellar ice analogues has recently been proven to induce desorption of molecules, thus being a potential source for the still-unexplained presence of gaseous organics in the coldest regions of the interstellar medium, especially in protoplanetary disks. The proposed desorption mechanism involves the Auger decay of excited molecules following soft X-ray absorption, known as X-ray induced electron-stimulated desorption (XESD). Aiming to quantify electron induced desorption in XESD, we irradiated pure methanol (CH3 OH) ices at 23 K with 505 eV electrons, to simulate the Auger electrons originating from the O 1s core absorption. Desorption yields of neutral fragments and the effective methanol depletion cross-section were quantitatively determined by mass spectrometry. We derived desorption yields in molecules per incident electron for CO, CO2 , CH3 OH, CH4 /O, H2 O, H2 CO, C2 H6 and other less abundant but more complex organic products. We obtained desorption yields remarkably similar to XESD values.

Electron-induced fragmentation mechanisms in organic monomers and their implications for photoresist optimization for EUV lithography.

Secondary electrons generated during the Extreme Ultraviolet Lithography (EUVL) process are predominantly responsible for inducing important patterning chemistry in photoresist films. Therefore, it is crucial to understand the electron-induced fragmentation mechanisms involved in EUV-resist systems to improve their patterning performance. To facilitate this understanding, mechanistic studies were carried out on simple organic EUV-resist monomers, methyl isobutyrate (MIB) and methacrylic acid (MAA), both in the condensed and gas phases. Electron-stimulated desorption (ESD) studies on MIB in the condensed phase showed desorption peaks at around 2 and 9 eV electron energies. The gas-phase study on MIB showed that the monomer followed the dissociative ionization (DI) fragmentation pathway, under single collision conditions, which opened up at electron energies above about 11 eV. No signs of dissociative electron attachment (DEA) were detected for MIB in the gas phase under single collision conditions. However, DEA was an active process in MAA in the gas phase under single collision conditions at around 2 eV, showing that slight modifications of the molecular structures of photoresists may serve to sensitize them to certain electron-induced processes.

Design for a high resolution electron energy loss microscope.

An electron optical column has been designed for High Resolution Electron Energy Loss Microscopy (HREELM). The column is composed of electron lenses and a beam separator that are placed between an electron source based on a laser excited cesium atom beam and a time-of-flight (ToF) spectrometer or a hemispherical analyzer (HSA). The instrument will be able to perform full field low energy electron imaging of surfaces with sub-micron spatial resolution and meV energy resolution necessary for the analysis of local vibrational spectra. Thus, non-contact, real space mapping of microscopic variations in vibrational levels will be made possible. A second imaging mode will allow for the mapping of the phonon dispersion relations from microscopic regions defined by an appropriate field aperture.

Strain relaxation and epitaxial relationship of perylene overlayer on Ag(110).

We present a room temperature STM study of perylene self-assembly on Ag(110) beyond the monolayer coverage regime. Coupling of the perylene aromatic boards yields π-π bonded stacks. The perylene stacks self-assemble into a continuous three-dimensional epitaxial overlayer of (3 × 5) symmetry. The self-assembly is driven by thermodynamic balance established under coupling of the intra- and intermolecular interactions and the molecule-substrate interaction all accommodating the short-range thermal motion of the constituent molecules. The balance bestows to the overlayer the unique ability to accommodate the underlying substrate morphology and to spread over the surface steps as a single structure preserving its lateral order and keeping epitaxial relationship with every surface terrace. The complete epitaxy is driven by (i) anchoring of half of the perylene stacks into specific adsorption sites on each terrace, (ii) interlacing of the perylene stacks across the steps within the entire H-bonded network, and (iii) relaxation of the overlayer strain via enhancement of the overlayer-specific vibrational modes and short-range thermal motion of the constituent molecules. This complete epitaxy phenomenon is described via (i) structural and statistical analysis of the molecularly resolved STM topographies, (ii) monitoring of the short-range molecular displacements under the strain relaxation, (iii) highlighting of specific intra-molecular and inter-molecular vibration modes through detailed analysis of HREELS spectra, and (iv) parametrization of the intermolecular interaction via pair potential calculation.

Response under low-energy electron irradiation of a thin film of a potential copper precursor for focused electron beam induced deposition (FEBID).

Background: Focused electron beam induced deposition (FEBID) allows for the deposition of free standing material within nanometre sizes. The improvement of the technique needs a combination of new precursors and optimized irradiation strategies to achieve a controlled fragmentation of the precursor for leaving deposited material of desired composition. Here a new class of copper precursors is studied following an approach that probes some surface processes involved in the fragmentation of precursors. We use complexes of copper(II) with amines and perfluorinated carboxylate ligands that are solid and stable under ambient conditions. They are directly deposited on the surface for studying the fragmentation with surface science tools. Results: Infrared spectroscopy and high-resolution electron energy loss spectroscopy (HREELS) are combined to show that the precursor is able to spontaneously lose amine ligands under vacuum. This loss can be enhanced by mild heating. The combination of mass spectrometry and low-energy electron irradiation (0-15 eV) shows that full amine ligands can be released upon irradiation, and that fragmentation of the perfluorinated ligands is induced by electrons of energy as low as 1.5 eV. Finally, the cross section for this process is estimated from the temporal evolution in the experiments on electron-stimulated desorption (ESD). Conclusion: The release of full ligands under high vacuum and by electron irradiation, and the cross section measured here for ligands fragmentation allow one to envisage the use of the two precursors for FEBID studies.

Electron Processing at 50 eV of Terphenylthiol Self-Assembled Monolayers: Contributions of Primary and Secondary Electrons.

Aromatic self-assembled monolayers (SAMs) can serve as platforms for development of supramolecular assemblies driven by surface templates. For many applications, electron processing is used to locally reinforce the layer. To achieve better control of the irradiation step, chemical transformations induced by electron impact at 50 eV of terphenylthiol SAMs are studied, with these SAMs serving as model aromatic SAMs. High-resolution electron energy loss spectroscopy (HREELS) and electron-stimulated desorption (ESD) of neutral fragment measurements are combined to investigate electron-induced chemical transformation of the layer. The decrease of the CH stretching HREELS signature is mainly attributed to dehydrogenation, without a noticeable hybridization change of the hydrogenated carbon centers. Its evolution as a function of the irradiation dose gives an estimate of the effective hydrogen content loss cross-section, σ = 2.7-4.7 × 10(-17) cm(2). Electron impact ionization is the major primary mechanism involved, with the impact electronic excitation contributing only marginally. Therefore, special attention is given to the contribution of the low-energy secondary electrons to the induced chemistry. The effective cross-section related to dissociative secondary electron attachment at 6 eV is estimated to be 1 order of magnitude smaller. The 1 eV electrons do not induce significant chemical modification for a 2.5 mC cm(-2) dose, excluding their contribution.

Physisorption and desorption of H2, HD and D2 on amorphous solid water ice. Effect on mixing isotopologue on statistical population of adsorption sites.

We study the adsorption and desorption of three isotopologues of molecular hydrogen mixed on 10 ML of porous amorphous water ice (ASW) deposited at 10 K. Thermally programmed desorption (TPD) of H2, D2 and HD adsorbed at 10 K have been performed with different mixings. Various coverages of H2, HD and D2 have been explored and a model taking into account all species adsorbed on the surface is presented in detail. The model we propose allows to extract the parameters required to fully reproduce the desorption of H2, HD and D2 for various coverages and mixtures in the sub-monolayer regime. The model is based on a statistical description of the process in a grand-canonical ensemble where adsorbed molecules are described following a Fermi-Dirac distribution.

D(2) desorption kinetics on amorphous solid water: from compact to porous ice films.

The desorption kinetics of D(2) from amorphous solid water (ASW) films have been studied by the temperature-programmed desorption (TPD) technique in the 10-30 K temperature range. Compact (and nonporous) films were grown at 120 K over a copper substrate. Ultra-thin porous films were additionally grown at 10 K over the compact base. The TPD spectra from compact and from up to 20 monolayers (ML) porous films were compared. The simulation of the TPD experimental traces provides the corresponding D(2) binding-energy distributions. As compared to the compact case, the binding-energy distribution found for the 10 ML porous film clearly extends to higher energies. To study the transition from compact to porous ice, porous films of intermediate thicknesses (<10 ML), including ultra-thin films (<1 ML), were grown over the compact substrate. The thermal D(2) desorption peak was found to shift to higher temperatures as the porous ice network was progressively formed. This behavior can be explained by the formation of more energetic binding sites related to porous films. TPD spectra were also modelled by using a combination of the two energy distributions, one associated to a bare compact ice and the other associated to a 10 ML porous ice film. This analysis reveals a very fast evolution of the binding-energy distribution towards that of porous ice. Our results show that few ML of additional porous film are sufficient to produce a sample for which the D(2) adsorption can be described by the energy distribution found for the 10 ML porous film. These experiments then provide evidence that the binding energy of D(2) on ASW ice is primarily governed by the topological and morphological disorder of the surface at molecular scale.