Primary Ion Energy Research Articles

Depth resolution during C60+ profiling of multilayer molecular films.

Time-of-flight secondary ion mass spectrometry is utilized to characterize the response of Langmuir-Blodgett (LB) multilayers under the bombardment by buckminsterfullerene primary ions. The LB multilayers are formed by barium arachidate and barium dimyristoyl phosphatidate on a Si substrate. The unique sputtering properties of the C60 ion beam result in successful molecular depth profiling of both the single component and multilayers of alternating chemical composition. At cryogenic (liquid nitrogen) temperatures, the high mass signals of both molecules remain stable under sputtering, while at room temperature, they gradually decrease with primary ion dose. The low temperature also leads to a higher average sputter yield of molecules. Depth resolution varies from 20 to 50 nm and can be reduced further by lowering the primary ion energy or by using glancing angles of incidence of the primary ion beam.

Static time-of-flight secondary ion mass spectrometry analysis of microelectronics related substrates using a polyatomic ion source

Recent time-of-flight secondary ion mass spectrometry studies using primary ion cluster sources such as Au n +, SF 5 +, Bi n + or C 60 + have shown the great advantages in terms of secondary ion yield enhancement and ion formation efficiency of polyatomic ion sources as compared to monoatomic ion sources like the commonly used Ga +. In this work, the effective gains provided by such a source in the static ToF-SIMS analysis of microelectronics devices were investigated. Firstly, the influence of the number of atoms in the primary cluster ion on secondary ion formation was studied for physically adsorbed di-isononyl phthalate (DNP) (plasticizer) and perfluoropolyether (PFPE). A drastic increase in secondary ion formation efficiency and a much lower detection limit were observed when using a polyatomic primary ion. Moreover, the yield of the higher mass species was much enhanced indicating a lower degree of fragmentation that can be explained by the fact that the primary ion energy is spread out more widely, or that there is a lower energy per incoming ion. Secondly, the influence of the number of Bi atoms in the Bi n primary ion on the information depth was studied using reference thermally grown silicon oxide samples. The information depth provided by a Bi n cluster was shown to be lowered when the number of atoms in the aggregate was increased.

Evaluation of sputtering rate change in the silicon transient region under medium energy O 2+ sputtering

A sputtering rate near the silicon surface is different from that in the bulk during secondary ion mass spectrometry (SIMS) analysis. It is important to evaluate the sputtering rate change in the transient region in order to obtain the correct depth profile. In this study, we report the sputtering rate change in the transient region under various oxygen bombarding conditions. A sample measured was multiple BN delta layers separated by Si spacing layer. We used MRI analysis applied to SIMS profiles in order to determine the accurate delta positions. In concluding, when the incident angle is smaller than 27° in which SiO 2 is formed, the sputtering rate at the surface is faster than that in the bulk and its difference becomes larger as the primary ion energy becomes larger. When the angle of incidence is 63°, the native oxide at the surface makes the sputtering rate slightly slower than in the bulk.

Ultralow-energy SIMS for shallow semiconductor depth profiling

We present a comprehensive secondary ion mass spectrometry (SIMS) study performed at ultralow energies using both oxygen and cesium primary ions, in positive and negative SIMS for silicon depth profiling, respectively. Variations in surface transient widths and depth resolution are reported as a function of primary ion energy (250 eV–1 keV) over a wide range of incidence angles (0–70°). The instrument used is the Atomika 4500 SIMS depth profiler and the sample was Si with 10 delta-layers of Si 0.7Ge 0.3. Optimum profiling conditions are found that are useful for silicon ultrashallow profiling.

High depth resolution SIMS analysis using metal cluster complex ion bombardment

SIMS depth profiles were measured using metal cluster complex ions of Ir4(CO)7+ as a primary ion beam in order to obtain high depth resolution. Depth resolution was evaluated as a function of primary ion species, energy and incident angle using a multiple boron delta-doped silicon sample. The depth resolution obtained using cluster ion bombardment was considerably better than that obtained by oxygen ion bombardment under the same bombardment condition due to reduction of atomic mixing in the depth. The best depth resolution was 0.9 nm under the bombardment condition of 5 keV, 45° with oxygen flooding, which approaches the value measured with state of the art SIMS analyses. However, depth resolution was not improved by decreasing the cluster ion energy (less than 5 keV), even though the roughness of the sputtered surface was suppressed. The limit of depth resolution improvement may be caused by a carbon cover-layer that prevents the formation of surface oxide that buffers atomic mixing. To overcome this issue, it will be necessary to eliminate carbon from the cluster ion.

On the emission of MoCs+ molecular ions from Cs+ irradiated molybdenum surface

The present paper deals with the emission of atomic and molecular ions from elemental molybdenum surface under Cs+ bombardment to explore the MCs+ formation mechanism with changing Cs surface coverage. Integrated count of MoCs+ shows a monotonic increase with increasing primary ion energy (1–5keV). Change in MoCs+ intensity is attributed to the variation of surface work function ϕ and cesium surface concentration cCs due to varying impact energies. Variation of cCs has been obtained from the expression, cCs∝1/(1+Y) where Y is the elemental sputtering yield estimated from TRIM calculations. Systematic study of the energy distributions of all species emerging from Mo target has been done to measure the changes in surface work function. Changing slopes of the leading parts of Cs+ energy distributions suggest a substantial depletion in surface work function ϕ with decreasing primary ion energies. Δϕ shows a linear dependence on cCs. The maximum reduction in surface work function Δϕmax=0.69eV corresponds to the highest value of cCs=0.5. A phenomenological model, based on the linear dependence of ϕ on cCs, has been employed to explain the MoCs+ data.

Energy Shift During Ion Extraction from an ICP Source

In this work, the deviation of energy distribution function of energetic ions from the predetermined value in an inductively coupled plasma (ICP) ion gun source is discussed. An abnormal plasma potential increase at an extraction voltage 400 V caused a beam energy shift of up to 50 eV compared to the preset value. The ion energy peak position was found to be more aected by pressure at higher extraction voltages on the acceleration grid. The energy of primary and secondary ions was measured with an ion energy analyzer. The evolution of the ion energy distribution function during the accelerating voltage change was studied in the pressure range from 3 ◊ 10 3 to 1.1 ◊ 10 2

Surface transient effects in ultralow‐energy Cs+ sputtering of Si

AbstractThis study examines the dependence of the sputter rate and the transient width (ztr) as a function of Cs+ primary ion energy (impact energy (Ep) = 320 eV, 500 eV and 1 keV) and incident angles between 0 and 70° . The instrument used was the ATOMIKA 4500 SIMS depth profiler and the sample was Si with ten delta layers of Si0.7 Ge0.3. We observed the narrowest transient widths of between 1.4 and 2.0 nm apparent depth. This was achieved at incident angles (θ) of 30–50° . An extended transient effect was observed when profiled at θ > 50° . Below this incident angle, the transient width is less than twice the penetration depth (ztr < 2Rnorm). At minimum ztr, ztr ≈ Rnorm. The detection sensitivity is best achieved at θ ≈ 30° for all energies investigated. The sputter rate is lowest at normal incidence, rising gradually to a maximum at θ ≈ 50–60° . This is similar to that observed with ultralow‐energy O2+ primary ion beams.1 At ultralow energies, reducing Ep does not have a significant effect in reducing ztr. We conclude that for Ep < 1 keV, the optimum condition to achieve minimum ztr while maintaining good sensitivity and high sputter rate is at θ ≈ 30° . Copyright © 2007 John Wiley & Sons, Ltd.

Measurement of sputtering yields and damage in C60 SIMS depth profiling of model organic materials

AbstractSputter‐depth profiles of model organic thin films on silicon using C60 primary ions have been employed to measure sputtering yields and depth resolution parameters. We demonstrate that some materials (polylactide, Irganox 1010) have a constant and high sputtering yield, which varies linearly with the primary ion energy, whereas another material (Alq3) has lower, fluence‐dependent sputtering yields. Analysis of multi‐layered organic thin films reveals that the depth resolution is a function of both primary ion energy and depth, and the sputtering yield depends on the history of sputtering. We also show that ∼30% of repeat units are damaged in the steady‐state regime during polylactide sputtering. Crown Copyright © 2006. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

A study of theoretical depth resolution of GaAs/AlAs reference material with low energy inert‐gas ions by Monte Carlo simulation

AbstractA Monte Carlo (MC) simulation program written in C++ has been newly developed to describe the dynamic processes of depth profiling with low energy ions. This MC simulation was applied to the depth profiling of GaAs/AlAs reference material for Ne+, Ar+, and Xe+ ions to elucidate the depth resolution attained by surface analytical techniques. The result clearly predicts that there is a considerable difference between the depth resolutions estimated from the leading and trailing edges of Ne+ and Xe+ ions, whereas the difference is quite small for Ar+ ions.Systematic investigation of the dependence of theoretical depth resolution on primary ion energy has revealed that the preferential sputtering primarily caused by the difference in energy transfer to target atoms through elastic collisions between incident ions and target atoms results in the difference between the leading and trailing edges.The inclusion of other factors, e.g. preferential sputtering effect caused by the metallization of Al atoms on the topmost surface, etc. for further improvement of the MC simulation modeling before accommodating quantitative arguments on the depth resolution is strongly recommended. Copyright © 2006 John Wiley & Sons, Ltd.

Experimental determination of particle range and dose distribution in thick targets through fragmentation reactions of stable heavy ions

In radiation therapy with highly energetic heavy ions, the conformal irradiation of a tumour can be achieved by using their advantageous features such as the good dose localization and the high relative biological effectiveness around their mean range. For effective utilization of such properties, it is necessary to evaluate the range of incident ions and the deposited dose distribution in a patient's body. Several methods have been proposed to derive such physical quantities; one of them uses positron emitters generated through projectile fragmentation reactions of incident ions with target nuclei. We have proposed the application of the maximum likelihood estimation (MLE) method to a detected annihilation gamma-ray distribution for determination of the range of incident ions in a target and we have demonstrated the effectiveness of the method with computer simulations. In this paper, a water, a polyethylene and a polymethyl methacrylate target were each irradiated with stable 12C, 14N, 16O and 20Ne beams. Except for a few combinations of incident beams and targets, the MLE method could determine the range of incident ions R MLE with a difference between R MLE and the experimental range of less than 2.0 mm under the circumstance that the measurement of annihilation gamma rays was started just after the irradiation of 61.4 s and lasted for 500 s. In the process of evaluating the range of incident ions with the MLE method, we must calculate many physical quantities such as the fluence and the energy of both primary ions and fragments as a function of depth in a target. Consequently, by using them we can obtain the dose distribution. Thus, when the mean range of incident ions is determined with the MLE method, the annihilation gamma-ray distribution and the deposited dose distribution can be derived simultaneously. The derived dose distributions in water for the mono-energetic heavy-ion beams of four species were compared with those measured with an ionization chamber. The good agreement between the derived and the measured distributions implies that the deposited dose distribution in a target can be estimated from the detected annihilation gamma-ray distribution with a positron camera.

Depth resolution improvement in secondary ion mass spectrometry analysis using metal cluster complex ion bombardment

Secondary ion mass spectrometry analyses were carried out using a metal cluster complex ion of Ir4(CO)7+ as a primary ion beam. Depth resolution was evaluated as a function of primary ion species, energy, and incident angle. The depth resolution obtained using cluster ion bombardment was considerably better than that obtained by oxygen ion bombardment under the same experimental condition due to reduction of atomic mixing in the depth. The authors obtained a depth resolution of ∼1nm under 5keV, 45° condition. Depth resolution was degraded by ion-bombardment-induced surface roughness at 5keV with higher incident angles.

Sputtering yields of PMMA films bombarded by keV C 60+ ions

A quartz crystal microbalance (QCM) has been used to determine total-mass sputtering yields of PMMA films by 1–16 keV C 60 +,2+ ion beams. Quantitative sputtering yields for PMMA are presented as mass loss per incident ion Y m . Mass-lost rate QCM data show that a 13 keV C 60 cluster leads to emission equivalent to 800 PMMA molecules per ion. The power law obtained for the increase in sputtering yield with primary ion energy is in good agreement those predicted by “thermal spike” regime and MD models, when crater sizes are used to estimate sputtering.

Interfacial dynamics of the rhomboidal pyramid pattern on ion-eroded Cu(110)

Consecutive transitions between regular and periodic nanostructures induced by ion bombardment on the Cu(110) surface have been studied as a function of the sputtering parameters (primary ion energy, substrate temperature, and ion flux). The morphologies can vary from the well-known ripple patterns oriented along the two main symmetry directions of the fcc(110) substrate, to mounded structures, and include the far from equilibrium rhomboidal pyramid motif recently observed on the Rh(110) surface. The dependence of the nanostructure facet slope and lateral separation from ion energy allows us to identify a morphological regime accessed for low ion energy, below $500\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, corresponding to the formation of rhomboidal pyramids. The dependence from the ion flux and substrate temperature bears strong similarities with a growth experiment, and is determined by the relaxation of isolated adatom and/or vacancy clusters created in the topmost surface layer by an ion impact. The selection of preferential step orientation and slopes follows from a delicate balance between the diffusion currents along the two main diffusion channels, the $⟨1\overline{1}0⟩$ and the ⟨001⟩.

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.

Ion beam effects on dust grains: 2—Influence of charging history

Dust grains in space are charged by various processes. Impacts of energetic ions lead to deposition of positive charge on the grain, increasing the grain potential and, as a consequence, the electric field at its surface. The accumulated charge is spontaneously released as an emission current when the electric field reaches a threshold. This discharging current is usually attributed to field ionization of any gas surrounding the grain or to ion field emission and would thus be predominantly a function of the surface potential. However, preliminary studies [Velyhan A, Z˘ilavý P, Pavlů J, S˘afránková J, Němeček Z. Ion beam effects on dust grains. Vacuum 2004;76:447–55] using melamine formaldehyde spheres have shown that the discharging current depends strongly on the energy of primary ions. The present paper continues these investigations with the motivation to understand the whole charging/discharging process. The experiment is based on the capture of a single dust grain in an electrodynamic quadrupole. The trapped grain is exposed to an ion beam with different energies up to 5 keV and its charge and surface potential are estimated from the frequency of its oscillations in the quadrupole. The charging/discharging currents are determined from temporal changes of the grain charge. Our results suggest that the grain charge is accumulated in a thick surface layer of non-conducting samples. The thickness of this layer depends on the mass and energy of primary ions. On the other hand, the beam ions probably recombine on the metallic surfaces and create an adsorbed layer there. We believe that the main discharging process is field desorption complemented in this particular case with post-ionization.

Identification of ion fragments produced from thymine and deuterated thymine by low energy ion impact in films and electron impact in the gas phase

The chemical composition of charged fragments desorbing from thymine-methyl-d 3-6-d (T d) films during 10–200 eV Ar + ion irradiation is studied by mass spectrometry of positive and negative ions. The resulting mass spectra are compared to those obtained from similar films of thymine (T), and from 70 eV electron impact on both T and T d in the gas phase. Ion impact on T and T d films produces numerous positive and negative ion fragments via endocyclic and exocyclic bond cleavage, even at ion energies well below 60 eV. The major cations desorbing from T films are identified as HNCH +, HN(CH)CCH 3 +, [T–OCN] +, OCNH 2 +, C x H y + ( x = 1–3 and y = 0–4), and [T + H] +. While ion impact on T and T d films produces a new fragment [T–O] +, [T d–O] +, not seen in gas phase electron impact, the latter yields CO + fragments that are not observed during ion impact on the films. Anion desorption is dominated by H −, O −, CN −, OCN −, and [T–H] − formation, with lesser dissociation channels leading to desorption of C 2 −, C 2H −, C 2CN −, NC 3H 2 −, HNC 3H 3 −, OC 3H 3 −, C 2OCN −, and C 3H x − ( x = 2 and 3). Measurements of the primary ion energy dependence of the fragment desorption yields show that positive ion fragments appear at energies near 15–20 eV, while generally the endocyclic fragments appear at lower energies than the exocyclic (anion and cation) fragments. Our results show that even at very low ion energies, thymine is sensitive to complete fragmentation, whereby the loss of the HNCH (or HNCD from T d) fragment, i.e., ring cleavage involving the N1 and C6 atoms, dominates; in cellular DNA, this would correspond to complete loss of the base.

Surface transient effects in ultralow‐energy O2+ sputtering of silicon

AbstractIt is known that by lowering the impact energy the sputter rate and surface transient width in SIMS will be reduced. However, few studies have been done at ultralow energies over a wide range of impact angles. This study examines the dependence of sputter rate and transient width as a function of O2+ primary ion energy (Ep = 250 eV, 500 eV and 1 keV) and incidence angles of 0–70°. The instrument used is the Atomika 4500 SIMS depth profiler and the sample was Si with 10 delta‐layers of Si0.7Ge0.3. We observed that the lowest transient width of 0.7 nm is obtainable at normal and near‐normal incidence with Ep ∼ 250 eV and Ep ∼ 500 eV. There is no significant improvement in transient width going down in energy from Ep ∼ 500 to ∼250 eV. The onset of roughening is also not obvious at Ep ∼ 250 eV over the whole angular range studied. Although the sputter rate during the surface transient is normally different from that at steady state, only at Ep ∼ 250 eV was it observed that the sputter rate remained fairly independent of depth. We conclude that the best working ranges to achieve a narrow transient width and accurate depth calibration are at Ep ∼ 250 eV/0° < θ < 20°and 500 eV/0°< θ < 10°. Copyright © 2005 John Wiley & Sons, Ltd.

Transmission electron microscopy study of low‐energy ion‐induced damaged layer

AbstractA low‐energy Ar+ ion‐induced damaged layer on a GaAs surface was investigated using transmission electron microscopy. The dependence of the thickness of the damaged layer on the incident energy and angle of primary ions was studied. The primary energies of Ar+ ions were 200, 100 and 50 eV, which were adjusted by setting the accelerating voltages at 200, 100 and 50 V, respectively. The energies of primary Ar+ ions were measured by a laboratory‐made compact retarding‐field analyser, confirming that the peak energies were ∼180, ∼88 and ∼44 eV for accelerating voltages of 200, 100 and 50 V, respectively. The angles of incidence were 90°, 60° and 30° from the sample surface. The thickness of the damaged layer was ∼4.3 nm for 200 eV Ar+ ion irradiation at the incident angle of 90°. By lowering the primary energy from 200 to 50 eV and setting the incident angle at 30°, the thickness of the damaged layer was reduced to ∼1.5 nm. The present results confirmed that lowering the primary energy of ions to 50 eV and a glancing incidence are significantly effective for achieving higher depth resolution in sputter depth profiling. Copyright © 2005 John Wiley & Sons, Ltd.

Electronic stopping of keV nitrogen ions interacting with a Pt(110)(1 × 2) surface—a tool to characterize electronic surfaces

AbstractIon channeling is used to investigate the electronic density corrugation at surfaces by analysing the electronic stopping behaviour of ions scattering grazingly off a clean single crystalline Pt(110)(1 × 2) surface. We use the fact that under these conditions the elastic contribution can be separated from the inelastic energy‐loss processes, which are closely related to the electronic density sampled along the projectiles trajectories. Therefore, we have a direct probe of the surface electronic density structure. Here, experiments and their theoretical interpretation for N+ ions at primary ion energies of 1–10 keV are presented. Multi‐peak features of the energy spectra, depending on the azimuthal surface orientation, are attributed to different particle trajectories. A detailed analysis of trajectory calculations affords an unambiguous assignment of the observed peak structures to three trajectory classes, each sampling specific energy losses. The comparison of the experimental energy‐loss values with results obtained from a surface‐adapted energy‐loss model for ion scattering that includes the electron density allows us to investigate the electronic surface of the sample. The results are compared with calculated surface electron density contours. Reasonable agreement is obtained, showing that the presented method is suitable to characterize the electronic density structure at crystalline surfaces. Copyright © 2005 John Wiley & Sons, Ltd.