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A stepped-frequency continuous-wave radar system has been designed and integrated into a drill head for use on midrange horizontal directional drills. Data from the forward-looking radar are acquired, processed, and displayed in near real time. A unique, computationally efficient target detection algorithm has been developed and tested to predict the position of potential obstacles in front of the drill head. This method is called derivative imaging, and it provides distance to target and radial offset information such that the drill operator may respond accordingly. Application to traditional ground-penetrating radar systems is also demonstrated.

We describe a holographic microscope with a spatial resolution approaching the diffraction limit. The instrument uses a tiny drop of glycerol as a lens to create the spherically diverging reference illumination necessary for Fourier-transform holography. Measurement of the point-spread function, which is obtained by imaging a knife edge in dark-field illumination, indicates a transverse resolution of 1.4 microm with wavelength lambda = 514.5 nm. Longitudinal resolution is obtained from the holograms by the numerical equivalent of optical sectioning. We describe the method of reconstruction and demonstrate the microscope's capability with selected biological specimens. The instrument offers two unique capabilities: (1) it can collect three-dimensional information in a single pulse of light, avoiding specimen damage and bleaching; and (2) it can record three-dimensional motion pictures from a series of light pulses. The conceptual design is applicable to a broad range of wavelengths and we discuss extension to the x-ray regime.

Scanning soft x‐ray microtomography was used to obtain high‐resolution three‐dimensional images of a microfabricated test object. Using a special rotation stage mounted on the scanning transmission x‐ray microscope at the X1A beamline at the National Synchrotron Light Source, we recorded nine two‐dimensional projections of the 3D test object over an angular range of −50° to +55°. The x‐ray wavelength was 3.6 nm and the radiation dose to the object per projection was approximately 2×106 Gy. The object consisted of two gold patterns supported on transparent silicon nitride membranes, separated by 4.75 μm, with 100‐ to 300‐nm‐wide and 65‐nm‐thick features. We reconstructed a volumetric data set of the test object from the two‐dimensional projections using an algebraic reconstruction technique algorithm. Features of the test object were resolved to ∼100 nm in transverse and longitudinal extent with low artifact in three‐dimensional images rendered from the volumetric set.

A high-resolution table-sized x-ray nanotomography (XRMT) tool has been constructed that shows the promise of nondestructively imaging the internal structure of a full IC stack with a spatial resolution better than 100 nm. Such a tool can be used to detect, localize, and characterize buried defects in the IC. By collecting a set of X-ray projections through the full IC (which may include tens of micrometers of silicon substrate and several layers of Cu interconnects) and applying tomographic reconstruction algorithms to these projections, a 3D volumetric reconstruction can be obtained, and analyzed for defects using 3D visualization software. XRMT is a powerful technique that will find use in failure analysis and IC process development, and may facilitate or supplant investigations using SEM, TEM, and FIB tools, which generally require destructive sample preparation and a vacuum environment.

We have constructed a high resolution scanning x-ray microscope 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 10{sup 8} ph/s/0.1% 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 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.

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.

The use of soft x-ray (about 1.8 KeV) 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 were 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 (< 10 pm3) could be imaged in 3-D using such an approach.

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{mu}m. A series of nine 2-D images of the object were recorded at angles between {minus}50 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 submicron 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 1-4KeY x-rays on a spherical grating monochromator beamline at the Advanced Photon Source. The microscope, which useszone plate optics, has an anticipated spatial resolution of 100 nm and an energy resolution of better than 1 eV.Keywords: x-ray microscopy, tomography, three-dimensional imaging, microprobe, x-ray optics, zone plates, beamlines 1. INTRODUCTION X-ray microtomography is a powerful technique for high-resolution three-dimensional imaging1'2 that offers elemental and chemical state contrast3 at greater resolution than that obtainable with visible light, and less radiation damage than electron probes.4 Applications of this method include 3D microscopy of biological, microelectronics, and materials sciencesspecimens. Use of 1-4 KeV x-rays for xnicrotomography is especially attractive due to the number of K, L, and M absorptionedges in the middle of the periodic table that are accessible, the capability to match the penetration depth to many interestingand industrially important samples, and possibilities for low-dose phase contrast imaging of biological specimens.5 Third-generation synchrotron facilities, such as the Advanced Photon Source, will open up new opportunities for x-ray microscopyin this energy region that, so far, have been relatively unexplored.

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 which were separated by ∼ 5μm. Depth resolution comparable to the transverse resolution was achieved by recording nine 2‐D images of the object at angles between −50 to +55 degrees with respect to the beam axis. The projections were then combined tomographically to form a 3‐D image using an algebraic reconstruction technique (ART) algorithm. We observed a transverse resolution of ∼1000 A. Artifacts in the reconstruction limited the overall depth resolution to ∼6000 A, however some features were clearly reconstructed with a depth resolution of ∼1000 A.

Ultrahigh-resolution three-dimensional images of a microscopic test object were made with soft x-rays collected with 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 micrometers. Depth resolution comparable to the transverse resolution was achieved by recording nine two-dimensional images of the object at angles between -50 and +55 degrees with respect to the beam axis. The projections were then combined tomographically to form a three-dimensional image by means of an algorithm using an algebraic reconstruction technique. A transverse resolution of approximately 1000 angstroms was observed. Artifacts in the reconstruction limited the overall depth resolution to approximately 6000 angstroms; however, some features were clearly reconstructed with a depth resolution of approximately 1000 angstroms.

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.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.