N. S. Faradzhev

Kinetics of electron-induced decomposition of CF2Cl2 coadsorbed with water (ice): A comparison with CCl4

The kinetics of decomposition and subsequent chemistry of adsorbed CF(2)Cl(2), activated by low-energy electron irradiation, have been examined and compared with CCl(4). These molecules have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal surfaces (Ru; Au) at low temperatures (25 K; 100 K). The studies have been performed with temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and x-ray photoelectron spectroscopy (XPS). TPD data reveal the efficient decomposition of both halocarbon molecules under electron bombardment, which proceeds via dissociative electron attachment (DEA) of low-energy secondary electrons. The rates of CF(2)Cl(2) and CCl(4) dissociation increase in an H(2)O (D(2)O) environment (2-3x), but the increase is smaller than that reported in recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with H(2)O, using 180 eV incident electrons, are measured (using TPD) to be 1.0+/-0.2 x 10(-15) cm(2) for CF(2)Cl(2) and 2.5+/-0.2 x 10(-15) cm(2) for CCl(4). RAIRS and XPS studies confirm the decomposition of halocarbon molecules codeposited with water molecules, and provide insights into the irradiation products. Electron-induced generation of Cl(-) and F(-) anions in the halocarbon/water films and production of H(3)O(+), CO(2), and intermediate compounds COF(2) (for CF(2)Cl(2)) and COCl(2), C(2)Cl(4) (for CCl(4)) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and the decomposition kinetics are similar to those observed in our recent experiments involving x-ray photons as the source of ionizing irradiation.

Electron-driven chemistry of halogenated compounds in condensed phases: Effects of solvent matrices on reaction dynamics and kinetics

The focus of this review is the effect of H2O on the electron-driven chemistry of condensed halogenated compounds. We present data with emphasis on results from the authors’ laboratories for halomethanes (CF2Cl2, CCl4, CH3I, CDCl3, CD2Cl2) and SF6. The halogenated species are suspended in or adsorbed on the surface of ultrathin films of amorphous solid water (ice) condensed on metal surfaces. Bombardment of the film by X-rays or energetic electrons leads to the release of low-energy secondary electrons; these are responsible for much of the rich electron-driven chemistry, which includes molecular decomposition, desorption of charged and neutral fragments, radical formation, anion solvation, and condensed-phase reactions. Potential implications of this work range from environmental remediation of toxic compounds to atmospheric ozone depletion.

Electron stimulated reactions of methyl iodide coadsorbed with amorphous solid water.

The electron stimulated reactions of methyl iodide (MeI) adsorbed on and suspended within amorphous solid water (ice) were studied using a combination of postirradiation temperature programmed desorption and reflection absorption infrared spectroscopy. For MeI adsorbed on top of amorphous solid water (ice), electron beam irradiation is responsible for both structural and chemical transformations within the overlayer. Electron stimulated reactions of MeI result principally in the formation of methyl radicals and solvated iodide anions. The cross section for electron stimulated decomposition of MeI is comparable to the gas phase value and is only weakly dependent upon the local environment. For both adsorbed MeI and suspended MeI, reactions of methyl radicals within MeI clusters lead to the formation of ethane, ethyl iodide, and diiodomethane. In contrast, reactions between the products of methyl iodide and water dissociation are responsible for the formation of methanol and carbon dioxide. Methane, formed as a result of reactions between methyl radicals and either parent MeI molecules or hydrogen atoms, is also observed. The product distribution is found to depend on the films initial chemical composition as well as the electron fluence. Results from this study highlight the similarities in the carbon-containing products formed when monohalomethanes coadsorbed with amorphous solid water are irradiated by either electrons or photons.

Resonance effects in photoemission from TiO2-capped Mo/Si multilayer mirrors for extreme ultraviolet applications

In the unbaked vacuum systems of extreme ultraviolet (EUV) lithography steppers, oxide formation and carbon growth on Mo/Si multilayer mirrors (MLMs) are competing processes leading to reflectivity loss. A major contribution to this mirror degradation is a series of surface reactions that are thought to be driven in large part by photoemitted electrons. In this paper, we focus on the resonance effects in photoemission from Mo/Si MLMs protected by thin TiO2 cap layers. In the vicinity of the resonant energy of the mirror, the energy flux of the EUV radiation forming standing wave oscillates throughout the multilayer stack. As a result, light absorption followed by the emission of photoelectrons becomes a complex process that varies rapidly with depth and photon energy. The electron emission is characterized as a function of the EUV photon energy, the angle of incidence, and the position of the standing wave with respect to the solid/vacuum interface. In our experiments, the position of the standing wave wa...

Measuring the EUV-induced contamination rates of TiO2-capped multilayer optics by anticipated production-environment hydrocarbons

The primary, publicly reported cause of optic degradation in pre-production extreme-ultraviolet (EUV) lithography systems is carbon deposition. This results when volatile organics adsorb onto optic surfaces and then are cracked by EUV-induced reactions. Hence the deposition rate depends on the adsorption-desorption kinetics of the molecule-surface system as well as the basic photon-stimulated reaction rates, both of which may vary significantly for different organic species. The goal of our ongoing optics-contamination program is to estimate the contamination rate of species expected in the tool environment by exposing samples to in-band 13.5 nm light from our synchrotron in the presence of fixed partial pressures of admitted gases. Here we report preliminary results of contamination rates on TiO2-capped samples for species observed in resist-outgassing measurements (benzene, isobutene, toluene and tert-butylbenzene) in the pressure range (10-6 to 10-4) Pa which all display an unexpected logarithmic dependence on pressure. This scaling is in agreement with previous EUV exposures of other species at NIST as well as independent measurements of coverage performed at Rutgers University. These results are consistent with a molecular desorption energy that decreases with coverage due to molecular interactions (Temkin model). Use of the proper scaling law is critical when estimating optic lifetimes by extrapolating over the 3-to-6 orders of magnitude between accelerated-testing and tool-environment partial pressures.

EUV Lithography optics contamination and lifetime studies

In this paper we discuss surface phenomena leading to contamination of multilayer optics designed for Extreme Ultraviolet (EUV) lithography. Experimental data supported by calculations indicate dramatic influence of resonance structure of EUV mirror on the secondary electron yield. These low energy electrons play an important role in radiation chemistry at the mirror surface. We also discuss the dependence on the partial pressure of admitted gas of equilibrium surface coverage and contamination rate under EUV irradiation.

A simple null-field ellipsometric imaging system (NEIS) for in-situ monitoring of EUV-induced deposition on EUV optics

We describe a null-field ellipsometric imaging system (NEIS) that provides for the real-time imaging of carbon deposition profiles on extreme-ultraviolet (EUV) optics in a vacuum system. NEIS has been demonstrated at NIST on a small chamber that is used for EUV optics lifetime testing. The system provides images of carbon deposition spots with sub-nanometer resolution thickness measurements that maintain good agreement with those from ex-situ spectral ellipsometry (SE) and x-ray photoelectron spectroscopy (XPS). The system will be implemented on several synchrotron beamlines for real-time monitoring of carbon film growth on optics during EUV irradiation.

Complex species and pressure dependence of intensity scaling laws for contamination rates of EUV optics determined by XPS and ellipsometry

The goal of our ongoing optics-contamination program is to estimate the magnitude and scaling laws of the contamination rates of optics exposed to extreme-ultraviolet (EUV) radiation in the presence of various contaminant species expected in the EUV-lithography-tool environment by exposing samples to in-band 13.5 nm light from our synchrotron in the presence of fixed partial pressures of admitted gases. We report contamination rate measurements on TiO2-capped samples for species observed in separate resist-outgassing measurements (benzene, isobutene, toluene and tert-butylbenzene) in the pressure range (10-8 to 10-5) Pa. We use two spatially-resolved surface probe techniques, spectroscopic ellipsometry and X-ray photoelectron spectroscopy, to determine the thickness of deposited carbon. The correlation and sensitivities of these techniques are discussed. The high sensitivity of ellipsometry shows that contamination rates for some species have a pronounced non-linear intensity dependence and can be strongly influenced by admixtures of water vapor, while the rates for other species are linear over the same intensity range and are less affected by ambient water. Understanding scaling laws is critical when estimating optic lifetimes or cleaning cycles by extrapolating over the 3-to-6 orders of magnitude between accelerated-testing and tool-environment partial pressures.

Effects of electron irradiation on structure and bonding of SF6 on Ru(0001)

Electron-stimulated desorption ion angular distribution (ESDIAD) and temperature-programmed desorption (TPD) techniques have been employed to study radiation-induced decomposition of fractional monolayer SF6 films physisorbed on Ru(0001) at 25 K. Our focus is on the origin of F+ and F− ions, which dominate ESD from fractional monolayers. F− ions escape only in off-normal directions and originate from undissociated molecules. The origins of F+ ions are more complicated. The F+ ions from electron-stimulated desorption of molecularly adsorbed SF6 desorb in off-normal directions, in symmetric ESDIAD patterns. Electron beam exposure leads to formation of SFx (x=0–5) fragments, which become the source of positive ions in normal and off-normal directions. Electron exposure >1016 cm−2 results in decomposition of the entire adsorbed SF6 layer.