Digital Image Acquisition and Presentation for High Resolution SEM

Images obtained from an analog SEM are traditionally viewed and recorded from a cathode-ray tube (CRT). Many laboratories use instant film (e.g. Polaroid #52, #55 instant film) to justify image quality and obtain permanent image quickly. Digital imaging provides an alternative approach for image acquisition and recording. One major advantage of digital SEM is image averaging that allows one to improve the signal-to-noise ratio (SNR) from a noisy quick-scan image to reduce charging. SEM signal yield is proportional to incident beam intensity, image acquisition time or duration of beam interaction with specimen (dwell time). The higher beam intensity, or longer the dwell time, the more signal generated. However, for high-resolution SEM imaging, the beam dose and dwell time are limited by drafting, radiation damage, and contamination. Therefore high-resolution biological SEM images invariably have poor SNR.

Correcting spatial distortions and nonuniform response in area detectors

Software and hardware methods have been developed to correct images for spatial and intensity distortions produced by optical and electro-optical components in X-ray area detectors. Spatial distortions are divided into two types: gross distortions produced by the inherent properties of the detector components and local distortions formed by irregularities in the components. Intensity distortions are separated into three types: those caused by background nonuniformity; those resulting from pixel-dependent nonuniform intensity response; and those resulting from time-dependent variations in background and incident beam intensity. From background, flat-field, reference and mask images, `forward' and `reverse' interpolation tables are generated to correct for spatial distortions and a lookup table is generated to correct for nonuniform sensitivity. The routines have been used successfully on four different area detectors to correct entire images or to correct intensities of individual Bragg peaks. The spatial-distortion correction is good to within 0.1 pixels and the nonuniformity correction to ≲ 2%.

Plasmon Scattering Intensity in Small Aluminum Spheres

It has generally been assumed that a relative thickness determination in thin samples can be made by observing the variation of the bulk plasmon scattering intensity. This method, of course, relies on the assumption that the plasmon scattering varies with the thickness,1 Here Ip is the plasmon intensity, I0 is the incident beam intensity, λ is the total scattering mean free path, t is the sample thickness, and ɛ is the bulk dielectric constant for the material being probed. It can be shown1 that the unscattered beam intensity is just , and therefore a simple ratio of the plasmon intensity to the unscattered beam intensity would seem to give t, provided ʵ is known. It has been shown by Ritchie2, however, that for thin films two more terms occur in addition to that shown in (1). He found for the scattered intensity, where f and g arc some functions periodic in the thickness t. The third term here is the normal surface plasmon scattering. The second is a surface interference term at the bulk plasmon energy and is a manifestation of a spatial quantization of the bulk plasmon in the direction perpendicular to the film surfaces. (2) can be evaluated at the plasmon frequency ωp,

Energy Dispersive XRF Composition Profiling Using Crystal Collimated Incident Radiation

In order to measure changes in composition as a function of distance (macrosegregation) in directionally solidified two phase samples, a well collimated incident x-ray beam is required for XRF analysis. This is accomplished using Bragg diffraction of AgKα radiation from a highly perfect Si crystal. Because the incident beam is also monochromatic, additional advantages are realized: a) the backgrounds caused by Compton and thermal diffuse scattering (TDS) of the incident beam are well localized in the energy spectrum and do not interfere with the fluorescent peaks, b) the TDS can be used as a monitor of the incident photon flux and hence eliminates often substantial errors caused by incident beam intensity fluctuations.Using several prepared standards, the ratio of PbL counts to TDS counts was found to be a function of the total Pb content of the two phase microstructure, with a reproducibility determined only by counting statistics. Furthermore, the function was found to be nearly linear over a wide range of compositions. Standard methods of absorption or enhancement correction can be employed using this ratio. The spatial resolution, determined by profiling a sharp discontinuity between two metals, was 0.5 mm.Macrosegregation data is presented for Pb-Sn two phase alloys whose compositions range from 35 wt % Pb to 70wt % Pb. Comparison of compositions with those determined by a titration method agrees to within 2 wt % for most of the metallurgical structures present in the work. Somewhat larger deviations were found for samples with high Pb contents with extremely coarse two phase microstructures.

The Use of Diffraction Pattern Information In Stem

For each position of the incident beam in a STEM instrument, a convergentbeam diffraction pattern is produced in the detector plane. Except in the idealized, impracticable case of single isolated atom specimens, the intensity distribution in this diffraction pattern contains information regarding the relative positions of atoms within the volume element of the specimen illuminated by the beam. If this intensity distribution is recorded, for example by use of a two-dimensional detector array (Fig. 1) rather than a single detector which integrates the intensity, the additional data available should allow additional information to be derived concerning the specimen. When radiation damage is not a problem, this should allow an improvement in resolution. For radiation sensitive specimens, this should allow the incident beam intensity to be spread over a larger area so that radiation damage is reduced.

Electron Radiation Damage on thin Films of Phenylalanine

Consecutive observation of the mass loss and change in electron energy loss spectra are being carried on In a simple field emission scanning mlcroscope. The specimens are prepared by slow sublimation onto thin carbon films on a copper grid.For mass loss measurement, unscattered electrons Nun are counted, W'lich Is related to Incident beam Intensity N0 by Nun = N0 exp(-(ai+ae)t), where tIs a specimen thickness, ae, ai the Inverse of the mean free path for elastic and Inelastic scattering, respectively. With 20 KeV and 12 mrad aperture angle, most of the elastically scattered electrons are stopped by the aperture. The Inelastically scattered electrons arefll tered by the spectrometer.