Ion-beam reduction of the surface of higher niobium oxide

X-ray photoelectron spectroscopy is used to study the process of reduction of the surface of the higher oxide Nb₂O₅ upon bombardment with inert gas ions (Ar⁺) and reactive gas ions (O₂⁺) with an energy of 1 and 3 keV in high vacuum at room temperature. It is found that, upon bombardment with Ar⁺ ions, the lower oxide NbO and the intermediate oxide NbO₂ are formed in the surface layers of the oxide Nb₂O₅. Bombardment with O₂⁺ ions leads to the formation of an extremely insignificant amount of the intermediate oxide NbO₂ in the surface layers of the oxide Nb₂O₅. It is revealed that the process of ion-beam reduction of the surface of the oxide Nb₂O₅ depends on the ion type, dose, and energy of exposure.     

Effect of oxide surface metallization under ion-beam irradiation

Changes in the surface chemical composition of WO₃, Ta₂O₅, MoO₃, and Nb₂O₅ oxides after Ar⁺ ion irradiation and those of the WO₃ surface after He⁺ ion irradiation under high vacuum were investigated by X-ray photoelectron spectroscopy. Upon Ar⁺ ion irradiation with an energy of 3 keV, the pronounced effect of ion-beam metallization was observed on the WO₃ oxide surface; a moderate effect was found for the Ta₂O₅ oxide surface; a weak one for the MoO₃ oxide surface; and no effect was discovered for the Nb₂O₅ oxide surface. At the saturation dose, 44 at % W, 12 at % Ta, and 2 at % Mo form on the oxide surfaces. Irradiation by light He⁺ ions with energies of 1 and 3 keV results in WO₃ surface metallization. At the saturation dose, 2 and 10 at % W (at 1 and 3 keV, respectively) forms on the oxide surface. The nature, mechanisms, and features of the oxide surface metallization effect induced by ion-beam irradiation are discussed.     

Sputtering of Ta2O5 by Ar+ ions at energies below 1 keV

The sputtering of anodically formed Ta₂O₅ layers of about 3500 Å thickness has been studied by Sputtered Neutral Mass Spectroscopy (SNMS). For perpendicular bombardment with Ar⁺ ions up to 900 eV the flux of ejected neutral particles is found to consist almost exclusively of metal atoms Ta and Oxide specific molecules TaO and TaO₂ with intensity ratios in the order 1 : 1 : 10^?1. From depth profiling measurements with SNMS, and from the intensity ratios in the SNMS spectra the total sputtering yield of Ta₂O₅ and the partial yields of Ta, TaO and of oxygen have been determined for normally incident Ar⁺ ions of 100 to 600 eV. After an initial increase the TaO intensity in the SNMS spectra remains constant during the sputter removal of the whole layer. A simple model is derived by which the preferred emission of TaO molecules, and the initial increase of the TaO intensity is referred to ion induced variations of the surface stoichiometry of Ta₂O₅. For optimum TaO production the model predicts equal atomic surface concentrations of Ta and O.     

Surface composition modification of tungsten higher oxide under He<Superscript>+</Superscript> ion bombardment

The surface reduction of higher oxide WO₃ under irradiation by He⁺ ions with the energies 1 and 3 keV in a high vacuum is investigated by X-ray photoelectron spectroscopy. It is found that lower WO₂ and intermediate WO _x (2 < x < 3) oxides form first in WO₃ surface layers under He⁺ ion bombardment, and with an increase in the irradiation dose metallic tungsten forms. It is shown that the degree of irradiated oxide surface metallization increases with an increase in the energy of the bombarding He⁺ ions. A comparison of WO₃ oxide surface composition modification under He⁺ and Ar⁺ ion irradiation is presented.     

Ion-beam reduction of the surface of higher oxides of molybdenum and tungsten

The process of reduction of the surface of oxides MoO₃ and WO₃ under irradiation by Ar⁺ and O ₂ ⁺ ions with an energy of 3 keV in high vacuum is investigated by X-ray photoelectron spectroscopy. It is shown that upon irradiation by Ar⁺ ions, lower and intermediate oxides and unoxidized metals are formed in the surface layers of higher oxides. Irradiation by O ₂ ⁺ ions mainly leads to formation of intermediate oxides with an insignificant content of lower oxides. It is found that the process of ion-beam reduction of the surface of oxides MoO₃ and WO₃ substantially depends on the ion type, irradiation dose, and difference in energy of the metal-oxygen bond in oxides.     

An XPS study on the surface reduction of V2O5(0 0 1) induced by Ar ion bombardment

The effect of the irradiation with Al Kα X-rays during an XPS measurement upon the surface vanadium oxidation state of a fresh in vacuum cleaved V₂O₅(0 0 1) crystal was examined. Afterwards, the surface reduction of the V₂O₅(0 0 1) surface under Ar⁺ bombardment was studied. The degree of reduction of the vanadium oxide was determined by means of a combined analysis of the O1s and V2p photoelectron lines. Asymmetric line shapes were needed to fit the V³⁺2p photolines, due to the metallic character of V₂O₃ at ambient temperature. Under Ar⁺ bombardment, the V₂O₅(0 0 1) crystal surface reduces rather fast towards the V₂O₃ stoichiometry, after which a much slower reduction of the vanadium oxide occurs.     

On the nature of ion bombardment-induced phase transformations in films of transition metals

Abstract

Electron diffraction studies have been made of polycrystalline Ni films irradiated with well separated beams of ions of different nature, namely ions of inert (He⁺, Ne⁺, Ar⁺, Kr⁺, Xe⁺) and reactive (N⁺ and O⁺) gases. The Ni films were prepared under vacuum conditions (P? 3·10^?6Pa during evaporation) preventing an appreciable contamination of the films with impurities. The samples were irradiated at T? 300 K with ion beams of energies from 10 to 100 keV in the dose range between 5·10¹⁶ cm^?2 and the value leading to sample destruction.

Irradiation with noble gas ions revealed no phase transitions in the Ni films. A similar result was obtained in irradiation of Fe and Cr films with He⁺ ions. The bombardment of Ni films with reactive gas ions does cause changes in the lattice structure of the samples under study, depending on the nature of the bombarding ions. The N⁺ ion bombardment gives rise to the hcp phase with the lattice parameters typical of the Ni₃N compound, and the O⁺ ion bombardment results in the fcc phase with the NiO-type parameter.

The conclusion is drawn on the chemical origin of the phase transformations in the Ni films under ion bombardment. The necessity of revising the concept about the polymorphous nature of phase transformations induced in the films of transition metals by ion bombardment is substantiated.     

High-resolution sputter depth profiling with a low pressure hf plasma

An arrangement for sputter-depth profiling by means of a low pressure hf plasma is described. Extremely plane bombarding craters are obtained when the bombarding voltage, the plasma parameters and the geometry of the target assembly are fitted appropriately. A quantitative relation for plane crater conditions is established, and verified by a comparison with crater profiles created in Ta₂O₅ layers by the bombardment with Ar⁺ ions of 100–600 eV. Under optimum conditions the flatness of the crater bottoms being investigated by optical interferometry was in the order of 10Å. The method has been applied for depth profiling of 3500Å thick Ta₂O₅ layers by Sputtered Neutral Mass Spectrometry (SNMS).     

ISS measurement of surface composition of AuCu alloys by simultaneous ion bombardments with Ar+ and He + ions

Surface composition of Au-Cu(43 at%) alloy under 1.5–5 keV argon ion bombardment has been investigated by ion scattering spectroscopy (ISS). In this experiment, we adopted a specific technique to use mixed He⁺ and Ar⁺ ions as primary beam in order to perform sputtering (Ar⁺) and ISS measurement (He⁺) simultaneously. The outermost atom layer of Au-Cu alloys under Ar⁺ ion bombardment is Au-rich leading to the conclusion that Ar⁺ ion bombardment of AuCu alloys causes the preferential sputtering of Cu atoms, resulting in a Au-rich outermost atom layer and a depletion layer of Au atoms beneath the outermost atom layer due to ion-beam-enhanced surface segregation. This result explains the experimental results obtained by AES as well.     

ESCA studies of metal-oxygen surfaces using argon and oxygen ion-bombardment

ESCA has been used to monitor alterations of catalytically and electrochemically important metal-oxygen surfaces following exposure to Ar⁺ and O₂⁺ ion bombardment. This treatment resulted not only in sputtering, but also, in many cases, in reduction to the corresponding metal or lower oxide. A model based on bulk thermodynamic free energy considerations Is proposed to explain this phenomenon. We have also exploited this approach to obtain an in-depth concentration profile of various oxidation states of an element, to selectively prepare desired surface oxide compositio and to aid in interpreting complex O ls spectra. Results obtained from metal-oxygen surfaces for Ni, Ru and Mo are presented. Ni₂O₃ and RuO₃, which are gross defect structures of the bulk species, are present on NiO and RuO₂ respectively, with the former being confined to the surface layers. The MoO₂, on the other hand, is covered with a surface layer of MoO₃ present as a regular crystal structure.     

低能氩离子束对多孔铝阳极氧化膜表面的刻蚀效应研究

用低能氩离子束(Ar⁺)处理了多孔铝阳极氧化膜(AAO)表面.扫描电子显微镜和原 子力显微镜结果表明，Ar⁺束刻蚀不仅可以有效地去除AAO反面阻挡层，还可使A AO表面产生多种特殊的形貌，如采用倾角入射可使其表面产生波纹，倾角入射同时旋转样品 台，可实现表面抛光.并结合Bradley和Harper提出的无定形材料表面波纹的形成和演化理论 解释了AAO表面波纹的特征. 关键词： 多孔铝阳极氧化膜 离子束刻蚀     

Temperature dependence of compositional changes of SiO2 surface during ion and electron bombardment

The influence of ion (Ar⁺ 0.5 keV, 2 microA/cm²) and electron (2 keV, 2 mA/cm²) bombardment on the elemental composition of SiO₂ was investigated in the temperature range of 270–790 K. Elemental composition was controlled by AES. It was found that both ion and electron bombardment resulted in an increasing amount of Si₉₂ (elemental silicon) and in decreasing amounts of both O₅₁₀ and Si₇₈ (silicon bound to oxygen). The temperature influence on the composition of SiO₂ is negligible under ion bombardment while the amount of Si₉₂ strongly increases under electron bombardment at temperatures exceeding 600 K. The mechanism of temperature dependence is discussed.     

Anomalous enhancement of negative sputtered ion emission by oxygen

Enhancement of negative sputtered ion yields by oxygen (either O⁺₂ bombardment or O₂ gas with Ar⁺ bombardment) is demonstrated for Si^?, As^?, P^?, Ga^?, Cu^? and Au^?, sputtered from a variety of matrices. Because oxygen also enhances positive ion yields of the same species, this effect cannot be simply explained on the basis of existing sputtered ion emission models. To rationalize these phenomena, a surface polarization model is developed which invokes localized electron emissive or electron retentive sites associated with differently oriented surface dipoles in the oxygenated surface. Such sites are considered to dominate the emission of negative and positive ions respectively. The model is shown to correctly predict that Au⁺ and Au^? ion yields are much more strongly enhanced by oxygen in dilute Au-Al alloys than in pure gold.     

Combined SIMS,AES, and XPS investigations of tantalum oxide layers

Thick layers of tantalum oxide prepared by thermal and anodic oxidation have been studied by combined SIMS, AES, and XPS during depth profiling by 3keV Ar⁺ ion sputtering. The chemical composition of these films is revealed by the OKLL and O 1s signals and by the “lattice valence” parameter determined from the TaO _n ^± intensities. Thus the anodic film consists of a contamination layer, an oxygen-rich reactive interface and a thick homogeneous oxide layer followed by an interface to the Ta metal. The thermal oxide shows an oxygen concentration decreasing with depth and a broad oxide-metal interface. In both cases, carbon contamination (carbide) prevents the application of the valence model to the clean Ta substrate. The sputtering yield of the oxides was found to be 0.6 Ta₂O₅/ion.     

Composition and properties of nanoscale Si structures formed on the CoSi<Subscript>2</Subscript>/Si(111) surface by Ar<Superscript>+</Superscript> ion bombardment

The variations in the composition and structure of CoSi₂/Si(111) surface layers under Ar⁺ ion bombardment with subsequent annealing has been studied. It has been demonstrated that nanocluster phases enriched with Si atoms form on the CoSi₂ surface at low doses D ≤ 10¹⁵ cm^–2, and a pure Si nanofilm forms at high doses.     

AES and EELS study of ErSi2 and its behaviour under ion bombardment and oxygen exposure

We report on Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS) investigations on ErSi₂ : the silicide behaviour under Ar⁺ bombardment (1–5 KeV) and oxygen exposure at room temperature was studied. The ion beam processed surface shows a Si enrichment, resulting in an ErSi-like stoichiometry. This result suggests a predominant role of the mass difference between Er and Si in the sputtering mechanism of ErSi₂ At exposure up to 10³ langmuirs, both components of the silicide react with oxygen. Firstly, Er oxide is formed, in an Er₂O₃-like state. After the consumption of the available Er atoms in the surface layer, SiO₂ starts to grow. These results are interpreted in terms of a greater heat of formation for Er oxide than for Si oxide.     

Ion-enhanced gas-surface chemistry: The influence of the mass of the incident ion

There are many examples of situations in which a gas-surface reaction rate is increased when the surface is simultaneously subjected to energetic particle bombardment. There are several possible mechanisms which could be involved in this radiation-enhanced gas-surface chemistry. In this study, the reaction rate of silicon, as determined from the etch yield, is measured during irradiation of the Si surface with 1 keV He⁺, Ne⁺, and Ar⁺ ions while the surface is simultaneously subjected to fluxes of XeF₂ or Cl₂ molecules. Etch yields as high as 25 Si atoms/ion are observed for XeF₂ and Ar⁺ on Si. A discussion is presented of the extent to which these results clarify the mechanisms responsible for ion-enhanced gas-surface chemistry.     

A SIMS study of ion beam induced migration of nitrogen in thermally oxidized silicon

The potential use of thin silicon nitroxide films as gate dielectrics in VLSI MOS devices has motivated much recent work. The present study shows that positive ion bombardment, as encountered in sputter depth profiling or ion implantation, can induce considerable movement of nitrogen in thin thermal oxide films on silicon. Low energy N⁺₂ implants are performed in-situ in a SIMS apparatus and are subsequently depth profiled. The effect of implant dose and oxide thickness are examined and comparisons are made to films prepared by rapid thermal nitridation and LPCVD. Profiles obtained under O⁺₂, O^-, and Cs⁺ bombardment are also compared. SIMS depth profiles of implanted 200 Å oxides using positive ion bombardment show a depletion of nitrogen near the surface, a shoulder in the nitrogen concentration near the Si-SiO₂ interface, and a peak in this concentration at the interface. Negative ion bombardment did not induce a shoulder-peak structure at the interface. The implications of these results are discussed.     

Simultaneous SIMS and EID investigation on the interaction of oxygen with a W (100) surface

A UHV system, containing a beatable tungsten ribbon target (showing [100] planes), an ion source (Ar⁺, 2 keV) with mass separator, an electron source (300 eV), a quadrupole secondary ion mass filter, and a quadrupole gas analyzer is used for the study of the interaction of O₂ with W (100) by simultaneous, i.e. fast interchanging, “static” SIMS (secondary [ion-induced] ion mass spectrometry) and EID (electron-induced [ion] desorption). Two different adsorptive binding states can be distinguished: β₂ and β₁. The O⁺ emission cross section under electron bombardment from the β₂ state is smaller by a factor of about 10³ than from β₁ and is found to be temperature-dependent. After the state β₂ has been saturated and before the occupation of β₁ begins, an oxide formation process starts. This oxidation can be interpreted by a two-stage model.     

Preferential sputtering of TIC under keV Ar+ ion bombardment: Simultaneous sputtering-ISS measurement with He+ and Ar+ ions

The composition change of the outermost atom layer of TiC(110) under ion bombardment with 1.5–3 keV He⁺ and He⁺ + Ar⁺ ions has been measured by ion scattering spectroscopy with He⁺ ions at different sample temperatures. It has been found that the preferential sputtering of C atoms takes place for both the He⁺ and Ar⁺ ion bombardment, however the preferred sputtering is more pronounced for Ar⁺ ions than for He⁺ ions. The ion bombardment with He⁺ ions at elevated sample temperatures hardly results in any change in surface composition below ～800°C, while Ar⁺ ion bombardment results in C enrichment for elevated temperatures as reported so far.