The sputtering mechanism for low-energy light ions

The computer simulation program MARLOWE which follows the trajectories of energetic ions and recoiling target atoms in solids has been used to calculate sputtering yields for low energy (0.1–10keV) light ions (H, D, T,⁴He). Recoil energy densities were calculated for comparison with analytical theories. The sputtering yields obtained for amorphous Fe agree within a factor of two with experimentally measured values for polycrystalline stainless steel, while the calculated yields for protons on amorphous molybdenum are more than twice the experimental values on polycrystalline material. The calculations show that in the parameter range investigated, ions backscattered in the solid contribute a major part to sputtering. This result confirms earlier calculations of the threshold energy for sputtering which are in agreement with recent measurements. Operated for the United States Department of Energy by the Union Carbide Corporation.     

A simple formula for low-energy sputtering yields

A simple formula for low-energy sputtering yields of elemental targets is presented. The formula follows directly from Sigmund's theory but includes a modified form of the function(M _t /M _p ) and an accurate expression for the nuclear stopping cross section. Good agreement with experimental results is obtained for the projectiles Ne, Ar, Kr sputtering not too reactive surfaces at energies 1 keV. Further experiments are suggested as well as theoretical work concerning the meaning of the surface binding energy and the effect of the surface on the sputtering process in general.     

On the influence of backscattered electrons on the lateral resolution in scanning auger microscopy

A definition of edge resolution is proposed, which is adapted to the pecularities of scanning Auger microscopy. Based on recent monte-Carlo computer simulations for scanning electron microscopy, the influence of backscattered electrons on the edge resolution is estimated for low-Z (Al) and high-Z materials (Au). The resolution is found to be of the order of 100nm and to be nearly independent of the atomic number of the sample.     

AES investigations of Ar+ ion retention in Si during Ar sputtering

Argon retention in silicon has been studied by AES in the energy range between 1 and 15 keV at bombardment fluences up to ∼10¹⁸ ions/cm². AES data of implanted argon in silicon near the surface region, as obtained during sputtering, can be interpreted qualitatively by a simple model of ion collection. Discrepancies between calculated and measured saturation values of collected argon ions indicate that during implantation at high fluences addition surface effects become important and that the simple model of ion collection has to account for this. Quantitative AES correlated with RBS indicates pronounced concentration gradients of argon in silicon near surface regions.     

Computer simulation of low-energy sputtering in the binary collision approximation

The sputtering of amorphous Cu targets by low-energy atoms has been investigated in the binary collision approximation using the computer program MARLOWE. Particular attention was given to the influence of the surface binding model on the results. Calculations were made of the dependence of the sputtering yield on the incident particle direction, energy, and mass. Angular-, energy-, and yield-distributions of the ejected atoms were evaluated. Comparisons with experimental results on polycrystalline targets show that the planar surface binding model is to be preferred over the isotropic surface binding model, especially with regard to the angular- and energy-distributions. Calculated yields are in reasonable agreement with experiment at energies below about 1 keV, but deviate at higher energies, apparently because of crystal correlation effects that are neglected in the amorphous model. Operated by Union Corporation under contract W-7405-eng-26 with the U.S. Department of Energy.     

Mechanisms for the reflection of light atoms from crystal surfaces at kilovolt energies

The computer program MARLOWE was used to investigate the backscattering of protons from the (110) surface of a nickel crystal. Grazing incidence was considered so that anisotropic effects originated mainly from the surface region. The contribution of aligned scattering was studied by comparing the results with similar calculations for an amorphous target. Energy distributions of backscattered particles were investigated for incident energies ranging from 0.1 to 5keV. The structure of these distributions was explained by making calculations for several target thickness. Specular reflection was found to depend on the structure of the first few atomic planes only. The (110) rows in the surface plane were responsible for focusing into surface semichannels. Focusing in these semichannels was found to be the strongest under total reflection conditions (below about 1.3 KeV) while the scattering intensity from surface rows increased with increasing incident energy. The orientation of the plane of incidence was found to have large influence on the relative contributions of the reflection mechanisms involved. Operated by Union Carbide Corporation for the Department of Energy.     

Angular distribution and differential sputtering yields for low-energy light-ion irradiation of polycrystalline nickel and tungsten

Angular differential and total sputtering yields of polycrystalline nickel and tungsten have been measured for 1 and 4 keV H⁺ and 4 keV He⁺ ion bombardment at incidence angles between 0° and 80°. The differential sputtering yields (dY/dΩ) were determined with the aid of the collector technique, whereas the total yieldY was determined from the weight loss of the target during irradiation. Asymmetric angular distributions are observed at oblique angles of incidence, the emission maximum being shifted in forward direction (with respect to the incident ions). Even more pronounced than the change in shape of the emission distribution is an increase in the differential yield:dY/dΩ rises with increasing incidence angle over the whole range of ejection angles, the increase being most prominent in the direction of primary recoil emission. This effect is therefore ascribed to emission of surface atoms in direct projectile-surface atom collisions. On short term leave from Institut für Plasmaphysik der Kernforschungsanlage Jülich GmbH, D-5170, Jülich, Fed. Rep. Germany.     

Energy and mass dependence of isotopic enrichment in sputtering

Silver, copper, and boron (from a boron nitride target) were sputtered with xenon ions. The isotopic composition of secondary ions of silver was measured at ion energies ranging from 300 eV to 3 keV and, for copper and boron, at 2.0, 2.5, and 3.0 keV. An ion gun was used to generate the ion beam. The secondary ions were detected at a small emission angle by a quadrupole mass spectrometer. The secondary-ion flux of silver was found to be enriched in heavy isotopes at lower incident-ion energies. The heavy-isotope enrichment was observed to decrease with increasing primary-ion energy. Beyond 500 eV, light isotopes of silver were sputtered preferentially with the enrichment increasing to a constant value of 1.018. The sputtered flux of copper and boron also indicated constant enrichments (1.008 and 1.281 for copper and boron respectively) in light isotopes at high ion energies. Received: 2 August 2002 / Accepted 9 August 2002: / Published online: 4 December 2002 RID="*" ID="*"Present address: Pacific Northwest National Laboratory, Richland, WA 99 352, USA RID="**" ID="**"Present address: Philips Display Components Company, Ottawa, OH 45 875 USA RID="***" ID="***"Corresponding author. Fax: +1-334/727-8090, E-mail: pkray@tusk.edu     

Energy distributions of low-energy noble gas ions backscattered from a single-crystal nickel surface

The energy distribution of noble gas ions in the energy range below 1 keV from a single-crystalline surface depends on the composition and the structure of the surface, the mass and energy of the impinging ions and the geometrical conditions (e.g. angle of incidence) of the experiment. In the case of He ions the kinematic binary collision theory can be applied. For Ne ions the scattering process is more complicated, good agreement between a multiple scattering model and the experiment is achieved. Deviations from this general behaviour are observed with heavier ions (Ar⁺) and at very low energies. The experimental evidence for the different processes is discussed.     

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.     

Molecular sputtering of fullerite by low-energy bismuth ions

Physics of the Solid State - The interaction of low-energy bismuth ions with a fullerite surface at ion energies in the range from 50 to 200 eV and at target temperatures from 100 to 270°C has...     

Investigation of the local atomic arrangement on surfaces using low-energy ion scattering

Ions with energies around 1 keV are well suited for detecting atoms on solid surfaces and for investigating their relative arrangement. This is due to the large scattering cross-sections which are of the order of 10^–2Ų/sr. The conceptually simple method is limited by the fact that interaction potentials and, more so, charge exchange processes are only approximately known. Progress in low-energy ion scattering has recently been made by applying special scattering geometries and by using alkali ions, in addition to noble gas ions.Among the successful applications there are studies of the arrangement of atomic layers on supported catalysts, ordered adsorption systems on metal surfaces, surface reconstruction, and surface disordering due to defects and thermal motion. Energy spectra of recoil atoms and ions convey additional information. The fundamental physical features of low-energy ion scattering are discussed on the basis of examples of recent results.     

On the chemical sputtering of oxygen-exposed molybdenum

The sputtering of oxygen-exposed molybdenum was studied by means of mass analysis of emitted neutral and charged particles. The irradiation was performed with 8 keV Ar⁺ ions at temperatures of 25° and 485°C. It was found that the enhanced sputtering yield at elevated temperature during oxygen exposure is due to beam-induced desorption of MoO₂ and cascade sputtering of MoO. At this temperature considerable oxygen incorporation also takes place owing to recoil mixing and diffusion.EURATOM Association     

Energy transfer to a copper surface by low energy noble gas ion bombardment

The energy transferred to a copper surface by bombardment with Xe⁺, Ar⁺, and He⁺ ions with kinetic energies in the range 100–4000 eV has been studied by our group in previous experiments. There were significant experimental uncertainties for that data at energies below about 200 eV. The present investigation overlaps the previous work, extends the energy range to 10 eV, and includes data for Ne⁺. Particular emphasis is placed on the energy range below 200eV. A specially designed ion source was employed in these experiments. A polycrystalline copper film deposited onto a highly sensitive calorimeter was used as the target material. The results show that the Xe⁺ ion deposits more than 97% of its energy over the entire range investigated whereas the lighter ions deposit a decreasing fraction of their energy below about 1 keV. The decrease is largest for the lightest ion (He⁺). In all cases the deposited energy is about or more than 70% of the incident energy. It will be shown that the present results are in agreement with previous measurements for copper and are qualitatively in good agreement with computer calculations using the TRIM.SP code.On leave from: Institut für Schicht- und Ionentechnik, Forschungszentrum Jülich GmbH, W-5170 Jülich, Fed. Rep. Germany     

Relative ion sputtering yield measurements by integration of secondary ion energy distribution using a retarding-dispersive Ion energy analyzer

The application of a retarding-dispersive energy analyzer as the pre-filter of a quadrupole mass analyzer has made it possible to combine a standard high-speed Secondary Ion Mass Spectrometer (SIMS) and a high-resolution secondary-ion energy analyzer into one instrument. Data taken with this instrument indicate the presence of very significant high-energy tails in the energy distribution of all observed secondary ions, even with relatively low (2 keV) primary ion energies. The shape of the energy distribution varies widely from element to element, for atomic compared to molecular species sputtered from a clean metal surface, and depends, for a given species sputtered from a metal surface, on the degree of surface oxidation. The variations established in the present work are large enough to introduce in many cases substantial discrepancies between published values of both relative ion sputtering yields and surface elemental concentrations and values obtained by considering the complete energy distribution. Methods of obtaining accurate secondary ion yields by integrating the energy distribution are discussed. Work performed under the auspices of the Division of Physical Research of the U.S. Energy Research and Development Administration.     

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.     

Comment on relative ion sputtering yield measurements by integration of secondary ion energy distribution using a retarding-dispersive ion energy analyzer

The location and width of the peak in the sputtered ion energy distributions of W⁺ from the clean surface can be explained by a simple power dependence on energy for the ionization probability. Research sponsored by NASA under Contract/Grant Number NGR 21-002-096     

On the direct observation of the gas-dynamics of laser-pulse sputtering of polymers

We bring together a wide range of ideas relating to the gas-dynamic effects that are now recognized to play a leading role in laser-pulse sputtering. The ideas are grouped according to three basic models. (a) In the effusion model one deals with particles which are released from a target surface, form a Knudsen layer (KL), and then enter an unsteady adiabatic expansion (UAE). When the release terminates at time t=_r there is an abrupt change at the surface from positive to zero flow velocity, which means that particles moving towards the surface are reflected. The flow breaks up into three regions and analytical solutions exist for all aspects of this flow. (b) In the recondensation model the comportment of the target is initially like that of the effusion model but when the release terminates at t=_r the change at the surface is from positive to negative flow velocity, meaning that particles which move towards the surface recondense. Only numerical solutions presently exist (due to Sibold and Urbassek) but they are sufficient to show that the flow breaks up into two (not three) regions. (c) The outflow model could be described as the escape of gas from a finite reservoir, a well-known problem since it describes some aspects of guns. In its application to laser sputtering it is assumed that bond-breakage occurs rapidly over a characteristic depth and the resulting gas-like particles then flow out in a UAE; there is no formal KL.For part II, in which explicit photographs of laser-sputtered particles are analyzed, see [1]     

Orientation dependence of the survival fraction of molecular ions reflected from a single crystal

The experiment aimed at studying the 30 keV molecular nitrogen ion reflection from the (110)Cu single-crystal face has shown that the reflected molecular ion survival fraction is a strong and non-monotonic function of target orientation relative to the primary beam incidence plane.     

Time-of-flight investigation of the intensity dependence of laser-desorbed positive ions from SrF2

The intensity dependence of the total and specific yields of positive ions desorbed from SrF₂ under 193 nm and 308 nm excimer-laser irradiation has been investigated by the time-of-flight method. The following positive ion species have been detected: F⁺, Sr⁺, Sr⁺⁺, SrF⁺⁺ and SrF ₂ ⁺ . The Sr⁺ and SrF⁺ emission yields are found to increase as E ⁿ, where E represents the laser energy per pulse. The exponent n is related to defect-initiated neutral particle emission and gas-phase ionization. The influence of surface damage on this power dependence is investigated. The F⁺ emission yield showed a quite different behaviour compared to that of the Sr⁺ and SrF⁺ emission. At both wavelengths the total positive ion emission yields saturate at a certain laser energy. In the saturation regime the SrF⁺ emission vanishes and alternative emission of F⁺ and Sr⁺ was observed at both wavelengths, but the total emission yield in the saturation regime (F⁺ + Sr⁺) remained constant. A Scanning Electron Microscope (SEM) was used to investigate the damage spots after laser irradiation for thermal effects.