The effect of incident angle on the C60 bombardment of molecular solids

The effect of incident angle on the quality of SIMS molecular depth profiling using C₆₀⁺ was investigated. Cholesterol films of ∼300 nm thickness on Si were employed as a model and were eroded using 40 keV C₆₀⁺ at an incident angle of 40° and 73° with respect to the surface normal. The erosion process was characterized by determining at each angle the relative amount of chemical damage, the total sputtering yield of cholesterol molecules, and the interface width between the film and the Si substrate. The results show that there is less molecule damage at an angle of incidence of 73° and that the total sputtering yield is largest at an angle of incidence of 40°. The measurements suggest reduced damage is not necessarily dependent upon enhanced yields and that depositing the incident energy nearer the surface by using glancing angles is most important. The interface width parameter supports this idea by indicating that at the 73° incident angle, C₆₀⁺ produces a smaller altered layer depth. Overall, the results show that 73° incidence is the better angle for molecular depth profiling using 40 keV C₆₀⁺.     

Polyethylene terephthalate (PET) bulk film analysis using C60, Au3, and Au primary ion beams

The damage characteristics of polyethylene terephthalate (PET) have been studied under bombardment by C₆₀⁺, Au₃⁺ and Au⁺ primary ions. The observed damage cross-sections for the three ion beams are not dramatically different. The secondary ion yields however were significantly enhanced by the polyatomic primary ions where the secondary ion yield of the [M + H]⁺ is on average 5× higher for C₆₀⁺ than Au₃⁺ and 8× higher for Au₃⁺ than Au⁺. Damage accumulates under Au⁺ and Au₃⁺ bombardment while C₆₀⁺ bombardment shows a lack of damage accumulation throughout the depth profile of the PET thick film up to an ion dose of ∼1 × 10¹⁵ ions cm⁻². These properties of C₆₀⁺ bombardment suggest that the primary ion will be a useful molecular depth profiling tool.     

Fundamental studies of molecular depth profiling and 3D imaging using Langmuir-Blodgett films as a model

Molecular depth profiling and three-dimensional imaging using cluster projectiles and SIMS have become a prominent tool for organic and biological materials characterization. To further explore the fundamental features of cluster bombardment of organic materials, especially depth resolution and differential sputtering, we have developed a reproducible and robust model system consisting of Langmuir-Blodgett (LB) multilayer films. Molecular depth profiles were acquired, using a 40-keV C₆₀⁺ probe, with LB films chemically alternating between barium arachidate and barium dimyristoyl phosphatidate. The chemical structures were successfully resolved as a function of depth. The molecular ion signals were better preserved when the experiment was performed under cryogenic conditions than at room temperature. A novel method was used to convert the scale of fluence into depth which facilitated quantitative measurement of the interface width. Furthermore, the LB films were imaged as a function of depth. The reconstruction of the SIMS images correctly represented the original chemical structure of the film. It also provided useful information about interface mixing and edge effects during sputtering.     

Molecular depth profiling of multi-layer systems with cluster ion sources

Cluster bombardment of molecular films has created new opportunities for SIMS research. To more quantitatively examine the interaction of cluster beams with organic materials, we have developed a reproducible platform consisting of a well-defined sugar film (trehalose) doped with peptides. Molecular depth profiles have been acquired with these systems using C₆₀⁺ bombardment. In this study, we utilize this platform to determine the feasibility of examining buried interfaces for multi-layer systems. Using C₆₀⁺ at 20 keV, several systems have been tested including Al/trehalose/Si, Al/trehalose/Al/Si, Ag/trehalose/Si and ice/trehalose/Si. The results show that there can be interactions between the layers during the bombardment process that prevent a simple interpretation of the depth profile. We find so far that the best results are obtained when the mass of the overlayer atoms is less than or nearly equal to the mass of the atoms in buried molecules. In general, these observations suggest that C₆₀⁺ bombardment can be successfully applied to interface characterization of multi-layer systems if the systems are carefully chosen.     

The effect of C60 cluster ion beam bombardment in sputter depth profiling of organic-inorganic hybrid multiple thin films

The effects of C₆₀ cluster ion beam bombardment in sputter depth profiling of inorganic-organic hybrid multiple nm thin films were studied. The dependence of SIMS depth profiles on sputter ion species such as 500 eV Cs⁺, 10 keV C₆₀⁺, 20 keV C₆₀²⁺ and 30 keV C₆₀³⁺ was investigated to study the effect of cluster ion bombardment on depth resolution, sputtering yield, damage accumulation, and sampling depth.     

Substrate effects on the analysis of biomolecular layers using Au, Au3 and C60 bombardments

Effects of platinum silicon, graphite and PET substrates on the secondary ion yield of sub-monolayer and multilayer samples of Cyclosporin A following 20 keV Au⁺, Au₃⁺and C₆₀⁺ impacts have been investigated. The obtained results of sub-monolayer samples show that platinum enhances the yield of the pseudo-molecular ion following Au⁺ and Au₃⁺ impacts due to the high density of the substrate that enables the energy of the primary ions to be deposited near the surface. C₆₀⁺ impacts on sub-monolayer samples are less effective, but there is an enhancement on PET substrates. Impacts of 20 keV Au⁺ and Au₃⁺ are not very efficient on multilayer samples. 20 keV C₆₀⁺ impacts enhance the yields significantly, especially for the relatively high molecular weight [M+H]⁺ ion.     

Depth profiling using C60 SIMS—Deposition and topography development during bombardment of silicon

A C₆₀⁺ primary ion source has been coupled to an ion microscope secondary ion mass spectrometry (SIMS) instrument to examine sputtering of silicon with an emphasis on possible application of C₆₀⁺ depth profiling for high depth resolution SIMS analysis of silicon semiconductor materials. Unexpectedly, C₆₀⁺ SIMS depth profiling of silicon was found to be complicated by the deposition of an amorphous carbon layer which buries the silicon substrate. Sputtering of the silicon was observed only at the highest accessible beam energies (14.5 keV impact) or by using oxygen backfilling. C₆₀⁺ SIMS depth profiling of As delta-doped test samples at 14.5 keV demonstrated a substantial (factor of 5) degradation in depth resolution compared to Cs⁺ SIMS depth profiling. This degradation is thought to result from the formation of an unusual platelet-like grain structure on the SIMS crater bottoms. Other unusual topographical features were also observed on silicon substrates after high primary ion dose C₆₀⁺ bombardment.     

Controlling energy deposition during the C60 bombardment of silicon: The effect of incident angle geometry

The profile of the energy deposition footprint is controlled during the C₆₀⁺ erosion of Si surfaces by varying the incident energy and/or incident angle geometry. Sputter yield, surface topography, and chemical composition of the eroded surfaces were characterized using atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS). The experiments show that the 10 keV, 40° incident C₆₀⁺ erosion of Si results in the formation of a C containing, mound-like structure on the solid surface. We find that the occurrence of this C feature can be avoided by increasing the incident energy of the C₆₀⁺ projectile or by increasing the incident angle of the C₆₀⁺ projectile. While both strategies allow for the Si samples to be eroded, the occurrence of topographical roughening limits the usefulness of C₆₀⁺ in ultra-high resolution semiconductor depth profiling. Moreover, we find that the relative effect of changing the incident angle geometry of the C₆₀⁺ projectile on the profile of the energy deposition footprint, and thus the sputter yield, changes according to the kinetic energy of the projectile and the material of the bombarded surface, a behavior that is quite different than what is observed for an atomic counterpart.     

Investigation of molecular weight effects of polystyrene in ToF-SIMS using C60 and Au primary ion beams

In secondary ion mass spectrometry, polyatomic primary ion sources are known to enhance yields from many surfaces including polymers. In order to understand the fundamental causes for these increases, the enhancement as a function of material type and molecular weight needs to be delineated. In this article, we report results from a systematic investigation of polymeric films of polystyrene (PS) with varying molecular weights to examine the influence of the primary ion beam on the secondary ion yields in time of flight secondary ion mass spectrometry (ToF-SIMS). The masses of the polymers investigated ranged from 1000 to 20,000 Da, or from about n = 10 to 200 where n indicates the number of polymeric units in a polymer chain. The polymers had a narrow molecular weight range (PDI < 1.07). The multilayer polymeric films (10-30 nm) characterized by AFM were prepared by spin-casting onto silicon substrates and were analyzed using Au⁺ and C₆₀⁺ primary ion beams. The analysis with the two beams provided a useful comparison between atomic and polyatomic primary ion sources. Information gathered from this study provides insight into the role of molecular weight on the observed yield enhancement from polyatomic ion sources.     

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%).     

Stretching the limits of static SIMS with C60

Pristine and Au-covered molecular films have been analyzed by ToF-SIMS (TRIFT™), using 15 keV Ga⁺ (FEI) and 15 keV C₆₀⁺ (Ionoptika) primary ion sources. The use of C₆₀⁺ leads to an enormous yield enhancement for gold clusters, especially when the amount of gold is low (2 nmol/cm²), i.e. a situation of relatively small nanoparticles well separated in space. It also allows us to extend significantly the traditional mass range of static SIMS. Under 15 keV C₆₀⁺ ion bombardment, a series of clusters up to a mass of about 20,000 Da (Au₁₀₀⁻: 19,700 Da) is detected. This large yield increase is attributed to the hydrocarbon matrix (low-atomic mass), because the yield increase observed for thick metallic films (Ag, Au) is much lower. The additional yield enhancement factors provided by the Au metallization procedure for organic ions (MetA-SIMS) have been measured under C₆₀⁺ bombardment. They reach a factor of 2 for the molecular ion and almost an order of magnitude for Irganox fragments such as C₄H₉⁺, C₁₅H₂₃O⁺ and C₁₆H₂₃O⁻.     

C60 sputtering of organics: A study using TOF-SIMS, XPS and nanoindentation

Sputtering of organic materials using a C₆₀ primary ion beam has been demonstrated to produce significantly less accumulated damage compared to sputtering with monatomic and atomic-cluster ion beams. However, much about the dynamics of C₆₀ sputtering remains to be understood. We introduce data regarding the dynamics of C₆₀ sputtering by evaluating TOF-SIMS depth profiles of bulk poly(methyl methacrylate) (PMMA). Bulk PMMA provides an ideal test matrix with which to probe C₆₀ sputter dynamics because there is a region of steady-state secondary ion yield followed by irreversible signal degradation. C₆₀ sputtering of PMMA is evaluated as a function of incident ion kinetic energy using 10 keV C₆₀⁺, 20 keV C₆₀⁺ and 40 keV C₆₀⁺⁺ primary ions. Changes in PMMA chemistry, carbon accumulation and graphitization, and topography as a function of total C₆₀ ion dose at each accelerating potential is addressed.     

Structural effects of C60 bombardment on various natural mineral samples—Application to analysis of organic phases in geological samples

Organic phases trapped inside natural mineral samples are of considerable interest in astrobiology, geochemistry and geobiology. Examples of such organic phases are microfossils, kerogen and oil. Information about these phases is usually retrieved through bulk crushing of the rock which means both a risk of contamination and that the composition and spatial distribution of the organics to its host mineral is lost. An attractive of way to retrieve information about the organics in the rock is depth profiling using a focused ion beam. Recently, it was shown that it is possible to obtain detailed mass spectrometric information from oil-bearing fluid inclusions, i.e. small amounts of oil trapped inside a mineral matrix, using ToF-SIMS. Using a 10 keV C₆₀⁺ sputter beam and a 25 keV Bi₃⁺ analysis beam, oil-bearing inclusions in different minerals were opened and analysed individually. However, sputtering with a C₆₀⁺ beam also induced other changes to the mineral surface, such as formation of topographic features and carbon deposition. In this paper, the cause of these changes is explored and the consequences of the sputter-induced features on the analysis of organic phases in natural mineral samples (quartz, calcite and fluorite) in general and fluid inclusions in particular are discussed.The dominating topographical features that were observed when a several micrometers deep crater is sputtered with 10 keV C₆₀⁺ ions on a natural mineral surface are conical-shaped and ridge-like structures that may rise several micrometers, pointing in the direction of the incident C₆₀⁺ ion beam, on an otherwise flat crater bottom. The sputter-induced structures were found to appear at places with different chemistry than the host mineral, including other minerals phases and fluid inclusions, while structural defects in the host material, such as polishing marks or scratches, did not necessarily result in sputter-induced structures. The ridge-like structures were often covered by a thick layer of deposited carbon.Despite the appearance of the sputter-induced structures and carbon deposition, most oil-bearing inclusions could successfully be opened and analysed. However, smaller inclusion (<15 μm) could potentially become entirely covered by sputter-resistant structures and therefore difficult to open. Therefore, it might become necessary, to for example increase the ion energy and rotate the stage to successfully open smaller inclusions for analysis.SIMS, C₆₀, carbon deposition, topography, mineral, fluid inclusions, geological samples, depth profiling.     

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.     

Cluster SIMS using metal cluster complex ions

Metal cluster complexes are chemically synthesized organometallic compounds, which have a wide range of chemical compositions with high molecular weight. Using a metal cluster complex ion source, sputtering characteristics of silicon bombarded with normally incident Ir₄(CO)₇⁺ ions were investigated. Experimental results showed that the sputtering yield at 10 keV was 36, which is higher than that with Ar⁺ ions by a factor of 24. In addition, secondary ion mass spectrometry (SIMS) of boron-delta-doped silicon samples and organic films of poly(methyl methacrylate) (PMMA) was performed. Compared with conventional O₂⁺ ion beams, Ir₄(CO)₇⁺ ion beams improved depth resolution by a factor of 2.5 at the same irradiation conditions; the highest depth resolution of 0.9 nm was obtained at 5 keV, 45° with oxygen flooding of 1.3 × 10⁻⁴ Pa. Furthermore, it was confirmed that Ir₄(CO)₇⁺ ion beams significantly enhanced secondary ion intensity in high-mass region.     

Sputtering of amorphous ice induced by C60 and Au3 clusters

Molecular dynamics simulations were performed to study the behavior of cluster SIMS. Two predominant cluster ion beam sources, C₆₀ and Au₃, were chosen for comparison. An amorphous water ice substrate was bombarded with incident energy of 5 keV. The C₆₀ cluster was observed to shatter upon impact creating a crater of damage approximately 8 nm deep. Although Au₃ was also found to both break apart and form a damage crater, it continued along its initial trajectory causing damage roughly 10 nm deep into the sample and becoming completely imbedded. It is suggested that this difference in behavior is due to the large mass of Au relative to the substrate water molecule.     

Depth profiling of cells and tissues by using C60 and SF5 as sputter ions

In the present study, SF₅⁺ and C₆₀⁺ were used as primary ions for sputtering and Bi₃⁺ was used as primary ions for analysis. The depth profiling procedure was utilized to make 3D images of the chemistry of single cultured cells and tissue samples of intact intestinal epithelium.The results show sputtering of organic material from cells and tissue with both SF₅⁺ and C₆₀⁺ sources. Cholesterol fragments were found in the superficial layers when sputtering with C₆₀⁺. Spectra were collected revealing the change in yield along the z-axis of the sample. 3D images of the localization of Na, K, phosphocholine and cholesterol were constructed with both ion sources for single cell cultures and the mouse intestine.Cryostate sections of mouse intestine were analysed in 2D and the results were compared with the 3D image of the intestine. The localization of cholesterol and phosphocholine was found to be similar in cryostate sections analysed in two dimensions and the sputtered, freeze-dried intestine analysed in 3D. The comparison of 2D and 3D images suggest that the phosphocholine signal faded with C₆₀⁺ sputtering. In conclusion, both C₆₀⁺ and SF₅⁺ can be used as primary ion sources for sputtering of organic material from cells and tissues. Consecutive analysis with a Bi₃⁺ source can be used to obtain image stacks that could be used for reconstruction of 3D images.     

Analysis of C60 and Cs sputtering ions for depth profiling gold/silicon and GaAs multilayer samples by time of flight secondary ion mass spectrometry

Time of flight secondary ion mass spectrometry (ToF-SIMS) depth profiles of several inorganic layered samples using Cs⁺ and C₆₀⁺ primary sputtering ions of different energies are compared to evaluate sputter yield and depth resolution. A gold/silicon model system is employed to study interfaces between metals and semiconductors, and multilayers of AlGaAs, Al, and InAs in GaAs are analyzed to explore the ability of C₆₀⁺ to analyze semiconductor interfaces in GaAs. Roughness measurements are reported to differentiate between different factors affecting depth resolution. The best depth resolution from all samples analyzed is achieved using 1 keV Cs⁺. However, C₆₀⁺ sputtering has advantages for analyzing conductor/insulator interfaces because of its high sputter yield, and for analyzing deeper heterolayers in GaAs due to lower sputter-induced roughness.     

Studies of the electrochemical reduction processes of Bi, HTeO2 and their mixtures

The reduction process of Bi³⁺, HTeO₂⁺ and their mixtures on Au electrode surface was studied by cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy and chronoamperometry. XRD and EDS methods were also used to measure the reductive products prepared under different potentials and provide the evidences of the reactions. The results indicate that the reduction of HTeO₂⁺ occurs at more positive potential than that of Bi³⁺, but its reduction rate is slower and adsorption phenomenon exists during its reduction process. Bi₂Te₃ compound can be obtained potentiostatically at a proper potential in all the mixed solutions with concentration ratio CHTe+O₂/CBi3+ in our research range (0.1-10). But pure Bi₂Te₃ compound can only be obtained at 42 mV in the solution with concentration ratio CHTe+O₂/CBi3+ equaling to 1. And the formation of Bi₂Te₃ compound is an inductive co-depositing process: (1) HTeO₂⁺ + 4e⁻ + 3H⁺ → Te⁰ + 2H₂O, (2) 3Te⁰ + 2Bi³⁺ + 6e⁻ → Bi₂Te₃.     

Depth profiling of taxol-loaded poly(styrene-b-isobutylene-b-styrene) using Ga and C60 ion beams

The surface of a triblock copolymer, containing a solid-phase drug, was investigated using 15 keV Ga⁺ and 20 keV C₆₀⁺ ion beams. Overall, the results illustrate the successful use of a cluster ion beam for greatly enhancing the molecular ion and high-mass fragment ion intensities from the surface and bulk of the polymer system. The use of C₆₀⁺ also established the ability to see through common overlayers like poly(dimethyl siloxane) which was not possible using atomic ion sources. Moreover, the use of C₆₀⁺ allowed depth profiles to be obtained using primary ion dose densities in excess of 6 × 10¹⁴ C₆₀⁺/cm². Resulting sputter craters possess relatively flat bottoms without the need for sample rotation and reached depths of ca. 2 μm. AFM results illustrate the more gentile removal of surface species using cluster ions. Specifically, phase contrast and topographic images suggest the relatively high ion doses do not significantly alter the phase distribution or surface topography of the polymer. However, a slight increase in rms roughness was noticed.