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.     

A model of the ion sputtering process

This paper proposes a new model of the ion sputtering process. According to this model: (1) Sputtering atoms or ions originate almost entirely from the surface layer. (2) Atoms initially leave the surface in the same charge state in which they exist in the crystal, and are largely neutralized in traversing a region near the surface. (3) Neutralization can be explained in terms of quantum mechanical electron transmission and reflection phenomena that occur when the atom is within about three lattice spacings from the surface. The model upon which the theory is based pictures a potential well at the metal surface caused by the electric field of the departing ion. This potential well completely cancels the effect of the potential barrier normally present at the surface of the undisturbed metal, and for certain distances of the ion from the surface, results in a very high probability of emission of an electron from the metal surface, which then immediately neutralizes the emitted ion. This high neutralization probability explains the very low (10^?2 to 10^?6) sputtered ion yields observed experimentally. The ion sputtering yields for a number of elements were computed and compared with values obtained experimentally. It is found that the theory gives results compatible with experiment, and also provides an, at least qualitative, explanation for some of the heretofore rather puzzling experimentally observed features of ion sputtering.     

Continuum light emission from sputtered species of graphite during ion beam irradiation

Light emission during sputtering of graphite targets with 1–10 keV Ne⁺, Kr⁺ and Xe⁺ beams has been investigated in the 180-600 nm wavelength range. Beside the characteristic lines of sputtered C₁ and C₁ ⁺, a continuum superimposed with a number of broad structures was observed in the 250-520 nm range, and having a maximum at 386 nm. Mass analysis of the sputtered flux confirmed the presence of negative carbon clusters C _m ^- , C₂ ^- being the dominant one. Ion beam parameters i.e. ion mass, energy, current density and ion dose were varied to identify the origin of the continuum emission. On the basis of the experimental results, it is suggested that the continuum is predominantly due to the overlapping of various band systems of sputtered C₂ with a small contribution from the heavier sputtered carbon clusters C_m (m>2). Received 24 September 1999 and Received in final form 11 February 2000     

On the influence of reactive gases on sputtering and secondary ion emission

As a continuation of earlier sputtering yield measurements in an ion microprobe, the influence of oxygen and nitrogen on sputtering yield, ionisation efficiency and depth resolutions has been studied. For inert gas bombardment the yield of Ti and V falls sharply at a certain oxygen exposure. While this decrease in yield can be correlated with an increase in surface binding energy in the case of titanium, cone formation causes the yield to drop for oxygen exposed vanadium. In contrast, during nitrogen bombardment the only effect of oxygen exposure is a drastic increase of the ionisation efficiency; the sputtering yield or the depth resolution Δz/z is hardly influenced by oxygen coverage. As was observed earlier in the case of Cu?Ni layers, Δz is essentially constant for erosion depthsz?800 Å, thus yielding better resolution at large depths than is to beexpected from a sequential layer removal model. The extent of the transition zone Δz, is determined by surface topography and thus depends on the target composition as well as its structure.     

Optical emission spectroscopy of the sputtering process in the triode system

The results of spectroscopic investigation of plane plasma discharge and sputtering processes in the triode system are presented. The forced electric discharge with currents of 1–4 A at an argon pressure of 1 mTorr was studied using the emission spectroscopy method. The spectra of plasma discharge were observed in the 200–1100 nm wavelength range. Two metal targets, gold and silver, were used for sputtering. It was found that a part of sputtered particles is ionized in plasma. The emission spectra of the ionized gold and silver species were observed as a function of target voltage while sputtering. It was shown that the number of ionized metal species depends on the energy of argon ions.     

The emission of neutral clusters in sputtering

The mass, angle, and energy resolved emission of neutral clusters in sputtering was studied for a variety of metals and semiconductors. The main phenomena and results are the following: (i) Cluster emission from a series of transition metals reveals a prominent contribution of clusters to the total flux of ejected particles but there is no simple scaling of cluster intensities with the average sputtering yields. With increasing number of constituents, relative intensities of neutral clusters decrease much faster than those of secondary-ion clusters. (ii) The relative intensities of clusters emitted from amorphous and crystalline semiconductors are identical, but the energy spectra of Ge _n -clusters (n = 1–4) sputtered from Ge (111) peak at a slightly higher energy (1 eV) as compared to spectra taken from amorphous Ge. The intensities of all Ge _n -clusters exhibit the same dependence on emission angle; this holds for both the amorphous and crystalline Ge-sample. (iii) The flux of neutral monomers, dimers, and trimers sputtered from Cu(111), Ni(111), and Ag(111) crystals shows a pronouncedly anisotropic emission along the 110 lattice directions which is ascribed to a momentum alignment in the anisotropic part of the collision cascade. Energy spectra taken along 110 peak at higher energies than those obtained from a random emission angle.     

Contribution of ion emission to sputtering of uranium dioxide by highly charged ions

Measurements of the cluster size (n) distribution of secondary (UOx)+n ions from sputtering of uranium dioxide (UO2) by Ne8+, Ar8+ and Xeq+ ions (q=10, 23) at fixed kinetic energy (81 keV) have been performed with a time-of-flight mass spectrometer. The cluster ion yields Y follow a power law Y(n)n with -2.1<<-1.5. This is in contrast to a statistical recombination of the constituents upon ejection, but in agreement with the predictions of collective ejection models. Such a power law was also observed in the electronic stopping regime with MeV/uions. The exponent is found to decrease with increasing projectile mass (and thus total sputter yield) at fixed kinetic energy. The ratio of emitted ionic clusters to monomers varies from 3 to 4.5 depending on the projectile. The contribution of positive ions to the total sputtering yield amounts to about 0.03%.     

A comment on the effects of Cs on photon and secondary ion emission during sputtering

The effect of Cs on photon and negative ion emission is discussed for situations where the sputtered atom interacts either very weakly or very strongly with the target surface. The experimental data seem to favor the strong interaction case.     

Secondary ion emission from polymer layers by atomic and molecular ion bombardment: Data evaluation based on linear-cascade sputtering theory

This paper constitutes an attempt to rationalise impact-energy dependent yields of molecular secondary ions emitted from polymer samples under bombardment with atomic and molecular primary ions. The evaluation was based upon a comparison with sputtering yields calculated from linear-cascade sputtering theory, including threshold effects. To explore general trends, sputtering yields for carbon, silicon and silver were calculated under impact of normally incident C, F, S, Ga, Xe, Au, SF₅, C_11, C₆₀ and Au₅. The yields of carbon, for example, bombarded with C₆₀ are larger than for Ga by factors of ∼5 and ∼10 at 10 and 100 keV, respectively. However, owing to the fact that the effective threshold energy for sputtering increases with the number of constituents of the projectile, the yields for molecular ion impact start to exceed the yields for atomic ions only at energies between 0.5 and 5 keV. The analysed experimental results relate to molecular ion emission from one monolayer (1 ML) and 9 ML films of polymethacrylate on silver bombarded with Xe and SF₅ ions at energies E between 0.5 and 10 keV. Comparison of (initial) secondary ion yields S⁺ (m/z 143) with calculated sputtering yields suggests that S⁺ constitutes the sum of two contributions. The first, labelled , prevails at low energies and appears to reflect molecule ejection due to the mean effect of nuclear energy deposition (“ordinary” linear-cascade sputtering). The second contribution, , dominates at E > 1.5 keV, increases with a high power of E, but does not correlate with the calculated sputtering yield. It is suggested that is a measure of those impact events which occasionally generate a very high energy density at the surface, thus providing optimum conditions for very efficient ejection of molecules that are located at the rim of the agitated area. The SF₅/Xe secondary ion yield ratios are distinctly different for the two contributions, only about 0.3 for but ∼4 (1 ML) and ∼20 (9 ML) for . The pronounced secondary ion “yield enhancement” frequently reported in the literature for molecular versus atomic ion bombardment appears to be due to an enlargement of the contribution, more so the higher the impact energy and the more massive the projectile. The total (integrated) secondary ion yields, estimated by making use of the reported damage cross sections, were found to be the same for SF₅ bombardment of the 1-ML and the 9-ML samples. This finding calls for more attention towards obtaining high secondary ion yields at minimum sample consumption.     

The effect of sputtering on field ion specimens

A theoretical three-dimensional model is developed for the effect of in-situ ion bombardment on the specimen of a field-ion microscope. The theory takes into account the crystallography of the surface, and some preliminary numerical results are presented for bodycentred cubic structures.     

离子引出收集的沉积与溅射研究

研究离子引发收集过程中的沉积和溅射特性,给出了离子沉积和溅射的数理模型,其中重点分析了结合能、捕获概率和溅射系数这几个参数的物理意义和计算公式,给出了收集板总收集量和损失量.并且用计算机模拟了收集板的收集,给出不同的离子入射能量下入射离子元素沉积厚度和不同元素靶对各入射离子溅射特性的影响.得出以下的结论:随着离子沉积在收集板表面涂层厚度的增加溅射率也增加;离子的引出电压不是越高越好;轻质量离子的总收集率比较小;入射离子沉积和溅射特性和收集板靶原子质量有关,质量轻的金属材料作收集板,有利于提高离子的收集率. 关键词： 溅射 捕获概率 溅射概率     

Angular dependences of sputtering ratio,ion-electron emission coefficient and ion-induced photon emission yield at oblique ion incidence on crystalline targets

The angular dependences of the sputtering ratio, ion-induced photon emission yield and the ion-electron emission coefficient for two faces of a copper'crystal at glancing angles of ion incidence on the targets has been experimentally studied.     

Secondary electron emission of sputtered alumina films

Secondary electron emission yieldδ was measured for thin films of alumina prepared byrf sputtering technique. Single pulse method was used along with 4-gridleed optics system to determineδ. Maximum value of 4·3 was obtained at primary energy of 350 eV. The Dionne’s theory was used to analyse the results and the emission probability escape depth and absorption coefficient of secondaries were also estimated. Fairly good correlation is observed between experimental and theoretical values ofδ for beam energies upto 1 keV.     

The role of sample rotation and oblique ion incidence on quantum-dot formation by ion sputtering

Starting from a theory recently proposed by Kahng et al. that explains the formation of ordered dots by ion sputtering under normal ion incidence, it was demonstrated that extending this theory to ion sputtering under oblique ion incidence with simultaneous sample rotation offers the self-organized formation of dots by ion sputtering for a large variety of different ion/material combinations. While for sputtering under normal incidence the shape of the collision cascade must be anisotropic, where the lateral straggling exceeds the longitudinal straggling, this constraint is not necessary under oblique incidence. Received: 16 May 2001 / Accepted: 10 September 2001 / Published online: 17 October 2001     

Surface morphology for ion-beam sputtered Al layer with varying sputtering conditions

We study the growth morphology of thin macrostructure films which is known to be largely affected by the deposition conditions as thin film nucleation and formation is dependent on the kinetic energy and chemical free energy of the atoms. The ion-beam sputtering technique used for depositing thin layers is due to the advantage over other techniques, e.g. the independent control of many process parameters, such as the pressure and/or the energy of the ion-beam and the substrate temperature. Therefore, the dependence of various sputtering parameters such as: (i) sputtering pressure and/or the rate of deposition and (ii) the effect of substrate temperature on the growth has been studied by depositing a single layer of Al. The variations show some interesting dependencies on the structural parameters for the Al layer deposited which has been understood in terms of thin film growth and nucleation theory.     

Playback performance and microstructure of sputtered tape media formed by facing targets sputtering

Sputtered tape media of a CoPtCr–SiO₂ magnetic layer with a Ru underlayer was fabricated on a thin Aramid film by a facing targets sputtering (FTS) system at room temperature with no cooling. Transition electron microscope (TEM) images show columnar epitaxial growth of hcp-CoPtCr(1 0 0) plane on hcp-Ru(1 0 0). Average grain diameter of 10.2 nm with dispersion of 20.2% was obtained from TEM images. Enrichment of Co and Pt inside grains and segregation of Cr and Si to boundary were confirmed by point energy dispersive spectroscopy (EDS) measurements. Playback performance test on sputtered sample indicated that SNR is higher and PW50 value is lower than that of commercial coated tapes. These good playback properties could come from fine and isolated grain structure of magnetic layer of sputtered sample, as observed by TEM.     

Hydrogen superpermeation resistant to ion sputtering

A nonmetallic monolayer on the surface of a metallic membrane gives rise to the superpermeation of suprathermal hydrogen. This means that virtually the whole implanted flux passes through the membrane, depending neither on membrane temperature nor on its thickness. The fact that superpermeation is degrading at a sufficiently high energy of the incident hydrogen particles was considered to be a ‘natural’ result of sputtering of the monolayer. This view was equally applied to Nb, one of the best metals for superpermeation. It is shown in this article that Nb containing O impurity will be normally superpermeable to suprathermal hydrogen at any incident energy, the earlier observed sensitivity to sputtering being conditioned by the NbC layer originally present at the ‘real’ Nb surface. The dynamic exchange between the surface and bulk O impurity is proven to be responsible for the superpermeability of Nb resistant to sputtering. When the NbC layer is present, it hinders such an exchange. PACS 34.50.Dy; 66.30.-h; 82.65.+r     

Secondary ion emission processes of sputtered alkali ions from alkali/Si(100) and Si(111)

The secondary alkali ion yield vs. the work function change (Δφ) of Na, K and Cs/Si(100) and Si(111) was measured to discuss the details of secondary ion emission processes. In the case of alkali/metal systems, the secondary ion emission is explained by the electron tunneling model. In this model, the ionization of the ejected atom occurs as a result of electron resonant tunneling through the potential barrier separating an atom and a metal, and the secondary ion yield depends on exponentially the work function change of metal surface. For alkali/Si(100) systems, the secondary ion emission processes are explained in terms of the electron tunneling model since the secondary alkali ion yield vs. the work function change (Δφ) follows the exponential manner. However, it is not easy to apply the simple electron tunneling model to our experimental results for alkali/Si(111) systems. There is the essential difference in surface structures between Si(100) and Si(111). Therefore, it is suggested that the local electronic environment around the adsorbates might be taken into consideration for alkali/Si(111) systems.     

Low-temperature ion emission

Technical Physics - The kinetics of alkali metal ion emission from metal surfaces heated to temperatures of 800–2500 K is studied. The ion current relaxation is described by the relationship...     

Optical emission spectroscopy by Ar ion sputtering of Ti surface under O2 environment

Visible light emission from atoms and ions sputtered on a polycrystalline Ti surface was observed under irradiation of 30 keV Ar³⁺ ions. A number of atomic lines of Ti I and II were observed in the wavelength of 250-850 nm. The intensity of Ti II emission increased 1.3-5.6 times by introducing oxygen molecules at a pressure of 5.8 × 10⁻⁵ Pa, whereas that of Ti I decreased 0.5-0.8 times. Factors enhancing or reducing photon intensities were plotted as a function of energy of the corresponding electrons in the excited states for Ti atoms and Ti⁺ ions.