Quantitative X‐ray projection microscopy: phase‐contrast and multi‐spectral imaging

Abstract

SummaryWe outline a new approach to X-ray projection microscopy in a scanning electron microscope (SEM), which exploits phase contrast to boost the quality and information content of images. These developments have been made possible by the combination of a high-brightness field-emission gun (FEG)-based SEM, direct detection CCD technology and new phase retrieval algorithms. Using this approach we have been able to obtain spatial resolution of < 0.2 µ m and have demonstrated novel features such as: (i) phase-contrast enhanced visibility of high spatial frequency image features (e.g. edges and boundaries) over a wide energy range; (ii) energy-resolved imaging to simultaneously produce multiple quasi-monochromatic images using broad-band polychromatic illumination; (iii) easy implementation of microtomography; (iv) rapid and robust phase/amplitude-retrieval algorithms to enable new real-time and quantitative modes of microscopic imaging. These algorithms can also be applied successfully to recover object-plane information from intermediate-field images, unlocking the potentially greater contrast and resolution of the intermediatefield regime. Widespread applications are envisaged for fields such as materials science, biological and biomedical research and microelectronics device inspection. Some illustrative examples are presented. The quantitative methods described here are also very relevant to projection microscopy using other sources of radiation, such as visible light and electrons.

Low-dose three-dimensional hard x-ray imaging of bacterial cells

“…To record projection propagation images in full field mode the sample (for an optical micrograph of the cells, see Figure 1(a)) was placed at a (defocus) distance z 1 = 8 mm from the WG exit plane, where the divergent WG beam has broadened to a field of view (FOV) of 40 × 40 μm 2 , as calculated from the measured far-field divergence angle of 5 mrad. As shown previously, the imaging experiment can then be described in a well-known equivalent parallel-beam geometry (Mayo et al 2002;Fuhse et al 2006) with a demagnified (effective) detector pixel size of D /M and a (de)magnification factor of M = (z 1 + z 2 )/z 1 as well as an effective sample-detector distance z eff = z 1 z 2 /(z 1 + z 2 ) = z 2 /M. Note that here z 2 z 1 , so that z eff z 1 = 8 mm and M z 2 /z 1 = 660, resulting in an effective detector pixel size of 83 nm.…”

A study on new approaches in coherent x-ray microscopy of biological specimens

“…It can be shown [96,110] that, to first order inφ andã, the 2D Fourier transform of the propagated intensity, I (r ⊥ ; z) = |ψ(r ⊥ ; z)| 2 , is given as…”

Dose efficient Compton X-ray microscopy

“…The brilliance provided by thirdgeneration synchrotron light sources [5] that enabled soft X-ray microscopy also facilitated the development of coherent X-ray imaging techniques at photon energies beyond 10 keV, such as Zernike phase-contrast microscopy [6], coherent diffractive imaging [7], ptychography [8,9], and projection microscopy [10]. Lensless techniques of ptychography and diffractive imaging are able to retrieve three-dimensional images with the desired resolutions of several nanometers by using spatially coherent hard X-ray beams with high penetration power and without limitations of depth of focus [11,12].…”