Biological serial block face scanning electron microscopy at improved z-resolution based on Monte Carlo model.

Serial block-face electron microscopy (SBEM) provides nanoscale 3D ultrastructure of embedded and stained cells and tissues in volumes of up to 107 µm3. In SBEM, electrons with 1-3 keV energies are incident on a specimen block, from which backscattered electron (BSE) images are collected with x, y resolution of 5-10 nm in the block-face plane, and successive layers are removed by an in situ ultramicrotome. Spatial resolution along the z-direction, however, is limited to around 25 nm by the minimum cutting thickness. To improve the z-resolution, we have extracted depth information from BSE images acquired at dual primary beam energies, using Monte Carlo simulations of electron scattering. The relationship between depth of stain and ratio of dual-energy BSE intensities enables us to determine 3D structure with a ×2 improvement in z-resolution. We demonstrate the technique by sub-slice imaging of hepatocyte membranes in liver tissue.

Monte Carlo modeling of ion beam induced secondary electrons.

Ion induced secondary electrons (iSE) can produce high-resolution images ranging from a few eV to 100keV over a wide range of materials. The interpretation of such images requires knowledge of the secondary electron yields (iSE δ) for each of the elements and materials present and as a function of the incident beam energy. Experimental data for helium ions are currently limited to 40 elements and six compounds while other ions are not well represented. To overcome this limitation, we propose a simple procedure based on the comprehensive work of Berger et al. Here we show that between the energy range of 10-100keV the Berger et al. data for elements and compounds can be accurately represented by a single universal curve. The agreement between the limited experimental data that is available and the predictive model is good, and has been found to provide reliable yield data for a wide range of elements and compounds.

Characterization of hydrogen responsive nanoporous palladium films synthesized via a spontaneous galvanic displacement reaction.

A model is presented regarding the mechanistic properties associated with the interaction of hydrogen with nanoporous palladium (np-Pd) films prepared using a spontaneous galvanic displacement reaction (SGDR), which involves PdCl(2) reduction by atomic Ag. Characterization of these films shows both chemical and morphological factors, which influence the performance characteristics of np-Pd microcantilever (MC) nanomechanical sensing devices. Raman spectroscopy, uniquely complemented with MC response profiles, is used to explore the chemical influence of palladium oxide (PdO). These combined techniques support a reaction mechanism that provides for rapid response to H(2) and recovery in the presence of O(2). Post-SGDR processing via reduction of PdCl(2)(s) in a H(2) environment results in a segregated nanoparticle three-dimensional matrix dispersed in a silver layer. The porous nature of the reduced material is shown by high resolution scanning electron microscopy. Extended grain boundaries, typical of these materials, result in a greater surface area conducive to fast sorption/desorption of hydrogen, encouraged by the presence of PdO. X-ray diffraction and inductively coupled plasma-optical emission spectroscopy are employed to study changes in morphology and chemistry occurring in these nanoporous films under different processing conditions. The unique nature of chemical/morphological effects, as demonstrated by the above characterization methods, provides evidence in support of observed nanomechanical response/recovery profiles offering insight for catalysis, H(2) storage and improved sensing applications.

Diverse and conserved nano- and mesoscale structures of diatom silica revealed by atomic force microscopy.

An outstanding example of biological pattern formation at the single cell level is the diversity of biomineral structures in the silica cell walls of the unicellular eukaryotic algae known as diatoms. We present a survey of cell wall silica structures of 16 diatom species, which included all major cell wall components (valves, girdle bands and setae), imaged across the nano-, meso- and microscales using atomic force microscopy. Because of atomic force microscopy's superior ability to image surface topology, this approach enabled visualization of the organization of possible underlying organic molecules involved in mineral structure formation. Diatom nanoscale silica structure varied greatly comparing the same feature in different species and different features within a single species, and frequently on different faces of the same object. These data indicate that there is not a strict relation between nanoscale silica morphology and the type of structure that contains it. On the mesoscale, there was a preponderance of linear structures regardless of the object imaged, suggesting that assembly or organization of linear organic molecules or subcellular assemblies that confine a linear space play an essential and conserved role in structure formation on that scale. Microscale structure imparted an overall influence over nano- and mesoscale structure, indicating that shaping of the silica deposition vesicle plays a key role in structure formation. These results provide insights into the design and assembly principles involved in diatom silica structure formation, facilitating an understanding of the native system and potentially aiding in development of biomimetic approaches.

Scanning electron microscope imaging in liquids - some data on electron interactions in water.

The electron backscattering coefficient of liquid water has been determined for electrons in the energy range 15-30 keV using Quantomix capsules. Values of the mean atomic number for water estimated from a fit to the backscatter yield, the mean ionization potential of water and from Monte Carlo simulations, show that the scattering behaviour of water is not anomalous despite the effects of hydrogen bonding. Computations of the electron range, and of the mean depth for backscattering, in water as a function of incident beam energy show that water and vitreous ice are good media for imaging purposes.

Experimental secondary electron spectra under SEM conditions.

Secondary electron spectra have been collected from both pure elements and from compounds examined under conditions approximating those found in a scanning electron microscope. Despite the presence of substantial surface contamination these spectra are found to be reproducible and characteristic of the underlying material. Typically the peak in such spectra is found to be at an energy of about 5 eV, and 50% of the total secondary electron emission falls within the range 0-12 eV. These data may be of value for the design of detectors for scanning microscopy and might have applications for microanalysis.

Experimental resolution measurement in critical dimension scanning electron microscope metrology.

By applying the basic principles of metrology we discuss how to define the standards that any experimental method to measure resolution has to obey. Our results clearly indicate the need to apply a calibration procedure when designing algorithms to estimate resolution to satisfy accuracy requirements. Similarly, the precision of an algorithm has to be clearly specified. We compare here the performances of a variety of commonly used implementations of published methods, with that of an algorithm based on an approach known to be reliable. Our results confirm that when an algorithm is designed with the clear intent of satisfying metrology requirements, it demonstrates excellent accuracy, precision, and lack of sensitivity to the noise level, as is desirable. As a consequence, the algorithm will have the ability to measure accurately the point spread function convoluted in the image, thus paving the way for quantitative deconvolution techniques.

Measurements of absolute x-ray generation efficiency for selected K, L, and M-lines.

The absolute efficiency of generation of a selection of K, L, and M- x-ray lines has been measured as a function of the incident electron beam energy. At an overvoltage U=2 this efficiency falls within the range 1E-4 to 1E-7, with K-lines being highest and L-Lines usually being the lowest. It is shown that for all three families of lines the efficiency has a functional variation which has the form A. (U- 1)n, as first suggested by Compton and Allison, where A and n are constants. Values of A and n for the K, L, and M shells are tabulated. The smoothly varying behavior of the efficiency makes it well suited for analytical use and spectrum simulation purposes.

SMART--a program to measure SEM resolution and imaging performance.

It is important to be able to measure the parameters, such as spatial resolution, astigmastism, signal-to-noise ratio, and drift and instability, that characterize the performance of a scanning electron microscope. These quantities can be determined most reliably by a Fourier analysis of digital micrographs from the instrument, recorded under conditions of interest. A program designed to implement all of the necessary steps in an automated manner has been developed as a 'macro' for the popular, and freely available, NIH Image and SCION Image programs.

Microanalysis using secondary electrons in scanning electron microscopy.

A recent study of secondary electron (SE) spectra in an Auger spectrometer demonstrated unique features indicative of the chemical nature of the tested material. The scanning electron microscope (SEM) naturally generates SEs; therefore, in this paper, we combine the concept of using differential voltage contrast (DVC) with SE spectroscopy to identify the chemical nature of a material. It is demonstrated that this method reveals the uniqueness of electron energy distribution in the conduction band of a solid or, what is the same, the uniqueness of a build-up of the outer electron shell system, and avoids errors due to the changes in the angular distribution or yield of the SE in the SEM. A theory of this new microanalytical method is developed. The experimental limitations of the SEM for this type of study are examined as well.

A study of electron beam-induced conductivity in resists.

The charging of polymeric resist materials during electron beam irradiation leads to significant problems during imaging and lithography processes. Charging occurs because of charge deposition in the polymer and charge generation/trapping due to formation of electron-hole pairs in the dielectric. The presence of such charge also results in the phenomena of electron beam-induced conductivity (EBIC). Electron beam-induced conductivity data have been obtained for three commercial e-beam resists under a variety of dose rate and temperature conditions. From the observed values of induced conductivity under varying conditions significant information about the generation of electron-hole pair and the transport of charge in the resist can be obtained. Three electron beam resists, EBR900, ZEP7000, and PBS are examined by an external bias method. The difference in resist chemistry is considered to play the role in the initial state EBIC behaviors among three resists even though the way that it affects the behaviors is not clear. A comparison of the power consumption comparison is proposed as a measure to give a preliminary estimate of the carrier concentration and carrier drift velocity differences among the resists. A simple single trap model with constant activation energy is proposed and provides good agreement with experiment.

The efficiency of X-ray production at low energies.

The absolute efficiency of X-ray production has been determined for the K-lines of Al, Si and Cu; for the L-lines of Fe, Co, Cu, Ge and As; and for the M-lines of Hf, Ir, Pt, Au and Bi, using overvoltage ratios in the range 1-10. These emissions, with the exception of the Cu K, have critical excitation energies below 2.6 keV and are therefore typical of the lines used for X-ray microanalysis at low beam energies. For K-lines it is found that the experimental results are in good agreement with a Bethe, or a Casnati, crosssection model. For the L- and M-lines a Monte Carlo technique has been used to find an effective X-ray generation cross-section for each of the elements. The functional forms of these cross-sections are found to be in general agreement with proposed theoretical models.

Effects of molecular entanglements during electrospray of high molecular weight polymers.

Concentration-dependent bimodal size distributions (comprised of single-molecule particles and multimolecule clusters) observed by microscopic examination of particles collected during electrospray (ES) of dilute solutions of high molecular weight polymers suggest that chain entanglement can interfere with the droplet subdivisions believed to be intrinsic to the electrospray process. The feasibility of such interference is discussed in the context of the spray model of Kebarle, along with its potential impact on the ES mass spectrometry of macromolecules.

Imaging thin and thick sections of biological tissue with the secondary electron detector in a field-emission scanning electron microscope.

A field-emission scanning electron microscope (FESEM) equipped with the standard secondary electron (SE) detector was used to image thin (70-90 nm) and thick (1-3 microns) sections of biological materials that were chemically fixed, dehydrated, and embedded in resin. The preparation procedures, as well as subsequent staining of the sections, were identical to those commonly used to prepare thin sections of biological material for observation with the transmission electron microscope (TEM). The results suggested that the heavy metals, namely, osmium, uranium, and lead, that were used for postfixation and staining of the tissue provided an adequate SE signal that enabled imaging of the cells and organelles present in the sections. The FESEM was also used to image sections of tissues that were selectively stained using cytochemical and immunocytochemical techniques. Furthermore, thick sections could also be imaged in the SE mode. Stereo pairs of thick sections were easily recorded and provided images that approached those normally associated with high-voltage TEM.

Measuring the performance of scanning electron microscope detectors.

A simple digital method of measuring the performance of detectors in the scanning electron microscope (SEM) is described. The value derived is absolute and can therefore be used to compare both different detectors on the same instrument as well as different detectors on different instruments. The technique can be applied to secondary electron, backscattered electron, and energy-dispersive x-ray detectors. Examples are given of measurements made on a variety of commercial detectors installed on a number of current SEMs.

High-resolution scanning electron microscopy.

The spatial resolution of the scanning electron microscope is limited by at least three factors: the diameter of the electron probe, the size and shape of the beam/specimen interaction volume with the solid for the mode of imaging employed and the Poisson statistics of the detected signal. Any practical consideration of the high-resolution performance of the SEM must therefore also involve a knowledge of the contrast available from the signal producing the image and the radiation sensitivity of the specimen. With state-of-the-art electron optics, resolutions of the order of 1 nm are now possible. The optimum conditions for achieving such performance with the minimum radiation damage to the specimen correspond to beam energies in the range 1-3 keV. Progress beyond this level may be restricted by the delocalization of SE production and ultimate limits to electron-optical performance.

High resolution SE-I SEM study of enamel crystal morphology.

Until recently high resolution TEM was the only imaging mode capable of probing the atomic lattice structure of crystals composing tooth enamel. Studies designed to determine the polyhedral shape of normal enamel crystals and initiation of carious lesions in enamel crystals were hampered and limited by interpretation of two-dimensional TEM images from thin section and freeze fracture replica specimens lacking depth of field. The newly developed SE-I signal mode for SEM (SE-I/SE-II ratio) can produce images of enamel crystals approaching beam diameter dimensions (0.7-2.0 nm), rivaling the resolution of the TEM technique and generating topographic contrasts for three dimensional imaging at very high magnification (approximately 1,000,000X). Ultrathin chromium (Cr) films generate enriched high resolution SE-I contrasts of enamel crystal surfaces and when imaged using an immersion lens field emission SEM operated at high voltage (20-30 KeV) produce unsurpassed topographic contrasts. Since the grain size of Cr is below the resolution of any SEM and is ultrathin (approximately 1 nm), then SE-I images can provide a more accurate representation of enamel crystal structure than TEM methodologies. Our SE-I SEM observations of normal human enamel crystals reveal fractured spicules which contain angled flat surfaces delineated by a prominent 2 nm wide SE-I edge brightness contrast. Although microscopic observations often show crystals which are hexagonal in cross-section, in both SEM and TEM many other growth habits, including rectangular or irregular crystals (30-40 nm in width) which contain "notches," are also observed. More detailed morphological studies are therefore required to determine the most likely habit planes and their relevance to the function of the enamel crystals.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical and instrumental analysis of ferrites.

More than thirty years since the manufacture of the first commercial ferrites, research and development efforts continue to produce ferrites with enhanced performance and new applications. Analytical chemistry has maintained a substantial role in the ferrite industry in the characterization of both raw materials and products, and the analytical literature of ferrites has grown accordingly. The continuing importance of ferrites to the electronic device industry requires further development of analytical methods suitable for characterization of ferrites so that their chemical composition may be related to performance and to the manufacturing processes used. As modem analytical techniques have been developed, their application to the characterization of ferrites and the detection of heterogeneity in these materials is increasing.