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.     

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.     

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.     

Experimental shift allowance in the deconvolution of SIMS depth profiles

We study the deconvolution of the secondary ion mass spectrometry (SIMS) depth profiles of silicon and gallium arsenide structures with doped thin layers. Special attention is paid to allowance for the instrumental shift of experimental SIMS depth profiles. This effect is taken into account by using Hofmann's mixing‐roughness‐information depth model to determine the depth resolution function. The ill‐posed inverse problem is solved in the Fourier space using the Tikhonov regularization method. The proposed deconvolution algorithm has been tested on various simulated and real structures. It is shown that the algorithm can improve the SIMS depth profiling relevancy and depth resolution. The implemented shift allowance method avoids significant systematic errors of determination of the near‐surface delta‐doped layer position. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

A new approach to express ToF SIMS depth profiling

We propose a new approach to express SIMS depth profiling on a TOF.SIMS‐5 time‐of‐flight mass spectrometer. The approach is based on the instrument capability to independently perform raster scans of sputter and probe ion beams. The probed area can be much smaller than the diameter of a sputter ion beam, like in the AES depth profiling method. This circumstance alleviates limitations on the sputter beam–raster size relation, which are critical in other types of SIMS, and enables analysis on a curved‐bottomed sputter crater. By considerably reducing the raster size, it is possible to increase the depth profiling speed by an order of magnitude without radically degrading the depth resolution. A technique is proposed for successive improvement of depth resolution through profile recovery with account for the developing curvature of the sputtered crater bottom in the probed area. Experimental study of the crater bottom form resulted in implementing a method to include contribution of the instrumental artifacts in a nonstationary depth resolution function within the Hofmann's mixing–roughness–information depth model. The real‐structure experiment has shown that the analysis technique combining reduction of a raster size with a successive nonstationary recovery ensures high speed of profiling at ~100 µm/h while maintaining the depth resolution of about 30 nm at a 5 µm depth. Copyright © 2015 John Wiley & Sons, Ltd.     

Round‐robin test for the measurement of layer thickness of multilayer films by secondary ion mass spectrometry depth profiling

An international round‐robin test (RRT) was performed to investigate a method to determine the interface location and the layer thickness of multilayer films by secondary ion mass spectrometry (SIMS) depth profiling as a preliminary study to develop a new work item proposal in ISO/TC‐201. Two types of reference materials were used in this RRT. A SiGe alloy (Si_52.4Ge_47.6) reference film was used to determine the relative sensitivity factors of Si and Ge. A Si/Ge multilayer reference film was used to determine the relative sputtering rates of the Si and Ge layers. The layer thicknesses were measured from the interfaces determined by a 50 atomic percent definition. Seven laboratories from 5 countries participated in this international RRT. The RRT reference expanded uncertainties for Si and Ge layers in a Si/Ge multilayer with similar thicknesses as the reference film were 0.76 and 1.17 nm, respectively. However, those in a thinner Si/Ge multilayer film were slightly larger at 1.04 and 1.59 nm, respectively. Most of the thickness ratios in the 2 Si/Ge multilayer films were consistent with the RRT reference value within their expanded uncertainties.     

Time‐of‐flight secondary ion mass spectroscopic characterization of the nitrogen profile of shallow gate oxynitride thermally grown on a silicon substrate

Silicon oxynitride has been used as a shallow gate oxide material for microelectronics and its thickness has been reduced over the years to only a few tens of angstroms due to device size scaling. The nitride distribution and density characteristic in the gate oxide thus becomes imperative for the devices. The shallow depth profiling capability using time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) has huge potential for the nitrogen characterization of the shallow gate oxide film. In this article, both positive and negative spectra of TOF‐SIMS on silicon oxynitride have been extensively studied and it was found that the silicon nitride clusters Si_xN^? (x = 1–4) are able to represent the nitrogen profiles because their ion yields are high enough, especially for the low‐level nitride doping in the oxide, which is formed by the annealing of nitric oxide on SiO₂/Si. The gate oxide thickness measured by the TOF‐SIMS profiling method using ¹⁸O or CsO profile calibration was found to correlate very well with transmission electron microscope measurement. The nitrogen concentration in the gate oxide measured using the TOF‐SIMS method was consistent with the results obtained using the dynamic SIMS method, which is currently applied to relatively thicker oxynitride films. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of size‐segregated aerosols using ToF‐SIMS imaging and depth profiling

Size‐segregated particles were collected with a ten‐stage micro‐orifice uniform deposit impactor from a busy walkway in a downtown area of Hong Kong. The surface chemical compositions of aerosol samples from each stage were analyzed using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) operated in the static mode. The ToF‐SIMS spectra of particles from stage 2 (5.6–10 µm), stage 6 (0.56–1 µm), and stage 10 (0.056–0.1 µm) were compared, and the positive ion spectra from stage 2 to stage 10 were analyzed with principal component analysis (PCA). Both spectral analysis and PCA results show that the coarse‐mode particles were associated with inorganic ions, while the fine particles were associated with organic ions. PCA results further show that the particle surface compositions were size dependent. Particles from the same mode exhibited more similar surface features. Particles from stage 2 (5.6–10 µm), stage 6 (0.56–1 µm), and stage 10 (0.056–0.1 µm) were further selected as representatives of the three modes, and the chemical compositions of these modes of particles were examined using ToF‐SIMS imaging and depth profiling. The results reveal a non‐uniform chemical distribution from the outer to the inner layer of the particles. The coarse‐mode particles were shown to contain inorganic salts beneath the organics surface. The accumulation‐mode particles contained sulfate, nitrate, ammonium salts, and silicate in the regions below a thick surface layer of organic species. The nucleation‐mode particles consisted mainly of soot particles with a surface coated with sulfate, hydrocarbons, and, possibly, fullerenic carbon. The study demonstrated the capability of ToF‐SIMS depth profiling and imaging in characterizing both the surface and the region beneath the surface of aerosol particles. It also revealed the complex heterogeneity of chemical composition in size and depth distributions of atmospheric particles. Copyright © 2014 John Wiley & Sons, Ltd.     

An investigation of hydrogen depth profiling using ToF‐SIMS

Hydrogen depth distributions in silicon, zinc oxide, and glass are of great interest in material research and industry. Time‐of‐flight SIMS has been used for hydrogen depth profiling for many years. However, some critical information, such as optimal instrumental settings and detection limits, is not easily available from previous publications. In this work, optimal instrumental settings and detection limits of hydrogen in silicon, zinc oxide, and common glass were investigated. The recommended experimental settings for hydrogen depth profiling using time‐of‐flight SIMS are: (i) keeping pressure in the analysis chamber as low as possible, (ii) using a cesium beam for sputtering and monitoring the H^– signal, (iii) employing monatomic ion analysis beams with the highest currents, and (iv) using interlace mode. In addition, monatomic secondary ions from a matrix are recommended as references to normalize the H^– signal. Detection limits of hydrogen are limited by the pressure of residual gases in the analysis chamber. The base pressure of the analysis chamber (with samples) is about 7 × 10^?10 mbar in this study, and the corresponding detection limits of hydrogen in silicon, zinc oxide, and common glass are 1.3 × 10¹⁸ atoms/cm³, 1.8 × 10¹⁸ atoms/cm³, and 5.6 × 10¹⁸ atoms/cm³, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Ultrashallow depth profiling using SIMS and ion scattering spectroscopy

The accuracy of ultrashallow depth profiling was studied by secondary ion mass spectrometry (SIMS) and high‐resolution Rutherford backscattering spectroscopy (HRBS) to obtain reliable depth profiles of ultrathin gate dielectrics and ultrashallow dopant profiles, and to provide important information for the modeling and process control of advanced complimentary metal‐oxide semiconductor (CMOS) design. An ultrathin Si₃N₄/SiO₂ stacked layer (2.5 nm) and ultrashallow arsenic implantation distributions (3 keV, 1 × 10¹⁵ cm^?2) were used to explore the accuracy of near‐surface depth profiles measured by low‐energy O₂⁺ and Cs⁺ bombardment (0.25 and 0.5 keV) at oblique incidence. The SIMS depth profiles were compared with those by HRBS. Comparison between HRBS and SIMS nitrogen profiles in the stacked layer suggested that SIMS depth profiling with O₂⁺ at low energy (0.25 keV) and an impact angle of 78° provides accurate profiles. For the As⁺‐implanted Si, the HRBS depth profiles clearly showed redistribution in the near‐surface region. In contrast, those by the conventional SIMS measurement using Cs⁺ primary ions at oblique incidence were distorted at depths less than 5 nm. The distortion resulted from a long transient caused by the native oxide. To reduce the transient behavior and to obtain more accurate depth profiles in the near‐surface region, the use of O₂⁺ primary ions was found to be effective, and 0.25 keV O₂⁺ at normal incidence provided a more reliable result than Cs⁺ in the near‐surface region. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

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: 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.     

Simultaneous SIMS/AES Measurements for the Characterization of Multilayer Systems

The interface region of silicon dioxide layers deposited on indium phosphide was investigated by simultaneous secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) depth profile measurements. The results of such measurements depend strongly on the ion species used for sputtering. With Ar⁺ primary ions an enhancement of the P- and In-SIMS signals occurs in the mixing zone at the interface. This effect can be explained by an increase of the ionization yield of In and P in the presence of oxygen from the SiO₂. The use of O₂ ⁺ as sputter ions enlarges the phosphorus peak at the interface while the enhancement of the In-signal diminishes. The simultaneously measured AES spectra give clear evidence of oxygen bonded In and P at the interface. Additionally, preferential sputtering of phosphorus occurs. The understanding of these effects which complicate the interpretation of SIMS and AES depth profile measurements of the system SiO₂/InP allows us to investigate the silicon dioxide layers and the interface region in order to optimize the SiO₂ deposition process, e.g. for surface passivation or MIS structures.     

ToF-SIMS Depth Profiling of Trehalose: The Effect of Analysis Beam Dose on the Quality of Depth Profiles

In static secondary ion mass spectrometry (SIMS) experiments, an analysis dose of 10(12) ions/cm(2) typically produces optimum results. However, the same dose used in dual beam depth profiling can significantly degrade the signal. This is because during each analysis cycle a high-energy beam is rastered across the same x-y location on the sample. If a sufficient amount of sample is not removed during each sputter cycle, the subsequent analysis cycle will sample a volume degraded by the previous analysis cycles. The dimensionless parameter R' is used to relate the amount of damage accumulated in the sample to the amount of analysis beam dose used relative to the etching beam. Depth profiles from trehalose films spin-cast onto silicon wafers acquired using Bi(1) (+) and Bi(3) (+) analysis beams were compared. As R' increased, the depth profile and the depth resolution (interface width) both degraded. At R' values below 0.04 for both Bi(1) (+) and Bi(3) (+), the shape of the profile as well as the depth resolution (9 nm) indicated that dual beam analysis can be superior to C(60) single beam depth profiling.     

Depth profiling in secondary ion mass spectrometry for ultra‐thin layer with nanometer order thickness by mesa‐structure fabrication

We attempted to make an accurate depth profiling in secondary ion mass spectrometry (SIMS) including backside SIMS for ultra‐thin nanometer order layer. The depth profiles for HfO₂ layers that were 3 and 5 nm thick in a‐Si/HfO₂/Si were measured using quadrupole and magnetic sector type SIMS instruments. The depth profiling for an ultra‐thin layer with a high depth resolution strongly depends on how the crater‐edge and knock‐on effects can be properly reduced. Therefore, it is important to control the analyzing conditions, such as the primary ion energy, the beam focusing size, the incidence angle, the rastered area, and detected area to reduce these effects. The crater‐edge effect was significantly reduced by fabricating the sample into a mesa‐shaped structure using a photolithography technique. The knock‐on effect will be serious when the depth of the layer of interest from the surface is located within the depth of the ion mixing region due to the penetration of the primary ions. Finally, we were able to separately assign the origin of the distortion to the crater‐edge effect and knock‐on effect. Copyright © 2012 John Wiley & Sons, Ltd.     

ToF‐SIMS characterisation of methane‐ and hydrogen‐plasma‐modified graphite using principal component analysis

Time of flight secondary ion mass spectrometry (ToF‐SIMS) has been used to determine the extent of surface modification of highly ordered pyrolytic graphite (HOPG) samples that were exposed to radio‐frequency methane and hydrogen plasmas. The ToF‐SIMS measurements were examined with the multivariate method of principal component analysis (PCA), to maximise the amount of spectral information retained in the analysis. This revealed that the plasma (methane or hydrogen plasma) modified HOPG exhibited greater hydrogen content than the pristine HOPG. The hydrogen content trends observed from the ToF‐SIMS studies were also observed in elastic recoil detection analysis measurements. The application of the ToF‐SIMS PCA method also showed that small hydrocarbon fragments were sputtered from the hydrogen‐plasma‐treated sample, characteristic of the formation of a plasma‐damaged surface, whereas the methane‐plasma‐treated surface sputtered larger hydrocarbon fragments, which implies the growth of a polymer‐like coating. Scanning tunnelling microscopy measurements of the modified surfaces showed surface features that are attributable to either etching or film growth after exposure to the hydrogen or methane plasma. Copyright © 2009 John Wiley & Sons, Ltd.     

Degradation of poly(acrylates) under SF5+ primary ion bombardment studied using time‐of‐flight secondary ion mass spectrometry. 1. Effect of main chain and pendant methyl groups

Polyatomic primary ions have been applied recently to the depth profiling of organic materials by secondary ion mass spectrometry (SIMS). Polyatomic primary ions offer low penetration depth and high damage removal rates in some polymers, but the relationship between polymer chemistry and degradation under polyatomic primary ion bombardment has not been studied systematically. In this study, positive and negative ion time‐of‐flight SIMS (ToF‐SIMS) was used to measure the damage of ～100 nm thick spin‐cast poly(methyl methacrylate) (PMMA), poly(methyl acrylate) (PMA) and poly(methacrylic acid) (PMAA), films under extended (～2 × 10¹⁴ ions cm^?2) 5 keV SF₅⁺ bombardment. These polymers were compared to determine the effect of the main chain and pendant methyl groups on their degradation under SF₅⁺ bombardment. The sputter rate of PMMA was approximately twice that of PMA or PMAA and the rate of damage accumulation was higher for PMA and PMAA than PMMA, suggesting that the main chain and pendant methyl groups played an important role in the degradation of these polymers under SF₅⁺ bombardment. These results are consistent with the literature on the thermal and radiation‐induced degradation of these polymers, which show that removal of the main chain or pendant methyl groups reduces the rate of depolymerization and increases the rate of intra‐ or intermolecular cross‐linking. Copyright © 2004 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.