An effect of residual gas component on detected secondary ions during TOF‐SIMS depth profiling and a method to estimate contained component

With regard to Secondary Ion Mass Spectroscopy (SIMS) measurement of atmospheric gas elements, a problem occurs that the detected signal includes background components caused by residual gas along with contained components. Relating to this issue, an available method to quantify the contained components by separating the background ones had been established for Dynamic SIMS. Time‐of‐Flight SIMS with sputtering ion gun has also applied for depth profiling as well as Dynamic SIMS. However, few studies have attempted to investigate the secondary ion behavior of the atmospheric gas elements for depth profiling by Time‐of‐flight SIMS, especially for low concentration levels. In this study, experimental examinations of the secondary ions of the atmospheric gas elements, such as oxygen, hydrogen, and carbon in the silicon substrate, has been conducted in various analytical conditions of TOF‐SIMS depth profiling mode. Under the analytical conditions of our study, it has been proved that the background intensity of these elements was correlated to the sputtering rate. For the analysis of Floating Zone Silicon substrate, the oxygen intensity of the background component was proportional to the inverse number of the sputtering rate. Based on these facts, the total detected intensity of the atmospheric gas elements was able to be separated into the contained components and background ones by changing the sputtering rate during TOF‐SIMS measurement. An experimental result has shown that the contained oxygen concentration in the Czochralsk Silicon substrate estimated by the “TOF‐SIMS Raster Change Method” has successfully agreed with the result by the Dynamic SIMS.     

Novel floating‐type low‐energy ion gun for high‐resolution depth profiling in ultrahigh vacuum

A floating‐type low‐energy ion gun (FLIG) has been developed for high‐resolution depth profiling in ultrahigh vacuum (UHV). This UHV‐FLIG allows Ar⁺ ions of primary energy down to 50 eV to be provided with high current intensity. The developed UHV‐FLIG was sufficiently compact, being ～30 cm long, to be attached to a commercial surface analytical instrument. The performance of the UHV‐FLIG was measured by attaching it to a scanning Auger electron microprobe (JAMP‐10, Jeol), the base pressure of which in the analysis chamber was ～1 × 10^?7 Pa. The vacuum condition of ～5 × 10^?6 Pa was maintained during operation of the UHV‐FLIG without a differential pumping facility. Current density ranged from 41 to 138 µA cm^?2 for Ar⁺ ions of primary energy 100–500 eV at the working distance of 50 mm. This ensures a sputtering rate of ～10 nm h^?1 with 100 eV Ar⁺ ions for Si, leading to depth profiling of high resolution in practical use. Copyright © 2003 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.     

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.     

Summary of ISO/TC 201 Standard: X ISO 17560:2002—Surface chemical analysis—Secondary ion mass spectrometry—Method for depth profiling of boron in silicon

This International Standard specifies a secondary ion mass spectrometric method using magnetic‐sector or quadrupole mass spectrometers for depth profiling of boron in silicon, and using stylus profilometry or optical interferometry for depth calibration. This method is applicable to single‐crystal, polycrystal or amorphous silicon specimens with boron atomic concentrations between 1 × 10¹⁶ and 1 × 10²⁰ atoms cm^?3, and to the crater depth of 50 nm or deeper. Optical interferometry is generally applicable to crater depths in the range 0.5–5 µm. Copyright © 2004 John Wiley & Sons, Ltd.     

Summary of ISO/TC 201 standard: XV. ISO 20341:2003—Surface chemical analysis—secondary ion mass spectrometry—method for estimating depth resolution parameters with multiple delta‐layer reference materials

This International Standard specifies procedures for estimating three depth resolution parameters, via the leading‐edge decay length, the trailing‐edge decay length and Gaussian broadening, in SIMS depth profiling using multiple delta‐layer reference materials. This International Standard is not applicable to delta‐layers where the chemical and physical state of the near‐surface region, modified by the incident primary ions, is not in the steady state. Copyright © 2005 John Wiley & Sons, Ltd.     

Ion‐beam‐induced morphology on the surface of thin polymer films at low current density and high ion fluence

The effect of Xe⁺ bombardment on the surface morphology of four different polymers, polystyrene (PS), poly(phenylene oxide), polyisobutylene, and polydimethylsiloxane, was investigated in ion energy and fluence ranges of interest for secondary ion mass spectrometry depth‐profiling analysis. Atomic force microscopy (AFM) was applied to analyze the surface topography of pristine and irradiated polymers. AFM analyses of nonirradiated polymer films showed a feature‐free surface with different smoothness. We studied the influence of different Xe⁺ beam parameters, including the incidence angle, ion energy (660–4000 eV), current density (0.5 × 10² to 8.7 × 10² nA/cm²), and ion fluence (4 × 10¹⁴ to 2 × 10¹⁷ ion/cm²). Xe⁺ bombardment of PS with 3–4 keV at a high current density did not induce any change in the surface morphology. Similarly, for ion irradiation with lower energy, no surface morphology change was found with a current density higher than 2.6 × 10² nA/cm² and an ion fluence up to 4 × 10¹⁶ ion/cm². However, Xe⁺ irradiation with a lower current density and a higher ion fluence led to topography development for all of the polymers. The roughness of the polymer surface increased, and well‐defined patterns appeared. The surface roughness increased with ion irradiation fluence and with the decrease of the current density. A pattern orientation along the beam direction was visible for inclined incidence between 15° and 45° with respect to the surface normal. Orientation was not seen at normal incidence. The surface topography development could be explained on the basis of the balance between surface damage and sputtering induced by the primary ion beam and redeposition–adsorption from the gas phase. Time‐of‐flight secondary ion mass spectrometry analyses of irradiated PS showed strong surface modifications of the molecular structure and the presence of new material. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 314–325, 2001     

Evaluation of the sputtering rate variation in SIMS ultra‐shallow depth profiling using multiple short‐period delta layers

We have developed multiple short‐period delta layers as a reference material for SIMS ultra‐shallow depth profiling. Boron nitride delta layers and silicon spacer layers were sputter‐deposited alternately, with a silicon spacer thickness of 1–5 nm. These delta‐doped layers were used to measure the sputtering rate change in the initial stage of oxygen ion bombardment. A significant variation of sputtering rate was observed in the initial 3 nm or less. The sputtering rate in the initial 3 nm was estimated to be about four times larger than the steady‐state value for 1000 eV oxygen ions. Copyright © 2003 John Wiley & Sons, Ltd.     

Extraction of hidden information of ToF‐SIMS data using different multivariate analyses

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) is a powerful tool for determining surface information of complex systems such as polymers and biological materials. However, the interpretation of ToF‐SIMS raw data is often difficult. Multivariate analysis has become effective methods for the interpretation of ToF‐SIMS data. Some of multivariate analysis methods such as principal component analysis and multivariate curve resolution are useful for simplifying ToF‐SIMS data consisting of many components to that explained by a smaller number of components. In this study, the ToF‐SIMS data of four layers of three polymers was analyzed using these analysis methods. The information acquired by using each method was compared in terms of the spatial distribution of the polymers and identification. Moreover, in order to investigate the influence of surface contamination, the ToF‐SIMS data before and after Ar cluster ion beam sputtering was compared. As a result, materials in the sample of multiple components, including unknown contaminants, were distinguished. Copyright © 2014 John Wiley & Sons, Ltd.     

100 keV H+ ion irradiation of as‐deposited Al‐doped ZnO thin films: An interest in tailoring surface morphology for sensor applications

We report the influence of 100 keV H⁺ ion beam irradiation on the surface morphology, crystalline structure, and transport properties of as‐deposited Al‐doped ZnO (Al:ZnO) thin films. The films were deposited on silicon (Si) substrate by using DC sputtering technique. The ion irradiation was carried out at various fluences ranging from 1.0 × 10¹² to 3.0 × 10¹⁴ ions/cm². The virgin and ion‐irradiated films were characterized by X‐ray diffraction, Raman spectroscopy, atomic force microscopy, and Hall probe measurements. Using X‐ray diffraction spectra, 5 points Williamson‐Hall plots were drawn to deduce the crystallite site and strain in Al:ZnO films. The analysis of the measurements shows that the films are almost radiation resistant in the structural deformation under chosen irradiation conditions. With beam irradiation, the transport properties of the films are also preserved (do not vary orders of magnitude). However, the surface roughness and the crystallite size, which are crucial parameters of the ZnO film as a gas sensor, are at variation with the ion fluence. As ion fluence increases, the root‐mean‐square surface roughness oscillates and the surface undergoes for smoothening with irradiation at chosen highest fluence. The crystallite size decreases initially, increases for intermediate fluences, and drops almost to the value of the pristine film at highest fluence. In the paper, these interesting experimental results are discussed in correlations with ion‐matter interactions especially energy losses by the ion beam in the material.     

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

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

A ToF‐SIMS investigation of a buried polymer/polymer interface exposed by ultra‐low‐angle microtomy

The interfacial region of a model multilayer coating system on an aluminium substrate has been investigated by high‐resolution time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Employing ultra‐low‐angle microtomy (ULAM), the interface between a poly(vinylidene difluoride) (PVdF)‐based topcoat and a poly(urethane) (PU)‐based primer ‘buried’ >20 µm below the PVdF topcoat's air/coating surface was exposed. Imaging ToF‐SIMS and subsequent post‐processing extraction of mass spectra of the ULAM‐exposed interface region and of the PVdF topcoat and PU primer bulks indicates that the material composition of the polymer/polymer interface region is substantially different to that of the bulk PVdF and PU coatings. Analysis of the negative ion mass spectra obtained from the PVdF/PU interface reveals the presence of a methacrylate‐based component or additive at the interface region. Reviewing the topcoat and primer coating formulations reveals that the PVdF topcoat formulation contains methyl methacrylate (MMA)–ethyl acrylate (EA) acrylic co‐polymer components. Negative ion ToF‐SIMS analysis of an acrylic co‐polymer confirms that it is these components that are observed at the PVdF/PU interface. Post‐processing extraction of ToF‐SIMS images based on the major ions of the MMA–EA co‐polymers reveals that these components are observed in high concentration at the extremities of the PVdF coating, i.e. at the polymer/polymer interface, but are also observed to be distributed evenly throughout the bulk of the PVdF topcoat. These findings confirm that a fraction of the MMA–EA acrylic co‐polymers in the formulation segregate to the topcoat/primer interface where they enhance the adhesive properties exhibited by the PVdF topcoat towards the underlying PU primer substrate. Copyright © 2004 John Wiley & Sons, Ltd.     

Investigating Pb diffusion across buried interfaces in Pb(Zr0.2Ti0.8)O3 thin films via time‐of‐flight secondary ion mass spectrometry depth profiling

The diffusion of Pb through Pb(Zr_0.2Ti_0.8)O₃(PZT)/Pt/Ti/SiO₂/Si thin film heterostructures is studied by using time‐of‐flight secondary ion mass spectrometry depth profiling. The as‐deposited films initially contained 10 mol% Pb excess and were thermally processed at temperatures ranging from 325 to 700°C to promote Pb diffusion. The time‐of‐flight secondary ion mass spectrometry depth profiles show that increasing processing temperature promoted Pb diffusion from the PZT top film into the buried heterostructure layers. After processing at low temperatures (eg, 325°C), Pb⁺ counts were low in the Pt region. After processing at elevated temperatures (eg, 700°C), significant Pb⁺ counts were seen throughout the Pt layer and into the Ti and SiO₂ layers. Intermediate processing temperatures (400, 475, and 500°C) resulted in Pb⁺ profiles consistent with this overall trend. Films processed at 400°C show a sharp peak in PtPb⁺ intensity at the PZT/Pt interface, consistent with prior reports of a Pt₃Pb phase at this interface after processing at similar temperatures.     

Evaluation of column length and particle size effect on the untargeted profiling of a phytochemical mixture by using UHPLC coupled to high‐resolution mass spectrometry

Liquid chromatography coupled to high‐resolution mass spectrometry is the technique of choice for the untargeted profiling of food matrices. Despite the high potential of high‐resolution mass spectrometry, when dealing with complex mixtures, an efficient separation technique is also needed. The novel core‐shell chromatographic columns packed with sub‐2 μm sized particles are claimed to show very good resolution. However, the analytes retention can be significantly altered when working under ultra‐high performance chromatographic conditions. In this work, an evaluation of four chromatographic systems, with either a single or two in‐series Kinetex™ C₁₈ columns, either packed with 2.6 or 1.7 μm particles, is presented for the targeted analysis of a standard mixture and the untargeted analysis of a strawberry extract. An ultra‐high performance chromatographic system coupled via an electrospray source to a hybrid quadrupole‐Orbitrap mass spectrometer was used. From the extensive comparison, a surprising result was obtained, namely, that the system identifying the largest number of features was the one with two in‐series connected columns with the larger particle size. The inconsistency among the theoretical assumptions and the applicative findings points out the importance of an extensive chromatographic evaluation for the comprehensive untargeted profiling of complex real samples.     

Hidden information in principal component analysis of ToF‐SIMS data: On the use of correlation loadings for the identification of significant signals and structure elucidation

In this paper, an improved approach to interpret results of principal component analysis (PCA) of time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) spectra is presented. Signals are typically observed in different intensity ranges in a single ToF‐SIMS spectrum due to different sensitivity factors and surface concentrations. This can complicate the PCA interpretation, because loadings are reported to be strongly affected by these intensity changes. In contrast, it is shown here that correlation loadings are unaffected by these differences. In particular, correlation loadings were successfully used to identify signals with relatively low intensity but high significance. These signals may be overlooked when only loadings are used. This is particularly true in failure analysis, where ToF‐SIMS is used to screen for initially unknown signals that may be relevant for the characteristics/failure of a product. As a model study, the concept was applied to investigate ageing of Li‐ion batteries by ToF‐SIMS. In this data set, the significance of impurities that affect the quality of Li‐ion batteries was identified only by correlation loadings, whereas the loadings were found to overestimate the influence of other matrix signals. In addition, correlation loadings aid in the chemical identification and helped to successfully assign unknown peaks.     

Chemical profiling approach to evaluate the influence of traditional and simplified decoction methods on the holistic quality of Da‐Huang‐Xiao‐Shi decoction using high‐performance liquid chromatography coupled with diode‐array detection and time‐of‐flight mass spectrometry

Da‐Huang‐Xiao‐Shi decoction, consisting of Rheum officinale Baill, Mirabilitum, Phellodendron amurense Rupr. and Gardenia jasminoides Ellis, is a traditional Chinese medicine used for the treatment of jaundice. As described in “Jin Kui Yao Lue”, a traditional multistep decoction of Da‐Huang‐Xiao‐Shi decoction was required while simplified one‐step decoction was used in recent repsorts. To investigate the chemical difference between the decoctions obtained by the traditional and simplified preparations, a sensitive and reliable approach of high‐performance liquid chromatography coupled with diode‐array detection and electrospray ionization time‐of‐flight mass spectrometry was established. As a result, a total of 105 compounds were detected and identified. Analysis of the chromatogram profiles of the two decoctions showed that many compounds in the decoction of simplified preparation had changed obviously compared with those in traditional preparation. The changes of constituents would be bound to cause the differences in the therapeutic effects of the two decoctions. The present study demonstrated that certain preparation methods significantly affect the holistic quality of traditional Chinese medicines and the use of a suitable preparation method is crucial for these medicines to produce special clinical curative effect. This research results elucidated the scientific basis of traditional preparation methods in Chinese medicines.     

Integrated strategy based on high‐resolution mass spectrometry coupled with multiple data mining techniques for the metabolic profiling of Xanthoceras sorbifolia Bunge husks in rat plasma,urine, and feces

An integrated strategy based on high‐resolution mass spectrometry coupled with multiple data mining techniques was developed to screen the metabolites in rat biological fluids after the oral administration of Xanthoceras sorbifolia Bunge husks. Mass defect filtering, product ion filtering, and neutral loss filtering were applied to detect metabolites from the complex matrix. As a result, 55 metabolites were tentatively identified, among which 45 barrigenol‐type triterpenoid metabolites were detected in the feces, and six flavonoids and four coumarins metabolites were in the urine. Moreover, eight prototype constituents in plasma, 36 in urine and 23 in feces were also discovered. Due to the poor bioavailability of barrigenol type triterpenoids, most of them were metabolized by intestinal flora. Phase I metabolic reactions such as deglycosylation, oxidation, demethylation, dehydrogenation, and internal hydrolysis were supposed to be their principal metabolic pathways. Coumarins were found in all the biosamples, whereas flavonoids were mainly in the urine. Unlike the saponins, they were mainly metabolized through phase II metabolic reactions like glucuronidation and sulfonation, which made them eliminated more easily by urine. This work suggested the metabolic profile of X. sorbifolia husks for the first time, which will be very valuable for its further development.