Specific features of the angular and energy distributions of overfocused sputtered atoms ejected from the (001) Ni face

Peculiarities of the overfocusing of atoms sputtered from the surface of (001) Ni face are studied with the use of molecular dynamics computer simulations. The multivaluedness of the signal of overfocused atoms with respect to initial azimuthalal angle of ejection φ₀ is discovered and found to be associated with different mechanisms of atom scattering. The overfocused atoms form a separate maximum and can be separated from the focused and “proper” atoms in experiments with angle and energy resolution.     

用自洽的蒙特卡罗—流体结合模型模拟溅射过程

 采用自洽的蒙特卡罗－流体结合模型对溅射过程进行模拟,以了解等离子体粒子行为与溅射参数的关系。溅射过程包括气体放电和溅射原子传输。对于气体放电,蒙特卡罗部分模拟快电子和快气体原子,而流体部分则描述离子和慢电子。对于溅射原子传输,蒙特卡罗部分模拟溅射原子的碰撞过程,而流体部分则描述溅射原子的扩散和漂移。模拟的结果包括：等离子体粒子的密度和能量分布;不同电离机制对气体原子和溅射原子电离的贡献;不同等离子体粒子对阴极溅射碰撞的贡献;溅射原子的密度分布;溅射场和溅射粒子相对于入射离子能量和角度的分布;溅射原子经碰撞后在整个等离子体区的分布。     

Effects of the azimuthal-angle focusing of atoms ejected from the (001) Ni and (001) Au faces

The features of the azimuthal-angle focusing of atoms sputtered from the surfaces of the Ni (001) and Au (001) faces are studied by molecular dynamics using computer simulation. The degree of anisotropy of the azimuthal-angle distribution for atoms sputtered from the (001) Au surface is found to be higher than for atoms sputtered from the (001) Ni face. It is shown that two maxima are observed at comparatively small polar observation angles in the azimuthal-angle distribution in the directions in which Wehner spots are formed, i.e., to the left and to the right of the center of the lens formed by two neighbors closest to the ejected atom. The maxima are shifted from the center with a decrease in the polar observation angle. This feature occurs due to the rainbow scattering effect. The contribution of atoms overfocused with respect to the lens center is studied. All overfocused atoms are found to be strongly blocked. When atoms are ejected from the (001) Au face, the interaction cross section strongly increases due to an increase in the atomic number, and the effects of blocking and focusing appear to be more pronounced.     

Angular distribution of copper atoms sputtered with energetic ions

Measurements of the angular distribution of copper atoms which are sputtered by noble gas ions within the energy range between 0.1 and 1 MeV have been carried out for different angles of ion incidence. The hemisphere over the target surface could be studied with a microphotometer inside the sputtering chamber and the distributions can be plotted in tri-dimensional diagrams. The results are in principle similar to those obtained at lower energies. The angle of maximum emission varies with ion energy and with the angle of incidence and can be related to the sputtering yield.     

Sputtered atom transport processes

It is noted that the transport of sputtered atoms can be described in terms of three pressure regimes: low pressure, where no collisions occur during the trajectory of the atom; intermediate pressure, where the atom undergoes perhaps several collisions but does not completely thermalize; and high pressure, where the sputtered atom effectively stops and begins a density-gradient-driven conventional gas-phase diffusion process. The intermediate region is the most complicated to model, given the dependence of the energy on the collision cross-section, the various distributions in energy and angle of the sputtered atoms, and the extended nature of most sputtering sources. Experimental studies reported here have measured the transport probability by observing the distribution of atoms around a chamber following sputtering. The transport is found to be quite dependent on the mass of both the sputtered atom and the background gas, as well as the particle density and geometry of the vacuum system. A strong effect of sputtered-atom-induced gas rarefaction has also been observed. This results in power-dependent transport of sputtered atoms, and as a result may also lead to power-dependent compositional variation in alloy depositions. The general result is that high discharge powers tend to correlate with lower power operation at a significantly lower operating pressure than had been assumed     

Analytical calculation of atom ejection from the Ni (111), Ni (001), and Au (001) surfaces in frames of a three-dimensional model

Atom ejection from lattice sites at the Ni (111) and Ni (001) surfaces in the azimuthal direction toward the center of a lens consisting of two nearest neighboring atoms in the surface plane is calculated using a developed analytical three-dimensional model. The types of scattering of ejected atoms are classified in frames of the constructed model. It is found that the first and second ejection cones are observed in the sputtering pattern in the case of atom ejection from the Ni (111) surface and that the contribution of strongly blocked atoms to sputtering is considerable. The focusing of sputtered atoms at some angle from the surface normal is observed. A maximum of the polar angular distribution of sputtered atoms is shifted nonmonotonically as the energy increases. It is shown that the energy spent by the ejected atom on the recoiling of the lens atoms can be larger than that spent by this atom to overcome the potential barrier. It is found that small changes in the potential hardness and the binding energy at the magnetic phase transition can lead to a qualitative change in the ejection pattern. The expressions for the final ejection angle and energy in the case of Ni in the f-state are found in the form of an expansion in terms of two small parameters. As one passes from the case of atom ejection from the Ni (001) face to the case of atom ejection from the Au (001) face, the interaction cross section increases significantly because of an increase in the atomic number and the effects of blocking and focusing turn out to be considerable.     

The dimer sputtering mechanism of Cu(001) at low bombarding energy

The emission of dimers during bombardment of a Cu(001) surface by Cu atoms of 300 and 1000eV energy is studied. A molecular dynamics simulation method based on many-body potentials is employed. At 300eV bombarding energy, around 81% of the sputtered dimers originate from second-nearest neighbor sites. 58% of these are ejected in a collision sequence correlated by the projectile. These dimers lead to a second maximum in the kinetic energy distribution of emitted dimers at around 8eV, besides a maximum at 4eV. Only the latter is found at 1000eV bombarding energy. As in this case mostly next-neighbor surface atoms are sputtered, the specific emission mechanism found at 300eV is irrelevant. Finally, we show that the direction of the angular momentum of sputtered dimers is correlated with the original surface site of the dimers.     

Sputtering—a review of some recent experimental and theoretical aspects

After a brief outline of the present sputtering theory for a random solid, recent results of the sputtering yieldS for polycrystalline targets are discussed, in particular in view of the influence of the projectile mass and the bombarding angle. The angle dependence ofS at low bombarding energies, and results on the angular distribution of sputtered particles for oblique ion incidence point out necessary modifications of present sputtering theories with respect to the anisotropy of the collision cascades in the solid and the influence of the target surface. The energy distribution of the neutral particles ejected along the target normals is related to the theoretically predictedE ^?2-distribution of low energy recoils in the Recent mass spectrometric studies of postionized sputtered neutrals are discussed in view of the formation of sputtered molecules and the application of sputtered neutral mass spectroscopy for surface analysis. Finally, the paper deals with ion-induced surface effects on non-elementary sputtering targets, and the protracted removal of foreign atoms from a matrix.     

On the energy distribution of sputtered dimers

A theoretical expression is obtained for the energy distribution of sputtered dimers from crystal surfaces. The derivation is based on a model where the atoms which constitute a dimer, are sputtered independently from the crystal according to their respective single particle energy distribution functions. Two neighbouring atoms are then supposed to form a dimer if the sum of their initial relative kinetic energy and potential energy is less than zero. A comparison with experimental results for K₂ and KI sputtered from polycrystalline K and KI surfaces respectively, shows a good agreement.     

Focusing of atoms sputtered from the (001) Ni surface for the Gaussian distribution of ejected atoms

Angular distributions of atoms sputtered from the (001) Ni surface for the Gaussian initial angular distributions of ejected atoms have been studied by the molecular dynamics method using computer simulation. Focusing of ejected atoms has been observed for the relatively wide initial distributions for the emission stage, while the polar angular defocusing has been observed for the narrow distributions. The shift character for the maximum of the polar angular distribution of sputtered atoms being the energy differential has been found to change from monotonous to nonmonotonous with an increase in the initial width of the distribution.     

Monte Carlo simulation of thermalization process of sputtered particles

The transport process of sputtered particles in plasma sputter deposition was studied by computer simulation using the Monte Carlo method, with particular attention to the understanding of the thermalization process. Due consideration was taken of the momentum loss as well as of the energy loss of sputtered particles colliding with sputter gas molecules. The results clearly showed that with increasing target-substrate distance, the energy distribution of sputtered copper atoms arriving at the substrate shifts toward lower kinetic energies, but still contains a considerable fraction of high energy particles. In addition, it was found that the arrival rate of sputtered copper atoms at the substrate decreases exponentially with the target-substrate distance, while the return rate to the target first increases and then becomes constant. It was concluded that the present Monte Carlo simulation can be successfully used for a quantitative estimation of the transport process of sputtered particles.     

A comparison of radiation from sputtered atoms with radiation from projectiles scattered on oxidized magnesium at varying angle of incidence

The intensity of light emitted from sputtered atoms and neutralized, scattered primary ions, excited during 4 keV Ne⁺ and Ar⁺ bombardment of oxidized magnesium has been measured as a function of the incidence angle. It was found that the photon yield of sputtered atoms increases with the angle of incidence much more rapidly than the theoretical sputtering yield and the photon yield of scattered projectiles. In order to explain the experimental results a numerical approach was made based on the following assumptions: (1) the sputtered atom can be excited when it crosses the surface after getting the momentum from the collision cascade; (2) at oblique incidence the sputtered excited atom can be directly emitted after a gentle collision between the incident ion and the surface atom; (3) the neutralized primary ion can only be excited in a violent collision with the surface atom.     

Simulation of the transport of sputtered atoms and effects of processing conditions

Sputter deposition is a complex process; it is obvious that the energy and direction of the particles arriving at the substrate is in close relation with the transport process from the target to the substrate, it is desirable to model this transport of atoms through the background gas. The transport of sputtered Ag atoms during sputter deposition through the gas phase in the facing targets sputtering system studied by Monte Carlo simulation is presented. The model calculates the flux of the atoms arriving at the substrate, their energy, direction and number of collisions they underwent. The dependence of the deposition rates of Ag atoms on the gas pressure and the distance between the targets and substrate were investigated.     

Yields and ionization probabilities of sputtered Inn particles under atomic and polyatomic Aum ion bombardment

The emission of neutral and charged atoms and clusters from a polycrystalline indium surface under bombardment with 5 and 10 keV Au, Au₂, Au₃ and Au₅ projectiles was investigated. Single photon laser postionization was utilized for the detection of sputtered neutral particles. Secondary ions were detected without the laser under otherwise exactly the same experimental conditions. The relative cluster yields were found to be enhanced under polyatomic projectile bombardment, more so the larger the number of atoms in the sputtered cluster. The ionization probability strongly increases with increasing cluster size, but is essentially independent of the projectile impact energy. At a fixed impact energy, the ionization probability of sputtered monomers was found to decrease with increasing number of constituent gold atoms per projectile, but there was no detectable effect for sputtered dimers and larger clusters.     

Determination of angle dependence of sputtering yield for a multicomponent target by the time characteristics of the sputtering process

A method for determining the variations of sputtering yield of multicomponent target based on studies of time characteristics of the sputtered particles flux has been proposed and tested on an example of the angle dependence of sputtering yield. Time dependences of variation in the fluxes of the sputtered atoms of multicomponent oxides at different incidence angles of Ar ions were studied. Angular dependences of the sputtering yield for these compounds were obtained by direct measurements and computations. It has been shown that the angular dependence of the sputtering yield in a stationary mode can be obtained from the time dependences of the fluxes of the sputtered particles.     

Emission Theory of the Sputtering of Amorphous Materials: Self-Sputtering

The dependences of the coefficient of self-sputtering on the type of accelerated ions, their energy, and the angle of incidence on a target are calculated. Satisfactory coincidence between the calculated and experimental results is obtained for С–С and W–W systems. Two mechanisms of the entry of secondary particles into a flow of sputtered atoms are proposed.     

On the velocity distribution of excited Fe-atoms by sputtering of iron

From the velocity distribution of excited sputtered particles detailed information on the excitation process can be obtained. In the present paper the first direct measurement of velocity distribution of excited atoms sputtered from a metal target is presented. The irradiation of the Fe-target was performed with 10keV Ar⁺-ions. The sputtered atoms were detected using pulsed laser induced fluorescence (LIF). The sputtered Fe atoms in the metastable statea ⁵ F ₅ at 0.86 eV shows a much broader distribution, than found for the ground-state atoms, but no energy threshold, implied in the statistical excitation models, was found.     

Depth of origin of sputtered atoms

Estimates are given for the distribution of the depth of origin of sputtered atoms in the low-fluence limit, as well as the corresponding distribution of atoms sputtered into a given energy interval. The former distribution is well described by an exponential profile, with the characteristic depth being consistent with previous results. The latter distribution is characterized by an energy-dependent depth scale and a shape that varies from exponential at low sputtered-atom energies to inverse-power form at higher energies.     

Sputtering studies with the Monte Carlo Program TRIM.SP

The Monte Carlo Program TRIM.SP (sputtering version of TRIM) was used to determine sputtering yields and energy and angular distributions of sputtered particles in physical (collisional) sputtering processes. The output is set up to distinguish between the contributions of primary and secondary knock-on atoms as caused by in- and outgoing incident ions, in order to get a better understanding of the sputtering mechanisms and to check on previous theoretical models. The influence of the interatomic potential and the inelastic energy loss model as well as the surface binding energy on the sputtering yield is investigated. Further results are sputtering yields versus incident energy and angle as well as total angular distributions of sputtered particles and energy distributions in specific solid angles for non-normal incidence. The calculated data are compared with experimental results as far as possible. From this comparison it turns out that the TRIM.SP is able to reproduce experimental results even in very special details of angular and energy distributions.