Ion Yield Variations Research Articles

Fast-ion-induced secondary ion emission from submicron droplet surfaces studied using a new coincidence technique with forward-scattered projectiles.

A mass spectrometric study of secondary ions emitted from droplet surfaces by MeV-energy heavy ion impact was performed to investigate fast-ion-induced molecular reaction processes on liquid surfaces. Herein, a new coincidence technique was developed between secondary ions and scattered projectile ions at a small forward angle. The advantages of this technique were demonstrated by measurement of the collision between 4-MeV C3+ and ethanol droplets. Secondary ion emission probabilities were obtained directly from the coincidence data. Notably, this technique enabled positive fragment ions that had not been identified in previous measurements to be observed by suppressing the strong background originating from gas-phase molecules more than 104-fold. H+, H3O+, C2H5 +, and C2H5O+ were found to be produced as major positive fragment ions, in addition to minor fragments H2 +, C2H3 +, and CH2OH+. Production of these ions suggests that competition between rapid hydrogen ion emission from multiply ionized states and intermolecular proton transfer accompanied by fragmentation through protonated ethanol occurs after fast heavy-ion collisions. Clarification of the positive fragment ions also revealed the characteristic features of negative ions. Negative ions were realized to exhibit higher degrees of fragmentation and reactivity compared with positive ions. Furthermore, the energy loss by forward-scattered ions during droplet penetration was used to evaluate the target thickness at a submicron level. Variations in secondary ion yield, mass distribution, and kinetic energies depending on the penetration length were observed below 1 µm. These results highlight the unknown mechanism of these "submicron effects" observed in secondary ion emission processes as a new phenomenon.

ToF-SIMS analysis of ion implanted standard to quantify insecticide in mosquito netting with cesium and argon gas cluster sputtering beams

Ion implantation has been investigated for quantification of species in organic materials. Quantitative analysis of two insecticides in mosquito netting was recently achieved by ion implantation and time-of-flight secondary ion mass spectroscopy analysis with a Cs+ sputtering beam [Zhou et al., J. Vac. Sci. Technol., B 34, 03H107 (2016) and Zhou et al., J. Vac. Sci. Technol., B 35, 031802 (2017)]. Gas cluster ion beams (GCIBs) are of increasing utility in depth profiling of organic materials, and it was of interest to try the ion implantation approach with argon GCIB. The study was conducted on permethrin treated mosquito netting and on the substrate material [high density polyethylene (HDPE)] which was ion implanted with chlorine. The negative ion mass spectrum of permethrin is dominated by Cl−. Analysis of the ion implanted HDPE with Cs+ provided the expected Gaussian distribution, but analysis with argon GCIB shows a very sharp change in matrix species intensity at the penetration depth of the ion implant. This is presumably due to the degradation of the polymer structure by the implant. Even with reduced chlorine implant dose, the ability to use the analysis of the standard to quantify chlorine in insecticide treated fiber with GCIB was limited because of secondary ion yield variations. Analysis of netting which was no longer effective against mosquitoes showed significantly less chlorine at the surface than fresh netting, similar to the results obtained with the Cs+ sputtering beam. The results are useful to understand the effectiveness of insecticide treated nets and have implications for the analysis of other organic materials with the use of ion implantation for quantification.

Experimental study of the organic ion intensity distribution in the ion imaging of coated polymer fibres with S-SIMS

Time-of-Flight Static Secondary Ion Mass Spectrometry excels in probing the molecular composition of the outer monolayer of flat samples with a lateral resolution in the sub-μm range. However, the method faces significant methodological problems in the case of non-conducting samples with high topography or surface curvature, such as fibres, yarns or fabrics. Specifically, the useful secondary ion yield in a given spot on the fibre depends on the local incidence angle, the height above the earthed sample holder, the position relative to the axis of the mass analyser and the extent of the local surface charging. This study has focused on the empiric reduction of the useful ion yield variations observed in the ion images of fibres with diameter of 25 and 100μm. Up to now, most literature data consider the analysis of fibres positioned along or perpendicular to the projection of the projectile beam in the plane of the sample surface because these specific geometries facilitate the interpretation of the ion images. However, it has been discovered that the diagonal orientation of the fibre in the field-of-view largely reduces the ion yield variations for fibres with a small diameter (25μm). The situation is different for fibres with a diameter of 100μm. In that case, the ion images contain no secondary ion counts for the pixels referring to a significant part of the fibre. In particular, the resulting lack of delineation between the shadow zone in the front of the fibre and the boundary of the fibre hampers the practical use of the ion images A fourfold decrease of the extraction voltage or a 20% increase of the distance between sample holder and extraction electrode is found to improve the detection of secondary ions from the part of the fibre facing towards the impinging primary ion beam. These observations have been tentatively related to the mass analyser acceptance and its dependence on the delicate balance between conflicting effects such as field strength and curvature of the field lines, secondary ion emission angle and initial kinetic energy and difference in local surface potential due to the position in the extraction field and charge build-up during analysis.

Ion energy distributions in laser-produced plasmas with two collinear pulses

The formation and expansion dynamics of laser-produced plasmas was studied by means of the effect of a second delayed laser pulse upon the ion kinetic energy of the plasma created by the first one. Two types of measurements were carried out: ion kinetic energy distributions and overall ion time of flight (t.o.f.). Ion energy distributions were obtained with an electrostatic energy analyser, which allowed the observation of the energy distributions of each charge state separately while the ion t.o.f. signal was measured with an ion probe. Laser power densities ranged from 2 × 108 to 2 × 109 W cm−2 with 532 nm photons, and studies were extended to Al, Cu and Co targets. The effects of the second laser pulse on the plasma created by the first were very different depending on the interpulse delay. At low delays (from 0 to 10 ns) the second pulse produced an increase of the plasma average charge state and the maximum ion kinetic energy, while at higher delays (>20 ns) it produced a strong enhancement on the low charge yield (especially the single-charge ions) leaving unaltered the energies and yields of the high charges. An analysis of variations in both ion yield and kinetic energy produced interesting results regarding fundamental understanding of plasma formation and expansion dynamics, as well as possible improvements in technological applications based on laser-produced plasmas.

Negative ion yield and sputter yield variations for Cs+ bombardment of Si with O2 gas flooding

AbstractWe have investigated whether negative secondary ion yields from Cs+ sputtered silicon might be improved by simultaneous O2 gas flooding, which lowers the sputter yield of the silicon, and thus, might increase the cesium content at the surface. The composition at the sputtered surface is derived from quantitative measurements of the ratios of O/Si and Cs/Si in the total emitted flux using the recently developed oxygen 18 method1 and sputter yield measurements, respectively (with XSi + XCs + XO = 1). The silicon sputter yield, Y, is lowered by adsorbed oxygen by as much as a factor of 2.7 and is found to scale almost linearly with the silicon atom fraction XSi at the surface, resulting in the cesium atom fraction XCs at the surface remaining nearly constant during oxygen flooding. We find that oxidation of the cesiated silicon surface influences differently the useful ion yields (ions detected/atom sputtered) of various elements. The yields of B− and Cs− are enhanced by factors of 3 and 34, respectively, the yield of O− remains roughly constant, and yields of other elements (As−, Cl−, P−, Sb−, Si−) are lowered by a factor of 2 or 3. Copyright © 2010 John Wiley & Sons, Ltd.

Ion Beam Analysis and PD-MS As New Analytical Tools for Quality Control of Pharmaceuticals: Comparative Study from Fluphenazine in Solid Dosage Forms

In order to evaluate the potential of accelerator based analytical techniques ((particle induced X-ray emission (PIXE), particle induced gamma-ray emission (PIGE), and particle desorption mass spectrometry (PD-MS)) for the analysis of commercial pharmaceutical products in their solid dosage form, the fluphenazine drug has been taken as a representative example. It is demonstrated that PIXE and PIGE are by far the best choice for quantification of the active ingredient (AI) (certification with 7% precision) from the reactions induced on its specific heteroatoms fluorine and sulfur using pellets made from original tablets. Since heteroatoms cannot be present in all types of drugs, the PD-MS technique, which makes easily the distinction between AI(s) and excipients, has been evaluated for the same material. It is shown that the quantification of AI is obtained via the detection of its protonated molecule. However, calibration curves have to be made from the secondary ion yield variations since matrix effects of various nature are characteristics of such mixtures of heterogeneous materials (including deposits from soluble components). From the analysis of solid tablets, (either transformed into pellets and even as received), it is strongly suggested that the physical state of the grains in the mixture is a crucial parameter in the ion emission and accordingly for the calibration curves. As a result of our specific (but not optimized) conditions the resulting precision is <17% with an almost linear range extending from 0.04 to 7.87 mg of AI in a tablet made under the manufacturer conditions (the commercial drug product is labeled at 5 mg).

Multiple photon excitation and ionization of NO in and on helium droplets

The photoexcitation of NO embedded in superfluid Hen nanodroplets having n approximately 10(4) has been examined. Two-photon excitation prepares electronically excited states (NO(*)), most notably, the embedded analog of the A 2Sigma state of gas phase NO. Vertical excitation to this low Rydberg state is blueshifted and broadened relative to its gas phase counterpart because of the repulsive electron-helium interaction. Transport to the droplet surface is believed to be facile in the superfluid. For example, NO* prefers (energetically) to reside at the droplet surface rather than at the droplet center, in contrast to NO. Photoionization of surface-bound NO* occurs over a significant photon energy range. This yields small cluster ions NO+Hek) with approximately 90% of these clusters having k< or =10. The variation of ion yield with photon energy displays a precipitous change in the region of 24 300-24 400 cm(-1) for all values of k. Possible photoionization mechanisms are discussed and it is suggested that intermediate levels with high-n Rydberg character play a role. This work underscores the important role played by transport in the photophysics of species embedded in the superfluid host.

Characterizing high- k and low- k dielectric materials for semiconductors: Progress and challenges

SIMS has been applied to the characterization of high- k and low- k materials used in the semiconductor manufacturing process. Profiles of thin high- k films, particularly HfO 2 and HfSiO, exhibit preferential sputtering that affects ion yields and sputter rates in the films as well as in the Si substrate. These artifacts make it difficult to quantify major constituents and dopants in the films and substrate. The ion yields of B were observed to vary by as much as 3× as a function of Si content in HfSiO films, while As ion yield variations are not as great. Evidence for B penetration from a highly-doped Si substrate into the high- k films was also observed and quantified. First generation low- k films are not as susceptible to charging as the newer, porous materials. Backside SIMS was used to show Ti migration into the open pores of a low- k film during the metal deposition step.

SIMS analysis of nitrogen in various metals and ZnO

In this paper, we report an optimized way to measure nitrogen in ZnO using a Cs + primary beam while monitoring (O + N) −. Because of ion yield variations and mass interferences, monitoring molecular ions of the type (matrix + N) − may not be useful for metal films such as Cu and Ni. In this work, based on 15N +-implanted samples, we also report N profile measurements in Cu and Ni using a Cs + primary beam and monitoring Cs-attached cluster ions: e.g. (Cs 2N) + in Cu and (CsN) + in Ni. For a given profiling energy such as 5.5 keV and using Cs 2 + as the “matrix” signal for quantification, it has been found that the ratio of (Cs 2N) + or (CsN) + ion yield to (Cs 2) + ion yield (i.e. the relative sensitivity factor) is independent of the chosen sputter rate, but depends on the matrix materials. The range data measured by SIMS are compared with the simulated data from SRIM. The agreements are found to be very good in ZnO, and good in Cu and Ni films.

Sputter rate variations in silicon under high- k dielectric films

SIMS depth profiles obtained from a B implanted Si sample capped with a thin HfO 2 film revealed the presence of several artifacts that make it problematic to determine the distribution and concentration of elements in the underlying Si substrate. The distribution of the B implant, normalized to 30 Si + , was much broader than expected. Variations in the intensities of several matrix species under the HfO 2 film were also observed. AFM measurements from the sputtered crater bottoms suggested that sputter-induced surface roughness is present under the high- k/Si interface. The roughness is partially responsible for reducing the sputter rate, which in turn lowers matrix ion intensities. The sputter rate variation can be corrected using the ratio of intensity loss of different matrix species. Use of the variation in the SiO + intensity to correct the depth scale of the normalized B profile provided the best agreement with the original B implant-only sample. Rotating the sample while profiling improves the shape of the normalized B profile but some sputter rate correction was still necessary. The presence of 10% Hf (atomic) in the Si leads to slight reduction in the SiO + intensity but the sputter rate does not need correcting. Under these conditions, point-by-point normalization of the B profile to a matrix species cancels out the ion yield variations. At Hf concentrations >10% the combination of ion yield and sputter rate variations complicates the depth profiles.

Transient effects in SIMS analysis of Si with Cs + at high incidence angles: Secondary ion yield variations

Secondary ion mass spectrometry (SIMS) depth profile analysis of Si wafers using 1 keV Cs + primary ions at large incidence angles (80°) is plagued by unusually strong transient effects (variations in both sputter and ion yields). Analysis of a native oxide terminated Si wafer with and without the aid of an O 2 leak, and an Ar + pre-sputtered wafer revealed correlations between the implanted Cs content and various secondary ion intensities consistent with that expected from a resonance charge transfer process (that assumed by the electron tunneling model). Cs concentrations were defined through X-ray photoelectron spectroscopy of the sputtered surface from SIMS profiles terminated within the transient region. These scaled with the surface roughening occurring under these conditions and can be explained as resulting from the associated drop in sputter rates. An O induced transient effect from the native oxide was also identified. Characterization of these effects allowed the reconstruction of the XPS derived Cs concentration profiles, which in turn, allowed enhancement effects resulting from O and Cs to be approximated.

Ion probe measurements of Os, Ir, Pt, and Au in individual phases of iron meteorites

We have developed a technique to measure abundances of Os, Ir, Pt, and Au in situ in Fe-Ni metal on a microscale using secondary-ion mass spectrometry. A Cs + beam is used to generate negative secondary ions, and interferences from molecular ions are eliminated by a combination of 40 eV of energy filtering and a mass resolution of ∼1900. Ion yields for Fe, Co, Ni, and the PGEs vary considerably from spot to spot in meteoritic metal samples. Ion yield variations for Os and Ir correlate strongly with the Fe ion yield, that for Pt is weakly correlated, and that for Au is essentially uncorrelated. From ion yields for meteorite standards and the correlations with Fe ion yield, it is possible to obtain concentrations of Os, Ir, Pt, and Au in meteorite samples. Using this technique, a Cs + beam current of 10 nA gives a spatial resolution of 10–20 μm and detection limits of less than 1 ppm for Os, less than 0.1 ppm for Ir, and 10–20 ppb for Pt and Au. With refinement, it should be possible to measure Rh, Pd, and Ag and to improve the spatial resolution. However, Re, Ru, W, and Hf can only be measured as positive secondary ions. We have measured the abundances of Os, Ir, Pt, and Au in kamacite, taenite, and/or plessite in five iron meteorites: Canyon Diablo (IA), Cape York (Agpalilik) (IIIA), Colomera (IIE), Cruz del Aire (Anom), and Wallapai (IID). Our measurements show that Os, Ir, Pt, and Au partition preferentially into taenite relative to kamacite during slow cooling. Measured abundance ratios (taenite/kamacite) range from ∼1.3 to ∼2.1 for Os, Ir, and Pt, and from ∼2 to ∼6 for Au. These ratios are consistent with those determined recently by laser-ablation ICPMS, but differ significantly from those determined by earlier workers. Low-temperature taenite/kamacite distribution coefficients inferred from our data are ∼2.1 for Os, ∼1.6 for Ir, ∼1.9 for Pt, and ∼6 for Au. PGEs are not enriched in taenite as much as Ni. Partitioning is controlled by the interplay of the size and electron configuration of Os, Ir, Pt, and Au and the crystal structures of kamacite and taenite. The ion probe can be used to further investigate the distribution and subsolidus redistribution of PGEs and Au in iron meteorites and in the metal in chondrites and stony irons.

Characterization of Thin Titanium and Titanium Nitride Layers Using Sims

ABSTRACTTitanium (Ti) and titanium nitride (TiN) films are widely used as barrier stack to prevent junction spiking. It is also an important material for an anti-reflection coating (ARC) on aluminum (Al) films to facilitate lithography processes during multilevel metallization for the manufacture of integrated circuits on silicon-based (Si) semiconductor devices. Secondary Ion Mass Spectrometry (SIMS) is proven to be very powerful analytical technique for the semiconductor materials. However, quantitative analysis of very thin structures using SIMS constitutes an ultimate challenge since a large fraction of the profile is located in the transient region where a stable concentration of primary beam species has not been established.This paper reports a SIMS technique for advanced characterization of very thin titanium and titanium nitride layers. Improvements in depth resolution were achieved by reducing the angle of incidence and the impact energy maintaining enhanced ionization yield associated with oxygen bombardment. Significant improvements in characterization of the film surface were developed by using oxygen flooding technique. Optimized oxygen pressure was used to achieve a stable ion yield due to the complete surface oxidation of titanium and titanium nitride layers during the analysis. The method was employed in the SIMS characterization of multiple Ti/TiN films deposited on silicon substrate. The example presents dramatic enhancement in depth resolution due to minimized matrix related ion yield variations at the interfaces.

Influence of O+2 energy, flux, and fluence on the formation and growth of sputtering-induced ripple topography on silicon

The formation of ripples on Si(100) by O+2 sputtering at an angle of incidence of 40° and energies from 1 to 9 keV has been studied using secondary ion mass spectrometry and scanning electron microscopy. At 1 keV no ripples are observed. Between 1.5 and 9 keV ripples are observed oriented perpendicular to the ion direction with average wavelengths that increase, from ∼100 to 400 nm, approximately linearly with O+2 energy. Two-dimensional fast Fourier transforms of secondary electron images are used to investigate the frequency distribution of the ripples. For the conditions studied, the distributions of frequencies appear approximately Gaussian. At 1.5 keV, the wavelength and growth rate with sputtered depth are independent of flux for fluxes from 15 to 150 μA/cm2. Accompanying ripple formation are changes in secondary ion yields. The changes occur abruptly at depths that increase, from ∼0.2 to 5.6 μm, with O+2 energy. In contrast, sputtering with Ar+ at 1.5 and 7 keV to depths 5–10 times those that produce ripples with O+2 produce no observable topography. These results are discussed using several existing theories for ripple formation and growth. Ripple growth and the variations in secondary ion yield are modeled by accounting for the change in local angles of incidence as the ripples grow. This model describes well the variation in secondary ion yield assuming an exponential growth rate. Ripple formation is discussed in terms of a balance between roughening (by sputtering-induced surface stress and by the dependence of the sputtering yield on surface curvature) and smoothing (by both diffusion and ion mixing). Variation in ripple wavelength with energy is not simply explained by these theories. Surface smoothing by cascade ion mixing can, however, make the wavelength, as observed, independent of ion flux. Finally, the possibility of formation of ripples by phase separation within the SiOx surface layer is discussed.

Three-dimensional characterization of conducting polymer arrays using secondary ion mass spectrometry

Image depth profiling with dynamic secondary ion mass spectrometry (SIMS), using ion microscopy and digital imaging, provided high spatial resolution 3D images of patterned polymer films prepared from [Ru(Me4bpy)2(vpy)2]2+ and {Fe[(CH2Br)2bpy]3}2+. Although 3D SIMS has been widely applied to semiconductor device characterization, this study represents a first attempt to characterize conductive polymer arrays. The SIMS studies were useful in localizing the polymer domains, and the extent of polymer mixing within the film structures. Volume-rendered images and reconstructed local area depth profiles indicated that electropolymerization of the poly-Fe complex was not limited to the channels in the poly-Ru resist produced by optical lithography. Quantification of the SIMS image depth profiles required consideration of detection system nonlinearities, ion yield variations (especially transients at the film/substrate interface), and native and sputter induced roughness of the polymer films.

SIMS of Y 1Ba 2Cu 3O 7− x and (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+δ superconductors

Secondary ion mass spectrometry (SIMS) of Y 1Ba 2Cu 3O 7− x and (Bi 0.92, Pb 0.17) 2Sr 1.91Ca 2.03Ca 2.03Cu 3.06 O 10+δ superconducting materials has been studied. It has been found that the variation of secondary ion yield with respect to the bombarding ion energy, in general, shows a broad peak, whose location depends on the ion-target combination. A similar nature was observed earlier for polycrystalline metals. The decline in the secondary ion yield beyond the maximum in the case of these high- T c materials is explained as being due to the ion-induced texturing, as already reported for metal targets. Emission of individual ionic species has been explained in the light of crystallographic structures of the high- T c materials.

Novel very sensitive analytical technique for compositional analysis of Hg1−xCdxTe epilayers

This letter reports a novel analytical method using negative secondary ion mass spectrometry (SIMS) of the Te matrix element with a Cs primary ion beam to analyze and determine the composition in HgCdTe epilayers with a high sensitivity (better than Δx of 0.0015) and a depth resolution determined by SIMS. The method is based on matrix-induced ion yield variations in the Te signal observed in SIMS. The correlation between the Te− secondary ion yield and Cd mole fraction is a linear function. The material composition can be calculated by this technique when the Te yield profile is known provided that the composition of any two layers of the structure are known, or the structure contains a CdTe layer, for which case composition of no Hg1−xCdxTe layer is needed.

Biological cryosection preparation and practical ion yield evaluation for ion microscopic analysis

A mounting method utilizing an indium substrate is described for preparing freeze-dried cryosections of biological tissue for ion microscopic analysis. Using this procedure, a qualitative comparison between cryosection, conventional chemical, and freeze-substitution specimen preparations is made with rat liver tissue. Practical ion yield variations in cryosections are found to be minimal (+/- 13% relative standard deviation) in tissue-containing areas of rat liver and small intestine.

Ion microprobe trace-element analysis of silicates: Measurement of multi-element glasses

At present, ion microprobe determination of trace elements in silicates is dependent upon the availability of homogeneous well-characterised standards of similar major-element composition to the unknown. Our ability to predict yields where no standard is available and finally move towards standardless quantitative analysis requires modelling of ion yield behaviour. A table of ion yield data for low- and high-energy ions of 60 elements measured in the NBS 610 standard glass is presented, and includes ion yields for doubly charged and oxide ions. The variations in ion yield with atomic number give relatively smooth patterns especially for high-energy ion yields. Notably, a periodic relationship exists in ion yield behaviour. Variations which occur under different analytical (beam density?) conditions are related to the mass of the ion. Corrections can be applied to remove these effects. Unfortunately Si +, while being the most obvious element to use for normalisation in silicates to remove operator/laboratory artifacts, is one of the most sensitive elements to changes in analytical conditions.

Secondary ion yield matrix effects in SIMS depth profiles of Si/Ge multilayers

AbstractThin multilayer samples of Si/Ge, with individual layer thicknesses of 4–33 nm, have been analyzed by secondary ion mass spectrometry (SIMS) using Ar+, O2+ and Cs+ primary ion beams. Bombardment with both Ar+ and O2+ produced positive secondary ion depth profiles in which pronounced distortions were observed. Similar effects were found in negative secondary ion depth profiles with Cs+ bombardment. In each case, the SIMS depth profiles were characterized by abrupt interfacial secondary ion signal variations and a shift in the secondary ion signal maxima indicating that the layers were superposed, a condition that was not consistent with sample preparation, as verified by Auger electron spectroscopy. Auger electron spectroscopy depth profiling was also used to quantify the level of oxygen in the films. From these data it was concluded that the distortions in the positive secondary ion depth profiles under Ar+ bombardment were the result of secondary ion yield variations induced by enhanced incorporation of ambient oxygen, during sample preparation, into the stronger oxide‐forming silicon layers. Under O2+ and Cs+ bombardment, the profile distortions were introduced by differential incorporation of the implanted primary species into the lower‐sputter‐yield silicon layers.