The use of secondary ion mass spectrometry in surface analysis

Secondary ion mass spectrometry (SIMS) is based on the bombardment of solids by ions and subsequent mass analysis of the sputtered ions or of the post-ionized neutrals. Various models have been proposed for the emission of secondary ions from the target. These models can roughly be grouped into two categories: (a) lonization takes place outside the target; the sputtered particles are assumed to leave the target in an excited or super-excited state; ionization can then result from such processes as Auger de-excitation or resonance ionization. (b) Ions are generated inside the target by collision cascades initiated by a primary impinging ion. Experimental data of the element- and matrix-dependent ion yield and estimates of the minimum detectable concentrations are shown as well as the rate of consumption of the target. Methods for efficient use of the sputtered material will be discussed. Examples of the wide range of applications of SIMS for determination of the chemical composition and structure of the surface layer and for imaging of the distribution of elements in the surface are given. The SIMS results are compared with those of Auger and Ion Scattering Spectrometry. The essentially non-destructive method of static SIMS is shown to be a powerful tool for the investigation of the outermost atomic layers of a solid.     

Compositional analysis in the nano-regime:A SIMS perspective

A serious problem in secondary ion mass spectrometry (SIMS) analysis is its "matrix effect" that hinders the quantification of a certain species in a sample and consequently, appropriate corrective measures are taken to calibrate the secondary ion currents into respective concentrations for accurate compositional analysis. Use of "calibration standards" is necessary for this purpose. Detection of molecular MCsn+ ions (M-element to be analyzed, n=1, 2, 3,....) under Cs+ ion bombardment is a possible mean to minimize such matrix effect, enabling one to quantify without the need of calibration standards. Our recent studies on MCsn+ molecular ions aim towards the understanding of their formation mechanisms, which are important to know their effects on SIMS quantification.In-depth quantitative analysis is a major strength of SIMS for which 'depth resolution' is of significant relevance. The optimal choice of the impact parameters during SIMS analyses can play an effective role in obtaining data with ultra-high depth resolution. SIMS is possible at depth resolution in the nm or even sub-nm range, with quantifiable data obtained from the top monolayer onwards into the material. With optimized experimental conditions, like extremely low beam current (down to ～10 nA), and low bombarding energy (below 1 keV), ultra-high depth resolution SIMS has enabled interfacial composition analysis of ultra-thin films, quantum wells, heterostructures, etc. and complex low-dimensional structures with high precision and repeatability.     

Molecular SIMS – A journey from single crystal to biological surface studies

The development of Static or Molecular secondary ion mass spectrometry (SIMS) is reviewed with particular reference to the journey made by the Manchester group and its collaborators. The earliest studies focussed on the application of static SIMS to single crystal surface studies. These studies successfully demonstrated that static SIMS delivered information on the delicate adsorbate state that mirrored that obtained by other surface science techniques. Subsequent application of the technique to studying the state and reactivity of bimetallic surfaces stimulated by collaboration with the Ertl group, demonstrated that static SIMS could be applied to the investigation of quite complex surface chemistry. This success stimulated the application of the technique to surface chemistry studies of much more complex systems such as polymers, ice mimics of polar stratospheric clouds, aerosols, culminating in biological systems. The need to enhance ion yields of the larger biological molecules led to the development and introduction of polyatomic primary ion beams, most notably based on C₆₀ buckminsterfullerene. This type of ion beam has transformed molecular analysis by SIMS. Not only have the yields of larger molecular ions been greatly increased, the bombardment induced damage that necessitated the static limit has been dramatically reduced such that for many materials the static limit requirement can be abandoned. A completely new analytical regime has opened up so that molecular depth profiling and 3D chemical imaging is possible. To fully realise the new capabilities for biological analysis a new generation of ToF-SIMS instrument is being developed that overcomes the compromises of pulsed beam instruments and that enables high mass resolution, high spatial resolution and high duty cycle to be attained simultaneously.     

The Storing Matter technique: Preliminary results on PS and PVC

The Storing Matter technique has been developed recently in SAM to perform quantitative analyses with high sensitivity: the sample to be analysed is sputtered by ion bombardment and up to a sub-monolayer of emitted particles is deposited on a collector. In the present work, this collector is subsequently analysed by static Secondary Ion Mass Spectrometry (SIMS). We concentrate on the conservation of the molecular information throughout the deposition and analysis processes. The experiments are realised on PS and PVC films. The deposits have been clearly identified on the collector. The molecular information is partially conserved throughout the deposition and analysis processes. Higher fragmentation in the mass spectra of the collectors is due to double fragmentation in the sputter-deposition and analysis steps.     

Energy dependence of the ionization probability of sputtered Cu and Ni

Despite its great sensitivity, the usefulness of secondary ion mass spectrometry (SIMS) for many applications has been limited by an inadequate understanding of the probability of sputtering an atom in an ionized state. To determine this ionization probability for clean Cu and Ni surfaces, I have measured the energy distribution of sputtered neutrals and ions by quadrupole mass filtering and retarding potential analysis using potential modulation differentiation. Analysis of sputtered neutrals was accomplished by electron impact ionization. Because the neutrals outnumber the ions by at least two orders of magnitude, the ratio of sputtered ions to neutrals is an accurate measure of the ionization probability. For energies below 20 eV the dependence of the ionization probability on energy goes as P(E) α Eⁿ, where n = 0.65 for clean Cu. The absorption of oxygen on the Cu surface increases the total ion yield while causing a reduction in the value of the exponent n. Similar results are found for nickel, where n = 0.54 for the clean surface.     

Investigation of surface layers by SIMS and SIIMS

In the Secondary Ion Mass Spectrometry (SIMS) the sample to be analysed is bombarded with a beam of primary ions. The secondary ions sputtered away from the sample, characteristic for its composition near the surface at any time, are mass selected and detected in a mass spectrometer. The yields of several elements in a Fe-matrix and in technically pure samples bombarded with positive oxygen and argon ions have been determined to study the influence of the matrix and the primary ions on the ion yields. The properties of SIMS and of two of its special modes viz. Static Secondary Ion Mass Spectrometry (SSIMS) and Secondary Ion Imaging Mass Spectrometry (SIIMS) with respect to the analysis of surface layers are discussed.     

Improved quantitative analysis of Cu(In,Ga)Se2 thin films using MCs+-SIMS depth profiling

The chalcopyrite semiconductor, Cu(InGa)Se₂ (CIGS), is popular as an absorber material for incorporation in high-efficiency photovoltaic devices because it has an appropriate band gap and a high absorption coefficient. To improve the efficiency of solar cells, many research groups have studied the quantitative characterization of the CIGS absorber layers. In this study, a compositional analysis of a CIGS thin film was performed by depth profiling in secondary ion mass spectrometry (SIMS) with MCs⁺ (where M denotes an element from the CIGS sample) cluster ion detection, and the relative sensitivity factor of the cluster ion was calculated. The emission of MCs⁺ ions from CIGS absorber elements, such as Cu, In, Ga, and Se, under Cs⁺ ion bombardment was investigated using time-of-flight SIMS (TOF-SIMS) and magnetic sector SIMS. The detection of MCs⁺ ions suppressed the matrix effects of varying concentrations of constituent elements of the CIGS thin films. The atomic concentrations of the CIGS absorber layers from the MCs⁺-SIMS exhibited more accurate quantification compared to those of elemental SIMS and agreed with those of inductively coupled plasma atomic emission spectrometry. Both TOF-SIMS and magnetic sector SIMS depth profiles showed a similar MCs⁺ distribution for the CIGS thin films.     

Molecular secondary ion formation under cluster bombardment: A fundamental review

A brief review is given regarding the application of cluster ion beams as desorption probes in molecular SIMS. The general observation is that the efficiency of secondary ion formation, particularly that of complex molecular species, is significantly enhanced if polyatomic projectiles are employed instead of atomic species. Apart from the sensitivity increase, cluster bombardment also appears to allow for molecular depth profiling studies without the accompanying damage accumulation normally associated with atomic projectiles. A few fundamental aspects are addressed in an attempt to highlight the physics behind these observations. It appears that much of the benefit associated with cluster bombardment is connected to the fact that these projectiles give access to very high sputter yields which are not accessible with atomic primary ions.     

Experimental and theoretical investigations into the origin of cross-contamination effects observed in a quadrupole-based SIMS instrument

The minimum-detection limits achievable in SIMS analyses are often determined by transport of material from surrounding surfaces to the bombarded sample. This cross-contamination (or memory) effect was studied in great detail, both experimentally and theoretically. The measurements were performed using a quadrupole-based ion microprobe operated at a secondary-ion extraction voltage of less than 200 V (primary ions mostly 8keV O ₂ ⁺ ). It was found that the flux of particles liberated from surrounding surfaces consists of neutrals as well as positive and negative ions. Contaminant species condensing on the bombarded sample could be discriminated from other backsputtered species through differences in their apparent energy spectra and by other means. The apparent concentration due to material deposited on the sample surface was directly proportional to the bombarded area. For an area of 1 mm² the maximum apparent concentration of Si in GaAs amounted to 5 × 10¹⁶atoms/cm³. The rate of contamination decreased strongly with increasing spacing between the bombarded sample and the collector. The intensities of backsputtered ions and neutrals increased strongly with increasing mass of the target atoms (factor of 10 to 50 due to a change from carbon to gold). The effect of the primary ion mass (O ₂ ⁺ , Ne⁺, and Xe⁺) and energy (5–10keV) was comparatively small. During prolonged bombardment of one particular target material, the rate of contamination due to species not contained in the sample decreased exponentially with increasing fluence. In order to explain the experimental results a model is presented in which the backsputtering effect is attributed to bombardment of surrounding walls by high-energy particles reflected or sputtered from the analysed sample. The level of sample contamination is described by a formula which contains only measurable quantities. Cross-contamination efficiencies are worked out in detail using calculated energy spectra of sputtered and reflected particles in combination with the energy dependence of the sputtering yield of the assumed wall material. The experimental findings are shown to be good agreement with the essential predictions of the model.     

Surface investigation of solids by the statical method of secondary ion mass spectroscopy (SIMS)

The physical and chemical properties of a solid surface are determined by its uppermost monolayers. Besides other methods like Auger electron spectroscopy (AES) and X-ray electron spectroscopy (ESCA) for example, the chemical composition of these uppermost monolayers can be investigated by the statical method of secondary ion mass spectroscopy (SIMS). In this method a relatively large target area (0.1 cm²) is bombarded with a small primary ion current density (10^?9 A cm^?2). Thus a sputtering time of several hours is achieved for an individual monolayer. A mass analysis of the emitted positive and negative secondary ions gives information about the chemical composition of the uppermost monomolecular layer of the bombarded surface. Important features of SIMS are: detection of chemical compounds; isotope sensitivity; detection of hydrogen and its compounds ; depth resolution in the range of a single monolayer ; low detection limits (< 10^?6 monolayer or < 10^?14 g) for many elements and compounds.The capacity of this method is demonstrated using as examples the initial surface oxidation of metals and semiconductors and adsorption phenomena on clean metal surfaces. In some special cases additional information on the chemical composition of the uppermost monolayer can be obtained by the electron induced ion emission from the surface.     

3D molecular imaging SIMS

Thin monolayer and bilayer films of spin cast poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(lactic) acid (PLA) and PLA doped with several pharmaceuticals have been analyzed by dynamic SIMS using SF₅⁺ polyatomic primary ion bombardment. Each of these systems exhibited minimal primary beam-induced degradation under cluster ion bombardment allowing molecular depth profiles to be obtained through the film. By combing secondary ion imaging with depth profiling, three-dimensional molecular image depth profiles have been obtained from these systems. In another approach, bevel cross-sections are cut in the samples with the SF₅⁺ primary ion beam to produce a laterally magnified cross-section of the sample that does not contain the beam-induced damage that would be induced by conventional focussed ion beam (FIB) cross-sectioning. The bevel surface can then be examined using cluster SIMS imaging or other appropriate microanalysis technique.     

Analysis of solids by secondary ion and sputtered neutral mass spectrometry

A mass spectrometer is described, which allows the analysis of sputtered neutral and charged particles as well as of residual gas composition. This combined SIMS, SNMS, and RGA instrument consists of a scanning primary ion beam column, an electron impact ionizer, an electrostatic energy filter and an rf quadrupole mass analyzer.Various examples of surface and bulk analysis are presented which demonstrate the beneficial complementary features of these techniques. These are, in particular: a substantial reduction of the matrix effect and fewer complications with samples of low electrical conductivity in SNMS, and the possibility of measuring the depth distribution of gases included in small cavities in the solid in the SNMS/RGA mode. SIMS, on the other hand, allows in many cases higher detection sensitivities.EURATOM Association     

Functionality of novel black silicon based nanostructured surfaces studied by TOF SIMS

A functionality of the novel black silicon based nanostructured surfaces (BS 2) with different metal surface modifications was tested by time-of-flight secondary ion mass spectrometry (TOF SIMS). Mainly two surface functions were studied: analytical signal enhancement and analyte pre-ionization effect in SIMS due to nanostructure type and the assistance of the noble metal surface coating (Ag or Au) for secondary ion formation. As a testing analyte a Rhodamine 6G was applied. Bi⁺ has been used as SIMS primary ions. It was found out that SIMS signal enhancement of the analyte significantly depends on Ag layer thickness and measured ion mode (negative, positive). The best SIMS signal enhancement was obtained at BS2 surface coated with 400 nm of Ag layer. SIMS fragmentation schemes were developed for a model analyte deposited onto a silver and gold surface. Significant differences in pre-ionization effects can play an important role in the SIMS analysis due to identification and spectra interpretation.     

Matrix-enhanced secondary ion mass spectrometry: The Alchemist's solution?

Because of the requirements of large molecule characterization and high-lateral resolution SIMS imaging, the possibility of improving molecular ion yields by the use of specific sample preparation procedures has recently generated a renewed interest in the static SIMS community. In comparison with polyatomic projectiles, however, signal enhancement by a matrix might appear to some as the alchemist's versus the scientist's solution to the current problems of organic SIMS. In this contribution, I would like to discuss critically the pros and cons of matrix-enhanced SIMS procedures, in the new framework that includes polyatomic ion bombardment. This discussion is based on a short review of the experimental and theoretical developments achieved in the last decade with respect to the three following approaches: (i) blending the analyte with a low-molecular weight organic matrix (MALDI-type preparation procedure); (ii) mixing alkali/noble metal salts with the analyte; (iii) evaporating a noble metal layer on the analyte sample surface (organic molecules, polymers).     

Secondary ion emission from Si under nitrogen ion bombardment

The yield and energy distribution of positive secondary ions emitted from Si under N₂⁺ ion bombardment were measured. The obtained mass peaks correspond to three types of secondary ion species, that is, physically sputtered ions (Si⁺, Si₂⁺), chemically sputtered ions (SiN⁺ Si₂N⁺) and doubly charged ions (Si²⁺). The dependence of secondary ion emission on the primary ion energy was studied in a range of 2.0–20.0 keV. The yields of physically and chemically sputtered ions were almost independent of the primary ion energy. The yield of the doubly charged ion strongly depended on the primary ion energy. The energy distribution of secondary ions of the three types showed the same dependence on the primary ion energy. The most probable energy of the distribution increased with the primary ion energy. On the other hand, for the energy distribution curves of sputtered ions, the tail factors N in E^?N were constant and showed a m/e dependence.     

Angular-resolved energy distribution of secondary ions emitted from a silicon target sputtered by a xenon ion beam

This paper reports preliminary results obtained on an experimental apparatus dedicated to the study of angular resolved energy distribution of particles emitted from a sputtered target. Secondary ions emitted during the bombardment of a silicon target by xenon ions at a primary energy of 10keV have been studied. In its low energy part the distribution reaches a maximum around 8eV, and then decreases according to an E ^–1 law. In the range 200eV to 1000eV, a second maximum appears whose height depends on the emission angle. Apart from this range, the angular distributions have a cosine square-like shape. On the contrary, the angular distribution of ions with energy between 200eV and 1000eV is pointed in a forward direction near the specular reflection direction of the ion beam. It is assumed that the measured ions correspond to two ionic populations: secondary ions sputtered according to the linear cascade theory and recoil silicon target ions.     

Primary ion beam energy effects on secondary ion emission from Ni(001)c(2 × 2)-CO: Classical dynamics calculations and sims

The variation of the SIMS spectra of Ni(001)c(2 × 2)-CO induced by bombardment of Ar⁺ primary ions with kinetic energy between 300 and 1200 eV has been examined in detail. The purpose of the study is to examine the influence of primary ion energy on a number of experimental observables and to test the accuracy of classical dynamics calculations for neutral particles in predicting the experimental results for ejected ions. The calculations were performed using two extreme forms of the Ar⁺ -substrate interaction potential to examine the sensitivity of the results to the parameters that need to be included in the model. We find excellent agreement between the NiCO⁺/Ni⁺ ion yield ratio measured between 300–1200 eV Ar⁺ ion energy and the computational results if the Molière form of the ion-substrate potential is used and if the calculated results are corrected by including an image force. The calculated angular distributions of the ejected particles also agree well with those observed experimentally. From the calculations we see that the extent of CO fragmentation relative to the amount of molecular CO ejection is roughly constant in the 300–1200 eV beam energy range with a slight increase seen at lower (~ 300 eV) energies. The implications of these results are discussed in terms of our ability to study the chemistry and structure of surfaces with SIMS.     

Metal-assisted SIMS and cluster ion bombardment for ion yield enhancement

In addition to structural information, a detailed knowledge of the local chemical environment proves to be of ever greater importance, for example for the development of new types of materials as well as for specific modifications of surfaces and interfaces in multiple fields of materials science or various biomedical and chemical applications. But the ongoing miniaturization and therefore reduction of the amount of material available for analysis constitute a challenge to the detection limits of analytical methods. In the case of time-of-flight secondary ion mass spectrometry (TOF-SIMS), several methods of secondary ion yield enhancement have been proposed. This paper focuses on the investigation of the effects of two of these methods, metal-assisted SIMS and polyatomic primary ion bombardment. For this purpose, thicker layers of polystyrene (PS), both pristine and metallized with different amounts of gold, were analyzed using monoatomic (Ar⁺, Ga⁺, Xe⁺, Bi⁺) and polyatomic (SF₅⁺, Bi₃⁺, C₆₀⁺) primary ions. It was found that polyatomic ions generally induce a significant increase of the secondary ion yield. On the other hand, with gold deposition, a yield enhancement can only be detected for monoatomic ion bombardment.     

Secondary Ion Mass Spectrometry Imaging

Secondary ion mass spectrometry (SIMS) is a chemical analysis technique that employs mass spectrometry to analyze solid and low volatility liquid samples [1]. Although there are numerous configurations of SIMS instrumentation, the fundamental basis of SIMS analyses is the measurement of the mass and intensity of secondary ions produced in a vacuum by sputtering the surface of the sample with energetic ion or neutral beams. The sputtering beam is referred to as the primary beam and typically has a kinetic energy of several thousand electronvolts (keV). The primary beam removes atomic or molecular layers at a rate determined principally by the intensity, mass, and energy of the primary species and the chemical and physical characteristics of the sample [2]. Particle sputtering at the kiloelectronvolt level produces a variety of products including electrons, photons, atoms, atomic clusters, intact molecules, and distinctive molecular fragments. A small fraction of these sputter products are ionized, and these ions are the secondary ions in secondary ion mass spectrometry.     

Die Emission zusammengesetzter negativer Sekundärionen aus kontinuierlich regenerierten Targetoberflächen

In Ref. a method was described by use of secondary ion emission to generate wellcollimated and intense beams of heavy negative ions. In this paper the processes are studied that govern the emission of molecular ions from continuously regenerated surfaces. A rotating iron target was bombarded by noble gas ions in the energy range from 17 to 30 keV. The secondary ion yield was measured as a function of primary current density, mass and energy of the primary ions, gas pressure and the rotation frequency of the target.