Barry L. Winn

Illumination for coherent soft X-ray applications: the new X1A beamline at the NSLS.

The X1A soft X-ray undulator beamline at the NSLS has been rebuilt to serve two microscopy stations operating simultaneously. Separate spherical-grating monochromators provide the resolving power required for XANES spectroscopy at the C, N and O absorption edges. The exit slits are fixed and define the coherent source for the experiments. The optical design and the operational performance are described.

X1A: Second-generation undulator beamlines serving soft x-ray spectromicroscopy experiments at the NSLS

The X1A undulator beamline is being rebuilt with two separate monochromators on its two branches. The new arrangement will deliver spatially coherent beams to imaging experiments, with spectral resolving power of up to 5000, and the capability to optimize the resolving power versus flux. The beamlines will operate simultaneously, and each will use 15 percent of the undulator beam, yet deliver high coherent flux. Because of the small beam divergence, the spherical grating monochromators can operate with fixed exit arms throughout the 250–800 eV range.

Methods to remove distortion artifacts in scanned projections

Artifacts induced by distortions which sometimes occur in two- dimensional projection images can appear in the resulting tomographic reconstructions. We describe a procedure for analyzing, correcting and removing experimental artifacts, and hence reducing reconstruction artifacts. Two-dimensional and three-dimensional images acquired with scanning transmission x-ray microscopy of a sample containing an integrated circuit interconnect show how these procedures can be successfully applied.

Scanning transmission x-ray microscope at the NSLS: from XANES to cryo

The Stony Brook scanning transmission x-ray microscope (STXM) has been operating at the X1A beamline at the NSLS since 1989. A large number of users have used it to study biological and material science samples. We report on changes that have been performed in the past year, and present recent results. To stabilize the position of the micro probe when doing spectral scans at high spatial resolution, we have constructed a piezo-driven flexure stage which carries out the focusing motion of the zone plate needed when changing the wavelength. To overcome our detector limitation set by saturation of our gas-flow counter at count rates around 1 MHz, we are installing an avalanche photo diode with an active quenching circuit which we expect to respond linearly to count rates in excess of 10 MHz. We have improved the enclosure for STXM to improve the stability of the Helium atmosphere while taking data. This reduces fluctuations of beam absorption and, therefore, noise in the image. A fast shutter has been installed in the beam line. We are also developing a cryo- STXM which is designed for imaging frozen hydrated samples at temperatures below 120 K. At low temperatures, radiation sensitive samples can tolerate a considerably higher radiation dose than at room temperature. This should improve the resolution obtainable from biological samples and should make recording of multiple images of the same sample area possible while minimizing the effects of radiation damage. This should enable us to perform elemental and chemical mapping at high resolution, and to record the large number of views needed for 3D reconstruction of the object.

Sealed cell for in-water measurements

We describe here the use of a very simple sealed cell to carry out absorption spectroscopy measurements on hydrated thin films, particular for oxygen K edge studies, where the sample thickness is limited by about 0.5 microns due to the strong water absorption. The cell is small enough to be mounted on a standard TEM holder, and has been used in the vacuum chamber of the Stony Brook cryo STXM. An application includes the measurement of the Si3N4 wetting properties and spectroscopy of amino acid solutions. Comparison with frozen hydrated glycine shows that special care must taken to avoid the drying of submicron water layer during sample preparation, since precipitation may take place and influence XANES spectra.

Considerations for a soft x-ray spectromicroscopy beamline

The X-1A soft x-ray undulator at the NSLS is the source for our experimental programs in spectromicroscopy. We require both spatial and temporal coherence. Due to the relatively large horizontal divergence of the electron beam in the low (beta) straight section of the x-ray storage ring, it has been possible to split the beam using a scraping mirror into two branches: X-1A used by our program and X-1B used for high resolution spectroscopy. We are now rebuilding the X-1A beamline to provide improved resolving power and essentially linear trade-off between photon rate at the zone plate and resolving power for the soft x-ray spectromicroscopy experiments. This new beamline will exploit both additional floorspace due to the NSLS building expansion and increases in the brightness of the x-ray ring. Our beam will be further split into two separate beamlines, both of which will use toroidal mirrors to focus the source on the monochromator entrance slits horizontally and to focus on the monochromator exit slits vertically. This separation comes at no loss of coherent flux and permits low thermal loading on the optics, since we need little more than the coherent fraction of the beam at the Fresnel zone plate for microfocusing. Because of the small angular acceptance for spatially coherent illumination of the zone plates and the use of an approximately satisfied Rowland condition, our monochromators have sufficient resolving power with fixed exit arms. Experiments can then be placed near the exit slits, with spatial coherence established by the exit slit size. Resolving power will be controlled by adjusting the entrance slit alone with no change of spatial coherence. The zone plates will be overfilled to be less sensitive to beam vibration and drift.

Calibration of High-Resolution X-Ray Tomography With Atomic Force Microscopy.

For two-dimensional x-ray imaging of thin films, the technique of scanning transmission x-ray microscopy (STXM) has achieved images with feature sizes as small as 40 nm in recent years. However, calibration of three-dimensional tomographic images that are produced with STXM data at this scale has not yet been described in the scientific literature, and the calibration procedure has novel problems that have not been encountered by x-ray tomography carried out at a larger scale. In x-ray microtomography, for example, one always has the option of using optical imaging on a section of the object to verify the x-ray projection measurements; with STXM, on the other hand, the sample features are too small to be resolved by light at optical wavelengths. This fact implies that one must rely on procedures with higher resolution, such as atomic force microscopy (AFM), for the calibration. Such procedures, however, generally depend on a highly destructive sectioning of the sample, and are difficult to interpret because they give surface information rather than depth information. In this article, a procedure for calibration is described that overcomes these limitations and achieves a calibration of an STXM tomography image with an AFM image and a scanning electron microscopy image of the same object. A Ge star-shaped pattern was imaged at a synchrotron with a scanning transmission x-ray microscope. Nineteen high-resolution projection images of 200 × 200 pixels were tomographically reconstructed into a three-dimensional image. Features in two-dimensional images as small as 40 nm and features as small as 80 nm in the three-dimensional reconstruction were resolved. Transverse length scales based on atomic force microscopy, scanning electron microscopy, x-ray transmission and tomographic reconstruction agreed to within 10 nm. Toward the center of the sample, the pattern thickness calculated from projection images was (51 ± 15) nm vs (80 ± 52) nm for tomographic reconstruction, where the uncertainties are evaluated at the level of two standard deviations.

A shutter–photodiode combination for UV and soft X-ray beamlines

A fast (∼12 ms) shutter for UHV beamlines is described. In the closed position the beam is blocked by an electrically isolated aluminium piece. The total yield photocurrent in this situation can be used to monitor the beam intensity.