Ion-target Combinations Research Articles

On the ion bombardment induced cone/pyramid apex angle

The cone/pyramid apex angle dependence on ion energy has been measured on 20 to 80 keV argon-sputtered copper having well-defined initial surface orientation. It has been found that the present experimental data are contained within the limits predicted by the various theoretical models. Our experiments have shown that the cone/pyramid apex angle is dependent not only on the ion-target combination and the ion energy but also on the crystal orientation of the bombarded sample. A critical assessment of the published formulae for estimating cone angles has been made.

On the intensity of secondary Cu+, Cu+2, and Mo+ ions as a function of bombarding ion energy

The effect of Zn+ and Cd+ primary ions with energies between 3 and 10 keV on the emission of secondary Cu+, Cu+2, and Mo+ ions from polycrystalline copper and molybdenum target surfaces has been investigated. It was found that secondary ion intensity increases almost linearly with bombarding ion energy, attains a broad maximum, and then decreases steadily. It has also been observed that the location of the maximum depends strongly on the ion-target combination. The positions of maxima were found to be around 7.5, 6.5, and 5.0 keV for Zn+-Cu, Cd+-Cu, and Cd+-Mo combinations, respectively. Our finding could be experimental evidence of the fact that the nontextured surfaces may become textured under ion bombardment.

Monte Carlo simulation of the interaction of energetic ions passing through gas filled systems

The behaviour of energetic heavy ions in gases is simulated by means of a Monte Carlo method taking into account scattering, charge-exchange, and energy loss of particles. By this program calculations have been performed to study the influence of a gas filling on the focusing properties of quadrupole triplets. Only for light gases such as He an improvement of the focusing qualities may be expected. Furthermore the energy straggling of charged particles in gas targets has been calculated for some ion-target combinations and compared with the theory of Tschalär and with experimental data. Taking into account fluctuations of the ionic charge, energy straggling of heavy ions computed by this Monte Carlo program exceeds values predicted by current theories to a high degree but fits to experimental data.

Influence of electronic straggling on ion range distributions

The range straggling of ions above the stopping power maximum is shown in a large class of ion-target combinations to include a strong contribution from electronic collisions. Existing calculations based purely on the statistics of nuclear collisions may therefore have underestimated the true range straggling.

A semiempirical formula for the energy dependence of the sputtering yield

Abstract We have derived a simple empirical formula for sputtering yields of monatomic metals and semiconductors, taking the threshold effect for knock-on of atoms into account in the Sigmund theory. It is found that this semi-empirical formula fits to the energy dependence of the sputtering yields over a wide range of ion-target combinations. The best fit values of the parameters for the formula are given as a function of the ratio of the mass of target atoms and the mass of incident ions.

Important sputtering yield data for tokamaks: A comparison of measurements and estimates

Ion and neutral atom sputtering is a potential source of impurity release from the first walls on tokamaks. Sputtering yield data for the relevant ion target combinations are therefore needed. Most extended sputtering data are available for the total yield at normal incidence. These values can be described by an empirical formula which is useful for a first guess at impurity release caused in tokamaks by sputtering. Detailed calculations of the impurity fluxes in tokamaks as given by different plasma transport codes call for more specific data, such as the yield dependence on the angle of incidence and the ejection conditions for the sputtered particles. These data are only available for very few ion target combinations. Comparison of these data with calculated sputtering values (MARLOWE, TRIM, ANISN) gives reasonably good agreement. The calculated data may therefore be used in cases where experimental data are not available. Most of the data referred to in this work are obtained at an irradiation fluence well above the corresponding value in today's tokamaks. However, the experimental yield data show strong influence of surface contaminations (oxygen). The data given in the literature are therefore only relevant for tokamaks under similar surface conditions (careful surface cleaning or quasi-continuous operation as in reactors). Also, the high limiter temperature may cause modification of the impurity release rate by either chemical or thermal effects. A few examples of chemical and thermal effects are discussed.

An analytical formula and important parameters for low-energy ion sputtering

Sputtering yields for different ions and materials at low ion energies have a similar energy dependence. Due to this similarity, yield data can be characterized by a normalized energy function and two parameters for each ion target combination. One of these parameters is the threshold energy. An energy scaling can be based on this parameter. The other parameter is a multiplication factor. Both parameters depend mainly on the ion and target mass M1 and M2 and on the surface binding energy EB. An analytic expression for the normalized functions and both the parameters is given. This empirical relation also allows an estimate of unknown sputtering data, if M1, M2, and EB are noted. A physical interpretation of the empirical relation is given for the case M1≪M2, as in this case special collision processes which dominate the sputtering can be identified.

Stopping ratios of 50–300 keV light ions in metals

With 50–300 keV H +, He +, 7Li +, and Be + stopping cross sections and stopping ratios are measured for carbon, aluminium, copper, silver, and gold. The thickness of the target foils is calibrated with 100–200 keV He +, using the stopping cross sections fitted in ref. 1. Based on this thickness calibration the stopping cross sections for H +, 7Li +, and Be + can be measured with an error smaller than 7%. Foil experiments prove to be a very appropriate method for determining ratios of stopping cross sections. These ratios can be evaluated to a precision of 2–4%. The stopping ratios were found to be independent of the target material or the ion within 7% for most ion-target combinations, i.e. the simplified stopping function can be verified within this limit.

Defect clustering in copper, silver and aluminum during heavy-ion irradiations at low temperatures

Abstract Point defect clustering during heavy-ion irradiation at low temperature was investigated in copper, silver, and aluminum using electrical-resistivity measurements. Clustering effects were studied by determining the average recombination volume per Frenkel defect, V, as a function of defect concentration. Within single unoverlapped displacement cascades, the clustering increased with increased energy density in the cascades. For Ar-irradiated aluminum no clustering was observed in individual cascades, whereas for Kr-irradiated silver V was deduced to be V 0.2 Vo, where Vo is the value of V for unclustered defects. Clustering was observed for all ion-target combinations at sufficiently high dose. For copper and silver at high doses, a regime was reached in which additional irradiation increased the Frenkel defect number N without increasing the total recombination volume N V. Defect clustering in copper and silver during isochronal annealing of damage produced by 150-keV proton irradiation was inve...

Low energy light ion sputtering of metals and carbides

Abstract For a great number of ion-target combinations similarities in low energy sputtering have been found. 11Near the threshold energy for sputtering E th, the energy dependence of the sputtering yield can be written as S = Y (E/E th)1/4 (1 – E h/E)7/2, E th and Y depend only on the surface binding energy E B and the ion-target atom mass ratio. Using this relation the energy dependence of the sputtering yield of B4C, SiC, TiC, TaC, WC is compared with the yields from S, Si, Ti, Ta, W, for H, D and He in the energy range of 50 eV to 8 keV. A strong similarity of the energy dependence, i.e. Eth , and Y, can be found between the carbide and the heavier component of the compound. This leads to the assumption, that the target surface is depleted of the light component due to preferential sputtering and the sputtering process is dominated by the heavy component.

Metastable Phases in Metals Produced by Ion Implantation

Metallic alloys are usually produced by equilibrium or quasiequilibrium methods. The constituents of the alloy including any desired impurities can be mixed together, melted, and the resultant liquid allowed to solidify and cool. Because the cooling rates are normally fairly slow, the final alloy is usually stable against thermal annealing at operating tempera­ tures and times. In some cases, an alloy may be rapidly cooled or quenched in order to produce a material with a particular physical property. This alloy is metastable with respect to thermal annealing, and the property will be lost if the material is heated to a sufficiently high temperature. The alloy was produced, however, from an equilibrium phase, perhaps a liquid, at a higher temperature. This limits conventional studies of metastable alloys to those that can be formed by quenching from an equilibrium phase. Ion implantation is a method for introducing impurities into the surface layer of a solid in a nonequilibrium fashion. Ions of the desired impurity are accelerated to an energy (generally in the range of 20-200 keY) related to the desired depth of the implanted layer and directed onto the surface of the material to be implanted. The ions penetrate the surface of the material and come to rest some distance below the surface. The impurity concentration distribution in depth is usually almost Gaussian with the peak of the distribution at a depth of a few hundred to a few thousand angstroms, depending on the ion-target combination and the

Measurement of the sputtering yield by auger electron spectroscopy

The ion-induced sputtering yields from monatomic solids at normal incidence are presented graphically for various ion–target combinations as a function of the incident ion energy. To supplement the experimental data, sputtering yields are calculated by the Monte Carlo simulation code ACAT for some ion–target combinations. Each graph shows the available experimental and ACAT data points, together with the sputtering yields calculated by an empirical formula whose parameters are determined from the best fit to available data.

The electronic and nuclear stopping of energetic ions

The electronic contribution to stopping of 127 ion-target combinations is calculated by the use of an expression for scaling from proton stopping powers. The scaling is found to be accurate, with a standard error of 4.5% relative to experimental measurements. Also, an evaluation is made of a recently proposed nuclear stopping formalism and evidence is found that it is more accurate than LSS nuclear stopping.

Secondary ion emission studies of the range profiles of implanted ions

Abstract The apparatus described provides a universal technique for determining the range profiles of any ion species which has been implanted in a target. In this work a previously implanted sample is bombarded in the target chamber of a stainless steel UHV system by a low energy (1 keV) ion beam. Typical beam intensity is 1 μA/cm2 on the target. The resulting sputtering process causes the target surface to recede at rates of the order of 2 A/min. A fraction of the sputtered particles, consisting of both implanted species and target species, is ejected in the ionized state so that by using a specially modified quadrupole mass spectrometer and monitoring the intensity of these secondary ions as the surface recedes, the profile with time of the implanted species can be determined. By calibrating, this can be converted to a depth profile. Results for various ion-target combinations are presented together with an investigation of the effects on ion yield of surface contamination. Also the sensitivity of the ...

Molecular-orbital theory forK-vacancy sharing in atomic collisions

We have calculated the projectile and target $K$-excitation cross sections and their ratio for a series of ion-target combinations using a basis of exact molecular eigenstates for a model system of one electron in the field of two effective nuclear charges. Except for very asymmetric high-velocity collisions, close agreement is found with Meyerhof's formula, which is based on an atomic two-state expansion. In this range, Meyerhof's formula may be considered an analytical fit to our ab initio results. We indicate that the apparent accuracy of Meyerhof's formula is due to mutual cancellation of effects arising from approximations and model assumptions used in his deduction of the cross-section ratio. The significant deviation found at high velocities results from interference between the coupling processes that govern $K$-vacancy production. This possibility was ruled out in Meyerhof's theory.

Metal ion injection into metals III. Irradiation system and experimental techniques

Abstract An irradiation system is described which allows the homogeneous irradiation in an electromagnetic isotope separator of targets over a fixed area independent of the ion energy, while the ion current is measured continuously. The final aperature, defining the irradiation area, is situated directly in front of the target, electrically insulated from it, but at the same potential. Secondary electron suppression is achieved by a variable high positive potential on the target. This same potential also acts as a retarding potential, regulating the energy of the impinging ions. The use of atomic absorption and x-ray fluorescence to determine small amounts of trapped material is also described. The system and methods have facilitated an investigation of trapping and sputtering for metal ion-target combinations. The system is also convenient for producing isotopic targets for nuclear physics experiments.

Metal ion injection into metals I. trapped amounts, sputtering coefficients and the sputtering model of saturation

Abstract The loss of weight of target material as a function of dose of 39 keV ions injected at room temperature, as well as the amount of projectile material trapped, has been measured for 13 metal ion-target combinations in simultaneous experiments. The results are correlated with the sputtering model of saturation which is based on the assumption of no movement of trapped atoms. It is shown that the model is completely inadequate in describing the results and that diffusion of trapped atoms in the target must be occurring. For some ions on Zn, Cd and Pb targets the trapped amount increases linearly with dose in spite of a high sputtering rate.

The dependence of sputtering efficiency on ion energy and angle of incidence

Abstract Earlier measurements of sputtering efficiency of polycrystalline targets (fraction of impinging ion energy leaving the target through sputtering and backscattering) have been extended to higher energies. Lead and copper targets were bombarded with several different projectiles with energies between 80 and 1200 keV. The sputtering efficiency decreases with increasing energy. This decrease is ascribed to the combined influence of changes in the scattering cross section with energy, and to electronic stopping. The results may be described as a function of the mass ratio M 2/M 1 and the reduced energy ∊ only. The sputtering efficiency was measured as a function of angle of incidence of the bombarding ions. To ensure complete collection of sputtered and backscattered particles, it was possible to cover only the region of incidence angle from 0° to 45°. Targets of copper, silver, and lead were investigated with 17 different ion-target combinations. The sputtering efficiency increased with angle of inci...