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

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

Measurement of sputtering yields and damage in C60 SIMS depth profiling of model organic materials

Sputter‐depth profiles of model organic thin films on silicon using C₆₀ primary ions have been employed to measure sputtering yields and depth resolution parameters. We demonstrate that some materials (polylactide, Irganox 1010) have a constant and high sputtering yield, which varies linearly with the primary ion energy, whereas another material (Alq₃) has lower, fluence‐dependent sputtering yields. Analysis of multi‐layered organic thin films reveals that the depth resolution is a function of both primary ion energy and depth, and the sputtering yield depends on the history of sputtering. We also show that ～30% of repeat units are damaged in the steady‐state regime during polylactide sputtering. Crown Copyright © 2006. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

VAMAS interlaboratory study on organic depth profiling

We present the results of a VAMAS (Versailles project on Advanced Materials and Standards) interlaboratory study on organic depth profiling, in which twenty laboratories submitted data from a multilayer organic reference material. Individual layers were identified using a range of different sputtering species (C₆₀ⁿ⁺, Cs⁺, SF₅⁺ and Xe⁺), but in this study only the C₆₀ⁿ⁺ ions were able to provide truly ‘molecular’ depth profiles from the reference samples. The repeatability of profiles carried out on three separate days by participants was shown to be excellent, with a number of laboratories obtaining better than 5% RSD (relative standard deviation) in depth resolution and sputtering yield, and better than 10% RSD in relative secondary ion intensities. Comparability between laboratories was also good in terms of depth resolution and sputtering yield, allowing useful relationships to be found between ion energy, sputtering yield and depth resolution. The study has shown that organic depth profiling results can, with care, be compared on a day‐to‐day basis and between laboratories. The study has also validated three approaches that significantly improve the quality of organic depth profiling: sample cooling, sample rotation and grazing angles of ion incidence. © Crown copyright 2010.     

Electron Flood Gun Damage Effects in 3D Secondary Ion Mass Spectrometry Imaging of Organics

Electron flood guns used for charge compensation in secondary ion mass spectrometry (SIMS) cause chemical degradation. In this study, the effect of electron flood gun damage on argon cluster depth profiling is evaluated for poly(vinylcarbazole), 1,4-bis((1-naphthylphenyl)amino)biphenyl and Irganox 3114. Thin films of these three materials are irradiated with a range of doses from a focused beam of 20 eV electrons used for charge neutralization. SIMS chemical images of the irradiated surfaces show an ellipsoidal damaged area, approximately 3 mm in length, created by the electron beam. In depth profiles obtained with 5 keV Ar₂₀₀₀ ⁺ sputtering from the vicinity of the damaged area, the characteristic ion signal intensity rises from a low level to a steady state. For the damaged thin films, the ion dose required to sputter through the thin film to the substrate is higher than for undamaged areas. It is shown that a damaged layer is formed and this has a sputtering yield that is reduced by up to an order of magnitude and that the thickness of the damaged layer, which increases with the electron dose, can be as much as 20 nm for Irganox 3114. The study emphasizes the importance of minimizing the neutralizing electron dose prior to the analysis. Figure

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Precise and fast secondary ion mass spectrometry depth profiling of polymer materials with large Ar cluster ion beams

We demonstrate depth profiling of polymer materials by using large argon (Ar) cluster ion beams. In general, depth profiling with secondary ion mass spectrometry (SIMS) presents serious problems in organic materials, because the primary keV atomic ion beams often damage them and the molecular ion yields decrease with increasing incident ion fluence. Recently, we have found reduced damage of organic materials during sputtering with large gas cluster ions, and reported on the unique secondary ion emission of organic materials. Secondary ions from the polymer films were measured with a linear type time‐of‐flight (TOF) technique; the films were also etched with large Ar cluster ion beams. The mean cluster size of the primary ion beams was Ar₇₀₀ and incident energy was 5.5 keV. Although the primary ion fluence exceeded the static SIMS limit, the molecular ion intensities from the polymer films remained constant, indicating that irradiation with large Ar cluster ion beams rarely leads to damage accumulation on the surface of the films, and this characteristic is excellently suitable for SIMS depth profiling of organic materials. Copyright © 2009 John Wiley & Sons, Ltd.     

Development of B‐doped Si multiple delta‐layer reference materials for SIMS profiling

B‐doped Si multiple delta‐layers (MDL) were developed as certified reference materials (CRM) for secondary ion mass spectrometry (SIMS) depth profiling analysis. Two CRMs with different delta‐layer spacing were grown by ion beam sputter deposition (IBSD). The nominal spacing of the MDL for shallow junction analysis is 10 nm and that for high energy SIMS is 50 nm. The total thickness of the film was certified by high resolution transmission electron microscopy (HR‐TEM). The B‐doped Si MDLs can be used to evaluate SIMS depth resolution and to calibrate the depth scale. A consistency check of the calibration of stylus profilometers for measurement of sputter depth is another possible application. The crater depths measured by a stylus profilometer showed a good linear relationship with the thickness measured from SIMS profiling using the calibrated film thickness for depth scale calibration. The sputtering rate of the amorphous Si thin film grown by sputter deposition was found to be the same as that of the crystalline Si substrate, which means that the sputtering rate measured with these CRMs can be applied to a real analysis of crystalline Si. Copyright © 2005 John Wiley & Sons, Ltd.     

Depth resolution in sputter profiling revisited

Based on a brief review of the well‐established framework of definitions, measurement and evaluation principles of the depth resolution in sputter profiling for interfaces, delta layers, single layers and multilayers, an extension to additional definitions is presented, which include the full‐width‐at‐half‐maximum of layer profiles and non‐Gaussian depth resolution functions as defined by the Mixing‐Roughness‐Information depth (MRI) model. Improved evaluation methods for adequate analysis of sputter depth profiles as well as improved definitions of depth resolution are introduced in order to meet new developments in ToF‐SIMS and GDOES, and in cluster ion sputtering of so‐called delta layers in organic matrices. In conclusion, the full‐width‐at‐half‐maximum definition and measurement of depth resolution, Δz(FWHM), is found to be more appropriate than the traditional Δz(16–84%) in order to characterize depth profiles of single layers and multilayers, because it is also valid for non‐Gaussian depth resolution functions. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparison of fullerene and large argon clusters for the molecular depth profiling of amino acid multilayers

A major challenge regarding the characterization of multilayer films is to perform high-resolution molecular depth profiling of, in particular, organic materials. This experimental work compares the performance of C₆₀ ⁺ and Ar₁₇₀₀ ⁺ for the depth profiling of model multilayer organic films. In particular, the conditions under which the original interface widths (depth resolution) were preserved were investigated as a function of the sputtering energy. The multilayer samples consisted of three thin δ-layers (~8 nm) of the amino acid tyrosine embedded between four thicker layers (~93 nm) of the amino acid phenylalanine, all evaporated on to a silicon substrate under high vacuum. When C₆₀ ⁺ was used for sputtering, the interface quality degraded with depth through an increase of the apparent width and a decay of the signal intensity. Due to the continuous sputtering yield decline with increasing the C₆₀ ⁺ dose, the second and third δ-layers were shifted with respect to the first one; this deterioration was more pronounced at 10 keV, when the third δ-layer, and a fortiori the silicon substrate, could not be reached even after prolonged sputtering. When large argon clusters, Ar₁₇₀₀ ⁺, were used for sputtering, a stable molecular signal and constant sputtering yield were achieved throughout the erosion process. The depth resolution parameters calculated for all δ-layers were very similar irrespective of the impact energy. The experimental interface widths of approximately 10 nm were barely larger than the theoretical thickness of 8 nm for the evaporated δ-layers.

Summary of ISO/TC 201 Standard: ISO 22415—Surface chemical analysis—Secondary ion mass spectrometry—Method for determining yield volume in argon cluster sputter depth profiling of organic materials

The International Standard ISO 22415 provides methods to measure sputtering yield volumes of organic test materials using argon cluster ions. The test materials should consist of thin films of known thicknesses between 50 and 1000 nm. The format of the test materials, the measurement of sputtering ion dose, sputtered depth, and reporting requirements for sputtering yield volumes are described.     

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.     

Systematic Temperature Effects in the Argon Cluster Ion Sputter Depth Profiling of Organic Materials Using Secondary Ion Mass Spectrometry

A study is presented of the effects of sample temperature on the sputter depth profiling of two organic materials, NPB (N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) and Irganox 1010, using a 5 keV Ar₂₀₀₀ ⁺ cluster ion beam and analysis by secondary ion mass spectrometry. It is shown that at low temperatures, the yields increase slowly with temperature in accordance with the Universal Sputtering Yield equation where the energy term is now modified by Trouton’s rule. This occurs up to a transition temperature, T _T, which is, in turn, approximately 0.8T _M, where T _M is the sample melting temperature in Kelvin. For NPB and Irganox 1010, these transition temperatures are close to 15 °C and 0 °C, respectively. Above this temperature, the rate of increase of the sputtering yield rises by an order of magnitude. During sputtering, the depth resolution also changes with temperature with a very small change occurring below T _T. At higher temperatures, the depth resolution improves but then rapidly degrades, possibly as a result first of local crater surface diffusion and then of bulk inter-diffusion. The secondary ion spectra also change with temperature with the intensities of the molecular entities increasing least. This agrees with a model in which the molecular entities arise near the crater rim. It is recommended that for consistent results, measurements for organic materials are always made at temperatures significantly below T _T or 0.8 T _M, and this is generally below room temperature.

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Molecular depth profiling with cluster ion beams

Peptide-doped trehalose thin films have been characterized by bombardment with energetic cluster ion beams of C60+ and Aux+ (x = 1, 2, 3). The aim of these studies is to acquire information about the molecular sputtering process of the peptide and trehalose by measurement of secondary ion mass spectra during erosion. This system is important since uniform thin films of approximately 300 nm thickness can be reproducibly prepared on a Si substrate, allowing detailed characterization of the resulting depth profile with different projectiles. The basic form of the molecular ion intensity as a function of ion dose is described by a simple analytical model. The model includes parameters such as the molecular sputtering yield, the damage cross section of the trehalose or the peptide, and the thickness of a surface layer altered by the projectile. The results show that favorable conditions for successful molecular depth profiling are achieved when the total sputtering yield is high and the altered layer thickness is low. Successful molecular depth profiles are achieved with all of the cluster projectiles, although the degree of chemical damage accumulation was slightly lower with C60. With C60 bombardment, the altered layer thickness of about 20 nm and the damage cross section of about 5 nm2 are physically consistent with predictions of molecular dynamics calculations available for similar chemical systems. In general, the model presented should provide guidance in optimizing experimental parameters for maximizing the information content of molecular depth profiling experiments with complex molecular thin film substrates.     

TOF‐SIMS matrix effects in mixed organic layers in Ar cluster ion depth profiles

Matrix effects are crucial for analyses using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) in terms of quantitative analysis, depth profiling and imaging. It is often difficult to predict how co‐existing materials will influence each other before such analysis. However, matrix effects need to be curtailed in order to assume the appropriate amount of a target material in a sample. First, matrix effects on different types of organic mixed samples, including a sample composed of Irganox 1010 and Irganox 1098 (MMK sample) and another composed of Irganox 1010 and Fmoc‐pentafluoro‐L‐phenylalanine (MMF sample), were observed utilizing ToF‐SIMS and the dependence of the secondary ion polarity of the matrix effects on the same sample was evaluated. Next, the correction method for the ToF‐SIMS matrix effects proposed by Shard et al. was applied to a comparison of the positive secondary ion results to the negative ones. The matrix effects on the positive ion data in both samples were different from those on the negative ion data. The matrix effect correction method worked effectively on both the negative and positive depth profiles. Copyright © 2017 John Wiley & Sons, Ltd.     

Sample rotation improves gas cluster sputter depth profiling of polymers

X‐ray photoelectron spectroscopy (XPS) was used in conjunction with gas cluster ion source etching to analyze polystyrene and polyvinylpyrrolidone multilayer samples, total thickness ~15 μm, to establish optimal conditions for depth profiles over many μm in depth. Using standard conditions, these samples demonstrate a reduction in depth resolution and sputtering yield, which is shown to be partly due to X‐ray–induced damage and partly due to roughening of the sputtered surface. By limiting the X‐ray exposure, it was possible to retain depth resolution to a depth of approximately 5 μm; to obtain useful depth profiles beyond this depth, it was necessary to use sample rotation. The use of optimized conditions allowed the chemical integrity of the polymer layers to remain intact during the etching process with relatively sharp interfaces over the full depth of the films. Both the elemental intensities in XPS and the line shape of the C 1s peak could be used to determine the differences in chemical structure of the films in the depth profile. Detailed analysis suggests that a “stepwise” rotation scheme can maintain depth resolution better than continuous rotation during sputtering.     

An investigation of secondary ion yield enhancement using Bi n 2+ (n=1, 3, 5) primary ions

We have investigated secondary ion yield enhancement using Bin2+ (n=1, 3, 5) primary ions impacting phenylalanine, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), cholesterol, Irganox 1010, and polymer films adsorbed on silicon and aluminum. Secondary ion yields are increased using Bi2+and Bi3(2+) primary ions for the molecular layers and polymers that can undergo allyl cation rearrangements. For Irganox 1010, the deprotonated molecular ion yields (m/z 1175; [M-H]-) are one to two times larger for Bi2+ and Bi(3)2+ primary ions than for Bi+ and Bi3+ at the same primary ion velocities. In the positive ion mode, the largest fragment ion yield (m/z 899) is 1.5 times larger for Bi2+ ions than for Bi+. For Bi3(2+) the largest fragment ion yield is only 70% of the ion yield using Bi3+, but the secondary ion yields of the fragment ions at m/z 57 and 219 are enhanced. For polymers that can undergo allyl cation rearrangement reactions the secondary ion yield enhancements of the monomer ions range from 1.3 to 4.3. For Bi(5)2+ primary ions, secondary ion yields were the same or slightly larger than for Bi5+ in the negative ion mass spectra for Irganox 1010, but lower in the positive ion mode. No secondary ion yield enhancements were measured on polymer samples for Bi5(2+). For all polymer films studied, secondary ion intensities from the oligomer regions are substantially decreased using Bin2+ (n=1, 3, 5). We discuss differences in the ionization mechanisms for doubly and singly-charged Bi primary ion bombardment.     

Cluster ion beam profiling of organics by secondary ion mass spectrometry – does sodium affect the molecular ion intensity at interfaces?

The use of cluster ion beam sputtering for depth profiling organic materials is of growing technological importance and is a very active area of research. At the 44th IUVSTA Workshop on “Sputtering and Ion Emission by Cluster Ion Beams”, recent results were presented of a cluster ion beam depth profile of a thin organic molecular layer on a silicon wafer substrate. Those data showed that the intensity of molecular secondary ions is observed to increase at the interface and this was explained in terms of the higher stopping power in the substrate and a consequently higher sputtering yield and even higher secondary ion molecular sputtering yield. An alternative hypothesis was postulated in the workshop discussion which may be paraphrased as: “under primary ion bombardment of an organic layer, mobile ions such as sodium may migrate to the interface with the inorganic substrate and this enhancement of the sodium concentration increases the ionisation probability, so increasing the molecular ion yield observed at the interface”. It is important to understand if measurement artefacts occur at interfaces for quantification as these are of great technological relevance – for example, the concentration of drug in a drug delivery system. Here, we evaluate the above hypothesis using a sample that exhibits regions of high and low sodium concentration at both the organic surface and the interface with the silicon wafer substrate. There is no evidence to support the hypothesis that the probability of molecular secondary ion ionisation is related to the sodium concentration at these levels. © Crown copyright 2008. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

Monitoring the degradation of stabilization systems in polypropylene during accelerated aging tests by liquid chromatography combined with atmospheric pressure chemical ionization mass spectrometry

Degradation pathways of three commonly used antioxidants were successfully studied by using accelerated aging tests for polymers. Additionally, thermal stability and resistance to discoloration of seven stabilizers were investigated by aging pure stabilizers dissolved in the polymer-mimicking solvent squalane. Methods based on high-performance liquid chromatography hyphenated with highly sensitive tandem mass spectrometric detection (HPLC-MS) were developed for structural elucidation of degradation products. Subsequent quantification was done using UV-detection. While Irganox 1330, Irganox 3114 and Cyanox 1790 showed a similar degradation mechanism with highly colored decomposition products, no corresponding oxidized species could be found for other stabilizers and less discoloration was observed. For Irganox 1010, hydrolysis was the preferred degradation mechanism, leading to products with an increased solubility in water. Therefore this stabilizer is less suitable for materials intended for water applications. In the aged materials previously unknown degradation mechanisms were observed for Irganox 1010 and Irgafos 168 which also contribute to the inhibition of autoxidation of the polymer.     

Chemically alternating langmuir-blodgett thin films as a model for molecular depth profiling by mass spectrometry

Langmuir-Blodgett multilayers of alternating barium arachidate and barium dimyristoyl phosphatidate are characterized by secondary ion mass spectrometry employing a 40 keV buckminsterfullerene (C60) ion source. These films exhibit well-defined structures with minimal chemical mixing between layers, making them an intriguing platform to study fundamental issues associated with molecular depth profiling. The experiments were performed using three different substrates of 306 nm, 177 nm, and 90 nm in thickness, each containing six subunits with alternating chemistry. The molecular subunits are successfully resolved for the 306 nm and 177 nm films by cluster ion depth profiling at cryogenic temperatures. In the depth profile, very little degradation was found for the molecular ion signal of the underneath layers compared with that of the top layer, indicating that the formation of chemical damage is removed as rapidly as it is formed. The resolving power decreases as the thickness of the alternating subunits decrease, allowing a depth resolution of 20 to 25 nm to be achieved. The results show the potential of LB films as an experimental model system for studying fundamental features of molecular depth profiling.     

Depth profiling organic/inorganic interfaces by argon gas cluster ion beams: sputter yield data for biomaterials,in‐vitro diagnostic and implant applications

Argon gas cluster ion beam sources are likely to become much more widely available on XPS and SIMS instruments in the next few years. Much attention has been devoted to their ability to depth profile organic materials with minimum damage. What has not been the focus of attention (possibly because it has been very difficult to measure) is the large ratio of sputter yield for organic materials compared with inorganic materials using these sources and the special opportunities this presents for studies of organic/inorganic interfaces. Traditional depth profiling by monatomic argon ions introduces significant damage into the organic overlayer, and because sputter rates in both organic and inorganic are similar for monatomic ions the interface is often ‘blurred’ due to knock‐on and other damage mechanisms. We have used a quartz crystal technique to measure the total sputter yield for argon cluster ions in a number of materials important in medical implants, biomaterials and diagnostic devices, including polymethyl methacrylate, collagen, hydroxyapatite, borosilicate glass, soda lime glass, silicon dioxide and the native oxides on titanium and stainless steel. These data fit a simple semi‐empirical equation very well, so that the total sputter yield can now be estimated for any of them for the entire range of cluster ion energy typical in XPS or SIMS. On the basis of our total sputter yield measurements, we discuss three useful ‘figures‐of‐merit’ for choosing the optimum cluster ion energy to use in depth profiling organic/inorganic samples. For highest selectivity in removing the organic but not the inorganic material the energy‐per‐atom in the cluster should be below 6 eV. A practical balance between selectivity and reasonably rapid depth profiling is achieved by choosing a cluster ion energy having between around 3 and 9 eV energy‐per‐atom. Copyright © 2013 John Wiley & Sons, Ltd.