Secondary-ion emission from clean and oxygen-covered beryllium surfaces: II. Energy dependence

The secondary-ion energy distribution obtained by sputtering clean and oxygen-covered Be has been analyzed in terms of competing processes in secondary ion emission. The ion energy distribution N⁺(E) has been separated into an ionization coefficient R⁺(E) and a total energy distribution, N(E), i.e. N⁺(E) = R⁺(E) N(E). Experimentally, the dependence of R⁺(E) on both energy and oxygen coverage indicated a linear superposition of adiabatic tunneling and resonanance ionization processes from clean and oxygen-covered portions of the surface with no contributions to the secondary-ion yield from regions of intermediate coverage. Total energy distributions of sputtered Be atoms have been deduced and the principal features agree with the predictions of the collision cascade sputtering model. Variations of the energy distributions with oxygen coverage are in accord with small changes expected in the surface binding energy as a result of surface oxidation.     

Ionization probability of sputtered particles as a function of their energy: Part II. Positive Si ions

In this paper we represent the experimental ionization probability of sputtered silicon atoms as a function of their energy, which has been obtained for positive Si⁺ ions sputtered from silicon by O₂⁺ ion beam. To explain the experimental data, we have considered ionization of an outgoing atom at a critical distance from the surface, which occurs due to the electron transition between this atom and the surface, and suggested the formation of a local surface charge with the polarity opposite to that of the outgoing ion that has just been formed. Then we have considered the interaction between those two charges, outgoing ion, and surface charge as a process of the particle passage through a spherical potential barrier; as a result, we have obtained the theoretical energy distribution of secondary ions. Together with the well-known Sigmund-Thompson energy distribution of sputtered atoms, the obtained ion energy distribution allowed us to derive the equation for the secondary ion yield versus the sputtered particle energy. Both equations derived have exhibited a quite good correlation with our experimental results and also with a large number of published experimental data.     

Probability of ionization of sputtered particles as a function of their energy: Part I: Negative Si ions

In this study we have investigated how the probability of ionization of sputtered Si atoms to form negative ions depends on the energy of the atoms. We have determined the ionization probability from experimental SIMS energy distributions using a special experimental technique, which included de-convolution of the energy distribution with an instrumental transmission function, found by separate measurements.We found that the ionization probability increases as a power law ∼E^0.677 for particles sputtered with energies of 0-10 eV, then becomes a constant value (within the limits of experimental error) for particles sputtered with energies of 30-100 eV. The energy distributions of Si⁻ ions, measured under argon and cesium ion sputtering, confirmed this radical difference between the yields from low and high-energy ions.To explain these results we have considered ionization mechanisms that are different for the low energy atoms (<10 eV) and for the atoms emitted with higher energy (>30 eV).     

Energy dependence of the ionization probability of sputtered Cu and Ni

Despite its great sensitivity, the usefulness of secondary ion mass spectrometry (SIMS) for many applications has been limited by an inadequate understanding of the probability of sputtering an atom in an ionized state. To determine this ionization probability for clean Cu and Ni surfaces, I have measured the energy distribution of sputtered neutrals and ions by quadrupole mass filtering and retarding potential analysis using potential modulation differentiation. Analysis of sputtered neutrals was accomplished by electron impact ionization. Because the neutrals outnumber the ions by at least two orders of magnitude, the ratio of sputtered ions to neutrals is an accurate measure of the ionization probability. For energies below 20 eV the dependence of the ionization probability on energy goes as P(E) α Eⁿ, where n = 0.65 for clean Cu. The absorption of oxygen on the Cu surface increases the total ion yield while causing a reduction in the value of the exponent n. Similar results are found for nickel, where n = 0.54 for the clean surface.     

Model of ionization of atoms sputtered from solids

A new microscopical quantum-mechanical model of ionization of sputtered particles is described which takes into account electron excitations due to the atomic motions in the substrate. The substrate is represented by a simple atomic, metal-like, chain. Numerical calculations yield ionization probabilities R⁺ which have magnitudes close to the experimental values and have realistic velocity dependence. The function of R⁺ versus the sputtered atom ionization energy can be well approximated by the Maxwell-Boltzmann law with an effective temperature of about 3500 K.     

The formation of Ta+ secondary ions at oxygen-covered Ta surfaces

Absolute ionization probabilities R for Ta⁺ secondary ions are determined both experimentally from comparative in-situ SNMS and SIMS measurements and theoretically on the basis of recent microscopic ionization theories as a function of the oxygen surface concentration c_O. The theoretically predicted c_O dependence of R for sputtered Ta particles is found to agree well with the experimental results. The comparison between experiment and theory gives quantitative estimates of factors which influence the oxygen induced ion yield enhancement.     

Mass spectrometry of neutral molecules sputtered from polycrystalline metals by Ar+-ions of 100–1000 eV

The flux of neutral particles sputtered from clean polycrystalline targets of 11 metals has been studied by mass spectrometry after being postionized in a low pressure hf plasma. Besides atoms Me₁, postionized metal molecules Me₂ and Me₃ have been detected. The energyE ₀ of the bombarding Ar⁺ ions has been varied between 100 and 1000 eV. Considering the different plasma influences, an attempt is made to obtain the initial composition of the beam of sputtered neutrals from the ratiosR ₂₁ andR ₃₁ of postionized molecules Me₂ and Me₃ to postionized atoms, Me₁, respectively. The influence of the bombarding energyE ₀ onR ₂₁ has been measured for all target elements, onR ₃₁ for Pd and Ag. The yields of Me₂ atE ₀=1 keV approximately appear to be proportional to the square of the total sputtering yieldS for the different target materials. This behaviour and the energy dependence ofR ₂₁ andR ₃₁ are discussed in view of a possible molecule formation process in sputtering.     

N2微空心阴极放电特性及其阴极溅射的PIC/MC模拟

采用两维PIC/MCC模型模拟了氮气微空心阴极放电以及轰击离子 (N₂⁺,N⁺) 的钛阴极溅射. 主要计算了氮气微空心阴极放电离子 (N₂⁺,N⁺) 及溅射原子Ti的行为分布, 并研究了溅射Ti 原子的热化过程. 结果表明: 在模拟条件下, 空心阴极效应是负辉区叠加的电子震荡; 在对应条件下, 微空心较传统空心放电两种离子 (N₂⁺,N⁺) 密度均大两个量级, 两种离子的平均能量的分布及大小几乎相同; 在放电空间N⁺的密度约为N₂⁺的1/6, 最大能量约大2倍; 在不同参数 (P, T, V)下, 轰击阴极内表面的氮离子(N₂⁺,N⁺)的密度近似均匀, 其平均能量几乎相等; 从阴极溅射出的Ti原子的初始平均能量约6.8 eV, 离开阴极约0.15 mm处几乎完全被热化. 模拟结果为N₂微空心阴极放电等离子体特性的认识提供了参考依据. 关键词： 微空心阴极放电 PIC/MC模拟 2等离子体')" href="#">N₂等离子体     

Secondary ion emission from Si under nitrogen ion bombardment

The yield and energy distribution of positive secondary ions emitted from Si under N₂⁺ ion bombardment were measured. The obtained mass peaks correspond to three types of secondary ion species, that is, physically sputtered ions (Si⁺, Si₂⁺), chemically sputtered ions (SiN⁺ Si₂N⁺) and doubly charged ions (Si²⁺). The dependence of secondary ion emission on the primary ion energy was studied in a range of 2.0–20.0 keV. The yields of physically and chemically sputtered ions were almost independent of the primary ion energy. The yield of the doubly charged ion strongly depended on the primary ion energy. The energy distribution of secondary ions of the three types showed the same dependence on the primary ion energy. The most probable energy of the distribution increased with the primary ion energy. On the other hand, for the energy distribution curves of sputtered ions, the tail factors N in E^?N were constant and showed a m/e dependence.     

高电荷态Krq+与Al表面碰撞发射可见光的研究

利用低速(V≈0.01 V_Bohr)高电荷态Kr^q+ (q=8, 10, 13, 15, 17)离子轰击金属Al表面, 获得了碰撞过程产生的300–600 nm的光谱. 实验结果表明: 低能大流强(μA/cm²量级)离子束入射金属表面, 可产生溅射原子、离子和入射离子中性化后发射的可见光. 随着入射离子势能(电荷态)增加, 碰撞过程中发射谱线的强度增强. 与激发态3d能级相比, 较高的势能可以有效地激发Al原子的电子到较高4s能级. 关键词： 高电荷态离子 可见光发射 离子与表面作用     

Mechanism of MCs+ formation in Cs based secondary ion mass spectrometry

Quantitative analysis using MCs⁺ secondary ions has long been impossible because sample loading with Cs could not be controlled adequately. Using recently developed, advanced instrumentation, it can be shown that, at low levels of Cs loading, ionization and formation probabilities of MCs⁺ ions arrive at a constant, maximum level. On the basis of results obtained in three independent studies, involving three vastly different methods of controlling the sample loading with Cs, evidence is provided that MCs⁺ ions are emitted as such, independent of the number of atoms sputtered in the same impact event. Taken together, these findings finally pave the way for quantification of secondary ion signals without matrix effects.     

Indium sputtering upon bombardment with cluster ions

A surface-ionization method is developed for measuring the integral yields of neutral particles sputtered under the effect of ion bombardment. An investigation is performed to compare the integral yield of particles sputtered upon bombarding indium with Bi _m ⁺ cluster ions (m = 1?C7) in the energy range of 2?C10 keV and the secondary ion emission under bombardment with Bi _m ⁺ cluster ions (m = 1?C5) in the energy range of 6?C18 keV. A nonadditive increase in the indium sputtering coefficient is observed with an increasing number of atoms in the bombarding clusters.     

Cluster emission under ion bombardment of metallic targets

Mass spectroscopic studies of the neutral particles sputtered by Ar⁺ ions at 8 keV from polycrystaline samples have been performed, using non-resonant laser ionization and subsequent time-of-flight mass spectroscopy. Besides sputtered atoms, also dimer and trimer contributions in the order of 10^–1 to 10^–2 and 10^–3 to 10^–4, respectively, are found in the sputtered flux. The data obtained here together with previously published data by other groups for different bombarding energies provide strong support for the validity of the recombination model.     

Mass spectroscopy of sputtered neutrals and its application for surface analysis

Mass spectroscopic studies of the neutral particles sputtered by Ar⁺ions at energies around 1 keV from polycrystalline targets of 11 different metals have been performed, using a hf plasma maintained in pure argon at some 10^?4 Torr by electron cyclotron wave resonance as an ionizing medium as well as an effective ion source. Besides sputtered atoms Me₁ and molecules Me₂ with intensity ratios R₂₁ between Me₂ and Me₁ in the order of 10^?2 to 10^?1, neutral triatomic complexes Me₃ with corresponding ratios R₃₁ of some 10^?4 are found. Possible applications of the experimental method for surface analysis are discussed for previously air exposed polycrystalline Ta and Nb surfaces.     

Penning ionization electron spectroscopy of CO,HCl, HBr and of triatomic molecules N2O,NO2, CO2, COS and CS2

The energy of electrons released in ionization of di- and triatomic molecules by He (2¹S, 2³S), Ne and Ar (³P_{0, 2}) metastable atoms was measured. Attention was concentrated on the determination of peak shifts and peak form in order to elucidate new features of the mechanism of ionization with respect to ionization of atoms. In ionization by He (2¹S) atoms, asymmetric peak forms and greater shifts towards lower electron energy were found than in ionization by He (2³S) atoms. A tentative explanation for peak shifts in terms of orientation of molecules during the collision and of molecular orbitals involved in ionization was proposed. Effects due to J values of metastable atoms and molecular ions were described. The ionization of electronegative molecules (NO₂) probably takes place already at large particle separation because of mixing with the X⁺ABC^? ionic state which, on recombination, ionizes into X + ABC⁺ + e.     

Strong Enhancement of Electron–Ion Recombination Owing to Free–Bound and Bound–Bound Resonance Transitions

The effect of free–bound and bound–bound resonance nonadiabatic transitions of an electron on electron–ion recombination rates in the plasma of a Ne/Xe and Ar/Xe inert gas mixture has been studied. A kinetic model of recombination has been proposed including energy relaxation in collisions with electrons, resonant electron capture to Rydberg states through three-body collisions of Xe⁺ ions with Ne or Ar atoms and dissociative recombination of NeXe⁺ or ArXe⁺ ions, and n → n' resonance transitions. It has been shown that effective resonance processes occurring in quasimolecular systems sharply increase both the recombination coefficient and the effect of collisions with neutral particles even at quite high degrees of ionization of the plasma.     

Relation between activity of sorbed water vapor and wettability on polymer surfaces

During Ne⁺ bombardment of Al, 8 different spectral lines of Al I were observed. The intensities were found to depend on the presence of oxygen, the oxygen being in part adsorbed at the surface and in part recoil-implanted beneath the surface. A model is presented in which the oxygen dependence is related to resonance electron transfer between sputtered Al atoms and the target surface. The same model also accounts for the oxygen dependence of the yields of the secondary ions Al⁺, Al²⁺, and Al³⁺.     

Charge state effect on K-shell ionization of aluminum by 600–3400 keV xenonq+ (12 < q < 29) ion collisions

K-shell X-ray spectra of Al were measured by the interaction of 600–3400 keV Xe ^q+ (q = 12–29) ions with Al surface. The X-ray yields per incident ion were deduced and the K-shell ionization cross-sections were obtained from the experimental yield data. With the same incident energy, the K-shell ionization cross-sections of Al excited by Xe ^q+ (q < 26) ions were of the same order of magnitude, while for q = 26 and 29 Xe ion collisions, they were, respectively, about two and ten times larger. Taking into account the binding-energy-modification and the recoil effect of target atoms, the binary encounter approximation (BEA) theory was consistent with the experimental data for q < 26 Xe ion collisions, but it underestimated those excited by q = 26 and 29 Xe ions. This indicates that the K-shell ionization of target induced by Xe ^q+(q < 26) ions was mainly due to the direct Coulomb excitation. However for q = 26 and 29 Xe ions collisions, the transfer of 3d vacancies of projectile to the 1s orbital of target via rotational coupling of the 3dπ, δ-3dσ molecular orbitals, which were formed in the ion-atom quasi-molecule, may cause a considerable contribution to the enhancement of ionization. In addition to the well known Auger and X-ray transition, our experiments proved that the molecular orbital transition (“side-feeding”) mechanism is also a significant channel for de-excitation of hollow atoms formed below the surface.     

Low energy sputtering of neutral Cu2 molecules

The ratios of relative yields of neutral sputtered Cu₂ molecules to neutral sputtered Cu atoms were found to be linearly proportional to the sputtering yield of Cu, from a Cu target under bombardment by Ar⁺ ions (energy 50–90 eV), as determined by secondary neutral mass spectrometry.     

Secondary-ion emission from oxygen and hydrogen-covered beryllium surfaces: I. Coverage dependence

Secondary ion emission from a beryllium surface is studied in the presence of hydrogen and oxygen. For submonolayer coverages of oxygen, the adsorption follows site-exclusion statistics. On this basis, the Be⁺ secondary ion yield and energy distribution can be separated into oxygen-dependent and oxygen-independent processes governed by the local properties of the surface. Ionic molecular species with hydrogen or oxygen present include Be₂O⁺, Be₂O^?, BeO^?, and BeH⁺ but not BeO⁺ or BeOH⁺. This result is inconsistent with emission in which the oxide molecule is formed in the near-surface vacuum region by agglomeration of individually sputtered surface atoms. For BeH⁺emission the opposite conclusion is reached.