Characteristic energy losses on carbon contaminated layers correlated with secondary electron,auger electron emission and contrasts in scanning microscope

The secondary electron (SE) spectrum (0 < E < 50 eV) has been analysed by means of a CMA. Samples were clean aluminum, aluminum becoming carbon contaminated, sintered graphite powder, electro chemically deposited polymer on platinum and monocrystals of silicon carbon contaminated. When the clean Al surface is becoming carbon contaminated a quick decrease of surface plasmon and bulk plasmon losses is observed whereas a main characteristic energy loss peak (ELS) at 20 eV and a secondary electron peak at 20 eV appear simultaneously. Both peaks are very sensitive general features of carbon contaminated surfaces. The main loss peak is attributed to the excitation of the carbon-carbon bounds (σ → σ¹) as already proposed in the transmission ELS. The few eV change of the loss peak energy of various carbon compounds may correspond to slightly different carbon-carbon distances. The 20 eV secondary electrons could be produced by the relaxation of the excited state (σ¹ → σ transition) via an Auger process. The cross section for molecular electronic excitation is higher than that of atomic ionization for inner level. The loss peak is as intense as the SE peak and higher by more than two orders of magnitude than the C KLL Auger peak. The modification of secondary emission under carbon contamination has been observed on a silicon sample by Scanning Electron Microscopy (SEM) in the Secondary Electron Image (SEI) mode.     

Electron energy-loss spectra of clean and gas-covered Ni(100) surfaces

Electron energy-loss spectra have been measured on Ni(100) surfaces, clean and following oxygen and carbon monoxide adsorption, at primary energies of 40–300 eV. The observed peaks at 9.1, 14 and 19 eV in the clean-surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electron, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Assignments of the loss structures on the gas-covered surfaces have been attempted.     

磁控溅射铂抑制镀银表面的二次电子发射

降低表面的二次电子产额是抑制微波部件二次电子倍增效应和提升功率阈值的有效途径之一,目前主要采用在表面构造陷阱结构和沉积非金属薄膜的方法降低二次电子产额,其缺点是会改变部件的电性能.针对此问题,采用在表面沉积高功函数且化学惰性的金属薄膜来降低二次电子产额.首先,采用磁控溅射方法在铝合金镀银样片表面沉积100 nm铂,测量结果显示沉积铂后样片的二次电子产额最大值由2.40降至1.77,降幅达26%.其次,用相关唯象模型对二次电子发射特性测量数据进行了拟合,获得了在40-1500 eV能量范围内能够准确描述样片二次电子产额特性的Vaughan模型参数,以及在0-50 eV能量范围内能够很好地拟合二次电子能谱曲线的Chung-Everhart模型参数.最后,将获得的实验数据和相关拟合参数用于Ku频段阻抗变换器的二次电子倍增效应功率阈值仿真研究,结果表明通过沉积铂可将部件的功率阈值由7500 W提升至36000 W,证实了所提方法的有效性.研究结果为金属材料二次电子发射特性的研究提供实验数据参考,对抑制大功率微波部件二次电子倍增效应具有参考价值.     

Low energy electron diffraction studies of chemisorbed gases on stepped surfaces of platinum

The chemisorption of hydrogen, oxygen, carbon, carbon monoxide and ethylene was studied by low-energy electron diffraction on ordered stepped surfaces of platinum which were cut at angles less than 10° from the (111) face. The chemisorption characteristics of stepped platinum surfaces are markedly different from those of low index platinum surfaces and they are also different from each other. Hydrogen and oxygen which do not chemisorb easily on the (111) and (100) crystal faces chemisorb readily and at relatively low temperatures and pressures on the stepped platinum surfaces used in this study. In contrast to the ordered adsorption of carbon monoxide and ethylene on low index faces, the adsorption was disordered on the stepped surfaces and there is evidence for dissociation of the molecule. Carbon formed several ordered surface structures and caused faceting on the stepped surface, which are not observed on low index platinum surfaces. There appears to be a much stronger interaction of chemisorbed gases with stepped surfaces than with low index planes that must be caused by the differing atomic structures at the steps. Evidence for the differing reactivities of the two stepped surfaces are also discussed.     

Difficulties in the detection of surface impurities on platinum using Auger electron spectroscopy

The difficulties in the detection of surface impurities on platinum by Auger electron spectroscopy are demonstrated by the analysis of samples which showed varying degrees of contamination in field emission microscope investigations. Spectra indicate that S contamination can easily be overlooked because of an overlap of the main S transition at 152 eV with the Pt transitions at 150 and 158 eV. A similar example involving the overlap of the 92 eV Si peak with the 93 eV Pt peak is investigated. The presence of P is found to be easily obscured by electron beam induced effects. Several oxygen-P cross transition signals can arise, each of relatively low intensity. Together, these signals give the appearance of background noise. The results of these analyses are applied to investigations of the so called “clean” reconstruction of Pt {100}. In reviewing over seventy papers on {100} reconstruction, including those of Ir and Au, not one Auger spectrum could be found providing unequivocal evidence of surface cleanliness.     

Ultraviolet photoemission studies of acetylene and ethylene adsorptions on the Pt(111) surface: Correlations with LEED studies

The chemisorption of acetylene and ethylene on platinum (111) surfaces for T ≥ 300 K has been studied with ultraviolet photoelectron spectroscopy (UPS) at 21.2 eV. An activated metastable-stable acetylene transition observed recently in low-energy electron diffraction (LEED) intensity-energy profiles has been seen with the UPS spectra. The upperlying electronic levels of the metastable acetylene state are related to a shifted gas-phase acetylene spectrum. The stable acetylene state appears to involve a stronger molecule-surface interaction and probable rehybridization, consistent with the LEED analysis showing the molecule to be situated in a triangular position at covalent Pt-C distances. Ethylene is founf to dehydrogenate at room temperature to the stable acetylene species on Pt(111) surfaces.     

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

Electron and positron work functions of Cr(100)

We report measurements of the electron and positron work functions of submonolayer contaminated single crystal surfaces of Cr(100) in ultra high vacuum. The positron work function ø₊ is obtained by measuring the spectrum of slow positrons reemitted by the Cr(100) surface when it is bombarded with keV energy positrons. The electron work function ø_- is measured relative to Al(100) by comparing the target biases at which the slowest emitted positrons are recollected by the target. We obtain ø₊ = ?1.76(10) eV and ø_- = 4.46(6) eV for our Cr(100) surface using the value ø_- = 4.41(3) eV for Al(100) reported by Grepstad, Gartland and Slagsvold. The ø₊ value is in agreement with the ?2.2 eV calculated by Nieminen and Hodges. The positronium work function for Cr implied by these results is ?4.10(10) eV; the positronium negative ion (Ps^-) work function for this surface is calculated to be + 0.37(7) eV. A search for Ps^- showed that at a 90% confidence level less than one in 10³ thermalized positrons reaching the Cr surface are emitted as Ps^-. The slow positron emission spectrum was observed not to change over the 70–300 K range in contrast to recent theoretical predictions.     

Some leed and electron spectroscopic observations on disordered EuO(100) surfaces

Three vacuum-cleaved surfaces of EuO have been studied using LEED, Auger electron spectroscopy, electron loss spectrocopy and mass spectroscopy. The (100) surface is stable in the electron beam but is disordered after cleavage at room temperature. The Auger spectrum below 150 eV is assigned to initial state holes in the 4d shell of europium and to autoionisation processes. The consequences of the observation of disorder upon the interpretation of various magnetic and optical experiments are discussed.     

Characteristic electron energy losses in germanium

The characteristic electron energy loss spectrum of germanium was studied in a transmission type experiment as a function of the changes in structure due to electron bombardment. The structure of the material was characterized by electron micrograph and diffraction techniques. The electron energy loss spectrum of germanium was studied up to 45 eV, and loss peaks were observed at 15.7 eV and 31.6 eV as well as a 6 eV carbon loss. The positions of the most intense characteristic energy loss peak at 15.7 eV and its first multiple were constant for a large variation in the lattice parameters for the individual films.     

Influence of the substrate structure on the bonding of chemisorbed acetylene to transition metal surfaces

The electronic spectrum of acetylene adsorbed on various transition metals has been measured by ultraviolet photoemission spectroscopy in various laboratories. At low temperatures (T < 150 K), all measurements concur in finding an electron spectrum that differs only moderately from the gas phase spectrum of acetylene. At room temperature, the electron spectrum of acetylene is reported to be similar to the low-temperature form on Ir(100) and Pt(100), but acetylene is reported to form an olefinic surface complex on Pd(111) and Pt(111) surfaces. In order to examine whether the surface structure of the substrate is responsible for the difference, we have measured the electronic spectrum of acetylene adsorbed on the Pd(100) and Ru(0001) surfaces. At 120 K, the spectrum of adsorbed acetylene is again a distorted gas phase spectrum on both surfaces. At 330 K, we find the acetylenic form (with a splitting of 2.5 eV of the σ-orbitals) on Pd(100) and an olefinic form on the basal plane of Ru. We conclude that the olefinic complex is proper to the threefold symmetry of the (111) and (0001) surfaces and the gas-like form is favored on the (100) surfaces of the fcc crystals.     

A study of the charging and dissociation of SiO2 Surfaces by AES

AES is used to determine the initial spectrum of a vacuum-broken SiO₂ surface and to follow its dissociation under the electron beam probe. Both Auger peaks heights and energies are affected by the irradiation. The change in stoichiometry is accompanied by a decrease of the surface charge by 5–8 V. The relation between stoichiometry and charge is explained by the influence of radiation-induced defects on secondary electron emission. The reduction of SiO₂ is characterized in terms of irradiation dose, dissociation cross-section and electron impact efficiency. Resistance to radiation damage is increased by surface carbon contamination. The chemical contribution to the Auger peak energy can be distinguished from the charging effect leading to a shift between element and compound of 12 eV for the silicon peak.     

Rydberg states of HCN observed by electron impact spectroscopy

The electron energy loss spectrum of HCN has been determined in the energy region 8–13.6 eV at impact energies of 100, 50 and 30 eV. It is shown that energy loss spectra of HCN at intermediate impact energies can be satisfactorily analysed unlike the diffuse unassigned optical absorption spectra that have previously been reported. Rydberg series have been assigned using term values and quantum defects together with ionization potentials obtained by photoelectron spectroscopy.     

Temperature dependence of the low energy auger spectrum of NiO(100)

The temperature dependence of the Auger electron spectrum of NiO(100) has been observed in order to assign 10 features found below 300 eV. Six features are attributed to diffraction of true secondary electrons generated within the crystal. The others are due to substrate or surface impurity Auger peaks. The difficulties of identifying impurities and diffraction features are emphasised.     

Piezo optic properties of nickel and platinum

The piezoreflectance spectra of nickel and platinum were measured over the range 1.25–5.2 eV with the direction of incident light polarised parallel and perpendicular to the strain axis. Strain amplitudes of 0.12 × 10^?3 and 0.6 × 10^?4 respectively were employed at a frequency of 68kHz. The results showed enhanced structure in the differential absorption spectra of nickel at 1.75, 3.75 and 5 eV. Weaker structure was also found in the range 2.0–2.5 eV. From the results it is possible to discount the assignment of the higher energy structures as due to symmetry point transitions at the X and L points. A summary of possible assignments for the observed structures is given.The results for platinum show an essentially featureless spectrum in the visible range from which it may be concluded that the earlier prediction of interband transitions in this metal in the visible regions are incorrect.     

Electronic spectra of potassium bromide containing thallium II. Emission spectrum excited at 2537 Å

The emission spectrum of KBr: Tl⁺ excited at 2537 Å has been measured in the temperature range 15–296°K. At low temperatures the spectrum consists of a prominent band at 4.01 eV, a much smaller band at 3.40 eV and a very small band at 3.15 eV. The last does not appear to change much with temperature and so could not be measured accurately. The temperature-dependence of the two main bands is complex. Between 60 and 100°K the low-energy band increases sharply in intensity, while the high-energy band decreases correspondingly. Above 110°K the situation reverses and the low-energy band decreases in intensity while the high-energy band grows. Both bands closely approximate symmetric Gaussians. The temperature-dependence of the intensity of these two bands is well-explained qualitatively by the existence of two kinds of minima on the adiabatic potential energy surface for the A state. However, predictions of the temperature-dependence of the two emission bands based on calculated adiabatic potential energy surfaces are not in quantitative agreement with the experimental results. Possible reasons for this are our lack of knowledge of precise values for the parameters which enter into the theory, namely the spin-orbit coupling constant, the exchange integral, and the electron lattice coupling constant. The possible role of the ³A_1u state in emission is discussed briefly.     

The initial oxidation of Cu(100) single crystal surfaces: an electron spectroscopic investigation

The total energy distribution of electrons emitted from clean Cu(100) and oxygen covered surfaces is analysed. A primary electron energy of 400 eV enabled the investigation of characteristic losses (ELS), Cu MVV Auger transitions and true secondary electrons in a single spectroscopic run. Oxygen exposure up to 10⁸ L at elevated temperature (~400 K) results in a Cu density of states (DOS) strongly affected by O(2p) electrons. The Auger lines of Cu, atomic-like for clean surfaces, reveal DOS effects after some 10⁷ L oxygen exposure: all MVV transitions shift down by ~2 eV in spite of a fixed M₂₃ level; the M₂₃VV Auger line splitting is vanishing due to a broadened valence band maximum allowing the deexcitation of the final two-hole state of intraatomic transitions. Heating the oxygen covered crystal to 820 K is accompanied by the removal of much surface oxygen and an electronic state resembling an earlier oxidation state without DOS effects in the Cu Auger spectrum.     

The adsorption of sulfur on the platinum (100) surface

Clean platinum (100) surfaces of 1× 1 and 5 × 20 structure were exposed to H₂S. Surface coverage with Sulfur followed Langmuir kinetics, which, together with LEED data, points to a repulsive interaction between sulfur atoms. Sulfur adsorption causes a decrease in the work function of platinum by 0.7 eV at saturation coverage. This is attributed to polarization, rather than ionization, of the adsorbed sulfur. Photoemission measurements are difficult to interpret because of two-dimensional periodicity and the overlap of electronic structure of the adsorbate with the platinum d band. We observe peaks due to sulfur at 6.3, 4.5, and 2.5 eV below the Fermi level for the c(2 × 2) overlayer and at 6.8, 4.5, and 2.0 eV below E_F for the p(2 × 2) surface. A tentative interpretation in terms of sulfur orbitals is given. The decrease in work function and analogy with the properties of PtS₂ lead us to propose covalent bonding of sulfur to platinum, in which every sulfur atom is bonded to four Pt neighbors in both structures. The repulsive interaction between sulfur atoms is indirect through the platinum substrate.     

Secondary electron yield measurements of carbon covered multilayer optics

Carbon contamination on extreme ultraviolet (EUV) optics has been observed in EUV lithography. In this paper, we performed in situ monitoring of the build-up and removal of carbon contamination on Mo/Si EUV multilayers by measuring the secondary electron yield as a function of primary electron energy. An electron beam with an energy of 2 keV was used to simulate the EUV radiation induced carbon contamination. For a clean EUV multilayer, the maximum secondary electron yield is about 1.5 electrons per primary electron at a primary electron energy of 467 eV. The maximum yield reduced to about 1.05 at a primary electron energy of 322 eV when the surface was covered by a non-uniform carbon layer with a maximum thickness of 7.7 nm. By analyzing the change in the maximum secondary electron yield with the final carbon layer thickness, the limit of detection was estimated to be less than 0.1 nm.     

The bonding of water molecules to platinum surfaces

Using high resolution electron energy loss spectroscopy in ultra-high vacuum we have studied the vibrational spectrum of submonolayer and multilayer quantities of water adsorbed on platinum (100) surfaces. For adsorbed multilayers the spectrum resembles the spectrum of ice I. For submonolayer quantities of H₂O we find three different OH stretching vibrations, 2850, 3380, and 3670 cm^?1. The highest frequency is attributed to free OH groups. The vibration around 3380 cm^?1 indicates H bonding between oxygen atoms. It is therefore concluded that the water molecules cluster even at low coverage. The lowest OH stretching frequency is attributed to H bonding to platinum. We find also evidence for additional oxygen lone pair orbital bonding to the surface which disappears however when the first monolayer is completed. The relation to currently considered models in electrochemistry of aqueous solutions is discussed.