Secondary Yield Research Articles

Characterization of sample preparation methods of NIH/3T3 fibroblasts for ToF-SIMS analysis

The information that is obtained from single cells during time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis is influenced by the method that was used to prepare the cells. The removal of extracellular media before analysis is necessary, but the rinsing technique should not damage the plasma membrane of the cell. The presence of intracellular salts reduced the secondary ion yield an average of 2.6-fold during Bi3+/C60++ depth profiles. Chemical fixation followed by rinsing removed a majority of the intracellular salts, “recovering” the positive secondary ion yields. The formaldehyde-fixation process removed a majority of the intracellular Cl-, but other key anions were not removed in significant amounts. The data presented here is consistent the anion neutralization mechanism largely responsible for the lower ion yields. All of the organic secondary ions that were detected in the freeze-dried cells were also detected in the formaldehyde-fixed cells, suggesting that the fixation process did not remove any molecular species to an extent that is detectable by ToF-SIMS. Compared to freeze dried cells, well preserved, frozen-hydrated cells showed little increase, or a decreased yield, for most low mass ions, but an increased yield for larger mass fragments. This is consistent with a reduced damage cross section at cryogenic analysis temperatures, although proton donation from water and reduction the salt effects in the presence of water likely also play roles. Numerous ions detected from the frozen-hydrated cells were not detected from the freeze dried cells, however many of these ions were attributed to chemical combinations of water, salts and the ammonium acetate rinsing solution.

Enhancing Secondary Ion Yields in Time of Flight-Secondary Ion Mass Spectrometry Using Water Cluster Primary Beams

Low secondary ion yields from organic and biological molecules are the principal limitation on the future exploitation of time of flight-secondary ion mass spectrometry (TOF-SIMS) as a surface and materials analysis technique. On the basis of the hypothesis that increasing the density of water related fragments in the ion impact zone would enhance proton mediated reactions, a prototype water cluster ion beam has been developed using supersonic jet expansion methodologies that enable ion yields using a 10 keV (H2O)1000+ beam to be compared with those obtained using a 10 keV Ar1000+ beam. The ion yields from four standard compounds, arginine, haloperidol, DPPC, and angiotensin II, have been measured under static+ and high ion dose conditions. Ion yield enhancements relative to the argon beam on the order of 10 or more have been observed for all the compounds such that the molecular ion yield per a 1 μm pixel can be as high as 20, relative to 0.05 under an argon beam. The water beam has also been shown to partially lift the matrix effect in a 1:10 mixture of haloperidol and dipalmitoylphosphatidylcholine (DPPC) that suppresses the haloperidol signal. These results provide encouragement that further developments of the water cluster beam to higher energies and larger cluster sizes will provide the ion yield enhancements necessary for the future development of TOF-SIMS.

Changes in the composition of the surface layer of silicon under sputtering by oxygen and cesium ion beams

The distribution profiles of primary ion beams of electroactive atoms (cesium and oxygen) in silicon samples have been investigated experimentally. It is demonstrated that the initial chemical composition of a sample is modified due to the implantation of primary ions at depths of up to 5 nm from its surface. A mechanism responsible for the enhancement of the secondary-ion yield during target sputtering by the chemically active elements has been proposed.

Oxygen accumulation effect for depth profiling of thin‐multilayered sample using low‐energy oxygen ion beam

This paper tries to discuss the change of the secondary ion yield, sputtered neutrals yield and the change of the internal energy of sputtered neutrals with O2+ beam bombardment. For this discussion, we researched the influence of the accumulation effect with O2+ beam bombardment by using the secondary ion mass spectrometry (SIMS) and the resonance‐enhanced multiphoton ionization sputtered neutral mass spectrometry (REMPI‐SNMS). In order to reveal the influence of different oxygen implantation depth, we used O2+ beam of 0.5, 1.0 and 2.0 keV and compared the amounts of secondary ions and sputtered neutrals for the nano‐multilayered samples. The results of the SIMS and SNMS measurement indicated that the accumulation effect depends on irradiation energy of O2+ beam because the surface oxygen concentration, which greatly affects secondary ion yield and sputtering neutrals yield, changed for different irradiation energies. In conclusion, it was found that the secondary ion yield, the sputtered neutrals yield and internal energy of sputtered neutrals were significantly changed by oxidation of the matrix elements. Copyright © 2012 John Wiley & Sons, Ltd.

Development of gas cluster ion beam irradiation system with an orthogonal acceleration TOF instrument

Surface damage induced on biomolecules with gas cluster ion beam (GCIB) irradiation is significantly lower than with atomic or small cluster ion beams, and for this reason, surface analysis techniques such as secondary ion mass spectrometry (SIMS) have become one of the most important applications of GCIB, particularly for microscale chemical imaging of biomolecular species. Because of the low duty‐cycle in time‐of‐flight (TOF)‐SIMS, only less than 0.1 % of the incident ion beam is used for analysis, meaning that analysis with high spatial resolution can practically be extremely lengthy. The duty cycle can be significantly improved with the orthogonal acceleration (oa) TOF method because with this method secondary ion mass spectra can be measured at high mass resolution without requiring a pulsed primary ion beam. In this study, we developed a gas cluster ion irradiation system mounted on an oa‐TOF instrument and investigated the sputtering yield and secondary ion yield of arginine. Copyright © 2012 John Wiley & Sons, Ltd.

Secondary ion emission under the bombardment of Si by multiply charged Si q+ ions

The spectra of secondary ion emission under the bombardment of a B-doped Si target by multiply charged Si q+ ions (q = 1–5) have been studied in the energy range of 1 to 10 keV per unit of charge. A multifold increase in the yield of secondary cluster Sk + ions, multiply charged Si q/+ ion (q = 1–3), and H+, C+, B+, Si2N+, Si2O+ is observed as the charge of the multiply charged ions increases. The increase in the yield of secondary ions with increasing charge of the multiply charged-ion charge is most significant for ions with relatively high ionization potentials.

SIMS using O−, F−, Cl−, Br− and I− primary ion bombardment

The success of secondary ion mass spectrometry (SIMS) analyses depends largely on the ionization probability of the analyzed elements. The chemical state of the surface changes with the chemical nature and the concentration of implanted ions. The positive ionization probability can be enhanced by bombarding the surface with electronegative elements. In view of such an enhancement of the positive secondary ion yield, we present SIMS analyses carried out with O−, F−, Cl−, Br− and I− primary ion beams. Useful yields were experimentally determined for metal (Al, Ni, Cu, Ag and Ta) and semiconductor samples (Si, Ge, InP and GaAs). For metal samples, an enhancement of the useful yield under halogen bombardment, compared with O− bombardment, was observed for Ni, Cu and Ag under F− bombardment (enhancement of up to two orders of magnitude). For semiconductors, lower useful yields are obtained under halogen bombardment as compared with O− bombardment. The observed results are discussed in terms of the surface concentration of the implanted primary ion species and their electronegativity. Copyright © 2012 John Wiley & Sons, Ltd.

Cellular metabolic profiling using ToF‐SIMS

Proteomic and metabolomic changes associated with cellular metabolism are important metabolic indicators that can give insight into cellular function. Metabolites are small molecular weight cellular components of mass typically <1500 Da and well suited for detection using ToF‐SIMS. The advent of cluster ion beams, such as bismuth (Bin) and C60, as primary ion sources, has improved the sputtering efficiencies, with higher secondary ion yields and lower damage cross sections, enabling better detection of these components. However, the spectral information is dominated by fragment peaks, matrix interference and preferential ionizations making it challenging to apply time‐of‐flight secondary ion mass spectrometry for non‐targeted analyses. In this investigation, the challenges in detecting metabolic changes in cells and tissues are discussed, and areas that need further development are highlighted. Copyright © 2012 John Wiley & Sons, Ltd.

Quantification of Ge in Si1‐xGex by using low‐energy Cs+ and O2+ ion beams

Several studies have been reported on the quantification of Ge in Si1‐xGex using SIMS with oxygen or cesium primary ions. Most of them were limited to lower Ge concentrations. In this paper, we discussed procedures to quantify Ge content in Si1‐xGex with x ranging from 0.11 to 0.70 by using low‐energy Cs+ and O2+ primary beams. The results showed that at impact energy of 250 eV, the Ge secondary ion yield changed dramatically against the SiGe matrix composition with a Cs+ primary beam, whereas the change was minimal with an O2+ primary beam. As a result, quantification by using the reference signal inside Si substrate became difficult as no linear correlation exists between the Ge intensity and the Ge concentration in the case of Cs+ primary ions. However, the correlation between the ratios of secondary intensities I(Ge)/I(Si) within the SiGe layers and the composition ratio [Ge]/[Si] is monotonic (although nonlinear). Hence, quantification based on the ratios of the secondary ion intensities versus the ratios of the corresponding contents worked for both Cs+ and O2+ primary beams to quantify the Ge content of an unknown SiGe sample. In addition to this, we reported the angular (incidence angles between 0° and 70°) dependence of the sputter yield of SiGe as a function of Cs+ primary ion energy (150–500 eV). Copyright © 2012 John Wiley & Sons, Ltd.

Primary ion implantation and recoil implantation effects in Cs depth profiling of thin metallic layers on LiNbO3

Depth profiles obtained under Cs bombardment of samples of crystalline lithium niobate (LiNbO3) covered with a thin layer of metal display some artifacts related to metal/LiNbO3 interface effects. The altered yields of Li and Nb secondary ions in the first 50 nm after erosion of the metal layer can be interpreted in terms of different stopping powers between metal and LiNbO3. In this study, two typical samples are investigated and the artifacts are qualitatively explained by comparing experimental profiles and simulations performed with the transport of ions in matter code. The similarity between experiment and simulation suggests that the ballistic interpretation of the artifacts is appropriate. Copyright © 2012 John Wiley & Sons, Ltd.

SU-D-BRB-02: Investigations of Secondary Ion Distributions in Carbon Ion Therapy Using the Timepix Detector.

Due to the high conformity of carbon ion therapy, unpredictable changes in the patient's geometry or deviations from the planned beam properties can result in changes of the dose distribution. PET has been used successfully to monitor the actual dose distribution in the patient. However, it suffers from biological washout processes and low detection efficiency. The purpose of this contribution is to investigate the potential of beam monitoring by detection of prompt secondary ions emerging from a homogeneous phantom, simulating a patient's head. Measurements were performed at the Heidelberg Ion-Beam Therapy Center (Germany) using a carbon ion pencil beam irradiated on a cylindrical PMMA phantom (16cm diameter). For registration of the secondary ions, the Timepix detector was used. This pixelated silicon detector allows position-resolved measurements of individual ions (256×256 pixels, 55μm pitch). To track the secondary ions we used several parallel detectors (3D voxel detector). For monitoring of the beam in the phantom, we analyzed the directional distribution of the registered ions. This distribution shows a clear dependence on the initial beam energy, width and position. Detectable were range differences of 1.7mm, as well as vertical and horizontal shifts of the beam position by 1mm. To estimate the clinical potential of this method, we measured the yield of secondary ions emerging from the phantom for a beam energy of 226MeV/u. The differential distribution of secondary ions as a function of the angle from the beam axis for angles between 0 and 90° will be presented. In this setup the total yield in the forward hemisphere was found to be in the order of 10-1 secondary ions per primary carbon ion. The presented measurements show that tracking of secondary ions provides a promising method for non-invasive monitoring of ion beam parameters for clinical relevant carbon ion fluences. Research with the pixel detectors was carried out in frame of the Medipix Collaboration.

Exact compositional analysis of SiGe alloys by matrix effect compensated MCs+-SIMS

SiGe alloy, owing to its high electron and hole mobility, has potential applications in high-speed microelec- tronic device technology. The optimization of such tech- nology requires the precise determination of Ge concentra- tion in the full range of composition and the understanding and control of the Ge-Si interdiffusion phenomenon. The most appropriate analytical technique with highest detection sensitivity (∼subparts per billion) for measuring elemental concentration is secondary ion mass spectrometry (SIMS). However, strong compositional dependence of secondary ion yield, i.e. "matrix effect," has always made SIMS quan- tification extremely difficult. A procedure for the accurate quantification of Ge concentration in molecular beam epi- taxy (MBE)-grown Si1−xGex (0 <x< 0.72) alloys based on MCs + -SIMS approach has been proposed. The "matrix effect" is shown to be completely suppressed for all Ge concentrations irrespective of impact Cs + ion energies. The novel methodology has successfully been applied for direct quantitative composition analysis of Si/Ge multilayer struc- ture.

Quantification of cesium surface contamination on silicon resulting from SIMS analysis

In order to improve the understanding of unintended cesium (Cs) contamination that occurs during SIMS depth profiling, Cs concentrations on sample surfaces were measured before analysis and at various distances from a depth profile crater after analysis. Cs concentrations in excess of 1 at. % were found directly adjacent to the depth profile analysis site. Cs was also detected at significant concentrations hundreds of micrometers from the depth profile measurement location. This Cs contamination can originate from a number of sources including Cs beam tails, Cs neutral beam, and secondary sputtering from instrument optics and other structures. Since the presence of cesium significantly affects the secondary ion yield of electronegative elements (e.g., phosphorus) in silicon, the unintended presence of cesium on the surface of a previously analyzed sample can strongly affect the reproducibility and accuracy of low dose electronegative element measurements, especially at the surface.

Signals of secondary ions and resonantly and nonresonantly ionized neutrals sputtered from binary alloys as a function of oxygen exposure

For the quantification of elements in different chemical matrices, in‐depth knowledge of the emission processes of atomic and molecular particles under ion bombardment is of great importance. This knowledge can be obtained by analyzing the flux composition of secondary ions and neutrals under ion bombardment. Flux composition, the mutual interaction of alloy components, and the relative sensitivity factors as a function of oxygen exposure and as a function of different redox potentials between alloy components were investigated. GdTi80 (20 at% Gd and 80 at% Ti), GdFe75, TiFe50, and TiCu50 binary alloys were used for the experiments. The data show that the mutual interaction of alloy components during oxygen exposure causes significant changes in the secondary ion and neutral yield. Also, significant differences between the relative sensitivity factors for postionized atoms sputtered from binary alloys as a function of oxygen exposure were observed. Measurements using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and resonant and nonresonant laser‐secondary neutral mass spectrometry (laser‐SNMS) show that the fluctuation of the yield ratios can be explained by strong variations in the secondary ion yield and by variations of the molecular and atomic fractions in the flux composition of secondary particles. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Surface spectrometry using large argon clusters

Large argon cluster ions can very successfully be applied as primary ion projectiles in SIMS, but until recently, these sources could not be used in combination with conventional TOF‐SIMS systems. The large cluster size distribution limited the possibility to pulse the primary ion beam, which is a prerequisite for high mass resolution surface spectrometry. We developed a unique 90° pulsing system which enables the generation of short primary ion pulses for high mass resolution surface spectrometry. The pulsing system allows the variation of the applied cluster size from 250 to 10000 atoms/cluster. In this contribution, we present and compare data about the influence of the cluster size on the spectra appearance, the fragmentation and the secondary ion yield on different organic sample systems. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Characteristics of positive and negative secondary ions emitted from [Formula: see text] and [Formula: see text] impacts.

The current limitation for SIMS analyses is insufficient secondary ion yields, due in part to the inefficiency of traditional primary ions. Massive gold clusters are shown to be a route to significant gains in secondary ion yields relative to other commonly used projectiles. At an impact energy of 520 keV, [Formula: see text] is capable of generating an average of greater than ten secondary ions per projectile, with some impact events generating >100 secondary ions. The capability of this projectile for signal enhancement is further displayed through the observation of up to seven deprotonated molecular ions from a single impact on a neat target of the model pentapeptide leu-enkephalin. Positive and negative spectra of leu-enkephalin reveal two distinct emission regimes responsible for the emission of either intact molecular ions with low internal energies or small fragment species. The internal energy distribution for this projectile is measured using a series of benzylpyridinium salts and compared with the small polyatomic projectile [Formula: see text] at 110 keV as well as distributions previously reported for electrospray ionization and fast atom bombardment. These results show that [Formula: see text] offers high secondary ion yields not only for small fragment ions, e.g. CN-, typically observed in SIMS analyses, but also for characteristic molecular ions. For the leu-enkephalin example, the yields for each of these species are greater than unity.

Correlation between molecular secondary ion yield and cluster ion sputtering for samples with different stopping powers

In static SIMS, the secondary ion yield, defined as detected ions per primary ion, can be increased by altering several primary ion parameters. For many years, no quantitative predictions could be made for the secondary ion yield enhancement of molecular ions. For thick samples of organic compounds, a power dependency of the secondary ion yield on the sputtering yield was shown. For this article, samples with thick molecular layers and (sub-)monolayers composed of various molecules were prepared on inorganic substrates such as silicon, silver, and gold, and subsequently analyzed. For primary ion bombardment, monoatomic (Ne+, Ar+, Ga+, Kr+, Xe+, Bi+) as well as polyatomic (Bin+, Bin++) primary ions were used within an energy range of 10–50keV. The power dependency was found to hold true for the different samples; however, the exponent decreased with increasing stopping power. Based on these findings, a rule of thumb is proposed for the prediction of the lower limit of the secondary ion yield enhancement as a function of the primary ion species. Additionally, effects caused by the variation of the energy deposition are discussed, including the degree of molecular fragmentation and the non-linear increase of the secondary ion yield when polyatomic primary ions are used.

Dramatically Enhanced Oxygen Uptake and Ionization Yield of Positive Secondary Ions with C60+ Sputtering

To explore C(60)(+) sputtering beyond low-damage depth profiling of organic materials, X-ray photoelectron spectrometry (XPS) and secondary ion mass spectrometry (SIMS) were used to examine metallic surfaces during and after C(60)(+) sputtering. During C(60)(+) sputtering, XPS spectra indicated that the degrees of carbon deposition were different for different metallic surfaces. Moreover, for some metals (e.g., Al, W, Ta, Ti, and Mo), the intensity of the O 1s photoelectron increased significantly during C(60)(+) sputtering, even though the instrument was under ultrahigh vacuum (<5 × 10(-7) Pa). This result indicated that the rate of oxygen uptake was greater than the rate of C(60)(+) sputtering. This behavior was not observed with the commonly used Ar(+) sputtering. To measure the oxygen uptake kinetics, pure oxygen was leaked into the chamber to maintain a 5 × 10(-6) Pa oxygen environment. The C(60)(+)-sputtered surface had a clearly increased rate of oxygen uptake than the Ar(+)-sputtered surface, even for moderately reactive metals such as Fe and Ni. For relatively nonreactive metals such as Cu and Au, a small amount of carbon was implanted and no oxygen uptake was observed. High-resolution XPS spectra revealed the formation of metal carbides on these reactive metals, and the carbon deposition and enhanced uptake of oxygen correlated to the carbide formation. Because oxygen enhances the secondary ion yield through surface passivation, the enhanced oxygen uptake due to C(60)(+) sputtering could be beneficial for SIMS analysis. To examine this hypothesis, C(60)(+) and Ar(+) were used as primary ions, and it was found that the intensity enhancement (because of the oxygen flooding at 5 × 10(-6) Pa) was much higher with C(60)(+) than with Ar(+). Therefore, oxygen flooding during C(60)(+) sputtering has a great potential for enhancing the detection limit due to the enhanced oxygen uptake.

Relative ion yields in mammalian cell components using C60 SIMS.

Time of flight secondary ion mass spectrometry has been used to better understand the influence of molecular environment on the relative ion yields of membrane lipid molecules found in high abundance in a model mammalian cell line, RAW264.7. Control lipid mixtures were prepared to simulate lipid-lipid interactions in the inner and outer leaflet of cell membranes. Compared with its pure film, the molecular ion yields of 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine are suppressed when mixed with 2-dipalmitoyl-sn-glycero-3-phosphocholine. In the mixture, proton competition between 1,2-dioleoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, and 2-dipalmitoyl-sn-glycero-3-phosphocholine led to lower ionization efficiency. The possible mechanism for ion suppression was also investigated with 1H and 13C nuclear magnetic resonance spectroscopy. The formation of a hydroxyl bond in lipid mixtures confirms the mechanism involving proton exchange with the surrounding environment. Similar effects were observed for lipid mixtures mimicking the composition of the inner leaflet of cell membranes. The secondary molecular ion yield of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine was observed to be enhanced in the presence of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine.

Oxidation effect on the SIMS analysis of samples sputtered and deposited by the Storing Matter technique

Storing Matter is a new technique developed to avoid the matrix effect appearing during secondary ion mass spectrometry (SIMS) analysis by decoupling the sputtering step of the sample to be analyzed from the subsequent analysis step by SIMS. Prior to the analysis, the emitted matter is deposited at a sub-monolayer level on a well known collector, whose oxidation state may influence the secondary ion yields of the deposit.The aim of this work is to study the influence of the oxidation of a Si collector on the secondary ion yields of In and Au deposits analyzed in positive and negative modes, respectively. We have observed in our study that extreme variations of the level of oxidation alter the ion yield, although the oxidation generated on a Si0 collector under vacuum is not enough to induce relevant changes.