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.     

Probing thin over layers with variable energy/cluster ion beams

A series of carbon-coated magnetic recording disks proved ideal for exploring sampling depth and ion formation trends as a function of variations in energy and cluster size (Au_x) of the primary ion beam, and variations in over coat thickness and type. Ion yield from the underlying metal layer increased with increasing energy and decreasing cluster size of the primary ions. The yields varied nearly linearly with over layer thickness. In contrast, M_xCs_y depth profiles were unaffected by changes in the primary ion. The samples were fortuitously dosed with dinonyl phthalate, allowing a study similar to prior GSIMS work [I.S. Gilmore, M.P. Seah, J.E. Johnstone, in: A. Benninghoven, P. Bertrand, H.-N. Migeon, H.W. Werner (Eds.), Proceedings of the 12th International Conference on SIMS, Elsevier, Brussels, 2000, p. 801]. Ions prominent in the EI mass spectrum, including even electron ions, were more consistently enhanced at lower energies and higher cluster sizes than the primary (M + H)⁺ ion. The total secondary ion count was inversely proportional to the film thickness. Secondary electrons, largely originating in the buried metal layer, may be inducing organic ion formation [A.M. Spool, Surf. Interface Anal. 36 (2004) 264].     

ToF-SIMS cluster ion imaging of hippocampal CA1 pyramidal rat neurons

Recent studies have demonstrated the power of time-of-flight secondary ion mass spectrometry (ToF-SIMS) cluster ion imaging to characterize biological structures, such as that of the rat central nervous system. A large number of the studies to date have been carried out on the “structural scale” imaging several mm² using mounted thin sections. In this work, we present our ToF-SIMS cluster ion imaging results on hippocampal rat brain neurons, at the cellular and sub-cellular levels. As a part of an ongoing investigation to examine gut linked metabolic factors in autism spectrum disorders using a novel rat model, we have observed a possible variation in hippocampal Cornu ammonis 1 (CA1) pyramidal neuron geometry in thin, paraformaldehyde fixed brain sections. However, the fixation process alters the tissue matrix such that much biochemical information appears to be lost. In an effort to preserve as much as possible this original information, we have established a protocol using unfixed thin brain sections, along with low dose, 500 eV Cs⁺ pre-sputtering that allows imaging down to the sub-cellular scale with minimal sample preparation.     

Evaluation of secondary ion yield enhancement from polymer material by using TOF-SIMS equipped with a gold cluster ion source

We investigated the enhancement of the secondary ion intensity in the TOF-SIMS spectra obtained by Au⁺ and Au₃⁺ bombardment in comparison with Ga⁺ excitation using polymer samples with different molecular weight distributions. Since the polymer samples used in this experiment have a wide molecular weight distribution, the advantages of the gold cluster primary ion source over monoatomic ion could accurately be evaluated. It was observed that the degree of fragmentation decreased by the usage of cluster primary ion beam compared with monoatomic ion beam, which was observed as a shift of the intensity distribution in the spectra. It was also found out that the mass effect of Au⁺ and Ga⁺ as monoatomic primary ion, resulted in about 10-60 times of enhancement for both samples with different molecular distributions. On the other hand, the Au₃⁺ bombardment caused intensity enhancement about 100-2600 compared with Ga⁺ bombardment, depending on the mass range of the detected secondary ion species. The cluster primary ion effect of Au₃⁺, compared with Au⁺, therefore, was estimated to be about 10-45.     

Ga primary ion ToF-SIMS fragment pattern of inorganic compounds and metals

Regularity of Ga⁺ primary ion ToF-SIMS fragment pattern of inorganic compounds is discussed. For an inorganic compound as formulated M–A, where the valence of cation M is +n and that of anion A is −p, the chemical composition of appeared ToF-SIMS fragment are M_xA_y, which satisfy the rule nx≥py+1 for positive ion fragments and nx≤py+1 for negative ones. For example, for oxide fragment of chemical composition, M_xA_y (valence of M is +n), the fragment obeys the rule nx≥2y+1 for positive ions and nx≤2y+1 for negative ones, respectively. The regularity of ToF-SIMS fragment patterns of sulfides, nitrates, sulfates etc. is discussed.     

Fundamental studies of the cluster ion bombardment of water ice

The fundamental sputtering properties of water ice are of interest for molecular depth profiling of biological samples in their native environment. We report on a method of studying amorphous water ice films of precise thicknesses in which pure water vapor is condensed onto a pre-cooled, silver-coated quartz crystal microbalance (QCM). This scheme allows for the determination of water ice sputter yields for any primary projectile as well as providing a means for studying escape depths of atoms and molecules beneath the deposited water ice layer. Specifically, we find a removal of approximately 2500 water molecule equivalents/20 keV C₆₀⁺ projectile with an underlying silver ion escape depth of 7.0 Å.     

Desorption of cluster ions from adsorbed methane under cryogenic condition by low-energy ion irradiation

We have investigated ion desorption from adsorbed methane following keV He⁺ ion irradiation. The thickness of the adsorbed layer was precisely controlled. For mono-layered methane, only monomer ions (CH_x⁺) were desorbed by 1 keV He⁺ ion irradiation. On the other hand, a large number of cluster ions (C_nH_x⁺) up to n = 20 were desorbed from multi-layered film. Among cluster ions, molecular ions with CC bonds were found, which indicates that chemical bonds are newly formed by ion irradiation. Based on the results for thickness dependences of the mass spectral patterns, it was elucidated that the monomer ions are desorbed from the top surface layer through single electron excitation. While the cluster ions are formed mainly in the inside of the layers along the nuclear track due to the high-density electronic excitation, which is produced by nuclear collision between incident He⁺ ions and frozen molecules.     

Relative quantification of cellular sections with molecular depth profiling ToF-SIMS imaging

We report the use of secondary ion mass spectrometry (SIMS) imaging to quantify the relative difference in the amount of lipid between two sections, the plasma membrane and the cytoplasm, of single cells from two different populations. Cells were each labeled with lipophillic dyes, frozen, fractured and analyzed in a ToF-SIMS mass spectrometer equipped with a 40 keV C₆₀⁺ ion source. In addition to identifying cells from separate populations, the lipophilic dyes can be used as a marker for the outer leaflet of the cell membrane and therefore as a depth finder. Here, we show that it is possible to compare the amount of lipids with particular headgroups in the cell membrane of a treated cell to the membrane of a control cell. Following erosion of the cell membranes, the amount of the two specific lipid head groups in the cytoplasm of the treated cell can be compared to those lipids in a control cell. Here we take the first step in this experimental design and display the ability to analyze multiple sections of frozen cells following a single fracture.     

Secondary ion emission from Si bombarded with large Ar cluster ions under UHV conditions

Si was bombarded with size-selected 40 keV Ar cluster ions and positive secondary ions were measured using the time-of-flight technique under high and ultra-high vacuum (HV and UHV respectively) conditions. Si⁺ ions were main species detected under the incidence of 40 keV Ar cluster ions, and the yields of Si cluster ions such as Si₄⁺ were also extremely high under both conditions. On the other hand, oxidized secondary ions such as SiO⁺ were detected with high intensity only under the HV condition. The yield ratios of oxidized ions decreased in UHV to less than 1% of their values in HV. The effect of residual gas pressure on Si cluster ion yields is relatively low compared to oxidized ions, and the UHV condition is required to obtain accurate secondary ion yields.     

Influence of primary ion species on the secondary cluster ion emission process from SAMs of hexadecanethiol on gold

In order to investigate the secondary cluster ion emission process of organo-metallic compounds under keV ion bombardment, self-assembled monolayers (SAMs) of alkanethiols on gold are ideal model systems. In this experimental study, we focussed on the influence of the primary ion species on the emission processes of gold-alkanethiolate cluster ions from a hexadecanethiol SAM on gold. For this purpose, we carried out time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements using the following primary ion species and acceleration voltages: Ar⁺, Xe⁺, SF₅⁺ (10 kV), Bi⁺, Bi₃⁺(25 kV), Bi₃²⁺, Bi₅²⁺, Bi₇²⁺ (25 kV).It is well known that molecular ions M⁻ and gold-alkanethiolate cluster ions Au_xM_y⁻ with M = S-(CH₂)₁₅-CH₃, x − 3 ≤ y ≤ x + 1, x, y > 0, show intense peaks in negative mass spectra. We derived yields Y_SI exemplarily for the molecular ions M⁻ and the gold-hexadecanethiolate cluster ions Au_y+1M_y⁻ up to y = 8 and found an exponentially decreasing behaviour for increasing y-values for the cluster ions.In contrast to the well-known increase in secondary ion yield for molecular secondary ions when moving from lighter to heavier (e.g. Ar⁺ to Xe⁺) or from monoatomic to polyatomic (e.g. Xe⁺ to SF₅⁺) primary ions, we find a distinctly different behaviour for the secondary cluster ions. For polyatomic primary ions, there is a decrease in secondary ion yield for the gold-hexadecanethiolate clusters whereas the relative decrease of the secondary ion yield ξ_Y with increasing y remains almost constant for all investigated primary ions.     

Molecular dynamics study of particle emission by reactive cluster ion impact

Molecular dynamics (MD) simulations of sputtering process with fluorine cluster impact onto silicon targets were performed. By iterating collisional simulations on a same target, accumulation of incident atoms and evolution of surface morphology were examined as well as emission process of precursors. When (F₂)₃₀₀ clusters were sequentially irradiated on Si(1 0 0) target at 6 keV of total incident energy, column-like surface structure covered with F atoms was formed. As the number of incident clusters increased, sputtering yield of Si atoms also increased because the target surface was well fluoridised to provide SiF_x precursors. Size distribution of emitted particles showed that SiF₂ was the major sputtered particle, but various types of silicon-fluoride compounds such like Si₂F_x, Si₃F_x and very large molecules consists of 100 atoms were also observed. This size distribution and kinetic energy distribution of desorbed materials were studied, which showed that the sputtering mechanism with reactive cluster ions is similar to that under thermal equilibrium condition at high-temperature.     

ToF-SIMS and SEM-EDS analysis of the surface of chosen bioindicators

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy with field emission equipped with energy dispersive spectrometer (SEM-EDS) were applied in order to investigate the surface of chosen biological indicators such as bark, needles and hair. All samples were collected from the urban areas of the Lodz city region, recognized as one of the most polluted in Poland. The high resolution of the applied techniques provides good separation of signals from different chemical species. High detection sensitivity enables an analysis of crucial elements and/or molecules. It was found that the surface of the materials studied was in general covered by many exogenous particles, suggesting, from the monitoring purpose point of view, their possible anthropogenic origin. The preliminary results obtained revealed that these techniques can be successfully used in environmental analysis of various biological samples and can provide detailed information about distribution and composition of airborne pollutants within the sample surfaces studied as well as initiate a renewed interest in the exposure study.     

High-intensity Si cluster ion emission from a silicon target bombarded with large Ar cluster ions

Secondary ions emitted from Si targets were measured with a quadrupole mass spectrometer under large Ar cluster and monomer ion bombardment. Incident ion beams with energies from 7.5 to 25 keV were used and the mean size of the Ar cluster ion was about 1000 atoms/cluster. Si_n⁺ ions with n values up to n = 8 were detected under Ar cluster ion bombardment, whereas Si cluster ions were scarcely detected under Ar monomer ion bombardment. These cluster ion yields showed the power law dependence on the cluster size.     

New developments in the surface analysis of solids

In both fundamental and applied surface physics, it is essential to know as much as possible about the chemical composition of the outer atomic layers of solids. Rapid progress has recently been made in the development of analytical methods which could be used in surface analysis. All utilize some type of emission (photons, electrons, atoms, molecules, ions), caused by excitation of the surface states. Both the “excitation” and emission processes must meet certain basic requirements as regards information depth, form in which the information is obtained, sensitivity, changes in the surface layer during analysis, etc. The more important of the methods that qualify, namely Auger-Electron Spectroscopy (AES), photo-Electron Spectroscopy for Chemical Analysis (ESCA) and the static method of Secondary-Ion Mass Spectrometry (SIMS), are discussed and their potentialities and limitations illustrated by characteristic examples.     

SSIMS analysis of organics, polymer blends and interfaces

This paper gives a critical review on the applications of ToF SIMS in the areas of polymer additive characterization and in the study of polymer blends and interfaces. Polymer additives can readily be identified by ToF SIMS using their parent molecular ions or characteristic fragments. This analytical capability has been successfully applied to monitor the migration or segregation of additives during polymer processing. ToF SIMS is an ideal analytical tool for the study of polymer blends and interfaces because it is able to provide information on both surface composition and morphology. In combination with other analytical techniques such as AFM and XPS, ToF SIMS chemical imaging capability has opened up new horizons in the investigation of complex polymer blend systems. Finally the main advantages and limitations of ToF SIMS in these application areas are also discussed.     

Thermal analysis of the CSNS H- ion source

There are two cooling systems to maintain the thermal stability of the CSNS H ion source during its operation: Air-cooling in the source body of the discharging chamber and water-cooling in the flange on which the discharging chamber is installed.The optimal cooling parameters to ensure the operation of the H-ion source are determined through a thermal analysis. In addition, a transient analysis is also performed to know exactly the transient temperature variation during the whole 40 ms period of the pulsed mode operation of the ion source.     

Thermal analysis of the CSNS H－ ion source

There are two cooling systems to maintain the thermal stability of the CSNS H^- ion source during its operation： Air-cooling in the source body of the discharging chamber and water-cooling in the flange on which the discharging chamber is installed. The optimal cooling parameters to ensure the operation of the H^- ion source are determined through a thermal analysis. In addition, a transient analysis is also performed to know exactly the transient temperature variation during the whole 40 ms period of the pulsed mode operation of the ion source.     

Characterization of drug-eluting stent (DES) materials with cluster secondary ion mass spectrometry (SIMS)

Secondary ion mass spectrometry (SIMS) employing an SF₅⁺ polyatomic primary ion source was utilized to analyze several materials commonly used in drug-eluting stents (DES). Poly(ethylene-co-vinyl acetate) (PEVA), poly(lactic-co-glycolic acid) (PLGA) and various poly(urethanes) were successfully depth profiled using SF₅⁺ bombardment. The resultant molecular depth profiles obtained from these polymeric films showed very little degradation in molecular signal as a function of increasing SF₅⁺ primary ion dose when experiments were performed at low temperatures (signal was maintained for doses up to ∼5 × 10¹⁵ ions/cm²). Temperature was determined to be an important parameter in both the success of the depth profiles and the mass spectral analysis of the polymers. In addition to the pristine polymer films, paclitaxel (drug released in Taxus™ stent) containing PLGA films were also characterized, where it was confirmed that both drug and polymer signals could be monitored as a function of depth at lower paclitaxel concentrations (10 wt%).     

Thermal analysis of the CSNS H~- ion source

There are two cooling systems to maintain the thermal stability of the CSNS H- ion source during its operation:Air-cooling in the source body of the discharging chamber and water-cooling in the flange on which the discharging chamber is installed.The optimal cooling parameters to ensure the operation of the H- ion source are determined through a thermal analysis.In addition,a transient analysis is also performed to know exactly the transient temperature variation during the whole 40 ms period of the pulsed ...     

SIMS analysis of residual gas elements with a Cameca IMS-6f ion microprobe

In this paper, we present experimental data for SIMS analysis of residual gas elements (RGEs) with a Cameca IMS-6f ion microprobe. We considered a simple experimental technique, which provides an effective separation of the secondary ions, sputtered from the bulk of a target, and from the molecules, adsorbed on the analyzed surface from the residual atmosphere. The technique needs the sputtering yield of one monolayer (ML) per second to be applied. The method improves (in more than one order of magnitude) the detection limit for RGEs in SIMS analysis, and simultaneously, provides information about the residual atmosphere at the sample surface and in the main chamber of the experimental instrument. The method provides a calibration method for an ion gauge, and can be used for SIMS analysis with a gas (O₂) flooding.