Depth profiling of Irganox‐3114 nanoscale delta layers in a matrix of Irganox‐1010 using conventional Cs+ and O2+ ion beams

Depth profiling of an organic reference sample consisting of Irganox 3114 layers of 3 nm thickness at depths of 51.5, 104.5, 207.6 and 310.7 nm inside a 412 nm thick Irganox 1010 matrix evaporated on a Si substrate has been studied using the conventional Cs⁺ and O₂⁺ as sputter ion beams and Bi⁺ as the primary ion for analysis in a dual beam time‐of‐flight secondary ion mass spectrometer. The work is an extension of the Versailles Project on Advanced Materials and Standards project on depth profiling of organic multilayer materials. Cs⁺ ions were used at energies of 500 eV, 1.0 keV and 2.0 keV and the O₂⁺ ions were used at energies of 500 eV and 1.0 keV. All four Irganox 3114 layers were identified clearly in the depth profile using low mass secondary ions. The depth profile data were fitted to the empirical expression of Dowsett function and these fits are reported along with the full width at half maxima to represent the useful resolution for all the four delta layers detected. The data show that, of the conditions used in these experiments, an energy of 500 eV for both Cs⁺ beam and O₂⁺ beam provides the most useful depth profiles. The sputter yield volume per ion calculated from the slope of depth versus ion dose matches well with earlier reported data. Copyright © 2013 John Wiley & Sons, Ltd.     

Subnanometre depth resolution in sputter depth profiling of Si layers using grazing‐incident low‐energy O2+ and Ar+ ions

The sputter damage profiles of Si(100) by low‐energy O₂⁺ and Ar⁺ ion bombardment at various angles of incidence were measured using medium‐energy ion scattering spectroscopy. It was observed that the damaged Si surface layer can be minimized down to 0.5–0.6 nm with grazing‐incident 500 eV Ar⁺ and O₂⁺ ions at 80°. To illustrate how the damaged layer thickness can be decreased down to 0.5 nm, molecular dynamics simulations were used. The SIMS depth resolution estimated with trailing‐edge decay length for a Ga delta‐layer in Si with grazing‐incident 650 eV O₂⁺ was 0.9 nm, which is in good agreement with the measured damaged layer thickness. Copyright © 2004 John Wiley & Sons, Ltd.     

Preferential sputtering observed in Auger electron spectroscopy sputter depth profiling of AlAs/GaAs superlattice using low‐energy ions

Auger electron spectroscopy (AES) sputter depth profiling of an ISO reference material of the GaAs/AlAs superlattice was investigated using low‐energy Ar⁺ ions. Although a high depth resolution of ～1.0 nm was obtained at the GaAs/AlAs interface under 100 eV Ar⁺ ion irradiation, deterioration of the depth resolution was observed at the AlAs/GaAs interface. The Auger peak profile revealed that the enrichment of Al due to preferential sputtering occurred during sputter etching of the AlAs layer only under 100 eV Ar⁺ ion irradiation. In addition, a significant difference in the etching rates between the AlAs and GaAs layers was observed for low‐energy ion irradiation. Deterioration of the depth resolution under 100 eV Ar⁺ ion irradiation is attributed to the preferential sputtering and the difference in the etching rate. The present results suggest that the effects induced by the preferential sputtering and the significant difference in the etching rate should be taken into account to optimize ion etching conditions using the GaAs/AlAs reference material under low‐energy ion irradiation. Copyright © 2005 John Wiley & Sons, Ltd.     

Use of C60 cluster projectiles for sputter depth profiling of polycrystalline metals

We have investigated the merits of fullerene cluster ions as projectiles in time‐of‐flight secondary neutral mass spectrometry (ToF‐SNMS) sputter depth profiling of an Ni:Cr multilayer sample similar to the corresponding NIST depth profiling standard. It is shown that sputter erosion under bombardment with C₆₀⁺ ions of kinetic energies between 10 and 20 keV provides good depth resolution corresponding to interface widths of several nanometres. This depth resolution is maintained during the complete removal of the multilayer stack with a total thickness of 500 nm. This finding is in contrast to the case where atomic Ga⁺ projectile ions of comparable kinetic energy are used, demonstrating the unique features of cluster projectiles in sputter depth profiling. Copyright © 2004 John Wiley & Sons, Ltd.     

An effect of measurement conditions on the depth resolution for low‐energy dual‐beam depth profiling using TOF‐SIMS

An effect of measurement conditions on the depth resolution was investigated for dual‐beam time of flight‐secondary ion mass spectrometry depth profiling of delta‐doped‐boron multi‐layers in silicon with a low‐energy sputter ion (200 eV – 2 keV O₂⁺) and with a high‐energy primary ion (30 keV Bi⁺). The depth resolution was evaluated by the intensity ratio of the first peak and the subsequent valley in B⁺ depth profile for each measurement condition. In the case of sputtering with the low energy of 250 eV, the depth resolution was found to be affected by the damage with the high‐energy primary ion (Bi⁺) and was found to be correlated to the ratio of current density of sputter ion to primary ion. From the depth profiles of implanted Bi⁺ primary ion remaining at the analysis area, it was proposed that the influence of high‐energy primary ion to the depth resolution can be explained with a damage accumulation model. Copyright © 2013 John Wiley & Sons, Ltd.     

Enhancement of the positive secondary ion yield during low‐energy,dual‐beam depth profiling of polytetrafluoroethylene with 1‐keV Cs+

This work documents the behaviour of the positive secondary ion yield of bulk polytetrafluoroethylene (PTFE) under dual‐beam depth profiling conditions employing 1 keV Ar⁺, Cs⁺ and SF₅⁺. A unique chemical interaction is observed in the form of a dramatic enhancement of the positive secondary ion yield when PTFE is dual‐beam profiled with 1 keV Cs⁺. The distinct absence of such an enhancement is noted for comparison on two non‐fluorinated polymers, polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS). The bulk PTFE was probed using 15‐keV, ⁶⁹Ga⁺ primary ions in dual beam mode under static conditions; 1‐keV Ar⁺ (a non‐reactive, light, noble element), Cs⁺ (a heavier metallic ion known to form clusters) and SF₅⁺ (a polyatomic species) served as the sputter ion species. The total accumulated primary ion dose was of the order of 10¹⁵ ions/cm², which is well beyond the static limit. The enhancement of the positive secondary yield obtained when profiling with 1‐keV Cs⁺ far exceeds that obtained when SF₅⁺ is employed. An explanation of this apparent reactive ion effect in PTFE is offered in terms of polarisation of C? F bonds by Cs⁺ in the vicinity of the implantation site thereby predisposing them to facile scission. The formation of peculiar, periodic Cs_xF_y⁺ (where y = x ? 1) and Cs_xC_yF_z⁺ clusters that can extend to masses approaching 2000 amu are also observed. Such species may serve as useful fingerprints for fluorocarbons that can be initiated via pre‐dosing a sample with low‐energy Cs⁺ prior to static 15‐keV Ga⁺ analysis. Copyright © 2005 John Wiley & Sons, Ltd.     

On the role of ion bombardment parameters in AES sputter depth profiling of Ta2O5/Ta with Ar+ and Xe+

Summary In order to study the influence of the ion bombardment parameters on the achievable depth resolution of AES sputter depth profiles, 500 Å thick Ta₂O₅-layers produced by anodic oxidation of polished polycrystalline Ta-substrates were sputter depth profiled with Ar⁺- and Xe⁺-ions in a Scanning Auger Microprobe. The 90%–10% interface widthsz were measured for bombarding ion energies from 0.5 to 5 keV and angles of incidence of 15°, 33° and 56°, respectively.z reduces from 48 Å for Ar⁺-bombardment at = 15° andE = 5 keV to 20 Å when bombarding at = 56° andE = 1 keV. The corresponding values for Xe⁺-bombardment are 31 Å and 18 Å. The influence of the ion bombarding energy and angle on the interface broadening is discussed by means of a simple model. From corresponding evaluations the maximum transportation length of layer species into the substrate is found to be proportional toE ^0.5.

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Zum Einfluß der Ionenbeschußparameter auf die Tiefenauflösung bei der AES-Sputtertiefenprofil-analyse von Ta₂O₅/Ta mit Ar⁺ und Xe⁺

     

Probing trapped ion energies via ion-molecule reaction kinetics: Fourier transform ion cyclotron resonance mass spectrometry

The kinetic energy-dependent Ar⁺+ N₂ ion-molecule reaction has been used as a chemical “thermometer” to determine the kinetic energy of ions produced by electron ionization and trapped by using a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The rate constant for this reaction obtained on the FTICR mass spectrometer was compared to previous work, which allowed a kinetic energy estimate to be made. In addition, the effects of varying parameters such as trapping voltage and pressure on ion kinetic energy were investigated. No evidence of the differing reactivity of higher energy electronic states of Ar⁺, such as ²P_1/2, was observed and the results of a model of this system are presented that support this observation. Pressure studies revealed that with an average of as few as 13 ion-molecule collisions, Ar⁺ ions are collisionally relaxed to an extent unaffected by additional collisions. Based on recent variable temperature selected ion flow drift tube measurements, FTICR ion energies are estimated to be slightly above thermal.     

Removal of Ar+ beam‐induced damaged layers from polyimide surfaces with argon gas cluster ion beams

An Ar Gas Cluster Ion Beam (GCIB) has been shown to remove previous Ar⁺ ion beam‐induced surface damage to a bulk polyimide (PI) film. After removal of the damaged layer with a GCIB sputter source, XPS measurements show minor changes to the carbon, nitrogen and oxygen atomic concentrations relative to the original elemental bulk concentrations. The GCIB sputter depth profiles showed that there is a linear relationship between the Ar⁺ ion beam voltage within the range from 0.5 to 4.0 keV and the dose of argon cluster ions required to remove the damaged layer. The rate of recovery of the original PI atomic composition as a function of GCIB sputtering is similar for carbon, nitrogen and oxygen, indicating that there was no preferential sputtering for these elements. The XPS chemical state analysis of the N 1s spectra after GCIB sputtering revealed a 17% damage ratio of altered nitrogen chemical state species. Further optimization of the GCIB sputtering conditions should lead to lower nitrogen damage ratios with the elemental concentrations closer to those of bulk PI. Copyright © 2010 John Wiley & Sons, Ltd.     

Probing trapped ion energies via ion-molecule reaction kinetics: Quadrupole ion trap mass spectrometry

We present a detailed study of the energies of the ions stored in a quadrupole ion trap mass spectrometer (QITMS). Previous studies have shown that the rate constant, k, for the charge exchange reaction Ar⁺ N⁺ ₂ →, N⁺ ₂+Ar increases with increasing ion-molecule center-of-mass kinetic energy (K.E._cm). Thus, we have determined k for this chemical “thermometer” reaction at a variety of Ar and N₂ pressures and have assigned K.E._cm values as a function of the q₂ of the Ar⁺ ion both with and without He buffer gas present in the trap. The K.E._cm energies are found to lie within the range 0.11–0.34 eV over the variety of experimental conditions investigated. Quantitative “cooling” effects due to the presence of He buffer gas are reported, as are increases in K.E._cm due to an increase in the q₂ of the Ar⁺ ion. “Effective” temperatures of the Ar⁺ ions in He buffer are determined based on a Maxwell-Boltzmann distribution of ion energies. The resulting temperatures are found to lie within the range ≈ 1700–3300 K. We have also examined the K.E._cm, values arising from the chemical thermometer reaction of O⁺ ₂ with CH₄, as previous assignments of effective ion temperatures based on this reaction have been called into question.     

A new approach to express ToF SIMS depth profiling

We propose a new approach to express SIMS depth profiling on a TOF.SIMS‐5 time‐of‐flight mass spectrometer. The approach is based on the instrument capability to independently perform raster scans of sputter and probe ion beams. The probed area can be much smaller than the diameter of a sputter ion beam, like in the AES depth profiling method. This circumstance alleviates limitations on the sputter beam–raster size relation, which are critical in other types of SIMS, and enables analysis on a curved‐bottomed sputter crater. By considerably reducing the raster size, it is possible to increase the depth profiling speed by an order of magnitude without radically degrading the depth resolution. A technique is proposed for successive improvement of depth resolution through profile recovery with account for the developing curvature of the sputtered crater bottom in the probed area. Experimental study of the crater bottom form resulted in implementing a method to include contribution of the instrumental artifacts in a nonstationary depth resolution function within the Hofmann's mixing–roughness–information depth model. The real‐structure experiment has shown that the analysis technique combining reduction of a raster size with a successive nonstationary recovery ensures high speed of profiling at ~100 µm/h while maintaining the depth resolution of about 30 nm at a 5 µm depth. Copyright © 2015 John Wiley & Sons, Ltd.     

Enhanced surface sensitivity in secondary ion mass spectrometric analysis of organic thin films using size‐selected Ar gas‐cluster ion projectiles

A size‐selected argon (Ar) gas‐cluster ion beam (GCIB) was applied to the secondary ion mass spectrometry (SIMS) of a 1,4‐didodecylbenzene (DDB) thin film. The samples were also analyzed by SIMS using an atomic Ar⁺ ion projectile and X‐ray photoelectron spectroscopy (XPS). Compared with those in the atomic‐Ar⁺ SIMS spectrum, the fragment species, including siloxane contaminants present on the sample surface, were enhanced several hundred times in the Ar gas‐cluster SIMS spectrum. XPS spectra during beam irradiation indicate that the Ar GCIB sputters contaminants on the surface more effectively than the atomic Ar⁺ ion beam. These results indicate that a large gas‐cluster projectile can sputter a much shallower volume of organic material than small projectiles, resulting in an extremely surface‐sensitive analysis of organic thin films. Copyright © 2010 John Wiley & Sons, Ltd.     

Repair of defects created by Ar+ sputtering on graphite surface by annealing as confirmed using ToF‐SIMS and XPS

Defects were created on the surface of highly oriented pyrolytic graphite (HOPG) by sputtering with an Ar⁺ ion beam, then characterized using X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) at 500°C. In the XPS C1s spectrum of the sputtered HOPG, a sp³ carbon peak appeared at 285.3 eV, representing surface defects. In addition, 2 sets of peaks, the C_x⁻ and C_xH⁻ ion series (where x = 1, 2, 3...), were identified in the ToF‐SIMS negative ion spectrum. In the positive ion spectrum, a series of C_xH₂^+• ions indicating defects was observed. Annealing of the sputtered samples under Ar was conducted at different temperatures. The XPS and ToF‐SIMS spectra of the sputtered HOPG after 800°C annealing were observed to be similar to the spectra of the fresh HOPG. The sp³ carbon peak had disappeared from the C1s spectrum, and the normalized intensities of the C_xH⁻ and C_xH₂^+• ions had decreased. These results indicate that defects created by sputtering on the surface of HOPG can be repaired by high‐temperature annealing.     

Nanoscale imaging of hydrogen and sodium in alteration layers of corroded glass using ToF-SIMS: Is an auxiliary sputtering ion beam necessary?

The hydrogen (H)/sodium (Na) interface is of great interest in glass corrosion research. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is one of the few techniques that can provide nanoscale H and Na imaging simultaneously. However, the optimized condition for ToF-SIMS imaging of H in glass is still unclear. In H depth profiling using ToF-SIMS, H background control is a key, in which an analysis ion beam and a sputtering ion beam work together in an interlaced mode to minimize it. Therefore, it is of great interest to determine if an auxiliary sputtering ion beam is also necessary to control H background in ToF-SIMS imaging of H. In this study, H imaging with and without auxiliary sputtering beams (Cs⁺, O₂⁺, and Ar_n⁺) was compared on a corroded international simple glass (ISG). It was surprising that the H/Na interface could be directly imaged using positive ion imaging without any auxiliary sputtering ion beam under a vacuum of 2 to 3 × 10⁻⁸ mbar. The H⁺ background was about 5% atomic percent on the pristine ISG glass, which was significantly lower than the H concentration in the alteration layer (~15%). Moreover, positive ion imaging could show distributions of other interesting species simultaneously, providing more comprehensive information of the glass corrosion. If an auxiliary O₂⁺ sputtering ion beam was used, the H⁺ background could be reduced but still higher than that in the depth profiling. Besides, this condition could cause significant loss of signal intensities due to strong surface charging.     

Are cluster ion analysis beams good choices for hydrogen depth profiling using time‐of‐flight secondary ion mass spectrometry?

For more than three decades, time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) has been used for elemental depth profiling. In recent years, cluster primary ion sources (principally, C₆₀⁺, Bi_n⁺, and Au_n⁺) have become widely available, and they can greatly enhance the signal intensity of molecular ions (10–1000 times). Understanding the performance of cluster ion analysis beams used in elemental depth profiling can greatly assist normal ToF‐SIMS users in choosing the optimal analysis beam for depth profiling work. Presently, however, the experimental data are lacking, and such choices are difficult to make. In this paper, hydrogen and deuterium depth profiling were studied using six different analysis beams—25 keV Bi⁺, Bi₃⁺, Bi₅⁺, 50 keV Bi₃²⁺, 10 keV C₆₀⁺, and 20 keV C₆₀²⁺. The effort shows that cluster primary ions do enhance H^? and D^? yields, but the enhancement is only about 1.5–4.0 times when compared to atomic Bi⁺ ions. Because the currents of atomic ion analysis beams are much stronger than the currents of cluster ion analysis beams for most commercial ToF‐SIMS instruments, the atomic ion analysis beams can provide the strongest H^? and D^? signal intensities, and may be the best choices for hydrogen and deuterium depth profiling. In addition, two representative nuclides, ³⁰Si and ¹⁸O, were also studied and yielded results similar to those of H^? and D^?. Copyright © 2011 John Wiley & Sons, Ltd.     

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

This study examines the dependence of the sputter rate and the transient width (z_tr) as a function of Cs⁺ primary ion energy (impact energy (E_p) = 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 Si_0.7 Ge_0.3. 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 (z_tr < 2R_norm). At minimum z_tr, z_tr ≈ R_norm. 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 E_p does not have a significant effect in reducing z_tr. We conclude that for E_p < 1 keV, the optimum condition to achieve minimum z_tr while maintaining good sensitivity and high sputter rate is at θ ≈ 30° . Copyright © 2007 John Wiley & Sons, Ltd.     

Variation of the electron inelastic mean free path during depth profiling of the Fe/Si interface as determined by quantitative REELS

The aim of this work is to determine the dependence of the electron inelastic mean free path (IMFP) at the Fe/Si interface during depth profiling by sputtering with 3 keV Ar⁺ ions. In order to estimate the variation of the IMFP at the interface, reflection electron energy‐loss spectroscopy (REELS) measurements were performed after different sputtering times at the Fe/Si interface with three different primary electron energies (i.e. 0.5, 1 and 1.5 keV). Even though it is highly likely that a compound (i.e. Fe_xSi) is formed at the interface, all the experimental REELS spectra could be analysed as a linear combination of those corresponding to pure Si and Fe. Using the model developed by Yubero and Tougaard for quantitative analysis of these REELS spectra we could estimate the IMFP values along the depth profile at the interface. The resulting IMFPs are observed to vary linearly with the average composition (as determined by REELS) at the Fe/Si interface as it is sputter depth profiled. The energy dependence of the IMFP for different compositions is presented and discussed. For completeness, we have determined the energy‐loss functions as well as the IMFPs of the pure elements (i.e. Fe and Si). Copyright © 2004 John Wiley & Sons, Ltd.     

ToF‐SIMS depth profiling of a complex polymeric coating employing a C60 sputter source

A complex poly(vinylidene difluoride) (PVdF)/poly(methyl methacrylate) (PMMA)‐based coil coating formulation has been investigated using time‐of‐flight SIMS (ToF‐SIMS). Employing a Bi₃⁺ analysis source and a Buckminsterfullerene (C₆₀) sputter source, depth profiles were obtained through the polymeric materials in the outer few nanometres of the PVdF topcoat. These investigations demonstrate that the PVdF coating's air/coating interface is composed principally of the flow agent included in the formulation. Elemental depth profiles obtained in the negative ion mode demonstrate variations in the carbon, oxygen and fluorine concentrations within the coating with respect to depth. All three elemental depth profiles suggest that the PVdF coating bulk possesses a constant material composition. The oxygen depth profile reveals the presence of a very thin oxygen‐rich sub‐surface layer in the PVdF coating, observed within the first second of the sputter/etch profile. Retrospectively, extracted mass spectra (from the elemental depth profile raw data set) of the PVdF coating sub‐surface and bulk layers indicates this oxygen‐rich sub‐surface layer results from segregation of the acrylic co‐polymers in the formulation towards the PVdF coating air/coating interface. Molecular depth profiles obtained in both the positive and negative secondary ion modes provide supporting evidence to that of the elemental depth profiles. The molecular depth profiles confirm the presence of a sub‐surface layer rich in the acrylic co‐polymers indicating segregation of the co‐polymers towards the PVdF topcoats air‐coating surface. The molecular depth profiles also confirm that the PVdF component of the topcoat is distributed throughout the coating but is present at a lower concentration at the air‐coating interface and in the sub‐surface regions of the coating, than in the coating bulk. Copyright © 2007 John Wiley & Sons, Ltd.     

Depth profiling of polymer samples using Ga+ and C60+ ion beams

In this contribution, we focus on the use of C₆₀⁺ ions for depth profiling of model synthetic polymers: polystyrene (PS) and poly(methylmethacrylate) (PMMA). These polymers were spin coated on silicon wafers, and the obtained samples were depth‐profiled both with Ga⁺ ions and C₆₀⁺ ions. We observed an important yield enhancement for both polymers when C₆₀⁺ ions are used. More specifically, we discuss here the decrease in damage obtained with C₆₀, which is found to be very sensitive to the nature of the polymer. During the C₆₀⁺ sputtering of the PMMA layer, after an initial decrease, a steady state is observed in the secondary ion yield of characteristic fragments. In contrast, for PS, an exponential decrease is directly observed, leading to an initial disappearance cross section close to the value observed for Ga⁺. Though there is a significant loss of characteristic PS signal when sputtering with C₆₀⁺ ions beams, there are still significant enhancements in sputter yields when employing C₆₀⁺ as compared to Ga⁺. Copyright © 2008 John Wiley & Sons, Ltd.