Secondary Yield Research Articles

Enhancement of Intact Ion Yields in ToF-SIMS Using Molecular Cluster Ion Beams

Recently, water and some other molecules are applied to the massive cluster projectiles for SIMS to improve the secondary ion yields by chemical effects. The ionization probability should be influenced by the properties of the primary ion beams as well as the chemical nature of the sample surface. However, the mechanisms of ionization by the large cluster collision is still unclear. In this study, we have generated water, methanol and methane molecular cluster ion beams and investigated the secondary ion emission from amino acid thin films bombarded by the molecular cluster ion projectiles.

Bevel depth profiling by high-spatial-resolution sputtered neutral mass spectrometry with laser postionization

Secondary-ion mass spectrometry (SIMS) sputter depth profiling is used for the quantitative depth profile analysis of impurities. However, SIMS suffers from a large quantitative uncertainty and depth-scale uncertainty at the interfaces of heteromultilayers and in the near-surface region, because the secondary ion yield and sputtering yield are significantly influenced by matrix effects and accumulation effects of the primary ion. In this paper, the authors report on the development of a new depth profiling method with good depth-scale accuracy and low matrix effects to overcome these problems. This was achieved through the combination of high-spatial-resolution bevel depth profiling and sputtered neutral mass spectrometry with laser postionization (laser-SNMS). The sample used to evaluate this new bevel depth profiling method was a silicon on insulator wafer obtained using the separation by implantation of oxygen technique and implanted with boron. Depth profiles were obtained using both SIMS and laser-SNMS and evaluated by comparison with the stopping and range of ions in matter (SRIM) simulation. Although both methods afforded quite good depth resolutions, in SIMS the secondary ion signal intensity for boron was amplified by the influence of the matrix effect and showed a discontinuous profile shape at the interfaces, whereas the profile for boron obtained using laser-SNMS was consistent with the SRIM results and exhibited high continuity. By using a combination of the bevel depth profiling method and laser-SNMS method, it was confirmed that an easy-to-analyze depth profile could be obtained for the dopant concentration in multilayer samples, which is difficult to obtain using the conventional SIMS method.

Relative secondary ion yields produced by vacuum-type electrospray droplet ion beams

The relatively new electrospray droplet impact method is based on electrospray at atmospheric pressure and has been successful in achieving efficient desorption/ionization of biological molecules, soft etching of polymers, and nonselective etching of metal oxides. However, the beam current and brightness of this method are not practical for surface analyses. To improve the performance of this method, the authors have developed a new droplet ion beam gun using an electrospray of aqueous solutions in a vacuum. This technique could be expected to be a high-intensity massive cluster ion beam and they evaluate the performance of the vacuum-type electrospray droplet ion (V-EDI) gun. In this study, V-EDI and gas cluster ion beam guns were installed in a time-of-flight analyzer to directly compare the relative secondary ion yields of several biomolecules produced by several cluster ion beams.

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.

Suppression of self-organized surface nanopatterning on GaSb/InAs multilayers induced by low energy oxygen ion bombardment by using simultaneously sample rotation and oxygen flooding

Time of flight secondary ion mass spectrometry (ToF-SIMS) is a well-adapted analytical method for the chemical characterization of concentration profiles in layered or multilayered materials. However, under ion beam bombardment, initially smooth material surface becomes morphologically unstable. This leads to abnormal secondary ion yields and depth profile distortions. In this contribution, we explore the surface topography and roughening evolution induced by O2+ ion bombardment on GaSb/InAs multilayers. We demonstrate the formation of nanodots and ripples patterning according to the ion beam energy. Since the latter are undesirable for ToF-SIMS analysis, we managed to totally stop their growth by using simultaneously sample rotation and oxygen flooding. This unprecedented coupling between these two latter mechanisms leads to a significant enhancement in depth profiles resolution.

An investigation of the beam damage effect on in situ liquid secondary ion mass spectrometry analysis.

During in situ liquid secondary ion mass spectrometry (SIMS) analysis, the primary ion beam is normally scanned on a very small area to collect signals with high ion doses (1014 to 1016 ions/cm2 ). As a result, beam damage may become a concern when compared with the static limit of SIMS analysis, in which the dose is normally less than 1012 ions/cm2 . Therefore, a comparison of ion yields in in situ liquid SIMS analysis versus traditional static SIMS analysis of corresponding dry samples is of great interest. In this study, a dipalmitoylphosphatidylcholine (DPPC) liposome solution was used as a model system. Both liquid sample and dry sample were examined. Secondary ion yields using three primary ion species (Bi+ , Bi3+ and Bi3++ ) with various beam currents were investigated. Usable ion yields for both positive and negative characteristic signals (including molecular ions and characteristic fragment ions) were achievable based on optimized experimental conditions for in situ liquid SIMS analysis. The ion yield of the key DPPC molecular ion was comparable to that of traditional static SIMS, and unexpected low fragmentation was observed. The flexible structure of the liquid plays an important role for these observations. Therefore, beam damage may not be a concern in in situ liquid SIMS analysis if proper experimental conditions are used.

Time-of-flight secondary ion mass spectrometry using a new primary ion beam generated by vacuum electrospray of a protic ionic liquid, propylammonium nitrate.

Protic ionic liquids have the potential to be useful materials for primary ion beams in terms of protonation, since they have active protons. Selecting protic ionic liquids suitable for primary ion beams is of great importance to increase molecular secondary ion yields. Propylammonium nitrate ([C3 H7 NH3 ][NO3 ]) seems promising in view of its proton affinity. It is likely that [C3 H7 NH3 ]+ cations can act as proton donors, and [NO3 ]- anions can work as proton acceptors. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiments have been performed to verify the usefulness of [C3 H7 NH3 ][NO3 ]. A primary propylammonium nitrate cluster ion beam was generated by vacuum electrospray, and then used to analyze amino acids (arginine, glutamic acid, aspartic acid), angiotensin II and polyethylene glycol. Positive and negative secondary ion mass spectra were obtained to study both protonation and deprotonation. The propylammonium nitrate cluster ion beam successfully generated protonated molecules [M+H]+ of all the analytes in positive ion mode. The primary ion beam also generated deprotonated molecules [M-H]- of glutamic acid, aspartic acid and angiotensin II in negative ion mode. Additionally, adduct ions related to [C3 H7 NH3 ][NO3 ] were detected in the case of arginine and polyethylene glycol. The TOF-SIMS experiments confirmed that the propylammonium nitrate cluster ion beam was useful in generating molecular secondary ions, demonstrating that it is well suited for a primary ion beam in TOF-SIMS.

Solar Wind Sputtering Rates of Small Bodies and Ion Mass Spectrometry Detection of Secondary Ions

AbstractSolar wind interactions with the surfaces of asteroids and small moons eject atoms and molecules from the uppermost several nanometers of regolith grains through a process called sputtering. A small fraction of the sputtered species, called secondary ions, leave the surface in an ionized state, and these are diagnostic of the surface composition. Detection of secondary ions using ion mass spectrometry (IMS) provides a powerful method of analysis due to low backgrounds and high instrument sensitivities. However, the sputtered secondary ion yield and the atomic composition of the surface are not 1‐to‐1 correlated. Thus, relative yield fractions based on experimental measurements are needed to convert measured spectra to surface composition. Here available experimental results are combined with computationally derived solar wind sputtering yields to estimate secondary ion fluxes from asteroid‐sized bodies in the solar system. The Monte Carlo simulation code SDTrimSP is used to estimate the total sputtering yield due to solar wind ion bombardment for a diverse suite of meteorite and lunar soil compositions. Experimentally measured relative secondary ion yields are analyzed to determine the abundance of refractory species (Mg+, Al+, Ca+, and Fe+) relative to Si+, and it is shown that relative abundances indicate whether a body is primitive or has undergone significant geologic reprocessing. Finally, estimates of the sputtered secondary ion fluxes are used to determine the IMS sensitivity required to adequately resolve major element ratios for nominal orbital geometries.

The use of high-mass clusters to measure the TOF SIMS profiles of implanted bismuth

Present study considers the possibility of using high-mass clusters ions detected by TOF-SIMS for the depth profiling of implanted atoms. The Te and Se samples were implanted with the Bi+ ions at the energy of 500keV to a fluence of 1×1015cm−2. SIMS depth profiles were measured with the TOF SIMS 5 working in the dual-beam mode (Bi+ primary beam and Cs+ sputter beam). The analysis of all bismuth-related profiles in Te demonstrates that Bi+ ions create just surface maxima, and BiTex− and CsBi+ clusters create profiles of a similar shape which repeat well the shape of the bismuth ion profiles predicted by SRIM (Simulation of the stopping and Range of Ions in Matter) with the appropriate implantation parameters. However the secondary ion yield of BiTex (3≤x≤7) clusters are higher than the yield of CsBi+ by one order of magnitude. Qualitatively similar results were obtained for Bi implanted Se, although the intensity of all clusters ions was substantially lower. Further experiments and thorough analysis are required to justify the use of high-mass clusters in obtaining the reproducible depth profiles with improved detection limits.

Ionization Probability in Molecular Secondary Ion Mass Spectrometry: Protonation Efficiency of Sputtered Guanine Molecules Studied by Laser Postionization

The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and postionization techniques. An important factor affecting the detection sensitivity in molecular SIMS and other desorption techniques as well is believed to be the poor ionization probability of a sputtered molecule, a value which is often assumed to be as low as 10–5 but at present is basically unknown. In order to estimate how much headroom there is for future developments toward strategies aimed at enhancing the ionization probability, we study the protonation efficiency of sputtered guanine molecules for formation of [M + H]+ secondary ions using strong field laser postionization (LPI) to detect the corresponding neutral molecules. In order to allow a quantitative comparison of secondary ion and neutral yields, the postionization signal is corrected for undersampling of the principally detectable plume of sputtered neutral particles by t...

Sputtering of sodium and potassium from nepheline: Secondary ion yields and velocity spectra

Silicates are the dominant surface material of many Solar System objects, which are exposed to ion bombardment by solar wind ions and cosmic rays. Induced physico-chemical processes include sputtering which can contribute to the formation of an exosphere. We have measured sputtering yields and velocity spectra of secondary ions ejected from nepheline, an aluminosilicate thought to be a good analogue for Mercury's surface, as a laboratory approach to understand the evolution of silicate surfaces and the presence of Na and K vapor in the exosphere. Experiments were performed with highly charged ion beams (keV/u–MeV/u) delivered by GANIL using an imaging XY-TOF-SIMS device under UHV conditions. The fluence dependence of sputtering yields gives information about the evolution of surface stoichiometry during irradiation. From the energy distributions N(E) of sputtered particles, the fraction of particles which could escape from the gravitational field of Mercury, and of those falling back and possibly contributing to populate the exosphere can be roughly estimated.

Influence of experimental parameters on secondary ion yield for MeV-SIMS

Megaelectronvolt-Secondary Ion Mass Spectrometry (MeV-SIMS) is an emerging ion beam analysis technique for molecular speciation and submicrometer imaging. Following the construction of different experimental setups a systematic investigation on the dependence of secondary ion yields on experimental parameters is crucial. Without this knowledge, results are hard to interpret as surface roughness, scan size and the position on the sample can influence the secondary ion count and misleading images can be obtained. Additionally, to achieve better reproducibility the optimal experimental conditions need to be well known. In this work, we present the results of investigations into the influence of the main experimental parameters on the secondary ion yield.

Graphene Oxide as a Novel Evenly Continuous Phase Matrix for TOF-SIMS

Using matrix to enhance the molecular ion signals for biomolecule identification without loss of spatial resolution caused by matrix crystallization is a great challenge for the application of TOF-SIMS in real-world biological research. In this report, graphene oxide (GO) was used as a matrix for TOF-SIMS to improve the secondary ion yields of intact molecular ions ([M + H]+). Identifying and distinguishing the molecular ions of lipids (m/z >700) therefore became straightforward. The spatial resolution of TOF-SIMS imaging could also be improved as GO can form a homogeneous layer of matrix instead of crystalline domain, which prevents high spatial resolution in TOF-SIMS imaging. Lipid mapping in presence of GO revealed the delicate morphology and distribution of single vesicles with a diameter of 800nm. On GO matrix, the vesicles with similar shape but different chemical composition could be distinguished using molecular ions. This novel matrix holds potentials in such applications as the analysis and imaging of complex biological samples by TOF-SIMS. Graphical Abstract ᅟ.

Secondary Ion Emission of High-Entropy Cr14.3Mn14.3Fe14.3Ni28.6Co14.3Cu14.3 Alloy

To understand the unique mechanical properties of high-entropy alloys, it is important to know the nature and strength of interatomic interactions between similar and dissimilar atoms. In this regard, the objective of this study is to use the phenomenon of secondary ion emission for Cr14.3Mn14.3Fe14.3Ni28.6Co14.3Cu14.3 alloy with fcc structure. The yield of secondary ions for all alloy components and corresponding pure metals is quantitatively compared for the first time and an equation is proposed to calculate the atomic bond energy based on the existing models of secondary ion emission mechanism. Compared to pure metals, the bond energy increases in the alloy for Cr and Fe atoms. The greatest decrease in the bond energy is observed for Mn atoms. Reduction in the bond energy for Co and Ni is insignificant. It is suggested that the atomic interaction energy is influenced by changes in the local electron density in fusion as compared with pure metals.

Secondary ion yields for vacuum-type electrospray droplet beams measured with a triple focus time-of-flight analyzer.

We previously developed a massive cluster ion beam gun for secondary ion mass spectrometry (SIMS) in which the primary beam source is a vacuum electrospray. The secondary ion yields produced by this method had not yet been measured with a commercial time-of-flight (TOF) secondary ion mass spectrometer, and the ionization performance was unknown. A vacuum-type electrospray droplet ion gun was connected to a triple-focus TOF analyzer. The flight time of the secondary ions was measured using a sample-bias pulsing method, because a short pulse of the electrospray droplet beam could not be obtained. The secondary ion yields of an amino acid sample produced by the electrospray droplet beams and atomic Ga ion beams were compared. TOF secondary ion spectra were measured for the amino acid and peptide samples with a mass resolution of ~500 using the sample-bias pulsing method. The secondary ion yield of the amino acid sample produced with the 10 kV vacuum-type electrospray droplet beams was much higher than that produced by 10 kV Ga ion beams. In addition, the secondary ion yields for the peptide sample and amino acid samples were almost similar. This is the first report on secondary ion yields produced with vacuum-type electrospray droplet ion beams and measured with a semi-commercial TOF analyzer. The enhancement of secondary ion yields, in particular for relatively high-mass molecules, would be very useful in the SIMS analysis of a wide variety of biological samples. Copyright © 2016 John Wiley & Sons, Ltd.

Analysis of Chemotherapeutic Drug Delivery at the Single Cell Level Using 3D-MSI-TOF-SIMS.

In this work, we show the advantages of label-free, tridimensional mass spectrometry imaging using dual beam analysis (25keV Bi3+) and depth profiling (20keV with a distribution centered at Ar1500+) coupled to time of flight secondary ion mass spectrometry (3D-MSI-TOF-SIMS) for the study of A-172 human glioblastoma cell line treated with B-cell lymphoma 2 (Bcl-2) inhibitor ABT-737. The high spatial (~250nm) and high mass resolution (m/Δm ~10,000) of TOF-SIMS permitted the localization and identification of the intact, unlabeled drug molecular ion (m/z 811.26 C42H44ClN6O5S2- [M - H]-) as well as characteristic fragment ions. We propose a novel approach based on the inspection of the drug secondary ion yield, which showed a good correlation with the drug concentration during cell treatment at therapeutic dosages (0-200μM with 4h incubation). Chemical maps using endogenous molecular markers showed that the ABT-737 is mainly localized in subsurface regions and absent in the nucleus. A semiquantitative workflow is proposed to account for the biological cell diversity based on the spatial distribution of endogenous molecular markers (e.g., nuclei and cytoplasm) and secondary ion confirmation based on the ratio of drug-specific fragments to molecular ion as a function of the therapeutic dosage. Graphical Abstract ᅟ.

Lipid specific molecular ion emission as a function of the primary ion characteristics in TOF-SIMS.

In the present work, the emission characteristics of lipids as a function of the primary ion cluster size and energy were studied using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Characteristic fragmentation patterns for common lipids are described, and changes in secondary ion (SI) yields using various primary ion beams are reported. In particular, emission characteristics were studied for pairs of small polyatomic and nanoparticle primary ion beams (e.g., Bi3+ versus Ar1000+ and Au3+ versus Au400+4) based on the secondary ion yield of characteristic fragment and intact molecular ions as a function of the lipid class. Detailed descriptions of the fragmentation patterns are shown for positive and negative mode TOF-SIMS. Results demonstrate that the lipid structure largely dictates the spectral presence of molecular and/or fragment ions in each ionization mode due to the localization of the charge carrier (head group or fatty acid chain). Our results suggest that the larger the energy per atom for small polyatomic projectiles (Bi3+ and Au3+), the larger the SI yield; in the case of nanoparticle projectiles, the SI increase with primary ion energy (200-500 keV range) for Au400+4 and with the decrease of the energy per atom (10-40 eV/atom range) for Arn=500-2000+ clusters. The secondary ion yield of the molecular ion of lipids from a single standard or from a mixture of lipids does not significantly change with the primary ion identity in the positive ion mode TOF-SIMS and slightly decreases in the negative ion mode TOF-SIMS.

Quantitative SIMS depth profiling of Al in AlGaN/AlN/GaN HEMT structures with nanometer-thin layers

The possibilities of quantitative secondary ion mass spectrometry (SIMS) depth profiling of Al in AlxGa1 − xN/AlN/GaN transistor heterostructures are shown. Using a series of test structures for a TOF.SIMS-5 time-of-flight mass spectrometer, we obtained a refined linear calibration dependence of the secondary-ion yield on the composition ×, namely, Y(CsAl+)/Y(CsGa+) = K × x/(1 − x), with a high linear correlation coefficient, Rl = 0.9996, which permits quantitative SIMS analysis of relatively thick AlGaN barrier layers. The method of profile reconstruction with allowance for the main artifacts of ion sputtering has been first applied for the analysis of GaN/AlGaN/AlN/GaN high electron mobility transistor structure. This method permits to perform quantitative analysis of the thickness and composition of a nanometer-thin AlN sublayer and to estimate the measurement error. For the structure being studied, the AlN sublayer is 1.2 ± 0.2 nm thick. Copyright © 2016 John Wiley & Sons, Ltd.

Effects of molecular weight and cationization agent on the sensitivity of Bi cluster secondary ion mass spectrometry.

Bi cluster secondary ion mass spectrometry (SIMS) is one of the most promising tools for precise analysis of synthetic polymers. However, the sensitivity in the high-mass region is still insufficient compared with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). Accordingly, the effects of metal assistance (cationization agents) were investigated in this study. To investigate the effects caused by varying the ionization agent, three different polyethylene glycol (PEG) samples were prepared, one with an Ag-deposited film, and two others mixed with Ag and Na, respectively. The measurements were performed by using a commercial Bi cluster SIMS and MALDI-TOFMS systems. The mass spectrum obtained with MALDI-TOFMS was used as a reference molecular weight distribution to evaluate the effects of molecular weight and primary ion species (Bi+ , Bi3+ , Bi32+ ) on the sensitivity of Bi cluster SIMS. The intensity of each secondary ion was the highest in Bi32+ irradiation, and the lowest in Bi+ irradiation. Regarding the cationization agents, the secondary ion yield was the highest for the sample mixed with Ag, while the degree of decay of sensitivity along with the increase in molecular weight was the smallest for the sample mixed with Na. It was suggested that the cationization mechanism consists of pre-formed ionization and gas-phase ionization processes. The sensitivity of Bi cluster SIMS decreases to approximately one-fiftieth in every 1000 u. These results might help in understanding the mechanism of cationization and further enhancement of secondary ion yields of polymers. Copyright © 2016 John Wiley & Sons, Ltd.

Applications of ToF-SIMS for imaging and depth profiling commercial materials

The development of cluster primary ion sources such as Aun+, Bin+, SF5+, C60+, and Arn+ has been an exciting advancement in SIMS analysis. Relative to atomic primary ion sources, cluster ion sources provide higher secondary ion yields. Furthermore, C60+ and Arn+ impart significantly less chemical damage to the sample thus enabling molecular depth profiling. Molecular depth profiling using cluster primary ion sources is routinely used to characterize a wide range of commercially important materials, including organic light emitting diode, biomaterials and pharmaceuticals, adhesives, and architectural paints and coatings. This paper highlights the application of time of flight secondary ion mass spectrometry (ToF-SIMS) to study contact lenses and acrylic-based paints. In the first application, ToF-SIMS was used to investigate the surface composition of two commercial contact lenses. Lens material I is composed of 2-hydroxy-ethyl methacrylate (HEMA) and glycerol methacrylate while lens material II is composed of HEMA and 2-methacryloxyethyl phosphorylcholine cross-linked with ethyleneglycol dimethacrylate. The ToF-SIMS data confirm the presence of the 2-methacryloxyethyl phosphorylcholine on the surface of lens material II. ToF-SIMS was also used to characterize a HEMA-based contact lens which had been worn for about 4 weeks. The analysis reveals the presence of N-containing species, fatty acids, phosphorylcholine, and dioctyldecyl dimethyl ammonium. Arn+ gas cluster ion beams (GCIB) depth profiling indicates the N-containing species, the fatty acids, and the dioctyldecyl dimethyl ammonium are concentrated at the surface. In the second application, a combination of O2+ and Arn+ GCIB depth profiling was used to study the pigment levels in acrylic-based paints. The O2+ beam was used to profile into the bulk of the dried paint film and Arn+ gas cluster beam was then used to remove the damaged material. ToF-SIMS analysis of the crater bottom reveals differences in pigment levels. The combined O2+ and Arn+ GCIB depth profiling is an effective way of characterizing materials composed of both organic and inorganic components.