Performance Characteristics of Beamline 6.3.1 from 200 eV to 2000 eV at the Advanced Light Source

Bend magnet beamline 6.3.1 at the Advanced Light Source operates from 200 eV to 2000 eV, primarily used for x-ray absorption fine structure investigations. The beamline optics consist of a compact, entrance-slitless, HettrickUnderwood type variable-line-spacing plane-grating monochromator and refocusing mirrors to provide a 25 μm x 500 μm spot at the focal point in the reflectometer end station. Wavelength is scanned by the simple rotation of the grating and illuminates a fixed exit slit. The LabView based beamline control and data acquisition computer code has been implemented to provide a convenient interface to the user. The dedicated end station is a reflectometer that is isolated from the beamline by a differential ion pump. The reflectometer can position samples to within 4 μm with an angular position of 0.002, has total electron and fluorescence yield detectors, and pumps down in about 30 minutes. External end stations can be mounted downstream of the reflectometer as well. The versatility and simplicity of beamline 6.3.1 have made it useful for a wide range of applications such as the characterization of optical components, reflective coatings, and the investigation of a diverse range of materials in both the solid state and in solution. INTRODUCTION AND DESCRIPTION OF BEAMLINE 6.3.1 The Center for X-Ray Optics of the Lawrence Berkeley National Laboratory has built a user-friendly bend magnet Beamline 6.3.1 at the Advanced Light Source (ALS), which operates between 200 eV 2000 eV. Beamline 6.3.1 is shown schematically in Fig. 1 and Table 1 summarizes the optical parameters. Beamline 6.3.1 has a dedicated precision reflectometer end station and also permits the use of an external end station downstream of the reflectometer. The scientific programs of beamline 6.3.1 are dedicated to EUV and soft x-ray reflectometry, scattering, and x-ray absorption fine structure (XAFS) measurements. The horizontal focusing mirror, M1, is a concave spherical mirror that images the bending magnet source at the focal point. The M1 mirror is side-cooled by a water-cooled stainless steel block with a thin film of Ga-In eutectic for thermal contact. The vertically focusing mirror, M2, is an uncooled concave spherical mirror and demagnifies the source five times on the exit slit. Both the mirrors are single crystal silicon substrates coated with ~500 Å of Ru. The four-jaw aperture at the front end consists of four, moveable water-cooled copper plates defining the horizontal and vertical acceptance of the beamline. The maximum horizontal and vertical acceptance as determined by the M1 and M2 mirrors are 2.4 mrad and 0.44 mrad, respectively. The monochromator is a compact, entrance-slitless, Hettrick-Underwood type variable-line-spacing planegrating (VLS-PGM) design [1,2]. The entrance-slitless design results in maximal flux collection and the stability of the ALS electron beam prevents wavelength shifts over long periods of time. Furthermore, it operates in converging light from M2 allowing the beamline to be very short, resulting in high mechanical stability. It also permits better fnumber matching for optics following the fixed exit slit. The wavelength is scanned by simple rotation of the grating about its center. The large spherical aberration of M2 is corrected by the coefficients describing the ∗ Corresponding author, Email: [email protected] Downloaded 26 Apr 2012 to 131.216.164.137. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions variation of grating ruling [3], which allows the monochromator to reach a resolution limited by the ALS vertical source size. The monochromator has four interchangeable, mechanically ruled blazed gratings (fused silica substrates coated with Au) with central grove densities of 240, 600, 1200 and 2400 lines/mm. A DC motor under the control of a closed loop servo system, using a Heidenhain encoder, drives the grating sine bar. The short exit arm of the monochromator results in a small linear dispersion at the exit slit and precision exit slits are required. Adjustments for position, parallelism, and tilt are required for the best performance. To meet these requirements, a mechanism was designed in which each of the four slit corners is independently actuated and position sensed to 0.1 μm. Downstream of the exit slits but upstream of M3, is a filter carousel, in which thin foils are used for higher harmonic rejection and calibration. FIGURE 1. Schematic of beamline 6.3.1 at the ALS along with the most basic characteristics. TABLE 1. Optical parameters of beamline 6.3.1. Optical element Distance (meters) Angle θ (degrees) Radius (meters) Length (mm) Width (mm) Thickness (mm) Comments Source 0.0 M1 10.95 1.5 487 1000 40 30 Bent M2 16.0 1.25 244 400 38 10 Bent and profiled Grating 18.0 2.5 (in ZO) Plane 100 40 18 VLS Exit slit 19.2 ----------≥5 μm M3 22.7 1.25 160 400 38 10 Bent Sample 26.2 ----------Focus of M1, M3 The final optical element, M3, is a vertically focusing concave spherical mirror with a variable curvature designed to image the exit slit at unit magnification at the center of the reflectometer chamber and also permits focusing further downstream. An unequal end couple mirror bender allows the mean radius of curvature of M3 to be varied and in addition provides a linear variation of the mirror radius and an image free from spherical aberration when M3 is used at other than unit magnification. M3 is a single crystal silicon substrate coated with ~500 Å of Ru. The main experimental facility of the beamline is the dedicated reflectometer end station, which is a two-axis, vacuum system employing two Huber goniometers. The θ-motion axis accommodates the sample at the center of the reflectometer and the φ-motion supports the detectors [total fluorescence yield (TFY) and total electron yield] on a rotating arm. Linear motion translators position the sample in three directions (x, y, z) within 4 μm with an angular position of 0.002. Although the reflectometer motions are primarily designed for measurement of precision optical components, they function as a sample manipulator with sufficient flexibility to allow a wide range of Downloaded 26 Apr 2012 to 131.216.164.137. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions experiments. The sample holder can be removed and re-inserted with high precision into the θ-motion kinematic holder of the goniometer. The reflectometer section is isolated from the beamline by a differential ion pump, therefore the end station can be operated at pressures as high as 10 Torr with the beamline remaining at 10 Torr. The upstream end station isolation valve has a glass window, thus samples can be aligned using zero order light with the reflectometer at atmosphere. The reflectometer is equipped with a vibration-isolated 4000 l/s cryopump that permits 30-minute pump-down and the interchange of sample holders that mount multiple specimens. An auxiliary end station for beamline 6.3.1 has been developed to perform XAFS measurements under atmospheric pressure and with liquids from 400 eV to 3000 eV using both transmission and TFY detection techniques [4]. Measurements using this end station are carried out under a He atmosphere at room temperature. The experimental setup consists of two optical shutters and a Si3N4 membrane at the end of the beamline, with vacuum on one side and atmospheric pressure on the other. Optical shutters are interlocked to the acrylic case that encloses the sample holder and whenever the case is opened, the optical shutters close to prevent exposure to x-rays. The beamline valves are interlocked to prevent venting the beamline should a Si3N4 membrane fail. For example, XAFS measurements of thin films deposited on Si3N4 membranes are done in transmission mode and the sample holder has a provision to hold the detector closely behind the sample. The sample holder is mounted on a stage where it can be moved along the beam direction; hence the sample is placed close to the Si3Ni4 membrane at the end of the beamline. For solutions, liquids, and solids, measurements are done in TFY mode using a photodiode. Solid samples are mounted at 45 to the x-ray beam using carbon tape. Liquids and solutions are placed in a 6 mm diameter plastic cylinder that has a 1.5 mm 2 mm slot. The liquid surface at the cylinder slot is exposed to the beam and the fluorescence is collected at 90 with respect to the beam. The sample and the detector are kept close to the Si3N4 membrane to minimize absorption from the He atmosphere of the apparatus. The beamline and reflectometer are controlled by a Sun workstation through a VXI crate. All monochromator, reflectometer (except sample holder interchange) and data acquisition functions are controlled from and displayed at the workstation. The software provides the capability for scanning wavelength (λ) or energy (E); scanning any of the reflectometer motions θ, φ, x, y, z; making special scans such as φ = 2θ; and 2-dimensional scans using any pair of motions. BEAMLINE 6.3.1 PERFORMANCE Beamline 6.3.1 has been installed, aligned, and operational since March 2000. Recently, the full recharacterization of the resolution, flux, and wavelength calibration has been carried out following the improvement of an optical element. The beamline photon flux is presented in Fig. 2(a) and is on the order of 1x10 to 2x10 photons/sec/0.1%BW from 200 eV to 2000 eV. For measuring the monochromator resolution, a gas cell was used in the experimental location downstream of the reflectometer. Figures 2(b) and 2(c) show the XAFS spectra at the L2,3-edge of Ar gas and K-edge of Ne gas collected using the 600 lines/mm and 1200 lines/mm gratings, respectively. These spectra have a spectral resolution of λ/∆λ ≈ 2000. The Mg K-edge extended-XAFS (EXAFS) spectrum from a Mg metal thin film capped with 10 nm of Pd using the auxiliary end station with the 2400 lines/mm grating and collected in transmission mode is shown in Fig. 2(d) [5]. The TFY XAFS spectra from single crystals of MgO and MgF2 collected in the reflectometer are also included in Fig. 2(d). The XAFS spectra show characteristic edge shifts based on the bonding environment in the Mg metal, MgO, and MgF2. The spectra of both MgO and MgF2 show a sharp near-edge peak at ~1311 eV and 1314 eV, respectively, which are attributed to the 1s to 3p transition whose intensity depends on the degeneracy of the 3p states. This feature occurs at 1304.5 eV for metallic Mg. The interatomic distances and the coordination determined from the Mg metal EXAFS are in good agreement with crystallographic data up to the second shell [5]. The Al K-edge EXAFS spectra for crystalline NaAlO2, Al2(SO4)3, Al(NO3)3, and Na[Al(EDTA)] collected in TFY mode using the auxiliary end station with the 2400 lines/mm grating are shown in Fig. 2(e) [4]. The spectra in the near-edge region are characterized by two main features at ~1568 eV and ~1572 eV for the compounds studied. An additional feature at ~1564.5 eV is present for NaAlO2. The feature at ~1568 eV is characteristic of six-fold Al coordination with oxygen for NaAlO2, hydrated Al(NO3)3, and Al2(SO4)3. Similarly, the feature at ~1564.5 eV for NaAlO2 is characteristic of Al in four-fold coordination. The feature at ~1572 eV for all of the compounds results from a long-range-order correlation beyond the nearest-neighbor shell. Detailed analyses of the Al K-edge EXAFS are in progress and will be published elsewhere. Downloaded 26 Apr 2012 to 131.216.164.137. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions 130