Depth resolution in sputter profiling

The dependence of depth resolution on sputter depth due to various parameters is theoretically estimated. Comparison with experimental work of different authors shows the validity of the proposed model.     

Phonon spectromicroscopy of carbon nanostructures with atomic resolution

The vibrational properties of single-wall carbon nanotubes have been probed locally with atomic-scale resolution by inelastic electron tunneling spectroscopy with a low-temperature scanning tunneling microscope. The high spatial resolution has allowed the unraveling of changes in the local phonon spectrum related to topological defects. We demonstrated that the radial breathing mode is suppressed within tube segments of lengths below approximately 3 nm, and that in the cap region phonon modes characteristic of the fullerene hemisphere are emerging. Phonon spectromicroscopy should lead to a better understanding of the mechanisms that limit the transport of heat or electrical charge inside nanostructured carbon materials.     

High resolution quantitative SIMS analysis of shallow boron implants in silicon using a bevel and image approach

Secondary ion mass spectrometry (SIMS) is frequently used as the preferred tool for dopant profiling due to its sensitivity and depth resolution. However, as dopant profiles become shallower most, if not all of the implant profile lies in the pre-equilibrium or transient region of an SIMS depth profile. In this region sputter yield and ionisation rate vary making accurate quantification of the implant profile very difficult. These problems can be reduced through the use of much lower beam energies or oxygen flooding of the sample. However, most SIMS instruments do not have these capabilities. In this paper an alternative technique for producing an accurate depth profile of a shallow implant, using existing SIMS technology is presented.Through the fabrication of bevels with very small slope angles on a shallow boron implanted silicon via a chemical etch, SIMS ion imaging is performed on the exposed surface. Ion image data is then summed, and in conjunction with accurate measurement of the bevel morphology, a shallow boron implant profile produced. The ‘bevel-image’ profile compares very well with a profile obtained using a 1 keV oxygen beam. To ensure a good dynamic range on the ‘bevel-image’ profile it is important to clean the bevel with a HF etch, prior to imaging.     

Depth resolution and narrow nuclear resonance profiling

The basic stochastic theory of narrow nuclear resonance profiling is presented. We show that nm depth resolution is achievable in some cases. It is shown that the usual notions of depth resolution such as the full width at half maximum of the instrument function, adequate for instrument functions which vary only slowly over the depth range corresponding to the depth resolution, are unsatisfactory for narrow resonance profiling in the near surface region. Here, the system response to a delta function concentration profile varies very rapidly. However, the shape of the excitation curve is very sensitive to certain kinds of small changes in the concentration profile in the surface region. We discuss some first efforts to define a more general concept for depth resolution, consistent with the usually used definitions, intended to adequately express the perceived sensitivity of a measured excitation curve to variations in the underlying concentration profile.     

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.     

Single-shot extreme ultraviolet laser imaging of nanostructures with wavelength resolution

We have demonstrated near-wavelength resolution microscopy in the extreme ultraviolet. Images of 50 nm diameter nanotubes were obtained with a single ~1 ns duration pulse from a desktop-size 46.9 nm laser. We measured the modulation transfer function of the microscope for three different numerical aperture zone plate objectives, demonstrating that 54 nm half-period structures can be resolved. The combination of near-wavelength spatial resolution and high temporal resolution opens myriad opportunities in imaging, such as the ability to directly investigate dynamics of nanoscale structures.     

Depth-profile investigations of triterpenoid varnishes by KrF excimer laser ablation and laser-induced breakdown spectroscopy

The ablation properties of aged triterpenoid dammar and mastic films were investigated using a Krypton Fluoride excimer laser (248 nm, 25 ns). Ablation rate variations between surface and bulk layers indicated changes of the ablation mechanisms across the depth profiles of the films. In particular, after removal of the uppermost surface varnish layers there was a reduction of the ablation step in the bulk that was in line with a significant reduction of carbon dimer emission beneath the surface layers as detected by laser-induced breakdown spectroscopy. The results are explicable by the generation of condensation, cross-linking and oxidative gradients across the depth profile of triterpenoid varnish films during the aging degradation process, which were recently quantified and established on the molecular level.     

Applications of nanostructures in wide-field,label-free super resolution microscopy

Super resolution imaging capable of resolving details beyond the diffraction limit is highly desired in many scientific and application fields, including bio-medicine, nanomaterial science, and opto-electronic integration. Up to now, many different methods have been proposed, among which wide-field, label-free super resolution microscopy is indispensable due to its good applicability to diverse sample types, large field of view(FOV), and high imaging speed. In recent years,nanostructures have made a crucial contribution to the wide-field, label-free subdiffraction microscopy, with various working mechanisms and configuration designs. This review summarizes the recent applications of the nanostructures in the wide-field, label-free super resolution microscopy, with the emphasis on the designs of hyperlens with hyperbolic dispersion, microsphere with "nano-jets", and nanowire ring illumination microscopy based on spatial frequency shift effect. The bottlenecks of the current techniques and possible solutions are also discussed.     

Beyond nanosizing: an approach to shape analysis of fluorescent nanostructures by SMI-microscopy

Using spatially modulated illumination (SMI) light microscopy it is possible to measure the sizes of fluorescent structures that have an extension far below the conventional optical resolution limit (“subresolution size”). Presently, the sizes are determined as the object extension along the optical axis of the SMI microscope. For this, however, “a priori” assumptions on the fluorochrome distribution (“shape”) within the examined fluorescent structure have to be made. Usually it is assumed that the fluorochrome follows a Gauss-distribution or a spherical distribution. In this report we overcome the necessity to make an assumption on the shape of the fluorochrome distribution. We introduce two new experimentally obtained parameters which allow the determination of a shape measure to describe the spatial distribution of the fluorescent dye. This becomes possible by independent measurements with different excitation wavelengths. As an example, we present shape parameter measurements on individual fluorescent microspheres with a nominal geometrical diameter (“size”) of 190 nm. In the case investigated, the experimental shape correlated well with a homogeneous fluorochrome distribution (“spherical shape”) but not with a variety of other “shapes”.     

Surface analysis of insulating materials by Secondary Ion Mass Spectrometry (SIMS)

For a non-conducting solid sample (TiO₂ powder) the conditions for compensation of the charge build-up at the surface caused by the ion impact in SIMS are experimentally investigated. The compensation is achieved by an additional auxiliary electron beam of low energy. The resolution and the intensities of the secondary ions were measured as a function of the ratio of the current densities of the electron and the ion beams. The compensation for negative secondary ions, and especially for those with higher masses, is more critical than for positive ones. The intensities are influenced by the different values of the mean emission energies and the form of the energy distributions. Examples of mass spectra by SIMS for some insulators are given.     

Hydrogen detection by SIMS: Hydrogen on polycrystalline vanadium

The interaction of hydrogen with polycrystalline vanadium has been studied by SIMS and flash-desorption. Two well defined vanadium surfaces with different amounts of residual oxygen were exposed to H₂-doses up to 1000 L. The investigations were carried out under UHV conditions. The experiments result in a better understanding of the vanadium-hydrogen interaction, especially with regard to the influence of residual oxygen. The residual oxygen concentration can be characterized by the O^? emission; it increases with the number of O₂ exposure/ heating cycles. Different groups of secondary ions and flash signals, characteristic of certain chemical environments of hydrogen, could be distinguished. The ratios $\text{VH}{}^{\mathrm{+}}\text{V}{}^{\mathrm{+}}$ and $\text{V}{}_2\text{H}{}^{\mathrm{+}}\text{V}{}^{\mathrm{+}}{}_2$ proved to be typical of the direct V-H bond concentration. This can be derived from their behaviour during H₂ and O₂ exposure, e.g. Bulk dissolved hydrogen is indicated by a dosedepending H₂ flash signal at 900 K. Bulk solution of H₂ is blocked by a monomolecular oxygen layer. An additional H₂ flash signal at 500 K together with increasing values of $\text{VH}{}^{\mathrm{+}}\text{V}{}^{\mathrm{+}}$ and $\text{V}{}_2\text{H}{}^{\mathrm{+}}\text{V}{}^{\mathrm{+}}{}_2$ is observed for relatively oxygen free targets after H₂ exposure. The detection limit for metal-bonded hydrogen was found to be less than 100 ppm of one monolayer.     

Cesium ion sputtering with oxygen flooding: Experimental SIMS study of work function change

We investigated the work function (WF) change of a silicon surface being under cesium ion bombardment and simultaneous oxygen flooding with various oxygen pressures at the sample surface. It was found that WF of Cs⁺ ion sputtered Si decreases under oxygen flooding. This decrease provides an essential grow of secondary ion yields of some negative ions, sputtered from Si. At the same time Si⁻ ion yield decreases approximately in two times. In the paper we have discussed possible explanations of our experimental data: we considered a surface composition change, formation of surface dipoles and work function change caused by oxygen adsorption, and their relationships between each other.     

SIMS depth profile analysis of sodium in silicon dioxide

The depth profile analysis of sodium in SiO₂ matrix using secondary ion mass spectrometry (SIMS) has been examined. Q-SIMS (SIMS with a quadrupole mass analyzer: PHI ADEPT1010) was utilized as the measurement apparatus, because the absolute extraction voltage of the analyzer is typically lower than that of the sector type. As a result, the adequate sodium profile could be obtained under the optimized condition with the use of Cs⁺ as the primary beam, the lower absolute extraction voltage of secondary ion, and the lower electron beam voltage. Under this condition, it was found that the sodium was implanted deeper in the SiO₂ thin film than that in SiO₂ glass.     

Dynamic SIMS analysis of cryo-prepared biological and geological specimens

The modification of a dynamic magnetic sector secondary ion mass spectrometry (SIMS) instrument to permit the analysis of frozen biological and geological samples is described. The SIMS instrument used for this study combines SIMS analysis with the generation of ion-induced secondary electron images, allowing direct superposition of the SIMS image onto the image of cellular structures. Secondary ion maps have been acquired with sub-micron resolution, permitting the characterisation of sub-cellular elemental distributions in plant cells and human fibroblast cells, as well as the distribution of chemical impurities at grain boundaries in polar ice samples. This cryo-preparation technique clearly extends the applicability of SIMS analysis to a wide range of samples.     

High-time resolution radiographic technique for studying nanostructures with the use of synchrotron radiation

A high-time resolution radiographic technique for studying nanostructures using synchrotron radiation is described. A new modification of the methods, which makes it possible to perform large-scale X-ray diffraction analysis of structures of organic crystals, is proposed. X-ray diffraction patterns obtained in test experiments on a number of biological samples are presented.     

Characterization of oxidized CdS by SIMS/XPS

Although adsorption of simple gases is not detected on CdS by analytical means, oxidation is induced, for example, by exposure to a negative corona discharge [1] as well as by electron stimulation [2–4]. In the former case it was found that O attaches only to S, but in the latter case Cd is also oxidized. In this paper we investigate these processes by SIMS and XPS in an attempt to characterize and understand further these differences in behavior.     

Cluster projectile ions used for the SIMS analysis of silicon

Comparative studies of the emission of Si _n ⁺ (n = 1–11) cluster ions and impurity-containing polyatomic ions under bombardment of B-doped single crystal silicon with A _m ^? (m = 1–5) cluster ions with an energy of E ₀ = 6–18 keV are carried out. The peculiarities of sputtering an adsorbed-particle layer with cluster ions are revealed. The possibility of determining the depth distribution of adsorbed particles by analyzing the yield of sputtered heteroatomic molecular ions upon bombardment is demonstrated.