Low Primary Ion Energies Research Articles

Depth resolution studies in SiGe delta-doped multilayers using ultralow-energy O2+ secondary-ion-mass spectrometry

Improvements in depth resolution using low primary ion energy secondary-ion-mass spectrometry have been demonstrated. This comprehensive study is done using a wide range of impact angles at ultralow energies. In this work, using Ge delta-doped Si samples, we confirm that depth resolution can be improved by lowering the primary ion impact energy at ultralow energy. By varying the angle of incidence from 0° to 70°, we noted that a better depth resolution is achievable not only at normal incidence but over a wider range of impact angles as the probe energy is reduced. The best depth resolution was observed using Ep∼250eV and θ∼0°–40° with full width at half maximum (FWHM) ∼1.5nm and λd<1nm throughout the depth profiled (120nm). Using Ep∼500eV, we observed a good depth resolution of FWHM ∼2.2nm and λd∼1.2nm throughout the depth evaluated at θ∼0°–30°. Using Ep∼1keV, a good depth resolution of FWHM ∼3.5nm and λd∼1.8nm was observed at θ∼0°–20°. The dynamic range was also evaluated, the best being achieved at θ∼50° for Ep∼250eV, θ∼40° for Ep∼500eV, and at θ∼30° for Ep∼1keV. Contributions from roughening and atomic mixing to the depth resolution of δ layers are discussed using the mixing-roughness-information depth model.

Angular distributions of gold sputtered from a (111) crystal: Dependence of spot shapes and of spot and background yields on the primary ion mass and energy and on the target temperature

Angular distributions of gold sputtered from a single crystal with a (111) surface have been investigated as a function of the target temperature (15–550 K), the primary ion energy (0.1–270 keV) and the ion mass (He, Ne and Xe). Using the collector technique in combination with backscattering analysis of the deposits, absolute angular-differential sputtering yields were measured in a {11̄0} plane as well as at various azimuthal angles off this plane. The angular distributions showed the characteristic so-called 〈110〉 and 〈001〉 spots superimposed on an apparently random background. The essential features of the emission pattern, i.e. the shape of the spots and their relative contribution to the total sputtering yield, were found to depend on all experimental parameters. For the first time we have observed, at low primary ion energies, characteristic spots at glancing angles of emission (presumably emission due to correlated sequences in 〈101〉 directions parallel to the (111) surface, terminated by a defocusing collision). The angular dependence of spot and background emission can be described by cosine power distributions, from which the partial sputtering yields due to the spots, ΔY 110 and ΔY 001, and the background, ΔY bg, are derived by integration. The (total) sputtering yields Y thus obtained by summation over all partial yields agree well with available literature data. At low primary ion energies the relative contribution of spot emission to the total sputtering yield amounts to as much as 50%, but decreases with increasing energy to about 25% (15%) for He and Ne (Xe) impact. This finding implies that the angular distributions of particles sputtered from single crystals are sensitive to the energy and momentum distributions in the collision cascade. The ratio of the partial sputtering yields, ΔY 110 ΔY 001 , increases with increasing mean range〈 x 〉 of the primary ions, from about unity for 〈 x〉 = 1 nm to a maximum of 4–5 between 10 and 20 nm and then decreases slowly for larger 〈 x 〉. This result suggests that the range of 〈110〉 focusons may amount to a few nm. Whereas the background distribution remains unaffected by variations of the target temperature, the 〈110〉 spot and the 〈001〉) spots become broader as the temperature increases. At 15 K the half-width of the 〈110〉 spot is only 5°, but twice as large at 550 K. Spot shape analysis suggests that the 〈110〉 spot is composed of two contributions, one being strongly dependent on temperature (“focuson”) and the other one independent thereof. At ion energies between a few keV and about 50 keV the yield ratio R 110 = ΔY 110 Y shows a clearly detectable temperature effect with d R 110/d T = −(2 ± 0.5) × 10 −4/K, which is attributed to a decreasing focuson contribution to the total sputtering yield. High excitation densities in the collision cascade apparently also reduce the relative 〈110〉 spot yields. This effect of “dynamic randomization” of the crystalline structure was observed under Xe bombardment at energies above about 3–10 keV.

Determination of the angle of incidence in a Cameca IMS‐4f SIMS instrument

AbstractAn improved model for the calculation of the angle of incidence of primary ions is presented for describing secondary ion mass spectrometry (SIMS) measurements in the low primary energy region (<7.5 keV) of a Cameca IMS‐4f with positive primary ions and a positive secondary detection mode. The model explains some of the inconsistencies observed between experiment and the approximative model used so far. Our improvement takes into account the effect of the deflection plates for beam positioning, which leads to a change in the angle of the primary ion with respect to the optical axis of the primary column. A comparison with experimental results is accomplished, showing the improvement of the new model. This new model also explains the slope of the crater bottom in the x‐direction at low primary ion energies, owing to a different sputter yield from one side to the other of the crater bottom.

Unimolecular decay measurements of secondary ions from organic molecules by time-of-flight mass spectrometry

Unimolecular dissociations of positive ions ejected by keV Cs + bombardment from para-substituted phenylalanine F-X (X = H, HO, OCH 3 and halides) were studied by time-of-flight mass spectrometry. The first-order rates of dissociation and the corresponding fragmentation pathways were determined with the aid of a movable 45° ion mirror. Variation of ion yields and fragmentation pathways for [M + H] +, [M + Na] +, [M + 2Na - H] +, [M-45] +, and [tropylium] + decompositions were observed for the various substituents. The effect of varying the primary ion energy from 3 to 13 keV was studied for the ion yields and decay constants from phenylalanine (X=H). Although the ion intensities varied significantly, especially at low primary ion energies, the rate constants remained unchanged.

Ion-beam-induced effects in secondary ion mass spectrometry of inorganic salts

In the work reported in this paper, the ion-beam-induced effects which occur (in addition to sputtering) during the bombardment of polyoxyanion salts have been studied. The effects on the secondary ion mass spectra (SIMS) of sulphites, sulphates, nitrites and nitrates, which are found even when using the static mode, are detailed. The dependence of the ion-induced artefacts upon the beam energy, the ion dose and the partial pressure of oxygen in the gas phase is analysed carefully. Experiments with labelled oxygen have also been useful in clarifying the various processes of radiation damage, of which preferential sputtering and recoil implantation of oxygen are the most important. Their relative extents as well as the total sputtering yield determine the equilibrium superficial oxygen content and consequently the relative intensities of the oxygenated molecular ions. Two kinds of recoil-implanted oxygen atoms have been found, one of which accounts for the ion-induced oxidation of sulphite into sulphate.p ]The work reported here also shows that reliable identification of the oxidation state of the non-metal can be achieved under the conditions of low-damage SIMS, i.e., using not only low ion doses but also low primary ion energy.

Low yield sputtering of monocrystalline metals

Sputtering of monocrystalline metals by light noble gas ions is studied experimentally and theoretically at low primary ion energy. Evidence is found for a multiple collision process in which surface atoms are sputtered by backscattered ions. The introduction of the maximum recoil energy E M in the sputtering yield relation enabled us to describe sputtering when small energy transfers are involved.

Combined UHV ion-scattering and secondary-ion mass spectrometer using magnetic analysis

A combined ion-scattering and secondary-ion mass spectrometer, for use in studies of low-energy ion bombardment of solids, is described. The secondary ions from the target (both sputtered and scattered) are analysed by a momentum analyser having a 14 cm radius and 90 degrees magnetic sector which provides their positive identification in mass and energy. It is an ultrahigh-vacuum instrument; inert-gas primary ions of energies in the range 15-2000 eV are used. The principle of operation, constructional details and capabilities are presented. The use of the instrument in secondary-ion mass spectrometry and ion-scattering studies is discussed. The magnetic analysis used allows the sputtered and scattered ions to be distinguished even at very low primary-ion energies (below 100 eV).

The oxidation of aluminum studied by sims at low energies

The adsorption of oxygen by aluminum was studied using secondary ion mass spectrometry at low primary ion energies (⩽ 500 eV) and low primary ion fluxes (⩽ 5 × 10 10 ions/ cm 2/ sec) in an ultrahigh vacuum system. The SIMS characteristics of a cleaned aluminum surface were measured and yield changes of positive and negative ions were measured as a function of oxygen exposure. The results strongly support a model of oxygen uptake in two stages, the first monolayer equivalent being mostly incorporated into the aluminum, probably in the second layer and the next half-monolayer equivalent being superficially chemisorbed. Selection of the correct model is important in calculations of the resonances in the electronic (valence band) structure of free-electron-like metals caused by oxygen adsorption.

Chemische Reaktionskinematik

The velocity spectra and cross sections of reactions of the type X*+D2 → XD++D (X+=Ar+, N2 +, CO+) have been found to be independent of the temperature of the target gas ( — 190° to 20°C). The additional forward scattering (as compared to spectator stripping) of the product ion at high energies cannot be explained by the thermal motion of the D2 molecules. A recoil stripping mechanism is proposed. At low energies, an intermediate XD2* is postulated that lives shorter than half a period of rotation and is forward scattered but quickly equilibrates the excess energy among the vibrational degrees of freedom. The reactions of Ar+ and N2 + with CD4 can well be understood by the stripping model over a wide range of energy. At low energies, no preferential forward scattering of the product ion is found as in the corresponding reactions with D2. The strongly unsymmetric broadening of the product ion band together with a small shift to lower velocities indicate a strong interaction of the incident ion with the CD3 group at low energies. The formation of an intermediate complex XCD4 + which isotropically decays is expected at energies of a few tenth of one eV. Direct measurements at such low primary ion energies have not yet been possible.

Ionization Cross Sections, Charge-Transfer Cross Sections, and Ionic Fragmentation Patterns of Some Paraffins, Olefins, Acetylenes, Chloroalkanes, and Benzene with 40–100-keV Protons

The simple and charge-transfer ionization cross sections for Kr, Ne, N2, CH4, C2H4, and n-C4H10 were measured with 40–100-keV protons. The simple ionization cross sections are almost constant in this range, while the charge-transfer cross sections are roughly 40% of the total ionization at 40 keV and decrease to about 10% at 100 keV. The relative ionization cross sections (both simple and charge transfer) of a variety of molecular compounds (see title) were measured with 100-keV protons and found to be linearly related to the polarizabilities. The mass spectra of a number of hydrocarbons were measured with 50- and 100-keV protons. The mass spectra are qualitatively very similar to spectra obtained with electrons of the same velocity (55 eV for 100-keV protons). This is to be expected from a qualitative extension of the Bethe—Born equation to ionization of molecular electrons. Since the mass spectra change little with velocity the American Petroleum Institute (A.P.I.) fragmentation patterns with 50–75-V electrons are a good guide to ionization by any (singly) charged particle with velocity higher than 108 cm/sec. Fragments of high appearance potentials and doubly charged ions are considerably more abundant in the 40–100-keV proton spectra than at higher proton energies or in the electron-impact spectra. Conclusions can be made as to the charge-exchange mass spectra produced by particles with velocities in the range 108 cm/sec. These mass spectra are very different from charge-transfer spectra obtained at low primary ion energies. Thus there is little dependence on the recombination energy of the primary ion. The charge-transfer spectra seem not very different from the spectra produced by simple ionization. This means that the distribution of internal energies imparted to the new ion cannot be very different from that in pure ionization.