A modified electron bombardment type ion source for the study of solids

A modified electron bombardment type ion source suitable for use with mass spectrometer is described. Ion formation occurs throughout a relatively large volume in the ionisation box, since no magnetic field is used to collimate the ionising electrons. A sensitivity of 2 × 10⁻⁵ amp/torr is obtained for an ion extraction energy of 2 keV and 200 mass resolution. Trajectory tracing has been used to study the operation of the ion source. Capability of the ion source to analyse solid samples in microgram quantity was tested by studying evaporation of BaO from tungsten.     

Erosion of the field emitter surface exposed to low-energy ions

The radiation-induced erosion of the tungsten field emitter surface exposed to low-energy ions is studied by field ion microscopy and electron microscopy. During the bombardment, surface atoms are displaced to sites with lower coordination numbers and nanoasperities, generating a locally enhanced electric field, arise on the surface in a jump-like manner, which modifies the characteristics of the emitters. The field evaporation of the asperities produces cavities; hence, the erosion can be described in terms of blistering. Quasi-static surface erosion mechanisms are considered. It is shown that nanoblistering can be related to helium absorption in metal surface layers.     

Sputtering of tungsten, tungsten oxide, and tungsten-carbon mixed layers by deuterium ions in the threshold energy region

An original experimental method is developed for determining the sputtering coefficients of electrically conducting materials during bombardment by light gas ions at threshold energies. This information is very valuable in both purely scientific and practical terms. The basis of the method is a special field-ion-microscopic analysis regime. The procedure for measuring the sputtering coefficients includes cleaning the surface by field desorption and evaporation, with the subsequent work on an atomically clean and atomically smooth surface. The method permits identification of single vacancies on the irradiated surface, i.e., it is possible to count individual sputtered atoms. The method is tested on commercially pure tungsten, tungsten oxide, and a W-C mixed layer on tungsten under deuterium ion bombardment. The energy dependences of the sputtering coefficients of these materials for sputtering by deuterium ions at energies of 10–500 eV are obtained and analyzed. An important relationship between the energy threshold for sputtering and the conditions for oxidation of tungsten is found. The energy threshold for sputtering of an oxidized tungsten surface is 65 eV. The energy threshold for sputtering of the W-C mixed layer is almost equal to the corresponding value for pure tungsten. Zh. Tekh. Fiz. 69, 137–142 (September 1999)     

Miniature electron bombardment evaporation source: evaporation rate measurement

Miniature electron beam evaporation sources which operate on the principle of vaporization of source material, in the form of a tip, by electron bombardment are produced by several companies specialised in UHV equipment. These sources are used primarily for materials that are normally difficult to deposit due to their high evaporation temperature. They are appropriate for special applications, like heteroepitaxial thin films growth that require very low and well controlled deposition rate. We propose a simple and easily applicable method of evaporation rate control. The method is based on the measurement of ion current produced by electron bombardment of evaporated atoms. In order to be able to determine the ion current – evaporation flux calibration curves we measured the absolute values of evaporation flux by means of Bayard-Alpert ion gauge.     

Combined field ion microscopy and transmission electron microscopy of heavy ion damage in tungsten

Abstract

A field ion microscopy (FIM) and transmission electron microscopy (TEM) investigation of radiation damage in tungsten after heavy ion bombardment has been carried out. Field ion specimens of tungsten were irradiated with 180–230 keV Xe⁺ ions. The irradiation doses were varied between 4 × 10¹¹ and 4 × 10¹² ions/cm². The irradiated specimens were examined in FIM. Experiments combining both TEM and FIM were performed in order to compare the results obtainable by these two methods. The distribution of defects visible by TEM was inhomogeneous. The influence of the imaging field in FIM on the defects visible in TEM is discussed.     

The in situ cleaning of specimens in the field ion microscope by argon ion bombardment

The extensive adoption of argon bombardment cleaning techniques for specimens used in LEED and Auger studies, and the frequent, and often difficult, requirement of preparing field ion emitters, and their supports, free from contamination, led to the investigation of in situ argon ion bombardment of specimens in the field of ion microscope, both from the point of view of the efficiency of the cleaning process, and the investigation of ensuing surface and lattice damage, a task to which the technique of field ion microscopy is particularly appropriate.Experiments were carried out in some detail for tungsten specimens, and subsequently extended to the hexagonal metal, rhenium, with a view to obtain information on the thermally annealed end forms of such metals. In both cases, very small radius clean thermally annealed specimens could be prepared.     

Changes in Ion-Current Composition in the Process of Field Evaporation of Tungsten at High Temperatures

Technical Physics - Field evaporation of tungsten at T ~ 2000 K was studied using a sector magnetic mass spectrometer with a field ion source and a field emission microscope. Only low-charge ions...     

Field evaporation: Model and experiment

The ability of the image potential and charge exchange models to describe the evaporation of metals in a high electric field is examined. At present only the field dependence of the evaporation rate can be compared with the predictions of the two models. An analysis of experiments on the evaporation of tungsten atoms from kink sites, as well as from sites on top of the (110), suggests that the image potential model gives the most satisfactory representation of the data. The quantity α, which indicates the difference between the polarizability of a neutral surface atom and an ion, is found to differ little from one kind of site to another when obtained from a least-squares analysis. For tungsten at a kink site α =4.80 ± ± 0.03A?³; for an adatom on the (110), α = 5.24 ±0.04ų.     

Field evaporation end forms of tungsten

The anomalous field ion images consisting of bright bands of variable width parallel to the 〈111〉 zones, previously reported for tungsten pulse evaporated in helium at 78 K, have also been observed with neon as imaging gas. For both gases the upper limit to the pulse lengths for production of the bands is about 1 ms. The influence of the ratio, R, of pulse height to the total evaporation voltage on the intensity distribution in field ion images of tungsten has been studied for helium and neon together with the effect of residual gases peculiar to ion pumped stainless steel UHV systems (H₂, CH₄ and CO). It is found that when R is raised to a certain level traces of residual gases if present in sufficient quantities, can produce, with tungsten, images with intensity distributions similar to those found with molybdenum after slow evaporation. These are attributed to preferential adsorption of the residual gases on different parts of the specimen and, depending on the nature of the adsorbate, leading to a promotion or a reduction of the ionization probability of the imaging gas. It is pointed out that before an image can be described as typical of a particulaelement or alloy, the conditions of prior evaporation should be carefully controlled.     

Effective radius of adatoms/single atom of polycrystalline W nanotip in gas field ion source

A gas field ion source (GFIS) has several advantages such as high current density, low energy spread, and high brightness. In this study, Helium gas was used in a GFIS, generated using the field ionization and field evaporation processes. The (110) oriented three adatoms tip (TAT) and single atom tip (SAT) were prepared with its ion emission stability 9.8% and 10.1% by the nitrogen field-assisted etching, respectively. The TAT and SAT were reproduced throughout the field evaporation and nitrogen field-assisted etching along the same crystal axis with its brightness 2.6 × 10⁷ A/(m² sr) and 1.6 × 10⁸ A/(m² sr), respectively. The field emission electron beam effective radii for three adatoms and single atom were calculated to be 2.90 Å and 1.47 Å, respectively, by Fowler-Nordheim plot, in which the single atom size was especially shown to be quite in good consistency with an atomic radius of tungsten.     

The effect of target texture and topography on the spatial distribution of material sputtered from tungsten

The spatial distributions of sputtered particles have been investigated both experimentally and by computer simulation using the TRIM.SP code for 30 keV argon-ion bombardment of tungsten in a wide range of primary-ion incidence angles (0°–80°). Two sets of the targets were used. One of them was prepared from fine-grained polycrystalline ingot, another one from rolled sheet W. It was found that the experimental results for these targets were different. For rolled tungsten a typical Wehner-spot picture, although smeared, is observed. For fine-grained tungsten the sputtered particle spatial distributions are practically cupola-shaped. Some differences between the experimental results and computer-simulation data can be attributed to the effect of surface topography either initial or developed during the ion bombardment.     

Operation of activated-fullerene-coated field emitters in technical vacuum

The operation of tungsten tip field emitters coated by activated fullerenes in technical vacuum is studied. Ways of emitter training to provide the self-reproducibility of the coatings and their long service life under intense ion bombardment in wide ranges of pressures, from 2 × 10^?8 to (5–7) × 10^?7 Torr, and emission currents, from (1.5–2.0) to (10–20) μA, are developed.     

The influence of surface coordination on field evaporation processes in tungsten

The sequence of evaporation of atoms on (111), (332), (433), and (411) surfaces of tungsten has been observed in the field ion microscope for the evaporation of successive layers. Atoms with many different types of surface coordination were observed to evaporate. The data were processed to give the relative probabilities of evaporation (ρ) for each type of atom using a statistical procedure adapted to give probability limits for the ρ values. The results showed that for (111) surfaces the ρ values were the same with helium or neon as image gas and that in general differences in ρ values between differently coordinated atoms at 78 K were observed to be greater than at 20 K. At both these temperatures however the atoms with high surface coordination generally had lower ρ values. With (111), (332) and (433) surfaces the atomic arrangement allowed prediction of surface diffusion paths and with atoms in (111) surfaces an easy diffusion path was essential before evaporation could occur. Amongst atoms which had a diffusion path available, ρ values were relatively lower when the path had a saddle point protruding from the surface and which allowed the atom to be subjected to a higher field. On (411) surfaces diffusion paths are less predictable and the correlation with ρ was less.     

碳纳米管薄膜场蒸发效应

在超高真空系统中对基于丝网印刷方法制备的碳纳米管薄膜的场蒸发效应进行实验研究. 实验发现, 碳纳米管薄膜样品存在场蒸发现象, 蒸发阈值场在10.0-12.6 V/nm之间, 蒸发离子流可以达到百皮安量级; 扫描电子显微镜分析和场致电子发射测量结果表明, 场蒸发会使碳纳米管分布变得更加不均匀, 会导致薄膜的场致电子发射开启电压上升(240→300V)、场增强因子下降(8300→4200)、蒸发阈值场上升(10→12.6V/nm), 同时使得薄膜场致电子发射的可重复性明显变好. 场蒸发也是薄膜自身电场一致性修复的表现, 这种修复并非表现在形貌上, 而是不同区域场增强因子之间的差距会越来越小, 这样薄膜场致电子发射的可重复性和稳定性自然会得到改善.     

Surface diffusion of tungsten adatoms induced by low-energy bombardment with He atoms

Using low-temperature field ion microscope techniques, we studied at the atomic level the elementary events of radiation-induced surface diffusion of tungsten adatoms on the ideally pristine surface. The experiments on surface-damage formation and adatom displacements have been performed in situ with a source of neutral helium atoms with an energy of 5?keV. It was demonstrated that the low-energy He atomic bombardment at grazing incidence was able to induce a substantial surface mobility of adatoms. Most of the radiation-induced adatom displacements were oriented along the direction of irradiation. The long impact-induced jumps of adatoms, spanning more than a nearest-neighbor distance, were revealed. Performed molecular dynamics simulations reproduce the general experimental trends and conclusions.     

硅锥场发射阵列中离子轰击现象的分析

离子轰击影响尖端场致发射器件的稳定性和工作寿命.阐述了数值模拟硅锥阴极离子轰击现象的基本理论,并以硅锥场发射阵列的一个单元结构为例模拟了气体-电子碰撞电离产生的正离子回轰尖端的全过程,对模型中的硅锥受损的位置和程度进行了分析,得出了一些结论.     

Use of ion processing to improve the quality of fullerene-coated field emitters

The efficiency of tip field emitters covered by activated fullerene coatings is studied in a wide range of emission currents and residual gas pressures. Main mechanisms behind the influence of the gaseous medium and ion bombardment on the emitter efficiency are determined. The feasibility of improving the homogeneity of the fullerene coating by potassium ion bombardment is demonstrated. From data on the emitter performance in a technical vacuum, a previously unknown effect is discovered: the structure of activated fullerene coatings is reproduced under intense ion bombardment. It is found that intense bombardment by residual gas ions increases a limiting current extracted from fullerene-coated tip field emitters.     

Effects of X-ray radiation on hydrogenated amorphous silicon thin-film transistors and characteristics of tungsten shield layer

Hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) which have tungsten shield layer for AMOLED were fabricated. E-beam evaporation is useful method to prepare cathode layer above organic layers with high throughput and no damage to the organic layers but TFTs are damaged by X-ray radiation generated during the e-beam process. When the thickness of the tungsten layer was increased to 2000 Å, it was found that the characteristics of TFTs, such as threshold voltage, field effect mobility and sub-threshold slope, were slightly varied by X-ray radiation.     

Precise atomic scale studies of material sputtering by light-ion gases in the prethreshold energy region

The basis and prospects of a new original technique of determining the yields of the sputtering of conductive materials and subatomic films on their surface by light ion gases in the prethreshold energy region (from 10 to 500 eV) are discussed. This information is of great importance both for science and applications. The technique is based on special modes of field ion microscopy and includes the cleaning of specimens by field-induced desorption and evaporation, and subsequent operations with the atomically clean and atomically smooth surface in a wide temperature range from cryogenic temperatures. The technique enables one to identify single surface vacancies, that is, to directly count single sputtered atoms. The original results obtained with the developed technique are briefly reviewed. The energy thresholds of sputtering and the energy dependences of the sputtering yields in the prethreshold energy region are presented and analyzed for beryllium, tungsten, tungsten oxide, mixed tungsten-carbon layers, three carbon materials, and subatomic carbon films on the surface of certain metals bombarded by hydrogen, deuterium, and/or helium ions.     

Non-vacuum field desorption ion source implemented under super-atmospheric pressure

Standard field desorption (FD) ionization is implemented under high vacuum condition. In this paper, non-vacuum FD is performed under a super-atmospheric pressure environment using untreated tungsten wires as FD emitter, and the ion source was coupled to a commercial linear ion trap mass spectrometer. The operating pressure of the ion source was 6 bars which was high enough to provide sufficient dielectric strength to the working gas so that the high voltage that was required for FD could be applied to the emitter without occurrence of electrical discharge. Non-volatile sample deposited on the bare tungsten wire FD emitter was heated by flowing direct current through the emitter. Similar to vacuum FD, the formation of conical protrusion of the liquefied sample layer under the strong electric field was also observed. Using the present ion source, high pressure field-desorption of polar neutral compounds, organic salts and ionic liquids is demonstrated. Copyright ? 2012 John Wiley & Sons, Ltd.