James H. Underwood

X-ray microprobe using multilayer mirrors

Abstract Multilayer reflectors for the X-ray region have now progressed beyond the experimental stage to the point where they can be relied upon as optics for experimental systems, in synchrotron radiation research as well as in other fields. This paper reviews the design considerations for an X-ray microprobe, and summarizes experience with prototypes tested at both SSRL and NSLS. The optical systems described employ multilayer-coated spherical mirrors arranged in the Kirkpatrick-Baez configuration to demagnify the X-ray source by a factor of several hundred. By this means a spot of X-rays less than 10 μm square can be produced. The optical aberrations and other factors that limit the performance are detailed, and possible ways to improve the performance are discussed. In the prototypes the spot is directed on the specimen which is carried on a stage that can be translated horizontally and vertically. The characteristic fluorescent X-rays excited by the focused 10 keV photons are analysed by an energy-dispersive Si(Li) detector, so that by scanning the stage an elemental concentration map of the specimen is built up. In a companion paper [A.C. Thompson, J.H. Underwood, Y. Wu, R.D. Giauque, K.W. Jones and M.L. Rivers, these Proceedings, p. 318] some experimental programs are described, and estimates of the elemental sensitivity are provided.

Resolving power of 35,000 (5 mA) in the extreme ultraviolet employing a grazing incidence spectrometer

The performance of a high-resolution spectrometer employing a varied line-space plane reflection grating is measured. (AIP)

A Soft X-Ray/EUV Reflectometer Based on a Laser Produced Plasma Source

A soft x-ray reflectometer is described which is based on a laser-produced plasma source and is continuously tunable over the range 40 A < λ < 400 A. The source is produced by focusing 0.532-μm light from a Q-switched Nd:YAG laser on a solid target. The x-ray wavelength is defined using a high throughput spherical grating monochromator with moderate resolving power (λΔλ ≈ 100 to 500). A time-averaged monochromatized flux of more than 109 photons/s in a 1% bandwidth at 100 eV is obtained. Photon “shot noise” limited measurements are obtained by the use of an I0 detector to normalize out the shot-to-shot variations in source intensity. Measurements with submillimeter spot sizes are readily obtainable. Various detectors have been used and the advantages and disadvantages of each are discussed. The higher order contamination of the monochromator output has been analyzed using a second grating for the purpose of making measurement corrections. The reflectometer thus provides the capability for precision absolute measurements of the reflectance of gratings and multilayer mirrors, the transmittance of thin film filters, or other properties of x-ray optical elements.

Molybdenum/beryllium multilayer mirrors for normal incidence in the extreme ultraviolet

We report on a series of normal-incidence reflectance measurements at wavelengths just longer than the beryllium K-edge (11.1 nm) from molybdenum/beryllium multilayer mirrors. The highest peak reflectance was 68.7 ± 0.2% at λ = 11.3 nm obtained from a mirror with 70 bilayers ending in beryllium. To our knowledge, this is the highest normal-incidence reflectance that has been demonstrated in the 1-80-nm spectral range.

Multilayer Optical Elements for Generation and Analysis of Circularly Polarized X-Rays

Abstract Calculations of the relative phase changes of σ and π electric field components on Bragg reflection from and transmission through multilayers are presented. Large relative phase changes can be calculated in certain cases, which may lead to utility of multilayers as quarter-wave plates for generation and analysis of circularly polarized soft X-radiation. Similar behavior may be expected for perfect crystals in the hard X-ray range.

Calibration and standards beamline 6.3.2 at the Advanced Light Source

This bending magnet beamline has been in operation since February 1995 for the characterization of optical elements (mirrors, gratings, multilayers, detectors, etc.) in the energy range 50–1000 eV. Although it was designed primarily for precision reflectometry of multilayer reflecting optics for EUV projection lithography, it has capabilities for a wide range of measurements. The optics consist of a monochromator, a reflectometer, and refocusing mirrors to provide a small spot on the sample. The monochromator is a very compact, entrance‐slitless, varied‐line‐spacing plane‐grating design in which the mechanically ruled grating operates in the converging light from a spherical mirror working at high demagnification. Aberrations of the mirror are corrected by the line spacing variation, so that the spectral resolving power λ/Δλ is limited by the ALS source size to about 7000. Wavelength is scanned by simple rotation of the grating with a fixed exit slit. The reflectometer has the capability of positioning th...

Tunable coherent radiation in the soft X-ray and extreme ultraviolet spectral regions

Undulator radiation, generated by relativistic electrons traversing a periodic magnet structure, can provide a continuously tunable source of very bright and partially coherent radiation in the extreme ultraviolet (EUV), soft X-ray (SXR), and X-ray regions of the electromagnetic spectrum. Typically, 1-10 W are radiated within a 1/N relative spectral bandwidth, where N is of order 100. Monochromators are frequently used to narrow the spectral bandwidth and increase the longitudinal coherence length, albeit with a more than proportionate loss of power. Pinhole spatial filtering is employed to provide spatially coherent radiation at a power level determined by the wavelength, electron beam, and undulator parameters. In this paper, experiments are described in which broadly tunable, spatially coherent power is generated at EUV and soft X-ray wavelengths extending from about 3 to 16 nm (80-430-eV photon energies). Spatially coherent power of order 10 /spl mu/W is achieved in a relative spectral bandwidth of 9/spl times/10/sup -4/, with 1.90-GeV electrons traversing an 8-cm period undulator of 55 periods. This radiation has been used in 13.4-nm interferometric tests that achieve an rms wavefront error (departure from sphericity) of /spl lambda//sub euv//330. These techniques scale in a straightforward manner to shorter soft X-ray wavelengths using 4-5-cm period undulators at 1.90 GeV and to X-ray wavelengths of order 0.1 nm using higher energy (6-8 GeV) electron beams at other facilities.

The renaissance of x‐ray optics

There has been a spectacular resurgence of interest in the soft x‐ray and extreme ultraviolet regions of the electromagnetic spectrum in the past few years. In part, this is due to the development of new x‐ray optical devices that have given us an x‐ray view of the universe on scales ranging from the microscopic to the astronomical (see figure 1).

Elemental measurements with an X-ray microprobe of biological and geological samples with femtogram sensitivity

Abstract An X-ray microprobe has been developed to measure the concentration and spatial distribution of many elements quickly and simultaneously with a beam spot size of less than 10 μm × 10 μm. This instrument uses a pair of multilayer coated concave spherical mirrors arranged in the Kirkpatrick-Baez geometry to focus the beam. The fluorescent X-rays from the sample are detected with a Si(Li) detector. Samples are scanned in a raster manner to create one- or two-dimensional maps of elemental concentrations. Since samples are not under vacuum, a wide variety of samples can be scanned. Femtogram sensitivities for trace elements from K to Zn can be achieved in a 60 s counting time.

Tarnishing of Mo/Si multilayer x-ray mirrors

Multilayer x-ray mirrors of molybdenum and silicon operating at normal incidence at energies just below the Si L(II,III) absorption edges are a key component in the development of soft-x-ray projection lithography. In this application high reflectivity is essential. Aging tests on such reflectors, with Mo as the last layer deposited, show that the structures decline in reflectivity with time when stored in air. Chemical analysis of a well-aged surface by photoelectron spectroscopy techniques reveals that the uppermost Mo layer eventually becomes completely oxidized to MoO(3) and MoO(2) and contaminated with carbonaceous materials. The oxidation can be prevented by storing the mirrors in an oxygen-free atmosphere or by depositing the silicon as the top layer. The reflectivity of tarnished mirrors can be restored by removing the oxides by argon-ion etching or wet chemical methods.