Secondary Ion Formation Research Articles

Fragmentation of molecular compounds on silicon wafers and low dielectric constant materials studied by time‐of‐flight secondary ion mass spectrometry using a polyatomic ion source

AbstractRecent time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) studies using primary cluster ion sources such as Aun+, SFn+, Bin+ or C60+ have shown polyatomic ions to be more appropriate for the detection of high mass molecular secondary ions than monoatomic ion sources like Ga+, thanks to secondary ion yield and ion formation efficiency enhancements.This work is part of a study aiming at improving molecular compound quantification at the surface of microelectronics industry substrates by taking advantage of a polyatomic ion source in ToF‐SIMS analysis. It focuses on the case of a phthalate‐related contamination. Fundamental parameters like secondary ion yield, damage cross section and secondary ion efficiency were studied as a function of the size and the energy of the primary ions. The substrate composition influence on the ToF‐SIMS results was also studied by comparing several substrates of interest in the microelectronics industry such as silicon wafers and low dielectric constant materials SiOxCyHz.Phthalate‐related secondary ions detected at the surface of the contaminated surfaces were compared and a fragmentation pathway of the di‐isononyl phthalate (DNP) molecule was determined. A drastic improvement of the secondary ion yield was provided when using a polyatomic ion source. Furthermore, a clear increase in damage cross section was observed as a function of the secondary ion size for a given Bin primary ion as well as an increase for a given fragment as a function of the number of Bi atoms in the Bin aggregate. This led to a lower static limit when taking into account higher phthalate fragments and primary ions of larger size. Copyright © 2008 John Wiley & Sons, Ltd.

Influence of different primary ion species on the secondary ion emission from PNA/DNA biosensor surfaces

AbstractIn recent publications, the use of time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) for a detailed examination of the immobilization process of peptide nucleic acid (PNA) and its hybridization capability to label‐free complementary DNA fragments has been described. Unlabeled single‐stranded DNA was hybridized to biosensor surfaces containing both complementary and noncomplementary PNA sequences. The hybridization of complementary DNA could readily be identified by detecting phosphate‐related molecules from the DNA backbone such as PO3−.Extending previous measurements on hybridized DNA surfaces using different primary ions and energies, additional ion species have been applied for a more detailed evaluation of the secondary ion emission behavior under static SIMS conditions. The large number of available primary ion species allowed for a comparison between results achieved with ‘classic’ monoatomic and polyatomic ions (such as Ar+, Ga+, Xe+ and SF5+) and bismuth primary ions up to cluster sizes of seven constituents. It was found that the use of polyatomic primary ions in general resulted in a significant increase of secondary ion yields and secondary ion formation efficiencies of DNA‐specific fragments, while the highest values could be achieved with large bismuth cluster ions. It can also be shown that nonlinear effects in projectile‐surface‐excitation are partly responsible for the observed enhancements of yield and efficiency under polyatomic bombardment.It can be concluded that large bismuth cluster ions are the best choice for detecting hybridized DNA on PNA biosensor chips. Their application optimizes analysis conditions under which TOF‐SIMS provides a useful method for label‐free DNA diagnostics. Copyright © 2008 John Wiley & Sons, Ltd.

Suppression and Enhancement of Secondary Ion Formation Due to the Chemical Environment in Static-Secondary Ion Mass Spectrometry

Through analyzing mixtures of compounds of known gas-phase basicities, the importance of this property on the secondary ions emitted from a surface under primary ion bombardment is investigated. The aim is to obtain a greater understanding of the ionization mechanisms that occur in secondary ion mass spectrometry (SIMS). The commonly used matrix assisted laser desorption/ionization (MALDI) matrix 2,4,6-trihydroxyacetophenone (THAP) and a range of low molecular weight biomolecules were used to investigate whether analyte/matrix suppression effects that have been observed in analogous MALDI experiments were also present in static-SIMS. The outcome of the experiments demonstrates that strong suppression of the quasi-molecular signal of one molecule in a mixture can occur due to the presence of the other, with the gas-phase basicity of the compounds being a good indicator of the secondary ions detected. It is also demonstrated that the suppression of the quasi-molecular ion signal of a compound in a two-component mixture can be minimized by the inclusion of a third compound of suitable gas-phase basicity.

Mass spectrometric investigation of buspirone drug in comparison with thermal analyses and MO-calculations

The buspirone drug is usually present as hydrochloride form of general formula C 21H 31N 5O 2·HCl, and of molecular weight (MW) = 421.96. It is an analgesic anxiolytic drug, which does not cause sedative or depression of central nervous system. In the present work it is investigated using electron impact mass spectral (EI-MS) fragmentation at 70 eV, in comparison with thermal analyses (TA) measurements (TG/DTG and DTA) and molecular orbital calculation (MOC). Semi-empirical MO calculation, PM3 procedure, has been carried out on buspirone both as neutral molecule (in TA) and the corresponding positively charged species (in MS). The calculated MOC parameters include bond length, bond order, particle charge distribution on different atoms and heats of formation. The fragmentation pathways of buspirone in EI-MS lead to the formation of important primary and secondary fragment ions. The mechanism of formation of some important daughter ions can be illuminated from comparing with that obtained using electrospray ESIMS/MS mode mass spectrometer through the accurate mass measurement determination. The losses of the intermediate aliphatic part (CH 2) 4 due to cleavage of N–C bond from both sides is the primary cleavage in both techniques (MS and TA). The PM3 provides a base for fine distinction among sites of initial bond cleavage and subsequent fragmentation of drug molecule in both TA and MS techniques; consequently the choice of the correct pathway of such fragmentation knowing this structural session of bonds can be used to decide the active sites of this drug responsible for its chemical, biological and medical reactivity.

ToF-SIMS analysis of bio-systems: Are polyatomic primary ions the solution?

We discuss the potential of cluster ion beams for overcoming difficulties experienced within molecular analysis in conventional static secondary ion mass spectrometry (SIMS). Consideration of secondary ion yield, surface damage and molecular depth profiling is included. Examples of the greatly increased secondary ion yields from ‘real world’ samples such as bacterial membranes are illustrated, the increase in information then benefiting multivariate analysis techniques such as principal component analysis (PCA). The increased efficiency of secondary ion formation with cluster beams, in conjunction with the reduced subsurface damage associated with C 60 bombardment is exemplified using a depth profile through a phospholipid layer, where molecular information is obtained even following doses of 2 × 10 15 ions/cm 2. This is developed further to the identification of molecular ions from the interior of etched cells. Challenges that cannot be solved solely by increased secondary ion yields and low sample damage have been considered and complications due to the influence of matrix effects along with the effect of salt concentration on the fate of molecular signals in depth profiles of biomaterials have been investigated.

Mass spectrometric characterization of DNA microarrays as a function of primary ion species

Recent studies have shown TOF-SIMS to be an appropriate method for the detailed examination of the immobilization process of PNA and its ability to hybridize to unlabeled complementary DNA fragments. Unlabeled single-stranded DNA was hybridized to Si wafer biosensor chips containing both complementary and non-complementary immobilized PNA sequences. The hybridization of complementary DNA could readily be identified by detecting phosphate-containing molecules from the DNA backbone. An unambiguous discrimination was achieved between complementary and non-complementary sequences. In order to optimize detection parameters, different primary ions were applied, including monoatomic ions (Bi +) as well as cluster ions (Bi 2 +, Bi 3 +, Bi 4 +, Bi 3 ++, Bi 5 ++), and secondary ion yield behavior and formation efficiencies were studied. It was found that cluster primary ions resulted in a significantly increased yield of DNA-correlated fragments, enabling higher signal intensities and better secondary ion efficiencies. TOF-SIMS is undoubtedly a highly useful technique for identifying hybridized DNA on PNA biosensor chips. It is suitable for studying the complexity of the immobilization and hybridization processes and may provide a rapid method for DNA diagnostics. With the absence of the labeling procedure and the simultaneous increase of the phosphate signal as a result of increasing DNA sequence length, this technique comes to be especially useful for the direct analysis of genomic DNA.

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.

Mechanisms of secondary ion emission from self‐assembled monolayers and multilayers

AbstractAlkanethiol self‐assembled monolayers/multilayers (SAMs) have been applied as model organic systems with which to investigate secondary ion formation and emission processes during kiloelectronvolt ion bombardment. Self‐assembled monolayer and multilayer films of 11‐mercaptoundecanoic acid capped with 1‐dodecanethiol were prepared on gold‐coated substrates. Samples with varying number of thiolate layers were studied using static secondary ion mass spectrometry to investigate the origin of molecular secondary ions and the influence of surface chemistry and structure. The nature of the thiolate bonding affects the type and abundance of the observed ions. The intensity of atomic and cluster ions derived from the substrate decreases exponentially with increasing number of thiolate overlayers because of losses in transmission through the organic overlayers. Intact molecular and cluster ions can escape from >100 Å below the surface of these structures. The variation of molecular‐ion yields with multilayer thickness suggests that a significant proportion of molecular ions originate from subsurface thiolate layers. The detection of ions comprised species from the substrate or bottom of the multilayer associated with species from the top layer supports the view that chemical association at or near the surface is a viable mechanism of formation for molecular secondary ions. Copyright © 2005 John Wiley & Sons, Ltd.

Energetic Ion Bombardment of Ag Surfaces by C 60+ and Ga + Projectiles

The ion bombardment-induced release of particles from a metal surface is investigated using energetic fullerene cluster ions as projectiles. The total sputter yield as well as partial yields of neutral and charged monomers and clusters leaving the surface are measured and compared with corresponding data obtained with atomic projectile ions of similar impact kinetic energy. It is found that all yields are enhanced by about one order of magnitude under bombardment with the C60+ cluster projectiles compared with Ga+ ions. In contrast, the electronic excitation processes determining the secondary ion formation probability are unaffected. The kinetic energy spectra of sputtered particles exhibit characteristic differences which reflect the largely different nature of the sputtering process for both types of projectiles. In particular, it is found that under C60+ impact (1) the energy spectrum of sputtered atoms peaks at significantly lower kinetic energies than for Ga+ bombardment and (2) the velocity spectra of monomers and dimers are virtually identical, a finding which is in pronounced contrast to all published data obtained for atomic projectiles. The experimental findings are in reasonable agreement with recent molecular dynamics simulations.

Identifying the Characteristic Secondary Ions of Lignin Polymer Using ToF−SIMS

The chemical structure of lignin, a complex, irregular polymer of phenylpropane units that occurs in plant cell walls, was investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The positive ToF-SIMS spectra of lignin isolated from pine and beech wood showed prominent secondary ions possessing guaiacyl (at m/z 137 and 151) or syringyl (at m/z 167 and 181) rings, which are the basic building units of lignin polymer. This shows that ToF-SIMS is a useful tool for lignin structural analysis. The peaks at m/z 137 and 167 were assigned as the C6-C1 ion, and the peaks at m/z 151 and 181 may be double-component, the C6-C1 ion and the C6-C2 ion. We confirmed the characteristic guaiacyl ions using a synthetic lignin model compound, dehydrogenation polymer (DHP), which was formed by polymerizing of unlabeled and deuterium-labeled coniferyl alcohols. The formation mechanism of the main secondary ions was deduced by labeling specific positions of coniferyl alcohols with a stable isotope to study the relationship between chemical structure and secondary ion formation in ToF-SIMS.

Depth profiling studies of multilayer films with a C 60+ ion source

A newly developed C 60 + primary ion beam source for time-of-flight secondary ion mass spectrometry has been employed for depth profiling analysis of organic and inorganic multilayer films. In particular, the C 60 + ion beam is used in the dc mode to sputter the surface for depth profiling while spectra are taken both with Ga + 15 keV and C 60 + 20 keV projectiles between sputtering cycles. From C 60 + bombardment of Langmuir–Blodgett films of barium arachidate, we find that cluster beams increase the secondary ion yields and ion formation efficiencies compared to monoatomic projectiles. For a 15-layer film, a barium arachidate fragment ion at m/ z=208.9 was monitored as a function of C 60 + dose to determine that the sputtering rate is about 1.54 nm/s and that the film interface position can be determined with a depth resolution of 16 nm. For comparison purposes, a depth resolution of 8.7 nm was measured for a sample consisting of 66 nm of Ni and 53 nm of Cr on Si(1 0 0) at a C 60 + beam energy of 20 keV. The neutral atom yield was monitored via laser postionization to avoid matrix effects. These experiments show great promise for the use of C 60 + for depth profiling studies of multilayer targets.

Photoionization pathways and free electrons in UV-MALDI.

The recently developed model for primary and secondary UV-MALDI ion formation (Knochenmuss, R. J. Mass Spectrom. 2002, 37, 867-877. Knochenmuss, R. Anal. Chem. 2003, 75, 2199.) is applied to questions regarding photoionization pathways and electron versus negative ion production. Two-photon ionization of the matrix in direct contact with analyte is possible under some circumstances (Kinsel, G.; Knochenmuss, R.; Setz, P.; Land, C. M.; Goh, S.-K.; Archibong, E. F.; Hardesty, J. H.; Marynik, D. J. Mass Spectrom. 2002, 37, 1131-1140.), and is added to the model. When analyte is present in large mole ratios (such as when matrix suppression is desired), this effect contributes modestly to the ion yield. Generally, matrix exciton pooling remains dominant. The interfacial layer of thin samples on a metal substrate may also be ionizable in a 2-photon process. A mechanism is proposed, and the correspondingly modified model gives excellent agreement with electron emission versus laser intensity data. Capture in, or escape of low-energy electrons from a thick sample (or on a nonmetallic substrate) is also examined. Because the mean free path for MALDI electrons in a solid matrix is on the order of 10 nm, below such depths, any electrons generated are captured to form negative ions. Only a surface layer can emit free electrons. This surface emission effect is also well reproduced by the model, up to a laser intensity limit caused by surface charging. This charging phenomenon is investigated and illustrated by molecular dynamics calculations.

Depth profiling studies of multilayer films with a C60+ ion source

A newly developed C 60 + primary ion beam source for time-of-flight secondary ion mass spectrometry has been employed for depth profiling analysis of organic and inorganic multilayer films. In particular, the C 60 + ion beam is used in the dc mode to sputter the surface for depth profiling while spectra are taken both with Ga + 15 keV and C 60 + 20 keV projectiles between sputtering cycles. From C 60 + bombardment of Langmuir–Blodgett films of barium arachidate, we find that cluster beams increase the secondary ion yields and ion formation efficiencies compared to monoatomic projectiles. For a 15-layer film, a barium arachidate fragment ion at m / z =208.9 was monitored as a function of C 60 + dose to determine that the sputtering rate is about 1.54 nm/s and that the film interface position can be determined with a depth resolution of 16 nm. For comparison purposes, a depth resolution of 8.7 nm was measured for a sample consisting of 66 nm of Ni and 53 nm of Cr on Si(1 0 0) at a C 60 + beam energy of 20 keV. The neutral atom yield was monitored via laser postionization to avoid matrix effects. These experiments show great promise for the use of C 60 + for depth profiling studies of multilayer targets.

Interpretation of static secondary ion spectra

AbstractA model is presented in which electron impact (EI)/electronic excitation plays a pivotal role in the formation of secondary ions in the SIMS experiment, especially those originating from discrete molecular species. Positive ions are formed by electron loss whereas negative ions are formed by electron capture. Collisions of the new ions with the surface and with other species directly above the sample, along with metastable decay events, reduce the number of odd electron ions detected and produce the changes that make SIMS spectra so different from EI mass spectra. Primary support for this model is gathered from static SIMS spectra themselves, which can be rationalized to a large degree by assuming that the same rearrangement and fragmentation mechanisms that are invoked to explain EI mass spectra take place at the surface after kiloelectron‐volt ion impacts. The static secondary ion spectra of a variety of simple discrete molecular species, of simple hydrocarbons, of monofunctional organic species and of more complicated multifunctional organic species are analyzed in this way and the utility of this model is demonstrated. Copyright © 2004 John Wiley & Sons, Ltd.

Formation of pseudorotaxane induced by electrochemical oxidation of ferrocene-containing axis molecule in the presence of crown ether.

The electrochemical oxidation of aminomethylferrocenes in the presence of dibenzo[24]crown-8 affords pseudorotaxanes containing a ferrocenyl group in its axis moiety. The formation of the pseudorotaxanes results from formation of secondary ammonium ions by rapid electron transfer from the N atom to the initially formed Fe(III) state, followed by hydrogen atom transfer from TEMPOH to the N+*.

Secondary ion formation of low molecular weight organic dyes in time-of-flight static secondary ion mass spectrometry.

Time-of-flight static secondary ion mass spectrometry (TOF-S-SIMS) was used to characterize thin layers of oxy- and thiocarbocyanine dyes on Ag and Si. Apart from adduct ions a variety of structural fragment ions were detected for which a fragmentation pattern is proposed. Peak assignments were confirmed by comparing spectra of dyes with very similar structures. All secondary ions were assigned with a mass accuracy better than 50 ppm. The intensity of molecular ions as well as fragment ions has been studied as a function of the type of organic dye, the substrate, the layer thickness and the type of primary ion. A large yield difference of two orders of magnitude was observed between the precursor ions of cationic carbocyanine dyes and the protonated molecules of the anionic dyes. Fragment ions, on the other hand, yielded similar intensities for both types of dye. As the dye layers deposited on an Ag substrate yielded higher secondary ion intensities than those deposited on a Si substrate, the Ag metal clearly acts as a promoting agent for secondary ion formation. The effect was more pronounced for precursor signals than for fragment ions. The promoting effect decreased as the deposited layer thickness of the organic dye layer was increased.

A C60 primary ion beam system for time of flight secondary ion mass spectrometry: its development and secondary ion yield characteristics.

A buckminsterfullerene (C60)-based primary ion beam system has been developed for routine application in TOF-SIMS analysis of organic materials. The ion beam system is described, and its performance is characterized. Nanoamp beam currents of C60+ are obtainable in continuous current mode. C60(2+) can be obtained in pulsed mode. At 10 keV, the beam can be focused to less than 3 microm with 0.1 nA currents. TOF-SIMS studies of a series of molecular solids and a number of polymer systems in monolayer and thick film forms are reported. Very significant enhancement of secondary ion yields, particularly at higher mass, were observed using 10-keV C60+ for all samples other than PTFE, as compared to those observed from 10 keV Ga+ primary ions. Three materials (PS2000, Irganox 1010, PET) were studied in detail to investigate primary ion-induced disappearance (damage) cross sections to determine the increase in secondary ion formation efficiency. The C60 disappearance cross sections observed from monolayer film PS2000 and self-supporting PET film are close to those observed from Ga+. The resulting C60 efficiencies are 30-100 times those observed from gallium. The cross sections observed from C60 bombardment of multilayer molecular solids are approximately 100 times less, such that essentially zero damage sputtering is possible. The resulting efficiencies are > 10(3) greater than from gallium. It is also shown that C60 primary ions do not generate any more low-mass fragments than any other ion beam system does. C60 is shown to be a very favorable ion beam system for TOF-SIMS, delivering high yield, close to 10% total yield, favoring high-mass ions, and on thick samples, offering the possibility of analysis well beyond the static limit.

Relationship between the structure of polymers with well‐defined fluorocarbon segmental lengths and the formation of secondary ions in SIMS

AbstractA series of fluorinated polymers were studied using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) to investigate the relationship between the structures of the polymers and secondary ions. The influence of fluorocarbon segment length on the formation of secondary ions in a ToF‐SIMS process was studied. The characteristic negative and positive ions were found to be related to the chemical structure of the repeat units of the polymers. The ToF‐SIMS results provided some useful information on the fragmentation mechanism of positive and negative secondary ions. The results indicate that recombination of the ions in the gas phase is unlikely in a static SIMS process. Copyright © 2002 John Wiley & Sons, Ltd.

Optical spectroscopy of Mg by Kr + ion sputtering in presence of O 2

Low energy ion beam sputtering of magnesium exhibits rich and intense optical spectrum containing a number of Mg(I) and Mg(II) lines. It has been found that the emission intensities of Mg(I) and Mg(II) behave differently against the backfilled oxygen pressure. No single explanation can be given for the two different oxygen dependences observed. Considering the similarities with that of secondary ions, it is suggested that the sputtered ions are the precursors for the excited neutral or ionic species formation. It is thought that while the bond-breaking model is appropriate to the excited neutral atom production, the dominant mechanism for excited secondary ion formation is the doubly charged secondary ion production by kinetic processes as proposed by Joyes followed by electron capture processes from the surface.

Secondary ion emission from polymethacrylate LB-layers under 0.5–11 keV atomic and molecular primary ion bombardment

Secondary ion yields Y(Xiq) increase considerably when changing from atomic to molecular primary ions, whereas the parallel increase in the corresponding damage cross sections σ(Xiq) is much smaller. This results in a net increase of ion formation efficiencies E(Xiq)=Y/σ. For a more detailed understanding of the complex sputtering and ion formation processes, in particular for molecular primary ion bombardment, the secondary ion emission of well-defined polymethacrylate LB mono- and multilayers on Ag was investigated. For characteristic secondary ions Xiq emitted from these overlayers Y(Xiq) and σ(Xiq) for 11 keV Ne+, Ar+, Xe+, O2+, SF5+, C7H7+, C10H8+, C6F6+ and C10F8+ bombardment were determined and compared. The influence of primary ion energy was investigated in the energy range between 0.5 and 10 keV for Xe+ and SF5+ bombardment.For multilayers we found yield increases up to nearly a factor of 1000, when changing from Ne+ to SF5+ bombardment. We found a more pronounced yield and efficiency enhancement for multi than for monolayer coverages, a saturation of Y, σ and E enhancement for primary ions made of more than 6 heavy constituents at constant primary ion energy, no chemical effect on the secondary ion yields under static SIMS conditions (SF5+ / C7H7+ e.g.), and a pronounced decrease in secondary ion yields and secondary ion formation efficiencies for SF5+ primary ions with impact energies below 2 keV.