On the road to high-resolution 3D molecular imaging

This contribution reviews the state-of-the-art in the domains of molecular imaging and depth profiling, the two methodological platforms required for 3D molecular imaging by secondary ion mass spectrometric (SIMS). Using molecular dynamics calculations, it also describes some of the mechanisms that make cluster projectiles such as C₆₀ so different for organic sample analysis. The discussion addresses issues that deserve proper attention on the way to 3D molecular imaging in SIMS, such as ultimate lateral resolution, limited molecular yields, chemical effects and damage, and highlights solutions currently in embryo in the many research teams concerned by 3D molecular imaging.     

Quantitative secondary ion mass spectrometry of fluorocarbon polymers

Secondary ion mass spectrometry (SIMS) is used to measure quantitatively the thickness of thin (6–160 Å) polyperfluoroether films on silicon and gold surfaces. Linear relationship between ellipsometrically measured thicknesses and integrated SIMS signals is demonstrated. Time dependence of SIMS signals indicates that the polymeric films have a uniform thickness down to the thinnest layers studied. In the lower limit, the fluorocarbon polymers have extended, flat conformation due to polymer-substrate interactions. Sputtering yield and effective sputtering depth of oxygen ions are determined for these liquid polymers. It is also shown that organic adsorbates reside between the solid surface and the low surface tension fluorocarbon films.     

Compositional analysis in the nano-regime:A SIMS perspective

A serious problem in secondary ion mass spectrometry (SIMS) analysis is its "matrix effect" that hinders the quantification of a certain species in a sample and consequently, appropriate corrective measures are taken to calibrate the secondary ion currents into respective concentrations for accurate compositional analysis. Use of "calibration standards" is necessary for this purpose. Detection of molecular MCsn+ ions (M-element to be analyzed, n=1, 2, 3,....) under Cs+ ion bombardment is a possible mean to minimize such matrix effect, enabling one to quantify without the need of calibration standards. Our recent studies on MCsn+ molecular ions aim towards the understanding of their formation mechanisms, which are important to know their effects on SIMS quantification.In-depth quantitative analysis is a major strength of SIMS for which 'depth resolution' is of significant relevance. The optimal choice of the impact parameters during SIMS analyses can play an effective role in obtaining data with ultra-high depth resolution. SIMS is possible at depth resolution in the nm or even sub-nm range, with quantifiable data obtained from the top monolayer onwards into the material. With optimized experimental conditions, like extremely low beam current (down to ～10 nA), and low bombarding energy (below 1 keV), ultra-high depth resolution SIMS has enabled interfacial composition analysis of ultra-thin films, quantum wells, heterostructures, etc. and complex low-dimensional structures with high precision and repeatability.     

Formation of niobium oxynitrides by rapid thermal processing (RTP)

The potential of rapid thermal processing (RTP) for the preparation of thin films of niobium oxynitrides was investigated. The 200 and 500 nm niobium films were deposited via sputtering on oxidized silicon(1 0 0)- and on sapphire(1 −1 0 2)-substrates. At first, oxidation of niobium films in molecular oxygen and then nitridation in ammonia using an RTP-system was performed. The films were characterized before and after the oxidation and nitridation processes by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS). The influence of the two different substrates, amorphous SiO₂ and single crystalline sapphire on the reactivity of the niobium films was studied in dependence of temperature, time of reaction and film thickness. The existence of niobium oxynitride formation was verified for some of the films. In some of the experiments, crack formation in the films or even delamination of the Nb-films from the substrates was observed.     

Application of characteristic secondary ion mass spectroscopy to phase identification in amorphous titanium carbide

Concentration-depth profiles of amorphous titanium carbide films were performed using secondary ion mass spectroscopy (SIMS). Investigations were carried out based on the concept of characteristic mass spectra (fingerprint spectra). The studies showed that the three phases TiO₂, TiO, and TiC coexist in the coatings, rather than forming a TiCO oxycarbide solid solution.     

The influence of Fermi energy on structural and electrical properties of laser crystallized P-doped amorphous silicon

A series of phosphorous-doped hydrogenated amorphous silicon films (a-Si:H) were crystallized using step-by-step laser crystallization process. The structural changes during the sequential crystallization process were detected by Raman measurements. The dehydrogenation was monitored by measuring the Si-H local vibrational modes using Raman spectroscopy and hydrogen effusion measurements. Interestingly, hydrogen bonding is affected by doping of the amorphous material. The influence of doping concentrations, thus the Fermi energy on electronic properties has been investigated employing secondary ion mass spectroscopy (SIMS), dark-conductivity- and Hall-effect measurements. The results from hydrogen effusion are consistent with the results obtained from Raman spectroscopy, Hall-effect- and dark-conductivity measurements.     

射频溅射沉积硅薄膜实验

本文描述采用射频溅射法在GH39不锈钢基体上沉积硅薄膜的实验。对直接溅射沉积硅薄膜的工艺进行了实验研究,并对硅膜进行了SIMS分析。用称重法测出无定形硅膜的最大厚度可达400—500nm,为HL-1装置等离子体边界层物理实验制作了所需的无定形硅膜收集探针。     

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.     

Highly stable carbon-doped Cu films on barrierless Si

Electrical resistivities and thermal stabilities of carbon-doped Cu films on silicon have been investigated. The films were prepared by magnetron sputtering using a Cu-C alloy target. After annealing at 400 °C for 1 h, the resistivity maintains a low level at 2.7 μΩ-cm and no Cu-Si reaction is detected in the film by X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations. According to the secondary ion mass spectroscopy (SIMS) results, carbon is enriched near the interfacial region of Cu(C)/Si, and is considered responsible for the growth of an amorphous Cu(C)/Si interlayer that inhibits the Cu-Si inter-diffusion. Fine Cu grains, less than 100 nm, were present in the Cu(C) films after long-term and high-temperature annealings. The effect of C shows a combination of forming a self-passivated interface barrier layer and maintaining a fine-grained structure of Cu. A low current leakage measured on this Cu(C) film also provides further evidence for the carbon-induced diffusion barrier interlayer performance.     

Elastic and inelastic processes in the interaction of 1–10 eV ions with solids: ion transport through surface layers

We review the escape depth of secondary ions (or neutrals) desorbing from solid surfaces under the impact of electrons, photons or ions. We survey ion (or neutral) transport through many materials, but most are wide band gap insulators such as rare-gas solids and molecular solids. We address the issue of low-energy (<10 eV) ion—solid interactions, and review experimental and theoretical studies that provide insight into the physical mechanisms of these interactions, such as elastic scattering, charge transfer and ion—molecule reactions. Although it is usually assumed that most of the secondary ions stem from the top surface layer, we show that this is not necessarily the case: In certain instances, 1–10 eV ions are able to transmit solid films which are several monolayers thick. The transport of low-energy ions through materials has very broad implications. We point out the importance of these results for electron or photon stimulated desorption (ESD/PSD), secondary ion mass spectrometry (SIMS), and ion-sputtering of surfaces, and discuss their relevance to other fields, such as ion beam deposition (IBD), low-energy ion implantation, and electrochemistry.     

Sputter depth profiling by secondary ion mass spectrometry coupled with sample current measurements

Ion-induced secondary electron emission determined via sample current measurements (SCM) was employed as a useful adjunct to conventional secondary ion mass spectrometry (SIMS). This paper reports on the results of 3-6 keV O₂⁺ SIMS-SCM sputter depth profiling through CrN/AlN multilayer coatings on nickel alloy, titanium dioxide films deposited on stainless steel, and corrosion layers formed onto surface of magnesium alloy after long-term interaction with an ionic liquid. For Au/AlNO/Ta films on silicon, in addition to SIMS-SCM profiles, the signal of mass-energy separated backscattered Ne⁺ ions was monitored as a function of the depth sputtered as well. The results presented here indicate that secondary electron yields are less affected by “matrix effect” than secondary ion yields, and at the same time, more sensitive to work function variations and surface charging effects. SCM depth profiling, with suppression of “the crater effect” by electronic gating of the registration system is capable of monitoring interfaces in the multilayer structure, particularly, metal-dielectric boundaries. In contrast to SIMS, SCM data are not influenced by the angle and energy windows of an analyser. However, the sample current measurements provide lower dynamic range of the signal registration than SIMS, and SCM is applicable only to the structures with different secondary electron emission properties and/or different conductivity of the layers. To increase the efficiency, SCM should be accompanied by SIMS measurements or predetermined by proper calibration using other elemental-sensitive techniques.     

Secondary ion mass spectrometry for quantitative surface and in-depth analysis of materials

Secondary ion mass spectrometry (SIMS) is a technique based on the sputtering of material surfaces under primary ion bombardment. A fraction of the sputtered ions which largely originate from the top one or two atomic layers of the solid is extracted and passed into a mass spectrometer where they are separated according to their mass-to-charge ratios and subsequently detected. Because the sputter-yields of the individual species, coupled with their ionization probabilities, can be quite high and the mass spectrometers can be built with high efficiencies, the SIMS technique can provide an extremely high degree of surface sensitivity. Using a particular mode like static SIMS where a primary ion current is as low as 10^?11 amp, the erosion rate of the surface can be kept as low as 1 Å per hour and one can obtain the chemical information of the uppermost atomic layer of the target. The other mode like dynamic SIMS where the primary ion current is much higher can be employed for depth profiling of any chemical species within the target matrix, providing a very sensitive tool (～ 1 ppm down to ppb) for quantitative characterization of surfaces, thin films, superlattices, etc. The presence of molecular ions amongst the sputtered species makes this method particularly valuable in the study of molecular surfaces and molecular adsorbates. The range of peak-intensities in a typical SIMS spectrum spans about seven to eight orders of magnitude, showing its enormously high dynamic range; an advantage in addition to high sensitivity and high depth-resolution. Furthermore, the high sensitivity of SIMS to a very small amount of material implies that this technique is adaptable to microscopy, offering its imaging possibilities. By using this possibility in static SIMS or dynamic SIMS mode of analysis, one can obtain a two-dimensional (2D) surface mapping or a three-dimensional (3D) reconstruction of the elemental distribution, respectively within the target matrix. Secondary ion yields for elements can differ from matrix to matrix. These sensitivity variations pose serious limitations in quantifying SIMS data. Various methods like calibration curve approach, implantation standard method, use of relative sensitivity factor, etc. are presently employed for making quantitative SIMS analysis. The formation of secondary ions by ion bombardment of solids is relatively a complex process and theoretical research in this direction continues in understanding this process in general. The present paper briefly reviews the perspective of this subject in the field of materials analysis.     

Improved quantitative analysis of Cu(In,Ga)Se2 thin films using MCs+-SIMS depth profiling

The chalcopyrite semiconductor, Cu(InGa)Se₂ (CIGS), is popular as an absorber material for incorporation in high-efficiency photovoltaic devices because it has an appropriate band gap and a high absorption coefficient. To improve the efficiency of solar cells, many research groups have studied the quantitative characterization of the CIGS absorber layers. In this study, a compositional analysis of a CIGS thin film was performed by depth profiling in secondary ion mass spectrometry (SIMS) with MCs⁺ (where M denotes an element from the CIGS sample) cluster ion detection, and the relative sensitivity factor of the cluster ion was calculated. The emission of MCs⁺ ions from CIGS absorber elements, such as Cu, In, Ga, and Se, under Cs⁺ ion bombardment was investigated using time-of-flight SIMS (TOF-SIMS) and magnetic sector SIMS. The detection of MCs⁺ ions suppressed the matrix effects of varying concentrations of constituent elements of the CIGS thin films. The atomic concentrations of the CIGS absorber layers from the MCs⁺-SIMS exhibited more accurate quantification compared to those of elemental SIMS and agreed with those of inductively coupled plasma atomic emission spectrometry. Both TOF-SIMS and magnetic sector SIMS depth profiles showed a similar MCs⁺ distribution for the CIGS thin films.     

3D molecular imaging SIMS

Thin monolayer and bilayer films of spin cast poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(lactic) acid (PLA) and PLA doped with several pharmaceuticals have been analyzed by dynamic SIMS using SF₅⁺ polyatomic primary ion bombardment. Each of these systems exhibited minimal primary beam-induced degradation under cluster ion bombardment allowing molecular depth profiles to be obtained through the film. By combing secondary ion imaging with depth profiling, three-dimensional molecular image depth profiles have been obtained from these systems. In another approach, bevel cross-sections are cut in the samples with the SF₅⁺ primary ion beam to produce a laterally magnified cross-section of the sample that does not contain the beam-induced damage that would be induced by conventional focussed ion beam (FIB) cross-sectioning. The bevel surface can then be examined using cluster SIMS imaging or other appropriate microanalysis technique.     

Comparing the chemical properties of evaporated and sputtered niobium films on oxidized Si(1 0 0) wafers - preparation of oxynitride films

Five hundred nanometers of niobium films have been deposited on silicon(1 0 0) wafers with 100 or 300 nm thermally grown oxide by electron beam evaporation and DC magnetron sputtering. SEM and AFM investigations revealed smaller crystallites and rougher surfaces for the evaporated films. The differences in film morphology resulted in lower reflection intensities in XRD for the as-deposited evaporated films. In order to investigate the influence of the structural properties on their chemical reactivities, in a first set of experiments the films were nitrided with molecular nitrogen by rapid thermal processing (RTP) at varying temperatures. In another set of experiments after nitridation in nitrogen at 1000 °C an oxidation step in molecular oxygen at varying temperatures followed. The films showed different reactivities, leading to different rates of nitridation and oxidation. Sputtered films were less reactive than the evaporated films, deduced from the sequence of reaction products dependent on reaction temperature. XRD data indicated that oxynitrides have formed. Elemental depth profiles were measured by secondary ion mass spectrometry (SIMS).     

Structure and tribological properties of amorphous carbon films deposited by electrochemical method on GCr15 steel substrate

Amorphous carbon films were deposited on GCr15 steel substrates by electrolysis of methanol, dimethylsulfoxide (DMSO) and the methanol-DMSO intermixture electrolytes, respectively, under high voltage and low temperature conditions. The microstructure and wear morphology of the deposited films were analyzed using X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM) combined with energy dispersive X-ray fluorescence spectrometer (EDX), respectively. The tribological properties of the films were evaluated using a ball-on-disk rotating friction tester under dry friction condition. The results show that the films deposited by electrodeposition technique on GCr15 steel substrates are amorphous carbon films. It is also found that the electrolytes have an obvious influence on the tribological properties of the deposited films with the electrodeposition method. The tribological properties of the films deposited with the intermixture electrolyte are better than those of the films deposited by other pure electrolytes. The related growth mechanism of the films in the liquid-phase electrodeposition is discussed as well in this study. Via the reaction of the CH₃ groups with each other to form carbon network and reaction of the CH₃ and SO²⁺ groups to achieve the doping of sulfur atom in the carbon network, respectively, in other words, amorphous carbon films can be obtained on GCr15 steel substrates by electrodeposition technique.     

Magnesium doping in InAlAs and InGaAs/Mg films lattice-matched to InP grown by MOVPE

Mg-doped InAlAs and InGaAs films were grown at 560 °C lattice matched to InP semi-insulting substrate by metalorganic vapor phase epitaxy (MOVPE) under various Cp₂Mg flow conditions. Hall effect, photoluminescence (PL), high-resolution X-ray diffraction (HR-XRD), and secondary ion mass (SIMS) were the tools used in this work. The crystalline quality and the n-p conversion of the InAlAs and InGaAs/Mg films are described and discussed in relation to the Cp₂Mg flow. Distinguishing triple emission peaks in PL spectra is observed and seems to be strongly dependent on the Cp₂Mg flow. SIMS is employed to analyze the elements in the epitaxial layers. The variation of indium and magnesium components indicates a decrease of magnesium incorporation during the growth of InAlAs layers leading to a contracted lattice. In addition, the magnesium incorporation in the InGaAs lattice during growth has been confirmed by SIMS.     

Multimolecule glasses and crystals of isotactic polystyrene

When spreading dilute solutions of isotactic polystyrene (iPS) on water, it was observed that different concentrations produce different morphologies in the resulting amorphous films. From 10^?1 to 10^?2 wt% solutions, ribbonlike films and circular platelets of multimolecule aggregates were obtained. After isothermal crystallization of these amorphous films, lamellae, incipient spherulites, and single-molecule crystals were observed. The formation of lamellar crystals lying flat on the substrate is an indication of the absence of epitaxy by the substrate, which would require the molecular chains to be in the surface plane and thus yield lamellae normal to the substrate. The dense packing in the center of sheaflike spherulites is caused by screw-dislocation growth, and it produces structures similar to shish kebabs. It was also observed that the lamellae grown from the center develop secondary growth spirals, causing the branching and splaying of the structure typical for spherulitic growth.     

X-ray diffraction study of the molecular propolis films deposited from an alcohol solution onto the cleavage surfaces of layered V<Subscript>2</Subscript>VI<Subscript>3</Subscript> compounds

The structures of the molecular propolis films deposited from an alcohol solution on the (0001) cleavage surface of layered bismuth selenide and telluride are studied by X-ray diffraction. Despite the chemical interaction between the semiconductor substrates and the organic-substance components, the molecular structural ordering of the propolis films is shown to be identical to that in the films of this substance on the surface of amorphous glass substrates. The chemical and deformation interaction between the organic substance and the layered V₂VI₃ compounds is found to result in the formation of an organic-inorganic sandwich nanostructure at a distance of ∼0.3 μm from the layered crystal-propolis film interface.     

Ion beam deposition of a-Si:H

The ion beam deposition (IBD) of hydrogenated amorphous silicon is described. Hydrogen incorporation and bonding with the silicon network is evident from SIMS and infrared spectra; the latter show absorption bands centered at 2000cm^?1 and 630cm^?1 typical of monosilicon hydride bonding. IBD a-Si:H thin films are found to be free of microvoids and trace metallic impurities. Four probe conductivity measurements show that the ion beam deposition process yields high resistivity, hydrogenated amorphous silicon (?≥10⁹Ωcm). All of these measurements suggest a low density of defects states in the band gap.