Ian McNulty, Sean Frigo, Cornelia Retsch, Yuxin Wang, Yipeng Feng, Yonglin Qian, Emil Trakhtenberg, Brian Tieman, B.-C. Cha, K. Goetze, Timothy Mooney, Waleed Haddad
We have constructed a high resolution scanning x-ray microscopy at the 2-ID-B beamline at the Advanced Photon Source for 1-4 keV x-ray imaging and microspectroscopy experiments. The microscope uses a Fresnel zone plate to focus coherent x-ray undulator radiation to a 150 nm focal spot on a sample. The spectral flux in the focus is 108 ph/s/0.1 percent BW. X- ray photons transmitted by the sample are detected by an avalanche photodiode as the sample is scanned to form an absorption image. The sample stage has both coarse and fine translation axes for raster scanning and a rotation axis for microtomography experiments. The incident x- ray beam energy can also be scanned and a rotation axis for microtomography experiments. The incident x-ray beam energy can also be scanned via the 2-ID-B monochromator while the sample is kept in focus to record spatially resolved absorption spectra. We have measured the performance of the instrument with various test objects. THe microscope hardware, software, and performance are discussed in this paper.
The use of soft x-ray nanotomography techniques for the evaluation and failure mode analysis of microchips was investigated. Realistic numerical simulations of the imaging process were performed and a specialized approach to image reconstruction from limited projection data was devised. Prior knowledge of the structure and its component materials was used to eliminate artifacts in the reconstructed images so that defects and deviations from the original design could be visualized. Simulated data sets wee generated with a total of 21 projections over three different angular ranges: -50 to +50, -80 to +80 and -90 to +90 degrees. In addition, a low level of illumination was assumed. It was shown that sub-micron defects within one cell of a microchip could be imaged in 3D using such an approach.
An x-ray tomography system is being developed for high resolution inspection of large objects. The goal is to achieve 25 micron resolution over object sizes that are tens of centimeters in extent. Typical objects will be metal in composition and therefore high energy, few MeV x-rays will be required. A proof-of-principle system with a limited field of view has been developed. Preliminary results are presented.
Ultra high resolution three-dimensional images of a microscopic test object were made with soft x rays using a scanning transmission x-ray microscope. The test object consisted of two different patterns of gold bars on silicon nitride windows that were separated by approximately 5 micrometer. A series of nine 2-D images of the object were recorded at angles between -5 to +55 degrees with respect to the beam axis. The projections were then combined tomographically to form a 3-D image by means of an algebraic reconstruction technique (ART) algorithm. A transverse resolution of approximately 1000 angstrom was observed. Artifacts in the reconstruction limited the overall depth resolution to approximately 6000 angstrom, however some features were clearly reconstructed with a depth resolution of approximately 1000 angstrom. A specially modified ART algorithm and a constrained conjugate gradient (CCG) code were also developed as improvements over the standard ART algorithm. Both of these methods made significant improvements in the overall depth resolution, bringing it down to approximately 1200 angstrom overall. Preliminary projection data sets were also recorded with both dry and re-hydrated human sperm cells over a similar angular range.
X-ray microtomography enables three-dimensional imaging at sub-micron resolution with elemental and chemical state contrast. The 1 - 4 KeV energy region is promising for microtomography of biological, microelectronics, and materials sciences specimens. To capitalize on this potential, we are constructing a tomographic scanning x-ray microscope for 1 - 4 KeV x rays on a spherical grating monochromator beamline at the advanced photon source. The microscope, which uses zone plate optics, has an anticipated spatial resolution of 100 nm and an energy resolution of better than 1 eV.
The quality of images reconstructed from projections obtained by transmission tomography depends on the range of angles over which measurements can be made as well as the number of projections. Conventional methods such as filtered backprojection suffer when the number of measurements is small, and methods such as ART produce noticeable artifacts when the angular range is limited. Another possible approach is the direct minimization of the squared error between the measurements and the projection of the reconstructed image onto the measurement space. Alternatively, the unfiltered backprojection of the data can be modeled as a linear blur of the desired image, and this blur can be removed with a deconvolution algorithm. One way to handle the latter approach is to minimize the squared error between the backprojection and the reconstructed image blurred by an appropriately chosen point spread function. These methods result in higher quality images when the angular range is limited and the number of projections is small. We use a conjugate gradient based constrained optimization algorithm to do the minimization. The available constraints on the variables are upper and lower bounds and a hyperplane constraint. Since the variables in this case are the image pixels, we can enforce known bounds on the pixel values, such as nonnegativity, as well as keep the sum of the pixels at its known value. These constraints greatly improve the reconstruction quality and increase the rate of convergence of the algorithm.
A camera system suitable for microholography has been constructed, tested, and applied to the imaging of biological materials. The design of this instrument is compatible with operation over a very wide spectral range spanning from visible to x-ray wavelengths. In order to evaluate its properties, visible light Fourier transform microholograms of biological samples and other test targets have been recorded and digitally reconstructed using a glycerol microdrop as a reference wave scatterer. Current results give a resolution of approximately 4 (lambda) with (lambda) equals 514.5 nm.
We review the technical advantages offered by x-ray holographic microscopy for imaging
the structure of living biological specimens. We discuss the wavelength, coherence, energy,
and pulse-length requirements and conclude that these could be met by free-electron laser
architectures of the near future. We also show that Fourier-transform holography using a
reference scattering sphere is the best optical configuration for a practical instrument.
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