Boris V. Yakshinskiy

Desorption of alkali atoms and ions from oxide surfaces: Relevance to origins of Na and K in atmospheres of Mercury and the Moon

This paper begins with a brief survey of the literature dealing with the adsorption and desorption of alkalis on oxide surfaces. Emphasis is on desorption phenomena: thermal desorption, electron- and photon-stimulated desorption, and ion-induced desorption (sputtering). Then the relevance of these data to the desorption of alkalis from mineral surfaces and to the origins of alkali vapors in tenuous planetary atmospheres is discussed. The data presented for Na and K indicate that desorption processes initiated by thermal or electronic excitations do not depend strongly on whether the Na returns to the surface or diffuses up through the regolith, and that neutral yields dominate ion yields in all cases. Although the desorbed neutral energy distributions are not well approximated by Maxwell-Boltzmann distributions, the mean energies of the desorbed neutral Na and K are seen to be consistent with the temperatures extracted for the “hot” component of the lunar atmosphere. This suggests that the “hot” component may be produced by electronically stimulated desorption (e.g., electron-stimulated desorption and/or photon-stimulated desorption). If this is the case, a possible “size effect” may be operative, in which desorbed neutral K atoms are somewhat more energetic than desorbed Na. In such desorption processes a low-energy component may be generated by scattering of desorbing atoms in the porous regolith; thermal desorption can also generate low-energy atoms. The data further indicate that thermal desorption should be rapid in the equatorial regions of Mercury, possibly depleting this region of alkalis, whereas thermal desorption should be less efficient on the Moon. Surface charging may be important at the surface of the Moon, by accelerating the solar electrons to energies above the threshold for initiating alkali desorption. Suggestions are made for future laboratory work.

THERMAL DESORPTION OF SODIUM ATOMS FROM THIN SiO2 Films

The adsorption and thermal desorption of Na from thin SiO2 films have been studied. X-ray photoelectron spectroscopy (XPS), angle-resolved XPS (ARXPS), low energy ion scattering (LEIS), temperature-programmed desorption (TPD), low energy electron diffraction (LEED) and work function measurements have been used to characterize the growth mechanism and properties of stoichiometric SiO2 films deposited onto a Re (0001) substrate. Upon deposition of Na onto SiO2 at 250 K, the first monolayer of Na exhibits ionic character, and evidence of metallic Na (plasmon features in XPS) is observed for higher coverages. TPD spectra for Na from SiO2 include a monolayer peak at ~700 K, and the multilayer peak due to sublimation of bulk Na at ~330 K. Penetration of Na into SiO2 can be induced by heating, or by He ion bombardment of a Na/SiO2 layer. The sticking probability for Na on SiO2 is ~0.5 at 250 K, and it decreases at higher substrate temperatures.

Carbon accumulation and mitigation processes, and secondary electron yields of ruthenium surfaces

Metallic ruthenium capping layers ~2 nm thick protect and extend the lifetimes of Mo/Si multilayer mirrors used in extreme ultraviolet lithography (EUVL) applications. However, Ru-capped mirrors experience a loss of reflectivity after prolonged exposure to EUV radiation. In the present work, we use ultrahigh vacuum surface science methods to address several aspects of Ru surface chemistry that may impact on Ru capping layer stability and mitigation processes. (1) We characterize the composition and stability of Ru surfaces that simulate surfaces of Ru-capped multilayer mirrors, under exposure to different background gases (water, methyl methacrylate (MMA)) and to electron irradiation. Evidence for some mitigation of carbon accumulation during electron bombardment in MMA + water vapor is found. (2) We report the photon-energy dependence of secondary electron yield (SEY) measurements for clean Ru, O-dosed and C-dosed Ru, and Ru-capped multilayer mirrors using synchrotron radiation near 13.5 nm at Brookhaven National Synchrotron Light Source (NSLS). Much of the radiation-induced chemistry on the surfaces of capping layers is induced by low-energy secondary electrons rather than direct photoexcitation, so the SEY is an important parameter affecting mirror lifetimes in EUVL.

Electron‐ and photon‐stimulated desorption of K from ice surfaces

Motivated by controversy concerning the origin of alkali metal vapor in the atmosphere of the Jovian satellite Europa, we present laboratory studies of the electron-stimulated desorption (ESD) and photon-stimulated desorption (PSD) of atomic K deposited on the surface of thin water ice films. These ice films of both crystalline and amorphous modifications simulate the surface of icy satellites. A pulsed low-energy electron gun and a mercury arc lamp serve as sources of electron and UV irradiation. X-ray photoelectron spectroscopy (XPS) is used for surface chemical state control. Both ESD and PSD of potassium atoms from ice exhibit appearance thresholds at ∼4 eV, which is more pronounced for the case of crystalline ice than for amorphous ice. The velocity distribution of desorbed K atoms, measured at 100 K, is peaked at ∼500 m/s (by comparison, the peak for Na is at ∼800 m/s), with the high-energy portion extended up to ∼3000 m/s. The mechanism of desorption is identified as an electronically excited charge transfer from ice to alkali ion, followed by desorption. We conclude that along with magnetospheric energetic ion sputtering processes, UV solar photons and electron fluxes with E > 4 eV may cause alkali metal desorption from Europas surface.

Accelerated lifetime metrology of EUV multilayer mirrors in hydrocarbon environments

The ability to predict the rate of reflectivity loss of capped multilayer mirrors (MLMs) under various conditions of ambient vacuum composition, intensity, and previous dose is crucial to solving the mirror lifetime problem in an EUV stepper. Previous measurements at NIST have shown that reflectivity loss of MLMs exposed under accelerated conditions of dose and pressure can be a very complicated function of these variables. The present work continues this effort and demonstrates that reflectivity loss does not scale linearly for accelerated exposure doses over the range of 0-350 J/mm2 either for partial pressures of MMA in the range 10-8-10-7 Torr or acetone in the range 10-7-10-6 Torr. We suggest that this nonlinear scaling may be the result of a varying damage rate as the surface of the growing contamination layer moves through the EUV standing wave created by exposure of any MLM to resonant radiation. To further investigate the potential influence of these resonance effects, we report new measurements showing large variations of the secondary electron yield as a function of thickness of carbon deposited on top of a MLM.

Photons, electrons, and acid yields in EUV photoresists: a progress report

This paper describes our initial investigation into building a greater understanding of the complex mechanism occurring during extreme ultraviolet (EUV) exposure of resist materials. In particular, we are focusing on the number and energy of photoelectrons generated and available for reaction with photoacid generators (PAGs). We propose that this approach will best enable the industry to develop resists capable of meeting resolution, line width roughness (LWR), and sensitivity requirements.

DIET of alkali atoms from mineral surfaces

Abstract To investigate mechanisms for the origin of alkalis in the atmosphere of the Moon, we are studying the electron- and photon-stimulated desorption (ESD and PSD) of K atoms from model mineral surfaces (SiO 2 films), and ESD and PSD of Na atoms from a lunar basalt sample. X-ray photoelectron spectroscopy demonstrates the existence of traces of Na in the lunar sample. To obtain an increased signal for detailed measurements of desorption parameters (appearance thresholds, yields), a fractional monolayer of Na is deposited onto the lunar sample surface. An alkali atom detector based on surface ionization and a time-of-flight technique are used for DIET measurements, together with a pulsed electron gun, and a mechanically chopped and filtered mercury arc light source. We find that bombardment of the alkali covered surfaces by UV photons or by electrons with energies E >4 eV causes desorption of “hot” alkali atoms. The results are consistent with the model based on charge transfer from the substrate to adsorbate which was developed to explain our previous measurements of sodium desorption from a silica surface and desorption of K atoms from water ice. The data support the suggestion that PSD by UV solar photons is a dominant source process for alkalis in the tenuous lunar atmosphere.

Interaction of benzene with TiO2 surfaces: Relevance to contamination of extreme ultraviolet lithography mirror capping layers

The authors focus on thermal and nonthermal (radiation-induced) surface processes that affect the reflectivity of TiO2-capped multilayer mirrors used in extreme ultraviolet (EUV) lithography. Low energy electron beams mimic excitations initiated by EUV radiation. Where appropriate, comparison is made with electron bombardment in the vapor of methyl methacrylate (C5H8O2). Benzene adsorbs and desorbs reversibly on TiO2, and the steady state coverage Θ is found to be proportional to the logarithm of the benzene pressure p. This behavior is described by the Tempkin adsorption isotherm, which has the form Θ=const+logp. This isotherm is a consequence of a linear dependence of benzene adsorption energy on Θ. In addition, measurements of cross sections σ (cm2) for electron-stimulated dissociation of benzene on clean and C-covered TiO2 in the range of 10–100eV reveal surprisingly large values (e.g., ∼3.5×10−17cm2 at 10eV primary energy). Thus, low energy secondary electrons excited by EUV lithography photons are e...

Adsorption and electron-induced polymerization of methyl methacrylate on Ru(1010).

The adsorption and electron irradiation of methyl methacrylate (MMA) on a Ru(1010) surface have been studied using x-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and low energy ion scattering. TPD analysis indicates that a monolayer of MMA chemisorbs and dissociates on the Ru(1010) surface. The reaction products observed upon heating include H(2), CO, CO(2), and a small amount of MMA. Physisorbed multilayers of MMA desorb at temperatures around 170 K. Electron irradiation of physisorbed MMA at 140 K leads to a modification of the MMA film: The XPS spectra show an increase in thermal stability of the film with retention of the MMA structure, and indicate that electron irradiation induces polymerization. An increase in the electron bombardment fluence induces a degradation of the formed polymerized species and leads to the accumulation of carbon on the Ru surface. These results are relevant to the accumulation of carbon on surfaces of Ru films that serve as capping layers on MoSi multilayer mirrors used in extreme ultraviolet lithography.

Transmission of low-energy (<10 eV)O+ ions through Au films on TiO2(110)

In an attempt to identify the fundamental processes that influence ion transport through metallic surface layers, we have studied the transmission of O+ ions through discontinuous Au films adsorbed on TiO2(110). A low energy (< 10 eV) O+ ion beam is generated via electron stimulated desorption when an Au-dosed TiO2(110) substrate is bombarded with a focused 250 eV electron beam. Low energy ion scattering data indicate that Au evaporated under ultrahigh vacuum conditions at ∼ 300 K forms three-dimensional clusters on TiO2(110). As the Au coverage increases, the formation of Au clusters on TiO2(110) blocks a fraction of the TiO2 surface and the O+ yield is attenuated. However, for high coverages (≥30% Au covered substrate) the O+ signal decreases at a faster rate than the TiO2 open area fraction. We attribute the attenuation of the O+ yield for high Au coverages mainly to blocking of O+ by Au clusters, to deflection of trajectories by the image force between ions and Au clusters, and to charge transfer between desorbing O+ and neighboring Au clusters.