Dan G. Legnini

NANOMETER FOCUSING OF HARD X RAYS BY PHASE ZONE PLATES

Focusing of 8 keV x rays to a spot size of 150 and 90 nm full width at half maximum have been demonstrated at the first- and third-order foci, respectively, of a phase zone plate (PZP). The PZP has a numerical aperture of 1.5 mrad and focusing efficiency of 13% for 8 keV x rays. A flux density gain of 121u200a000 was obtained at the first-order focus. In this article, the fabrication of the PZP and its experimental characterization are presented and some special applications are discussed.

Hard x-ray phase zone plate fabricated by lithographic techniques

A Fresnel phase zone plate with an unprecedented focusing efficiency of 33% was fabricated using an x‐ray lithographic technique and was tested using synchrotron x rays. Contributions by the zeroth‐order x ray to the focus were minimal. Spatial resolution in the micrometer range was achieved. The measured spot size was dominated by geometric demagnification of the source. It should be possible to obtain submicrometer resolution by aperturing the source. Experimental results of focusing efficiency measurements, intensity distribution at the focal plane, and spatial resolution tests are reported.

DEVELOPMENT OF A HARD X-RAY IMAGING MICROSCOPE

A hard x‐ray imaging microscope based on a phase zone plate has been developed and tested. The zone plate, with a 5 cm focal length and a 0.2 μm smallest linewidth, was used to image 8 keV x rays from the samples. The imaging microscope can be used to obtain nearly diffraction‐limited resolution over the entire imaging field, and its resolution is almost independent of source size and source motions. We have tested such an imaging microscope, and a resolution of about 0.4 μm was obtained. The images were obtained with an exposure time of less than 1 min, for a magnification factor of 30 in the x rays. The x rays were then converted into visible light, and another 7 times magnification were obtained by using a lens system coupled to a charge coupled device camera. The results from the imaging microscope, and possible applications, will be discussed.

Synchrotron x-ray microdiffraction diagnostics of multilayer optoelectronic devices

Synchrotron-based x-ray microbeam techniques have been used to map crystallographic strain and multilayer thickness in micro-optoelectronic devices produced with the selective area growth technique. Our main results show that growth enhancements in InGaAsP multilayer device material are different for well and barrier material. Comparison with a vapor-phase model for selective area growth suggests that this difference is due to different vapor-phase incorporation rates for the group III metals.

Performance comparison of hard x-ray plates fabricated by two different methods

Two types of fabrication methods have been developed to fabricate Fresnel zone plates for focusing x-rays in the 5 - 25 keV energy region. These two fabrication methods are discussed in terms of spatial resolution and focusing efficiency, which are two important parameters that characterize the performance of a Fresnel zone plate. Experimental characterization of the zone plates fabricated by the two methods are described and the results are discussed.

Hard x-ray microimaging techniques based on phase zone plates

Phase zone plates of high focusing efficiency and submicron resolution have been demonstrated in the hard x-ray region. A scanning microscope based on these focusing optics will create many new applications. Preliminary results in the applications of the microscope are reported here. In the area of imaging, we have utilized absorption contrast to clearly identify the locations of Au and Ni constituents in a sample of two interleaved grids. Micro- EXAFS spectra has also been obtained on a Ni foil. Fluorescence from a nuclear fuel sample, as an example of microanalysis, has revealed the elemental distribution at the interfaces. Lastly, microdiffraction from AgBr crystallites has been studied.

Using x-ray microprobes for environmental research

Understanding the fate of environmental contaminants is of fundamental importance in the development and evaluation of effective remediation strategies. Among the factors influencing the transport of these contaminants are the chemical speciation of the sample and the chemical and physical attributes of the surrounding medium. Characterization of the spatial distribution and chemical speciation at micron and submicron resolution is essential for studying the microscopic physical, geological, chemical, and biological interfaces that play a crucial role in determining contaminant fate and mobility. Hard x-ray spectroscopy and imaging are powerful techniques for the element-specific investigation of complex environmental samples at the needed micron and submicron resolution. An important advantage of these techniques result from the large penetration depth of hard x-rays in water. This minimizes the requirements for sample preparation and allows the detailed study of hydrated samples. This paper discuses some current problems in environmental science that can be addressed by using synchrotron- based x-ray imaging and spectroscopy. These concepts are illustrated by the results of recent x-ray microscopy studies at the Advanced Photon Source.

Advanced zone plate microfocusing optics

Fresnel zone plates (ZP) have gained popularity as the optics of choice for advanced microfocusing applications. The main virtues of ZP are high resolution, high efficieny, low background, coherence preservation, and ample working distance. Zone plates are also unique because they are a normal incidence x-ray optics, which are much easier to align and use compared to other grazing incidence optics. We will report here recent progress that has drastically enhanced the performance of ZPs in 1) higher spatial resolution, 2) higher focusing efficiency, and 3) extension to higher energies. With the new developments, zone plates have proven to be one of the best microfocusing optics for monochromatic x-ray beams.

Beam size measurement of the stored electron beam at the APS storage ring using zone plate optics and undulator radiation

Beam sizes of the stored electron beam at the Advanced Photon Source storage ring were measured using zone-plate optics and undulator radiation. A gold Fresnel zone plate (3.5 {micro}m thick) located 33.9 meters from the x-ray source focused radiation of 18 keV, selected by a cryogenically cooled Si(111) crystal in horizontal deflection, and formed a source image in a transverse plane 2.41 m downstream. The sizes of the source image were determined from measured intensity profiles of x-ray fluorescence from a smooth nickel edge (1.5 {micro}m thick), fabricated using a lithographic technique, while the nickel edge was scanned across over the beam in the transverse plane. The measured vertical and horizontal sizes of the electron beam were 60 {+-} 4.3 {micro}m and 300 {+-} 13 {micro}m, respectively, in reasonable agreement with the expected values.

X-ray microdiffraction study of martensite-retained austenite microstructures at the tip of a steel blade

The x-ray microdiffraction technique was used to study lattice-strain field and phase concentration near the surface of a carburised steel blade on a micron-length scale. Our results show larger compressive lattice strains and more completed phase transformation at the location near the tip of the blade than that the locations away from the tip.