Spatial resolution, X-ray

Composition Profile Measurement. Results of Zieba et al. (1997) will be given as an example of the measurement of solute distribution in an alloy undergoing a phase transformation. They studied discontinuous precipitation in cobalt-tungsten alloys, in which a Co-32 wt% W alloy was aged in the temperature range 875 K to 1025 K, and high spatial resolution X-ray microanalysis of thin foils by STEM was used to measure the solute distribution near the reaction front. 

The valence states of metal cations in such materials can certainly be determined chemically using the redox titration, but it is inapplicable to nanophase or nanostructured materials, such as thin films. The wet chemistry approaches usually do not provide any spatial resolution. X-ray photoelectron spectroscopy (XPS) can provide information on the average distribution of cation valences for nanostructured materials with certain spatial resolution, but the spatial resolution is nowhere near the desired nanometer scale, and the information provided is limited to a surface layer of 2 to 5 nm in thickness. 

Boone MN, Garrevoet J, Tack P et al (2014) High spectral and spatial resolution X-ray transmission radiography and tomography using a color X-ray camera. Nucl Instmm Methods Phys Res A 735 644-648. doi 10.1016/j.nima.2013.10.044... 

The single crystal sample must obviously be of a size and quality suitable for high resolution X-ray diffraction. Hence, a structural reaction must be initiated uniformly and promptly through the whole of the crystal. Also, in such a process the crystal must not be damaged. It is essential that spatial uniformity be achieved in less time than the lifetime of all subsequent molecular intermediates whose imaging is required. 

Fundamental understanding of structure-function relationships is central for the design of improved selox catalysts, and has been greatly assisted by the development of new analytical tools with which to probe active sites at subnanometer spatial resolution [36] and subsecond time resolution. X-ray-based methods in particular can provide detailed insight into chemical composition and environment of active components and reacting adsorbates [140-142]. Quick and dispersive XAS have the capabihty to monitor dynamic changes in catalyst structure under reaction conditions (so-caUed operando spectroscopy) and have been applied to alcohol selox over Pd [96, 143-146], Pt [67, 147], and Ru [147] nanoparticles. 

The sensitivity of the luminescence IP s in the systems employed here decreases with increasing x-ray energy more strongly than in the case of x-ray film. Therefore, this phenomenon must be compensated by using thicker lead front and back screens. The specific contrast c,p [1,3] is an appropriate parameter for a comparison between IP s and film, since it may be measured independently of the spatial resolution. Since the absorption coefficient p remains roughly constant for constant tube voltage and the same material, it suffices to measure and compare the scatter ratio k. Fig. 2 shows k as a function of the front and back screen thickness for the IP s for 400 keV and different wall thicknesses. The corresponding measured scatter ratios for x-ray films with 0,1 mm front and back screens of lead are likewise shown. The equivalent value for the front and back screen thicknesses is found from the intersection of the curves for the IP s and the film value. 

Recently commercially available X-ray systems for laminography have a spatial resolution limited to hundred microns, which is not enough for modem multilayer electronic devices and assembles. Modem PCBs, flip-chips, BGA-connections etc. can contain contacts and soldering points of 10 to 20 microns. The classical approach for industrial laminography in electronic applications is shown in Fig.2. 

This approach is more close to X-ray stereo imaging and caimot reach enough depth resolution. There are also several systems with linear movement (1-dimensional) through the conical beam [5] as shown in Fig.4. In this case usable depth and spatial resolution can be achieved for specifically oriented parts of the object only. 

Laminographical approaches can be used for layer-by-layer visualization of the internal microstructure for the flat objects (multilayers, PCBs etc.), that caimot be reconstructed by computerized tomography because of the limited possibilities in rotation. Depth and lateral spatial resolutions are limited by the tube, camera and rotation accuracy. Microfocus X-ray tubes and digital registration techniques with static cameras allow improving resolution. Precision object manipulations and more effective distortion corrections can do further improvement. 

A much better way would be to use phase contrast, rather than attenuation contrast, since the phase change, due to changes in index of refraction, can be up to 1000 times larger than the change in amplitude. However, phase contrast techniques require the disposal of monochromatic X-ray sources, such as synchrotrons, combined with special optics, such as double crystal monochromatics and interferometers [2]. Recently [3] it has been shown that one can also obtain phase contrast by using a polychromatic X-ray source provided the source size and detector resolution are small enough to maintain sufficient spatial coherence. 

One of the more recent advances in XPS is the development of photoelectron microscopy [ ]. By either focusing the incident x-ray beam, or by using electrostatic lenses to image a small spot on the sample, spatially-resolved XPS has become feasible. The limits to the spatial resolution are currently of the order of 1 pm, but are expected to improve. This teclmique has many teclmological applications. For example, the chemical makeup of micromechanical and microelectronic devices can be monitored on the scale of the device dimensions. 

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