Sputtering yields for light ions as a function of angle of incidence

The sputtering yield of Ni, Mo, and Au have been measured at oblique angles of incidence for H⁺-, D⁺-, and⁴He⁺-ion irradiation in the energy region from 1 to 8 keV. The yields were determined from the weight loss of the targets. For Ni and Mo the dependence of the sputtering yield on the angle of incidence was found to be much stronger for H⁺- and D⁺-ion than for⁴He⁺-ion irradiation. In all cases the maximum in the yield was found at angles of incidence ϑ≧80°, where ϑ is the angle measured from the surface normal. Furthermore the ratio of the maximum yield to the yeild at normal incidence increases with increasing surface binding energy of the target material as well as with increasing ion energy in the energy region inveestigated. The results are discussed qualitatively in view of a model for the sputtering mechanism for light ions.     

高电荷态离子与Si(110)晶面碰撞的沟道效应研究

不同电荷态低速离子(Ar^q+,Pb^q+)轰击Si(110)晶面,测量不同入射角情况下的次级粒子的产额. 通过比较溅射产额与入射角的关系,证实沟道效应的存在. 高电荷态离子与Si相互作用产生的沟道效应说明溅射产额主要是由动能碰撞引起的. 在小角入射条件下,高电荷态离子能够增大溅射产额. 当高电荷态离子以40°—50°入射时,存在势能越高溅射产额越大的势能效应. 关键词： 高电荷态离子 溅射 沟道效应     

Carbon sputtering yield measurements at grazing incidence

In this investigation, carbon sputtering yields were measured experimentally at varying angles of incidence under Xe⁺ bombardment. The measurements were obtained by etching a coated quartz crystal microbalance (QCM) with a low energy ion beam. The material properties of the carbon targets were characterized with a scanning electron microscope (SEM) and Raman spectroscopy. C sputtering yields measured under Ar⁺ and Xe⁺ bombardment at normal incidence displayed satisfactory agreement with previously published data over an energy range of 200 eV-1 keV. For Xe⁺ ions, the dependence of the yields on angle of incidence θ was determined for 0° ≤ θ ≤ 80°. Over this range, an increase in C sputtering yield by a factor of 4.8 was observed, with the peak in yield occurring at 70°. This is a much higher variation compared to Xe⁺ → Mo yields under similar conditions, a difference that may be attributed to higher scattering of the incident particles transverse to the beam direction than in the case of Xe⁺ → C. In addition, the variation of the yields with θ was not strongly energy dependent. Trapping of Xe in the surface was observed, in contrast to observations using the QCM technique with metallic target materials. Finally, target surface roughness was characterized using atomic force microscope measurements to distinguish between the effects of local and overall angle of incidence of the target.     

Isospin symmetry violation, meson production and η-nucleus interaction studies

We have studied isospin symmetry violation in nuclear reactions by measuring simultaneously the cross-section of the following two reactionsp +d →³H π⁺ andp +d →³He π⁰. The experiment was perfomed at the cooler synchrotron accelerator COSY, Jülich at several beam energies close to the correspondingη production threshold. We also have ongoing programmes onη-nucleus final-state interaction studies viap+⁶ Li →⁷ Be +η reactions, high resolution search for dibaryonic resonances and lambda-proton final state interaction studies. The experimental details and results obtained so far are presented here     

Relation between surface structures and sputtering ratios of copper single crystals

Polished (100) Cu crystals have been bombarded at target temperatures of 204 K, 294 K and 456 K by 10 and 20 keV Ne⁺ ions up to a total dose of 1.7 × 10¹⁹ ions/cm². The plane of incidence was chosen to be a {100} plane perpendicular to the surface. Measurements have been performed for incident angles between 36° and 44° with respect to the surface normal. In this angular interval the sputtering ratio and the surface structure have been studied by weightloss and replica electron microscope techniques respectively. At target temperatures of 204 K and 294 K an anomaly was observed in the curve of the sputtering ratio versus angle of incidence. A small peak appears where the curve slopes towards the 〈110〉 minimum. The position and height of the peak is a function of target temperature and ion energy.

This sputtering submaximum is accompanied by the formation of {100} orientated furrows perpendicular to the ion beam. The nucleation of this relief is tentatively discussed in terms of local deviations from perfection of the surface, which might be due to a singularity in the production of focusing collisions influencing the damage structure. The growth of the furrows and the submaximum in the sputtering ratio are discussed in terms of the angle between the ion beam and the characteristic {110} side of the furrows.

These sputtering and faceting phenomena have not been observed at 20 keV Ar⁺ ion bombardment nor generally under bombardment at a target temperature of 456 K.     

Scalar-isovector K¯ production close to threshold

The reaction pp → dK ⁺ˉ has been investigated at excess energies Q = 47.4 and 104.7MeV above the K ⁺ˉ threshold at COSY Jülich. Coincident dK⁺ pairs were detected with the ANKE spectrometer, and subsequently ∼ 2000 events with a missing ˉ invariant mass were identified, which fully populate the Dalitz plot. The joint analysis of invariant mass and angular distributions reveals s-wave dominance between the two kaons, in conjunction with a p-wave between the deuteron and the kaon pair, i.e. Kˉ production via the a ₀ ⁺(980) channel. Integration of the differential distributions yields total cross-sections of σ(pp → dK ⁺ˉ) = (38±2_stat±14_syst)nb and (190±4_stat±39_syst)nb for the low and high Q value, respectively.     

The effect of incident angle on the C60 bombardment of molecular solids

The effect of incident angle on the quality of SIMS molecular depth profiling using C₆₀⁺ was investigated. Cholesterol films of ∼300 nm thickness on Si were employed as a model and were eroded using 40 keV C₆₀⁺ at an incident angle of 40° and 73° with respect to the surface normal. The erosion process was characterized by determining at each angle the relative amount of chemical damage, the total sputtering yield of cholesterol molecules, and the interface width between the film and the Si substrate. The results show that there is less molecule damage at an angle of incidence of 73° and that the total sputtering yield is largest at an angle of incidence of 40°. The measurements suggest reduced damage is not necessarily dependent upon enhanced yields and that depositing the incident energy nearer the surface by using glancing angles is most important. The interface width parameter supports this idea by indicating that at the 73° incident angle, C₆₀⁺ produces a smaller altered layer depth. Overall, the results show that 73° incidence is the better angle for molecular depth profiling using 40 keV C₆₀⁺.     

Angular distributions of sputtered atoms for low-energy nitrogen irradiation of silicon

We studied the angular distributions of silicon and nitrogen atoms emitted from a Si target subjected to reactive sputtering by N ₂ ⁺ ions at primary energies of 0.5 and 2keV. The composition of the deposited material does not depend strongly on the substrate position. From a comparison with nonreactive sputtering, we show that the observed shift of the Si angular distribution is mainly due to the contribution of collision events occurring in the first monolayer. Contrary to the case of noble gas ions, the sharpness of the Si distribution depends on the N ₂ ⁺ energy. The behavior of the differential sputtering yield of silicon indicates that this effect is likely to be due to a loss of recoil atoms out of the preferential direction. A possible explanation of the observed phenomena consists in assuming an anisotropic emission of Si _x N _y radicals. This hypothesis is very attractive as it could satisfactorily explain the similarity we observed between the angular distributions of silicon and nitrogen.     

Effect of surface steps on sputtering and surface defect formation: molecular-dynamics study of 5 keV Xe bombardment of Pt(1 1 1) at glancing incidence angles

Using molecular-dynamics simulation, we study sputtering and defect formation induced by 5 keV Xe⁺ ion impact on a Pt(1 1 1) surface at oblique and glancing incidence angles. Impact on a terrace produces yield maxima at ?=60-65° incidence angle towards the surface normal. Beyond 75-80°, no damage is produced due to projectile ion reflection. Impact on a dense-packed step, however, produces defects in sizeable numbers up to glancing incidence, ?=85°. The dependence of the yields on the incidence angle and distance of the impact point of the projectile to the step are discussed.     

Structural and Morphological Changes of Carbon Fiber Surfaces,Produced via Sputtering by Noble Gas Ions

SEM, laser goniophotometry, and Raman spectroscopy are used to analyze a modification of the carbon PAN fiber shell of KUP-VM composite upon irradiation with 30 keV Ne⁺ and Ar⁺ ions at normal incidence and temperatures of RT to 600°C. It is found that the formation of corrugated submicron structures in the composite upon irradiation at elevated temperatures (≥125°C for neon and ≥250°C for argon) displays certain features at temperatures of 400–500°C. The corrugated faces’ angles of inclination and the fraction of the corrugated structure on the fiber surface at these temperatures are minimal. Together with regularities established earlier, the observed patterns allow us to relate ion-induced corrugation to anisotropic radiation- induced plastic processes of dimensional changes in carbon materials affected by ion sputtering of their surfaces.     

Sodium halide sputtering by H+, He+, Ar+ ions

Sputtering experiments were performed with 70 to 300 keV H⁺, He⁺ and Ar⁺ ions impinging on KC1, KBr and Kl. The alkali halide samples are prepared as polycrystalline layers of about 2500 Å thickness, deposited on carbon-aluminium backings. During the ion bombardment the targets are kept at elevated temperatures between 50 and 300°C, in order to study the temperature dependence of sputtering. During the irradiation the removal of halogen and sodium is simultaneously observed by Rutherford backscattering.

The present results are (i) preferential sputtering of the halogen atoms, (ii) temperature dependent sputtering yields with 0.2 eV activation energy, (iii) sputtering yields proportional to the electronic stopping power, rather than the nuclear stopping power, and (iv) sputtering yields orders of magnitude higher than estimated by elastic collision cascade theories. These findings can be interpreted by a Pooley process with subsequent migration of the interstitial halogen atom to the surface.     

Investigation of the Reaction 9Be(d,t0)8Be in the Deuteron Energy Range 0.9÷2.5 MeV

The reaction ⁹Be (d, t₀) ⁸Be has been investigated in the deuteron energy range 0.9 ÷ 2.5 MeV. The excitation functions were measured at laboratory angles 45° and 120° and showed no sharp resonances. The angular distributions (differential cross sections in mb/sr) were measured in 200 keV intervals. An analysis of the data has been made using the Butler and the distorted wave theories.     

共面不对称条件下Ag+(4p,4d)(e,2e)反应三重微分截面的理论研究

采用扭曲波玻恩近似(DWBA)理论计算了共面不对称几何条件下Ag⁺(4p⁶) 及Ag⁺(4d¹⁰)在不同入射电子能量和散射电子角度下(e,2e)反应的三重微分截面. 散射电子角度为4°, 10°和20°. 计算结果表明, Ag⁺(4p⁶)(e,2e)反应的三重微分截面其binary峰峰位或劈裂峰的谷位与动量转移方向有较大差别, 这可能是由于一种两次两体碰撞造成的. 另外, 还发现Ag⁺(4p⁶)(e,2e)反应三重微分截面的binary峰出现了反常劈裂现象, 这表明离子靶内壳层电离(e,2e)反应过程较外壳层更为复杂.对Ag⁺(4p⁶)及Ag⁺(4d¹⁰), 除binary峰和recoil峰以外, 在其他敲出电子角度出现了新的峰, 本文用几种两次两体碰撞过程对这些新的峰进行了解释.     

Sputtering of highly oriented pyrolytic graphite with 30 keV Argon ions

The influence of the incidence angle of 30 keV Ar⁺ ions, ion fluence and target temperature on the sputtering yield and surface microgeometry of highly oriented pyrolytic graphite (UPV-1T) samples was experimentally studied. It was found that at fluences more than 5 × 10¹⁹ ion cm^?2 the sputtering yield at room temperature in the range of the ion incidence angle from 0° to 80° is twice as small as the corresponding experimental data for both polycrystalline graphite and glassy carbon. The analysis of ion-induced relief permits us to suppose the topographical suppression mechanism of highly oriented pyrolytic graphite sputtering.     

Angular distribution of sputtered alloy. Experimental and simulated study

Angular distribution of Cu and Be atoms sputtered from Cu98 Be2 alloy under 5 Kr⁺ ion bombardment were measured at different angle of incidences (0, 30, 70 and 80^○). The sputtered material is collected on a Mylar^TM foil surrounding the target. The used collector was mounted at different ejection angles and analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). The SRIM-code program was employed to obtain the sputtering yield of Cu and Be from simulated Cu98 Be2 target. The experiment was combined with simulations and revealed that both of them were in a fairly good agreement. Furthermore, angular distribution of differential sputtering yields of both Cu and Be showed over-cosine tendency.     

Energy and charge distributions of fast hydrogen ions and atoms reflected from Cu in the case of grazing incidence

The energy and charge distributions of protons and hydrogen atoms reflected from the Cu surface in the case of grazing incidence angles are measured at energies of incident particles (H⁺ and H⁰) of 200 and 250 keV. The charged fractions of reflected particles are analyzed. A weak dependence of the neutral fraction of reflected particles on the scattering angle is discovered for incidence angles of 1°–2° and an energy of scattered particles of 60 keV or less. It is shown that the neutral fraction of reflected particles with an energy of 60–80 keV or more is independent of the scattering angle and is determined by the ratio of the cross sections for the electron capture and loss by ions in the material.     

离子轰击引起的表面形貌对Ag的溅射产额的影响

用捕获膜技术和卢瑟福背散射(RBS)谱仪测定Ag靶在27keV Ar⁺离子轰击下的溅射原子角分布,从而确定不同剂量下Ag的溅射产额,并对其靶点表面形貌进行扫描电子显微镜(SEM)观察。结果发现,所有的角分布都呈over-cosine形状,但其溅射产额却随着表面形貌不同而不同。根据溅射产额Y与轰击离子入射角φ变化关系,讨论不同轰击剂量下溅射产额的差别,肯定了表面形貌是影响溅射产额的一个重要因素,并由此提出“表观产额”的新概念。 关键词：     

Cluster ion emission from LiF induced by MeV N<Superscript>q+</Superscript> projectiles and <Superscript>252</Superscript>Cf fission fragments

Ion cluster desorption yields from LiF were measured at PUC-Rio with ≈0.1 MeV/u N ^q+ (q = 2,4,5,6) ion beams by means of a time-of-fight (TOF) mass spectrometer. A ²⁵²Cf source mounted in the irradiation chamber allows immediate comparison of cluster emissions induced by ≈65 MeV fission fragments (FF). Emission of (LiF) _n Li⁺ clusters are observed for both the N beams and the ²⁵²Cf fission fragments. The observed cluster size n varies from 1 to 6 for N ^q+ projectiles and from 1 to ≈40 for the ²⁵²Cf-FF. The size dependence of the Y(n) distributions suggests two cluster formation regimes: (i) recombination process in the outgoing gas phase after impact and (ii) emission of pre-formed clusters from the periphery of the impact site. The corresponding distribution of ejected negative cluster ions (LiF) _n F⁻ closely resembles that of the positive secondary (LiF) _n Li⁺ ions. The desorption yields of positive ions scale as Y(n) ∼ q ⁵. A calculation with the CASP code shows that this corresponds to a cubic scaling ∼S _e ³ with the electronic stopping power S _e , as predicted by collective shock wave models for sputtering and models involving multiple excitons (Frenkel pair sputtering). We discuss possible interpretations of the functional dependence of the evolution of the cluster emission yield Y(n) with cluster size n, fitted by a number of statistical distributions.     

A new technique for the measurement of sputtering yields

We have measured the yields of 90 keV ⁴⁰Ar⁺ and V⁴He⁺ sputtering of Mo and V samples by the use of a new radio-tracer technique. This technique involves activating the samples by high-energy charged-particle irradiation before sputtering, and using conventional γ-ray counting methods to analyze the material subsequently sputtered onto collector foils. We have also measured angular distributions of the sputtered material, and compared these results and our total sputtering yields with the predictions of Sigmund's sputtering theory. Further comparisons between our radiotracer results and those obtained for ⁴⁰Ar⁺ sputtering of unactivated Mo and V samples, determined from elastic backscattering measurements using 12 MeV ¹⁶O ions, show that the techniques give consistent results.     

Ion sputtering of minerals and glasses: a first step to the simulation of solar wind erosion

Selected rockforming minerals (plagioclase, augite, olivine, ilmenite, silicate and metal phases of the meteorite “Brenham”) as well as silicate and phosphate glasses were irradiated with heavy ions (⁴He⁺, ¹⁴N+, ²⁰Ne⁺, ⁴⁰Ar+, ⁵⁶Fe+, Xe⁺ _nat) in the energy range of 50-130 keV in order to study ion-induced sputtering. Sputtering yields were measured independently by means of multiple beam interferometry and particle track autoradiography.

The theory of sputtering by Sigmund, modified by Smith, was used to convert experimental heavy ion sputtering yields to H⁺- and He⁺-sputtering yields of the same target. Taking into account solar wind irradiation conditions at the lunar surface, an estimate of lunar erosion rates due to solar wind sputtering is given for the targets studied.