Monoatomic Projectiles Research Articles

Large cluster ions: soft local probes and tools for organic and bio surfaces.

Ionised cluster beams have been produced and employed for thin film deposition and surface processing for half a century. In the last two decades, kiloelectronvolt cluster ions have also proved to be outstanding for surface characterisation by secondary ion mass spectrometry (SIMS), because their sputter and ion yields are enhanced in a non-linear fashion with respect to monoatomic projectiles, with a resulting step change of sensitivity for analysis and imaging. In particular, large gas cluster ion beams, or GCIB, have now become a reference in organic surface and thin film analysis using SIMS and X-ray photoelectron spectroscopy (XPS). The reason is that they induce soft molecular desorption and offer the opportunity to conduct damageless depth-profiling and 3D molecular imaging of the most sensitive organic electronics and biological samples, with a nanoscale depth resolution. In line with these recent developments, the present review focuses on rather weakly-bound, light-element cluster ions, such as noble or other gas clusters, and water or alcohol nanodroplets (excluding clusters made of metals, inorganic salts or ionic liquids) and their interaction with surfaces (essentially, but not exclusively, organic). The scope of this article encompasses three aspects. The first one is the fundamentals of large cluster impacts with surfaces, using the wealth of information provided by molecular dynamics simulations and experimental observations. The second focus is on recent applications of large cluster ion beams in surface characterisation, including mass spectrometric analysis and 2D localisation of large molecules, molecular depth-profiling and 3D molecular imaging. Finally, the perspective explores cutting edge developments, involving (i) new types of clusters with a chemistry designed to enhance performance for mass spectrometry imaging, (ii) the use of cluster fragment ion backscattering to locally retrieve physical surface properties and (iii) the fabrication of new biosurface and thin film architectures, where large cluster ion beams are used as tools to transfer biomolecules in vacuo from a target reservoir to any collector substrate.

Indium Phosphide Surface Modifications By Electrochemistry and Ionic Sputtering

The electrochemistry of III-V semiconductors such as Indium Phosphide (InP) has been studied in great detail, in particular for its reproducible electrochemical signatures, both in the dark and under illumination where photocurrents appear. However, little is known about the electrochemical behavior of such materials when it has been modified by a physical treatment as ionic bombardment. Such treatments are currently knowing an increasing interest for many studies where chemical depth-profile analysis is wanted. The electrochemistry of InP before and after sputtering with different parameters (energy, projectile size...) will be presented. The study of i-V curves, Open Circuit Potential, Mott Shottky plots and Nyquist plots of samples obtained with different conditions would indicate the relationship between such modification and changes in band bendings, interface states, etc. The coupling of electrochemical studies with XPS analyses during bombardments is an original tool which would become in the future a key approach for several systems, especially heterostructures. The evolution of the electrochemical behavior induced by perturbation can be exploited as very accurate surface probing. Another really promising interest of such experiments is the ability to recover with electrochemical or chemical treatments the initial properties of the damaged semiconductor surfaces. The purpose of this work is to study in detail the effects of different bombardments on InP, from monoatomic projectiles known to strongly modify crystallinity of surfaces to argon clusters, promising for characterization of sensitive materials thanks to a low energy by atom in the cluster (no ion implantation). While XPS profiling of crystalline materials by ionic bombardment is performed, a core level peak broadening is generally observed. In the case of binary semiconductors, changes in the atomic ratios may also appear. These effects can be interpreted as the appearance of disorder in a crystalline structure and a preferential etching. The objective here is the correlation between XPS and interfacial electrochemistry. Such approach allows to know the overall changes in the properties of the electrode induced by various treatments and therefore highlights the limits of interpretations in XPS profiling study (under vacuum) and the possible reorganizations when delivered to the air. Modifications of the electrode could be followed by its electrochemical modification toward hydrogen evolution leading to different cathodic decomposition of the material. After monoatomic sputtering, indium enrichment and electrochemical modification are observed and related to the level of perturbation. The photocurrent is no longer detectable, showing formation of a film that modifies the semiconductor response. The Mott-Schottky plot strongly flattens over the entire potential gap of InP. Nyquist plots also show an increase in the charge transfer resistance at the open circuit potential. Using cluster of thousands of argon atoms allows better control of the induced modifications. A regime where the characteristics of the semiconductor can be retrieved by anodic dissolution or chemical etching will be particularly investigated.

Conductive tracks of 30-MeV C60 clusters in doped and undoped tetrahedral amorphous carbon

In insulating tetrahedral amorphous carbon (ta-C), the irradiation with 30-MeV C60 cluster ions leads to the formation of well conducting tracks. While electrical currents through individual tracks produced with monoatomic projectiles (e.g. Au or U) often exhibit rather large track to track fluctuations, C60 clusters are shown to generate highly conducting tracks with very narrow current distributions. Additionally, all recorded current–voltage curves show linear characteristics. These findings are attributed to the large specific energy loss dE/dx of the 30-MeV C60 clusters. We also investigated C60 tracks in ta-C films which were slightly doped with B, N or Fe during film growth. Doping apparently increases the ion track conductivity. However, at the same time the insulating characteristics of the pristine ta-C film can be reduced. The present C60 results are compared with data from earlier experiments with monoatomic heavy ion beams. The investigations were performed by means of atomic force microscopy including temperature dependent conductivity measurements of single ion tracks.

TOF-S-SIMS molecular depth profiling of organic bilayers using mechanical wear test methodology

Recent publications on static secondary ion mass spectrometry (S-SIMS) focus on molecular depth profiling by using polyatomic or ultra-low energy monoatomic projectiles. Since their applicability depends on the relationship between the ion yield and the depth, which is hard to obtain without extensive studies, a combination of a wear test method with S-SIMS surface analysis was performed in the current study. Using this non-sputtering procedure, the relation between the signal intensity and the local concentration remains in principle the same as that at the surface (which is easy to determine). Mechanical erosion was successfully applied to expose sub-surface material from organic multilayers. Through surface analysis with S-SIMS on the gradually exposed deeper planes, molecular depth profiles could be obtained. The study was conducted on a model system relevant to offset printing, consisting of two polymer layers, containing dyes and a surfactant, cast on an Al substrate.

Ion yield improvement for static secondary ion mass spectrometry by use of polyatomic primary ions

Static secondary ion mass spectrometry (S-SIMS) is one of the potentially most powerful and versatile tools for the analysis of surface components at the monolayer level. Current improvements in detection limit (LOD) and molecular specificity rely on the optimisation of the desorption-ionisation (DI) process. As an alternative to monoatomic projectiles, polyatomic primary ion (P.I.) bombardment increases ion yields non-linearly. Common P.I. sources are Ga+ (liquid metal ion gun (LMIG), SF5+ (electron ionisation) and the newer Au(n)+, Bi(n)q+ (both LMIG) and C60+ (electron ionisation) sources. In this study the ion yield improvement obtained by using the newly developed ion sources is assessed. Two dyes (zwitterionic and/or thermolabile polar functionalities on a largely conjugated backbone) were analysed as a thin layer using Ga+, SF5+, C60+, Bi+, Bi3(2+) and Bi5(2+) projectiles under static conditions. The study aims at evaluating the improvement in LOD, useful and characteristic yield and molecular specificity. The corrected total ion count values for the different P.I. sources are compared for different instruments to obtain a rough estimate of the improvements. Furthermore, tentative ionisation and fragmentation schemes are provided to describe the generation of radical and adduct ions. Characteristic ion yields are discussed for the different P.I. sources. An overview of the general appearances of the mass spectra obtained with the different P.I. sources is given to stress the major improvement provided by polyatomic P.I.s in yielding information at higher m/z values.

Scanning force microscopy of surface damage created by fast C60 cluster ions in CaF2 and LaF3 single crystals

Single crystals of CaF2 and LaF3 were exposed to 30-MeV C60 clusters from the tandem accelerator in Orsay, extending earlier irradiation experiments with monoatomic swift heavy ions to larger energy loss values. The irradiated crystal surfaces were investigated by scanning force microscopy (SFM) in contact mode. Topographic images reveal nanometric hillock-like structures protruding from the surface at each cluster impact site. The hillock diameter and height are analyzed and compared to monoatomic projectiles.

Comparison of primary monoatomic with primary polyatomic ions for the characterisation of polyesters with static secondary ion mass spectrometry

Static secondary ion mass spectrometry (S-SIMS) emerges as one of the most adequate methods for the surface characterisation of polymers with an information depth of essentially one monolayer. The continuing search for increased analytical sensitivity and specificity has led to exploring the use of polyatomic primary ions as an alternative to the traditionally applied monoatomic projectiles. As part of a systematic investigation on polyatomic bombardment of organic and inorganic solids, this paper focuses on selected polyesters. Mass spectra and ion yields are compared for layers deposited on silicon wafers by spincoating solutions with different concentrations of poly(epsilon-caprolactone) (PCL), poly(butylene adipate) (PBA) and poly(ethylene adipate) (PEA). Accurate mass measurements have been used to support the assignment of the ions and link the composition of the detected ions to the analyte structure. Use of polyatomic projectiles increases the yield of structural ions with a factor of +/-15, +/-30 and +/-10 for PCL, PBA and PEA, respectively, in comparison to bombardment with Ga+ primary ions, while the molecular specificity is improved by the detection of additional high m/z ions.

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.

Sputtering simulations of organic overlayers on metal substrates by monoatomic and clusters projectiles

This paper reviews our recent work on computer simulations of monoatomic and cluster bombardment of metal and organic surfaces. The investigated surfaces are irradiated with keV monoatomic (C, Ar, Ga) and polyatomic (C 60) projectiles that are recognized as valuable sources for desorption of high mass particles in secondary ion and neutral mass spectrometry (SIMS/SNMS) experiments. The analysis of the results reveals that the C 60 ion beam enables to perform chemical imaging with higher sensitivity, better depth resolution, and lower contamination than monoatomic projectiles with similar kinetic energy. For monoatomic projectiles, the development of a linear collision cascade is the predominant mechanism responsible for the ejection of particles. In contrast, strongly nonlinear, collective processes occur during C 60 bombardment. These nonlinear processes lead to the enhanced removal of material and the formation of a macroscopic crater. This paper presents theoretical insight into possible mechanisms responsible for the observed behavior with the emphasis on the phenomena important to the SIMS/SNMS community.

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.

High electronic excitation-induced crystallization inFe73.5Cu1Nb3Si13.5B9 amorphous alloy: I. Irradiation with swift heavy monoatomic projectiles

We present a detailed study of the crystallization induced in an amorphous alloy bymonoatomic projectiles in the electronic energy deposition regime. The crystallizationphenomenon was investigated by transmission electron microscopy (TEM) for different leadion energies (0.7–5 GeV), irradiation temperatures (80–300 K) and ion fluences (from1 × 1011 upto 2 × 1013 ions cm−2).

Characterisation by various microscopic techniques of the damage created by MeV C60 ions in amorphous Ni3B

In the work presented here, we study the damage resulting from the high rates of energy deposition in electronic processes during the slowing down of energetic projectiles in amorphous Ni3B. More precisely, we study the structural modifications of the targets irradiated at 300 K in a wide range of fluences (109–5×1011 cm−2) with 30 MeV C60 ions. The experimental results mainly concern the surface features and were obtained by Transmission Electron Microscopy (TEM) using in particular phase contrast and topographical contrast imaging. In the case of C60 ions, TEM reveals that surface damage consists of approximatively 20 nm diameter hollows, which are surrounded by protuding rims. This seems to indicate that the anisotropic growth process, which is specific of amorphous structures, is induced by each individual C60 cluster ion. Previous results obtained using GeV heavy monoatomic projectiles showed that the plastic deformation of the targets started only above an incubation fluence, i.e. after a significant spatial overlap of the damaged regions.

Surface characterization with keV clusters and MeV ions

Desorption of atomic and molecular species from solids can be achieved by bombardment with keV and MeV projectiles. In the studies described here, the secondary ions (SIs) are identified by time-of-flight mass spectrometry (TOF-MS) in an event-by-event counting mode. In this paper, desorption induced by two types of projectiles, viz. particle desorption mass spectrometry (PDMS) and secondary-ion mass spectrometry with keV clusters as projectiles (cluster-SIMS), is discussed. PDMS uses fast heavy ions as projectiles (i.e., ions of E ≥ 0.1 MeV/amu and m ≥ 40 amu fission fragments are well suited for PDMS). The features of PDMS can be summarized as follows: it is suitable for isotopic and molecular analysis by virtue of the TOF-MS procedure; the desorption yield (ratio [%] of ions desorbed vs primary projectiles) can reach and even exceed 100%; thus mass spectra are obtained with a very low total primary-ion dose of ≤ 10 6 ions. PDMS has a shallow sampling depth (5–100 monolayers, depending on the target), i.e. the technique is suitable for outermost-surface analysis without interference from the substrate. Cluster-SIMS opens new perspectives for surface characterization. Clusters are attractive as projectiles because they are quasineutral, yet they can be accelerated, steered and focused. Their size, mass velocity and chemical composition can be selected to yield maximum SI emission. A new experimental approach has been devised to implement cluster-SIMS. We have observed up to 30-fold enhancement in the SI yield for cluster- vs monoatomic projectiles of equal velocities. Generally, the SI yields increase with the mass of the projectile and its velocity. The technique has particular promise for the surface mapping of organics. Event-by-event coincidence counting can be combined with TOF-MS. This allows one to determine which masses are desorbed together from a primary impact size. Surface homogeneity is thus revealed with high planar resolution.

Experimental study of molecular and cluster effects in secondary electron emission.

We have measured the ion-induced secondary-electron-emission (SEE) yields in forward and backward directions from thin sputter-cleaned foils in ultrahigh vacuum. C, Al, Ti, Ni, and Cu have been bombarded with ${\mathrm{H}}^{+}$, ${\mathrm{H}}_{2}^{+}$, and ${\mathrm{H}}_{3}^{+}$ (0.3--1.2 MeV/amu), and C and Al have been bombarded with ${\mathrm{C}}^{+}$, ${\mathrm{O}}^{+}$, and ${\mathrm{CO}}^{+}$ (15--85 keV/amu). The yields induced by molecular and cluster ions are compared to those induced by the corresponding isotachic monoatomic projectiles. We observe molecular effects as yield reductions at low projectile velocities (${\mathit{v}}_{\mathit{p}}$\ensuremath{\approxeq}${\mathit{v}}_{\mathit{o}}$) and yield enhancements at higher velocities (${\mathit{v}}_{\mathit{p}}$\ensuremath{\gg}${\mathit{v}}_{\mathit{o}}$). The results are discussed in the framework of the extended kinetic-emission model by Sternglass and the energy-loss model for clusters by Brandt and Ritchie. The velocity dependence of the molecular effect in SEE follows the velocity dependence of the molecular effect in Brandt's energy-loss calculations. Thus it seems that the energy loss is also proportional to SE yields for molecular projectiles at velocities around and above the Bohr velocity ${\mathit{v}}_{0}$.