Two topics in surface science: morphological changes of NiA(111) induced by oxygen : secondary electron yield studies of Ru and TiO2 surfaces related to extreme ultraviolet lithography
This dissertation reports on two surface science projects: (1) one focuses on a study of oxygen-induced faceting of a NiAl(111) single crystal surface as a potential support for model catalysts; and (2) the other concerns secondary electron yield studies of TiO2(011) and Ru(0001), which are models of ruthenium and titanium dioxide capping layers on Mo/Si multilayer mirrors designed for applications in Extreme Ultraviolet Lithography (EUVL). It is known that monolayer films of oxygen can induce faceting of some atomically rough but planar metal surfaces. We extend our knowledge from surfaces of elemental metals to atomically rough metal alloy surfaces, such as NiAl(111). We discovered that the NiAl(111) surface exhibits an unusual behavior upon interaction with oxygen, including nanometer scale facet formation and growth of micrometer scale dendritic features. A series of experiments aimed at understanding the adsorption of oxygen and oxygen-induced faceting of NiAl(111) employing a variety of ultrahigh vacuum surface characterization methods. The atomically rough NiAl(111) surface remains planar at room temperature when exposed to oxygen. However, the oxygen-covered surface changes its morphology and forms nanometer scale facets upon annealing in the temperature range of ~1050 K to 1200K. Covered with one monolayer-thick γ-Al2O3 film, three-sided facets of {110} orientation appear. These facets coexist with the planar (111) surface. The surface becomes planar upon annealing in UHV above ~1250K. After prolonged exposure to oxygen at elevated temperatures three dimensional features exhibiting three-fold symmetry erupted from the surface; their dimensions are several micrometers in length, and ~300 nm high; their orientation is along low index directions in the plane of the NiAl(111) substrate. SEM X-ray mapping and EDS measurements indicate that these are spinel (NiAl2O4) structures; further investigation with SPEM revealed that these structures consist of γ-Al2O3 and NiAl2O4 oxides. Finally, TEM studies of the cross section of dendrites-covered NiAl(111) surface detected that γ-Al2O3 [13 ] is aligned with the NiAl[111] direction with a mismatch angle of 6˚. Ruthenium and titanium dioxide capping layers ~2 nm thick protect and extend the lifetimes of Mo/Si multilayer mirrors (MLMs) used in EUVL. The magnitude of secondary electron yield (SEY) at EUV wavelengths (13.5nm) is a major factor in determining contamination rates of MLMs in EUV projection optics. Low energy secondary electrons (0 to ~ 20 eV) cause dissociation of adsorbed hydrocarbons from the background gas, and lead to carbon film growth on MLM surfaces. In this dissertation, we investigate SEY for model EUV optics cap layer materials -TiO2 and Ru single crystals (clean, O-covered, C-covered, air exposed) and compare them with measurements for Mo/Si multilayer films capped with Ru, TiO2, and RuO2. SEY measurements were performed using synchrotron radiation over the range 40 eV to 180 eV at three different beamlines (U4A, U5UA, and U3C) at NSLS. For photon beams incident at 45˚, the shapes of the curves for Ru MLMs, especially the maxima at ~ 65 eV due to the Ru 4p excitation, are very similar to the data for pure Ru; such similarities are found also for a TiO2 crystal and TiO2-capped MLMs. The observation that the cap layer properties dominate the SEY characteristics agrees with theory. For near normal incidence, and for photon energies ~92 eV, dramatic energy- and angle-dependent resonances in SEY are observed for the capped MLMs, with SEYs 2 to 3 times higher than off-resonance. Calculations show excellent correlations between the photon electric field strength on the surface of a cap layer and the angular-dependent SEYs.Ph.D.Includes abstractVitaIncludes bibliographical referencesby Elena Loginov
