M.T. Browne

Scanning soft X-ray imaging at 10 nm resolution

Abstract The potential imaging and selected-area near-edge X-ray absorption fine structure spectroscopic (NEXAFS) performance of a new scanning probe X-ray microscope (SPXM) for soft X-rays from synchrotron radiation is examined by computer modelling; an optical design for the microscope is also given. Acceptable dwell times per pixel in image collection are predicted. The microscope is expected to have a spatial resolution for “water-window” X-ray wavelengths (2.3–4.4 nm) of about 10 nm for specimens up to 200 nm thick. The central component of the optical system is a tube collimator forming a probe a few wavelengths in diameter above a scanned specimen. The collimator is illuminated at near-normal incidence by a Fresnel zoneplate condenser, and its exit aperture is positioned a few wavelengths above the specimen. Physical understanding is gained by a two-dimensional (2D) waveguide approach, and by calculations using the full-vector theory of Maxwells equations. The calculations, because of computational limitations, are carried out mainly in 2D. The vector results agree well with a multislice scalar calculation in 2D which is then applied to 3D to describe the 3D probe imaging and to estimate the energy throughput.

Studies of the spatial coherence of an x-ray laser

We describe the use of a dispersive Youngs slits coherence diagnostic for determining the transverse coherence length of a soft x-ray laser source. W have studied the emission from the Ne-like germanium x-ray lasing lines at 23.2/23.6 nm for comparison with the 19.6 nm lasing line obtained with the use of a prepulse. The effective source size has been determined for both these output modes from the calculated coherence length. We conclude that the use of a prepulse leads to a significant reduction in effective source size at 19.6 nm.

Optical source model for the 23.2-23.6 nm radiation from the multielement germanium soft X-ray laser

Abstract Distributions of source intensity in two dimensions (designated the source model), averaged over a single laser pulse, based on experimental measurements of spatial coherence, are considered for radiation from the unresolved 23.2/23.6 nm spectral lines from the germanium collisional X-ray laser.The model derives from measurements of the visibility of Young slit interference fringes determined by a method based on the Wiener–Khinchin theorem. Output from amplifiers comprising three and four target elements have similar coherence properties in directions within the horizontal plane corresponding to strong plasma refraction effects and fitting the coherence data shows source dimensions (FWHM) are ∼26 μm (horizontal), significantly smaller than expected by direct imaging, and ∼125 μm (vertical: equivalent to the height of the driver excitation).

Microstructural investigation of aggregates formed from aged silica sols by STXM

The scanning transmission X-ray microscope at the Daresbury Laboratory has been used to form direct high-resolution images of gels prepared from aged silica sols at different pH values. The pH plays an important role in the determination of both the gelation rate and the microstructures observed in the aggregates of the silica gel. The fractal dimension of the structures can be used to study differences between the resulting aggregates.

X-ray imaging of aggregation in silica and zeolitic precursors

The resolution available in the Kings College London scanning transmission x-ray microscope (STXM) can be exploited to study aggregate structures over a length scale from 100 nm to 10 micrometers that overlaps with and complements that available from small-angle x-ray scattering (SAXS) data. It is then possible to use these combined sets of data to test between different growth models for the aggregates, using the fractal dimension of the structures as a way of distinguishing the different models. In this paper we show some of the first transmission x-ray images taken of silica gels and zeolite precursors, materials that are of great practical and economic importance for certain selective catalytic processes in the chemical industry, and yet for which there is still only limited understanding of the complicated processes involved in their preparation. These images reveal clearly the fractal aggregates that are formed by the specimens.

Development and first applications of an imaging microscope using the Vulcan x-ray laser

An imaging microscope, comprising a Schwarzschild condenser and zone plate optical arrangement, has been established on the Vulcan Nd-glass laser system at the Rutherford Appleton Laboratory (RAL). Images of simple test structures have been taken in x-ray transmission using doublet x-ray laser radiation at 23.2 nm and 23.6 nm from collisionally pumped Ne-like germanium. Image resolutions of about 0.15 micrometers have been measured. The results are intended as a proof of principle and demonstrate both that images can be taken successfully using the Vulcan x-ray laser, and of specimen regions which are destroyed on passage of the x-ray beam.

Combined near-field STXM with cylindrical collimator and AFM

The SNXM [1] was first installed at the European Synchrotron Research Facility in November 1998. It has been designed for water-window operation at a spatial resolution of about 10 nm and in its final form will comprise a Zone Plate focusing X-ray onto a cylindrical collimator 10–20 nm in diameter, made by drilling an AFM tip, with its exist aperture within a few nm of the specimen surface. The operation of the microscope may be loosely defined to be in near-field in analogy with the optical SNOM e.g. [2]. Point to point resolution equal to the collimator diameter is expected for specimens up to 200 nm thick. The collimator to surface separation is also monitored by the AFM scanning tip. Simultaneous signals are available from X-ray transmission and surface topography. A progress report is given limited by the current availability of high energy X-rays (3–6 Kev).