First results on the reflectometry beamline on Indus1

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First results on the reflectometry beamline on Indus1
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  SPECIAL SECTION: INDUS-1 SYNCHROTRON CURRENT SCIENCE, VOL. 82, NO. 3, 10 FEBRUARY 2002 298 First results on the reflectometry beamline on Indus-1 R. V. Nandedkar*, K. J. S. Sawhney, G. S. Lodha, A. Verma, V. K. Raghuvanshi, A. K. Sinha, M. H. Modi and M. Nayak Centre for Advanced Technology, Indore 452 013, India We report here the first reflectometry beamline in soft X-ray and vacuum ultra violet regime on the Indian synchrotron radiation source Indus-1 along with some commissioning results on this beamline. This reflectometry beamline is installed on one of the bending magnets of Indus-1, and has been in opera-tion since November 2000. This beamline is being used for the characterization of optical elements (mirrors, gratings, thin films, multilayers, detectors, absorption edge filters, etc.) in the soft X-ray/vacuum ultra violet regimes in the wavelength range 40–1000 Å. Though it is designed primarily for precision reflectivity measure-ments on thin films and multilayer reflecting optics, the beamline is flexible to be used for a wide range of other experiments in this wavelength range. The beamline optics consists of a toroidal pre-mirror, a toroidal grating monochromator, a toroidal post-mirror for focusing the beam on the sample, and a high precision reflectometer. The beamline is capable of giving high flux at moderate energy resolution. The reflectometer has a capability of positioning the sample to within 10 microns and the angular position of the sample can be set within 0.01°. Results presented here demonstrate the performance and capabilities of this beamline. S OFT X-ray/vacuum ultra violet (VUV) region of the electromagnetic spectrum offers great opportunities in science and technology. The wavelengths in this region are considerably shorter than those in the visible region, thus allowing one to see smaller features in microscopy and to write finer patterns in lithography. Photon energies for use in this region are well matched to primary reso-nances of most of the elements in the periodic table. Optical elements for use in this region are continuously  being developed, for various applications connected with synchrotron radiation, plasma research, astronomy, litho-graphy and material science. For this, one needs a versatile facility for measuring reflectivity of mirrors, trans-mission in thin films, efficiency of gratings and detectors. Experimental measurements of optical constants in this wavelength region are scarce. These studies provide fundamental information on the electronic and geo-metrical structure of materials and their surfaces. The k-space resolution is significantly higher compared to X-ray scattering, thus allowing structural information in the range of 500 to 5000 Å. This spatial regime is of importance in polymer science since electron density fluctuations are often encountered in concentration profile, crystallization and phase separation in soft condensed matter. The first Indian synchrotron radiation source facility, the 450 MeV Indus-1, became operational recently at the Centre for Advanced Technology (CAT), Indore. Synchrotron radiation (SR) is produced by Indus-1 by acceleration of relativistic electrons in the dipole magnetic field of its four bending magnets. The quasi-continuous spectrum of electromagnetic radiation extends from far-infra red to soft X-rays with a critical wavelength of 61 Å (ref. 1). The reflectivity beamline is amongst the first  beamlines that have been recently made operational. This  beamline is based on a grazing incidence toroidal grating monochromator (TGM) and employs toroidal mirrors for  pre- and postfocusing optics. The entire beamline of more than 12 m length is maintained in ultra high vacuum (UHV) and comprises a variety of hardware including  in situ  precise alignment devices of optical mirrors, beam diagnostic devices, higher-diffraction-order suppression filters, etc. The experimental station on this  beamline is a high vacuum reflectometer that is capable of performing angle and wavelength-dependent reflecti-vity measurements. In addition to reflectivity measure-ments, the beamline is designed for multipurpose applications such as, for study of materials (metals, semiconductors, thin films, multilayers, etc.) in VUV and soft X-ray regimes. The first commissioning results of this beamline in terms of photon flux, spectral resolution and reflectivity data are very encouraging. The details of the beamline along with the initial commissioning and characterization results are presented here. Optical design The detailed beamline design was carried out earlier using ray tracing simulations 2,3  and only a brief descri- *For correspondence. (e-mail:  SPECIAL SECTION: INDUS-1 SYNCHROTRON CURRENT SCIENCE, VOL. 82, NO. 3, 10 FEBRUARY 2002 299  ption of the optical scheme is given here. The schematic of the optical configuration is given in Figure 1. This beamline is installed on a 50 °  port of the bending magnet (BM-2) of Indus-1. The r.m.s. electron source size at this port is 0.8 mm × 0.1 mm (horizontal × vertical). The beamline acceptance is 10 mrad × 5 mrad and the beamline is designed to cover the 40–1000 Å  photon wavelength range. The first optical element in the  beamline is a vertical deflecting toroidal mirror (M 1 ) that images the source at 2 : 1 demagnification onto the entrance slit (S 1 ) of the monochromator. The grazing angle of incidence on this mirror is 4.5º that is suffi-ciently small to have adequate reflectivity at the lowest  photon wavelength of interest. The monochromator emp-loyed in this beamline is a constant-deviation grazing-incidence TGM that performs the twin tasks of dispersion and focusing. The TGM covers the desired wavelength range with moderate spectral resolution and high photon flux with the use of three toroidal gratings that are interchangeable  in situ without breaking the vacuum. The toroidal gratings are holographically ruled and ion-etched. The parameters of the TGM are derived so that the monochromator is optimized to give good spectral resolution in the middle of the wavelength range of each grating. The constant deviation for the monochromator is 162º and the monochromator is used in first positive order. The monochromatic image of the source at the exit slit (S 2 ) is imaged onto the sample position by a vertical deflecting toroidal mirror (M 2 ) with 1 : 1 demagnification to get a synchrotron radiation (SR) beam spot of ~ 1 mm × 1 mm. The grazing angle of incidence on M 2  is also 4.5º All the optical elements are gold-coated and the  beamline optical scheme is so chosen that the reflected  beam from the post-mirror comes out in horizontal direc-tion. Various parameters of the beamline optical elements are given in Table 1. Beamline hardware The VUV–soft X-ray radiation from Indus-1 gets highly absorbed in any material and hence no window can be used to separate the storage ring from the beamline. To avoid build up of contamination on the reflecting 3210 Monochromator Toroidal Grating 4500 5.9 mradS9° 18363210 M1S2M29°162° S1  1 0 0 0 2 2 5 0 1 4 1 4 1 8 3 6  Entrance slit S1Premirror  M1S2M2 Postmirror Exit slit10 mradSOURCE M1M2 TGMDetector RDRELEVATIONPLANS D (TGM)Reflectometer    Figure 1. Optical layout of the reflectometry beamline. Table 1. Parameters of the optical elements of the reflectivity beamline Parameter Pre-mirror Post-mirror Monochromator Entrance arm length (mm) 4500 1836 1000 Exit arm length (mm) 2250 1836 1414 Angle of incidence 85.5 o  85.5 o  162 o * Meridional radius, R (mm) 38236 23400 7977 Sagittal radius (mm) 235.4 144.1 182.3 Demagnification ratio 2 : 1 1 : 1 § Size (mm × mm) 340 × 60 280 × 50 75 × 20 Coating Au Au Au Gratings Grating 1 (1800 grooves/mm) Grating 2 (600 grooves/mm) Grating 3 (200 grooves/mm)  –  –  –  –  –  – Wavelength range 40–120 Å 120–360 Å 360–1000 Å *Total deviation angle. § Varies with photon wavelength.  SPECIAL SECTION: INDUS-1 SYNCHROTRON CURRENT SCIENCE, VOL. 82, NO. 3, 10 FEBRUARY 2002 300 surfaces of the optical elements, the beamline has to  be operated in UHV conditions with pressure < 5 × 10  –9  mbar. The beamline is built in a modular fashion with each section of the beamline separated from the other by a UHV gate valve. Gate valves having a window made up of glass or sapphire fixed onto the sealing flange of the valve are installed at some places in the beamline. This enables transmission of visible SR from one beam-line section to another section even when some sections are not evacuated. The visible SR is useful in the alignment of the beamline in air. The modular design is helpful in the initial setup as well as in the routine maintenance of the beamline. Storage ring vacuum is protected from any vacuum failure in the experimental station by a combination of a fast shutter and a UHV gate valve placed just down-stream of the bending magnet output port. The high vacuum sensor for the fast shutter is mounted near the experimental station so that any vacuum breach in the experimental station is sensed by this sensor which thereby triggers the closing of the fast shutter placed ~ 10 m upstream. The fast shutter closes before the  pressure shock wave can reach the storage ring. The closing time of the fast shutter is ~ 10 ms but has poor vacuum leak tightness. Therefore, simultaneously the UHV gate valve is triggered to close in 1–2 s. A precision mirror movement mechanism has been developed for the movement of mirrors M 1  and M 2  (ref. 4). This mechanism is UHV compatible and is based on kinematic coupling providing precision movements in all the six degrees of freedom, three linear and three rota-tional. With this kinematic coupling the mirrors can be  positioned linearly in the three orthogonal directions within the range of 0 to + 10 mm and with a repeatability of 10 microns. It can also align angularly the mirrors about the three coordinate axes in the range 0 to ± 1° with a reproducibility of 10 arcsec. The TGM used in the beamline is a commercial mono-chromator (M/s Jobin Yvon, France). It has three holo-graphically made gold-coated gratings having 200, 600 and 1800 lines/mm. Wavelength scan in the TGM is done using a sine drive mechanism that rotates the grating around a horizontal axis passing through the surface of the grating. The wavelength is proportional to the per- pendicular displacement of the sine bar from zeroth order  position that is measured using a commercial linear encoder. The grating drive allows the TGM to be run with or without a computer. The gratings can be inter-changed horizontally in vacuum at any wavelength  position. Horizontal and vertical adjustable apertures in the entrance arm of the TGM permit masking the various regions of the grating for improving the aberration limited spectral resolution. The apertures can also be used to reduce stray light. Both the entrance and the exit slits of the monochromator are continuously variable from 0 to 1.8 mm with a resolution of 1 µ m in the dispersive (vertical) direction. In the non-dispersive direction, four discrete slit sizes varying from 0.3 mm to 3 mm are available. The higher diffraction order content in a grazing incidence monochromator is generally quite high and is more severe for the low energy gratings. A filter wheel mechanism has been incorporated in the beamline just after the exit slit of the monochromator to introduce an appropriate transmission filter in the SR beam path. The filter wheel has provision to install up to eight filters. Two filters, viz. of Al and Si, are already installed and more filters like Sn, B, C and In would be installed shortly. These filters are in the form of ~ 1000 to 1500 Å thick metal foils mounted on an 87% transmitting Ni mesh. An electromagnetically operated beam viewer  5  is  placed between M 1  and S 1  in which a solenoid-activated gold-coated glass screen can be brought into the beam  path at 45 °  and the visible part of the SR can be observed through a viewport. The shape, size and location of the focal spot of mirror M 1  are readily seen with the help of this beam viewer and by making appropriate adjustments using the motion-controls provided on M 1 , the focal spot, and its location can be precisely adjusted. A detector station is installed between S 2  and M 2 in which two soft X-ray detectors are mounted on an UHV-compatible linear translation stage. Any one of the two detectors can be brought into the beam path to measure the photon flux at various wavelengths. One detector is an absolute calibrated IRD AXUV 100 Si pin detector while the other is a windowless Hamamatsu GaAsP Schottky photodiode. While the wavelength-dependent quantum yield of the former is calibrated absolutely by the manufacturer, the latter is calibrated by Physikatisch-Technische-Bundesantalt (PTB), Berlin 6 . The photodiode current is measured using a commercial picoammeter (M/s Keithley, USA). These detectors are very handy in determining photon flux at various wavelengths and to  periodically check and if need be, maximize the photon flux reaching the sample station. Various sections of the beamline are provided with  pumping ports. Pre-pumping in each section is done using a 240 l/s turbo molecular pump and the ultimate vacuum is achieved with the help of 240 l/s sputter-ion  pumps. The beamline is maintained in UHV environment with pressure < 5 × 10  –9  mbar. A differential pumping station is used in between the  beamline and the experimental station so that the UHV in the beamline does not deteriorate because of the high vacuum (10  –7  –10  –8  mbar) environment of the reflectometer. The reflectometer station is isolated from the post-mirror section by a differential pumping station. The reflectometer section can be operated at 10  –6  mbar range with beamline at 10  –9  mbar range. The reflectometer station is equipped with an 880 l/s turbo molecular pump. Vacuum of 10  –6  mbar can be obtained within two hours  SPECIAL SECTION: INDUS-1 SYNCHROTRON CURRENT SCIENCE, VOL. 82, NO. 3, 10 FEBRUARY 2002 301 and nearly 10  –8  mbar can be obtained by overnight pump-ing. To monitor any contamination to the beamline optical components, residual gas analysers are mounted in the beamline and in the reflectometer station. Experimental station: Reflectometer Figure 2 shows the schematic assembly of the reflecto-meter station 7 . The reflectometer consists of a commer-cial two-axes high-vacuum compatible goniometer. The scattering geometry shown in Figure 2 is in the vertical  plane. This geometry is for the s-polarized reflectivity measurement, as the SR radiation is plane polarized in the horizontal plane. The axes of the two stages have  been pre-aligned (< 50 microns) on a coordinate measur-ing machine. The sample and the detector are mounted on the two axes respectively. For moving the sample in and out of the beam, a high vacuum compatible linear translation stage is mounted on the sample rotation. All motions are provided using vacuum compatible stepper motors and are computer-controlled. The sample is spring loaded to the reference surface of the sample holder. The existing sample holder can accommodate a sample of size up to 80 mm length, 50 mm width and 5 mm height. The maximum weight of the sample in existing configuration can be about 1 kg. Larger size samples can be accom-modated by fabricating a new sample holder. Detector distance from the axis of rotation is 200 mm. The reflectometer is mounted inside a high vacuum chamber of diameter 700 mm and a height of 700 mm. The gonio-meter in the reflectometer station can also be mounted in the horizontal plane for measurement of p-polarized reflectivity. For precise alignment of the optical axis of the SR beam to the center of the axis of rotation of the reflectometer, the chamber is mounted on a movable  plate and can be aligned in all six degrees of freedom. The reflectometer has a capability of positioning the sample to within 10 microns and the angular position of the sample can be set within 0.01 ° . While the sample motions are primarily designed for the measurement of reflectivity, the reflectometer can be used as a sample manipulator for undertaking a variety of other experi-ments. Sufficient number of additional ports has been  provided for this purpose. A silicon XUV photodiode and a GaAsP windowless  photodiode are mounted on the detector rotation axis. Using these detectors, reflectance can be measured over five dynamic ranges at nominal electron beam currents (in Indus-1) of a few tens of milliamp. A glass window gate valve separates the experimental station from the  beamline. This helps in using the visible part of the synchrotron radiation from the window of the gate valve to position and align the sample, keeping the reflecto-meter at the atmospheric pressure. Efforts are underway to mount additional detectors such as channel electron multipliers, micro channel plates and ultra thin window gas flow proportional counters. Various size pinholes can  be inserted just before the sample. A quadrant silicon detector with a central pinhole of 3 mm can be inserted in the incident beam to monitor the incident beam. Incident  beam intensity can be monitored continuously by insert-ing a gold wire mesh in the incident beam and monitoring the photoelectron current from this mesh. Beamline commissioning The beamline was set-up in a phased manner and many alignment and diagnostics tools like a digital level, a Figure 2. Schematic of the reflectometer station. The sample and detector scans are in the vertical plane. The geometry shown is for measurement of s-polarized reflectivity, as the SR beam is plane polarized in the horizontal plane. It is possible to mount the goniometer in the horizontal plane for the measurement of p-polarized reflectivity. In this case the sample and detector scans will be in the horizontal plane.  SPECIAL SECTION: INDUS-1 SYNCHROTRON CURRENT SCIENCE, VOL. 82, NO. 3, 10 FEBRUARY 2002 302 theodolyte, a He–Ne laser, a CCD camera, a visible light detector, a soft X-ray detector, etc. were used. After the installation of the front-end, the visible SR was taken out through a glass viewport and was used to align the optical components of the beamline. The entire beamline was initially setup in air without creating vacuum, which gives much flexibility for fine adjustment of various components of the beamline. After the preliminary alignment, vacuum was created in the whole beamline to < 5 × 10  –9  mbar and precise alignment was finally done. The stability of the electron beam orbit in the Indus-1 storage ring was determined by monitoring the height of the emitted SR beam from the ground level and it was observed over several days, over several injections and different electron-beam currents. Figure 3 shows that the electron-beam orbit is stable within ± 25 µ m over a  period of 10 days. The stability of the electron-beam in the same injection, over a period of 2 h – from 100 mA to 10 mA ring currents, was observed to be better than ± 12 µ m. This level of electron-beam stability is quite acceptable for the TGM type of moderate spectral-resolution beamlines. The beamline is quite stable mechanically and gene-rally no realignment of the beamline optics is required on a day-to-day basis. Flux reduction of the order of 10– 20% was observed over 5–10 days period, due to small deviations in the mirror settings, that is readily maxi-mized by slight adjustment of mirror M 1  using the motion controls provided therein. After commissioning the beamline, detailed measure-ments were carried out to characterize the performance of the beamline with respect to spectral resolution, photon flux, etc. The spectral resolution was determined at Si and Al L-edges at various slit settings (Table 2). Measure-ments also show that reducing the grating aperture improves the spectral resolution due to reduction of aberration-limited resolution. The improvement in spectral resolution due to masking will be even higher at smaller slit settings when aberration-limited resolution dominates over the slit-limited resolution. This improvement in the resolution is, of course, at the cost of reduction in the  photon flux. Photon flux has also been measured with all the three gratings (Figure 4); photon flux of the order of 10 11  –10 12  photons/s/100 mA is available over most of the wavelength range. Reflectivity measurements: Selected results Some representative results obtained during the first few months of operation of this beamline, are presented here. The beamline is being used for studying surfaces and interfaces, characterization of soft X-ray multilayer reflectors and for determination of optical constants in the VUV/soft X-ray region. A representative reflectivity measurement of Pt/C (2d = 90 Å,  N   = 30 layer pairs) X- Table 2.   Measurements of spectral resolution of the monochromator Photon wavelength (Å) Filter Grating aperture Slit size* ( µ m) Resolution ( λ / ∆ λ ) 2000 100 1000 220 500 303 170.5 Al Full H × full V 200 420 1000 116 500 200 124.5 Si Full H × full V 200 255 Full H × full V 500 303 Full H × Reduced V 500 333 170.5 Al Reduced H × full V 500 341 H, Horizontal; V, Vertical. *Non-dispersive (horizontal) slit size is 3 mm. 100100010 11 10 12 10 13  Grating: 1800 l/mm Grating: 600 l/mm Grating: 200 l/mm Photon Wavelength(Å)    P   h  o   t  o  n   f   l  u  x   (  p   h  o   t  o  n  s   /  s  e  c   /   1   0   0  m   A   )  Figure 4. Photon flux of Indus-1 on reflectometry beamline using the three gratings of the monochromator. Jul 9Jul 11Jul 13Jul 15Jul 17Jul 191251.01251.11251.21251.31251.4 July 2000 Stability of beam position over several days 50 µ m    B  e  a  m   p  o  s   i   t   i  o  n   (  m  m   ) Date  Figure 3. Stability of the electron beam in Indus-1 over a long period of time.
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