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.     

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.     

Model multilayer structures for three-dimensional cell imaging

The prospects for SIMS three-dimensional analysis of biological materials were explored using model multilayer structures. The samples were analyzed in a ToF-SIMS spectrometer equipped with a 20 keV buckminsterfullerene (C₆₀⁺) ion source. Molecular depth information was acquired using a C₆₀⁺ ion beam to etch through the multilayer structures at specified time intervals. Subsequent to each individual erosion cycle, static SIMS spectra were recorded using a pulsed C₆₀⁺ ion probe. Molecular intensities in sequential mass spectra were monitored as a function of primary ion fluence. The resulting depth information was used to characterize C₆₀⁺ bombardment of biological materials. Specifically, molecular depth profile studies involving dehydrated dipalmitoyl-phosphatidylcholine (DPPC) organic films indicate that cell membrane lipid materials do not experience significant chemical damage when bombarded with C₆₀⁺ ion fluences greater than 10¹⁵ ions/cm². Moreover, depth profile analyses of DPPC-sucrose frozen multilayer structures suggest that biomolecule information can be uncovered after the C₆₀⁺ sputter removal of a 20 nm overlayer with no appreciable loss of underlying molecular signal. The experimental results support the potential for three-dimensional molecular mapping of biological materials using cluster SIMS.     

Formation of Sim and SimCn clusters by C60 sputtering of Si

The secondary ion mass spectrum of silicon sputtered by high energy C₆₀⁺ ions in sputter equilibrium is found to be dominated by Si clusters and we report the relative yields of Si_m⁺ (1 ≤ m ≤ 15) and various Si_mC_n⁺ clusters (1 ≤ m ≤ 11 for n = 1; 1 ≤ m ≤ 6 for n = 2; 1 ≤ m ≤ 4 for n = 3). The yields of Si_m⁺ clusters up to Si₇⁺ are significant (between 0.1 and 0.6 of the Si⁺ yield) with even numbered clusters Si₄⁺ and Si₆⁺ having the highest probability of formation. The abundances of cluster ions between Si₈⁺ and Si₁₁⁺ are still significant (>1% relative to Si⁺) but drop by a factor of ∼100 between Si₁₁⁺ and Si₁₃⁺. The probability of formation of clusters Si₁₃⁺-Si₁₅⁺ is approximately constant at ∼5 × 10⁻⁴ relative to Si⁺ and rising a little for Si₁₅⁺, but clusters beyond Si₁₅ are not detected (Si_m≥16⁺/Si⁺ < 1 × 10⁻⁴). The probability of formation of Si_m⁺ and Si_mC_n⁺ clusters depends only very weakly on the C₆₀⁺ primary ion energy between 13.5 keV and 37.5 keV. The behaviour of Si_m⁺ and Si_mC_n⁺ cluster ions was also investigated for impacts onto a fresh Si surface to study the effects that saturation of the surface with C₆₀⁺ in reaching sputter equilibrium may have had on the measured abundances. By comparison, there are very minor amounts of pure Si_m⁺ clusters produced during C₆₀⁺ sputtering of silica (SiO₂) and various silicate minerals. The abundances for clusters heavier than Si₂⁺ are very small compared to the case where Si is the target.The data reported here suggest that Si_m⁺ and Si_mC_n⁺ cluster abundances may be consistent in a qualitative way with theoretical modelling by others which predicts each carbon atom to bind with 3-4 Si atoms in the sample. This experimental data may now be used to improve theoretical modelling.     

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.     

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⁺.     

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.     

Sputtering yields of PMMA films bombarded by keV C60 ions

A quartz crystal microbalance (QCM) has been used to determine total-mass sputtering yields of PMMA films by 1-16 keV C₆₀^+,2+ ion beams. Quantitative sputtering yields for PMMA are presented as mass loss per incident ion Y_m. Mass-lost rate QCM data show that a 13 keV C₆₀ cluster leads to emission equivalent to 800 PMMA molecules per ion. The power law obtained for the increase in sputtering yield with primary ion energy is in good agreement those predicted by “thermal spike” regime and MD models, when crater sizes are used to estimate sputtering.     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

Energy distributions of atomic and molecular ions sputtered by C60 projectiles

In the process of investigating the interaction of fullerene projectiles with adsorbed organic layers, we measured the kinetic energy distributions (KEDs) of fragment and parent ions sputtered from an overlayer of polystyrene (PS) oligomers cast on silver under 15 keV C₆₀⁺ bombardment. These measurements have been conducted using our TRIFT™ spectrometer, recently equipped with the C₆₀⁺ source developed by Ionoptika, Ltd. For atomic ions, the intensity corresponding to the high energy tail decreases in the following order: C⁺(E^−0.4) > H⁺(E^−1.5) > Ag⁺(E^−3.5). In particular, the distribution of Ag⁺ is not broader than those of Ag₂⁺ and Ag₃⁺ clusters, in sharp contrast with 15 keV Ga⁺ bombardment. On the other hand, molecular ions (fragments and parent-like species) exhibit a significantly wider distribution using C₆₀⁺ instead of Ga⁺ as primary ions. For instance, the KED of Ag-cationized PS oligomers resembles that of Ag⁺ and Ag_n⁺ clusters. A specific feature of fullerene projectiles is that they induce the direct desorption of positively charged oligomers, without the need of a cationizing metal atom. The energy spectrum of these PS⁺ ions is significantly narrower then that of Ag-cationized oligomers. For characteristic fragments of PS, such as C₇H₇⁺ and C₁₅H₁₃⁺ and polycyclic fragments, such as C₉H₇⁺ and C₁₄H₁₀⁺, the high energy decay is steep (E⁻⁴ − E⁻⁸). In addition, reorganized ions generally show more pronounced high energy tails than characteristic ions, similar to the case of monoatomic ion bombardment. This observation is consistent with the higher excitation energy needed for their formation. Finally, the fraction of hydrocarbon ions formed in the gas phase via unimolecular dissociation of larger species is slightly larger with gallium than with fullerene projectiles.     

Analysis of TOF-SIMS spectra from fullerene compounds

We analyzed TOF-SIMS spectra obtained from three different size of fullerenes (C₆₀, C₇₀ and C₈₄) by using Ga⁺, Au⁺ and Au₃⁺ primary ion beams and investigated the fragmentation patterns, the enhancement of secondary ion yields and the restraint of fragmentation by using cluster primary ion beams compared with monoatomic primary ion beams. In the TOS-SIMS spectra from C₇₀ and C₈₄, it was found that a fragment ion, identified as C₆₀⁺ (m/z = 720), showed a relatively high intensity compared with that of other fragment ions related to C₂ depletion. It was also found that the Au₃⁺ bombardment caused intensity enhancement of intact molecules (C₆₀⁺, C₇₀⁺ and C₈₄⁺) and restrained the fragmentation due to C₂ depletion.     

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.     

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.     

Molecular ion emission from single large cluster impacts

We investigated the emission of the secondary ions stimulated by single impacts of 136 keV Au₄₀₀⁴⁺ projectiles. The study was carried out on targets of glycine, phenylalanine, and C₆₀. In addition, a target of C₆₀ was examined with 18 keV C₆₀⁺ projectiles. The experiments were performed in the event-by-event bombardment/detection mode. The secondary ions were identified with linear time-of-flight mass spectrometer equipped with an 8-anode detector. The Au₄₀₀⁴⁺ projectile induces abundant multi-ion emission, for instance the average number of detected ions (atomic, fragment, molecular and cluster ions) emitted per event from glycine target is 12.5. The glycine intact molecular ion (Gly⁻) yield is 1.14. The bombardment of a C₆₀ target results in the efficient emission of multiple intact C₆₀⁻ (total yield is 0.15).     

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.     

SiO2 film electret with high surface potential stability

The plasma surface treatment and ion implantation were utilized to improve the stability of charge storage in the SiO₂ film electret. It was found that the SiO₂ films treated by argon plasma with the arcing at 700 V for 15 min, or implanted by 150 keV (kilo electron volt) Ar⁺ with a dose of 2 × 10¹¹ cm⁻², after being negatively charged, showed a remnant negative potential as large as 90% of the primary value after being stored in a glass container with desiccant for 10 days. It was also found that after being negatively charged at room temperature and aged at 200-350 °C for several times, the SiO₂ films implanted by 150 keV Ar⁺ had a relatively high remnant potential and it did not decay significantly even after being heated at the aging temperature of 200-350 °C in room atmosphere for 60 min.     

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.     

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.