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Soft x-ray reflectivity beamline (BL-3) [an error occurred while processing this directive]

The soft x-ray energy region available from Indus-2 offers a great opportunity to pursue fundamental studies and carryout new technological developments. Surfaces and near-surface interfacial layers can be probed using soft x-ray wavelengths. Wet cell and ultra-high vacuum capabilities will further extend the range of systems that can be investigated as the soft x-ray band of "water window" region (23 - 44Å) offers to do microscopy of biological tissues in their native environment. The presence of numerous atomic resonances in this wavelength region enables one to do experiments of elemental identification as well. In the soft x-ray region x-ray reflectivity becomes very powerful tool to probe low contrast system such as carbonaceous soft condensed matter films as it enables one to enhance the contrast among low Z elements near the absorption edges of respective elements. A reflectometry facility has been setup at Indus-2 synchrotron source to pursue various pure and applied research programs.

FIGURE 1: Optical layout of soft x-ray reflectivity beamline (side view).

X-ray reflectivity is a non-destructive tool to study surfaces and interfaces of thin films and multilayers. Soft x-ray resonant reflectivity is a powerful tool to study buried interfaces due to its increased sensitivity around the absorption edges of the constituent materials. Measurement of accurate optical constants is an important need to model and predict the actual performance of x-ray optical elements. A VUV reflectivity beamline on 450 MeV electron storage ring Indus-1 fulfils the above experimental needs in the energy range 10-300 eV. Indus-2 is a 2.5 GeV machine with a critical wavelength of 1.98 Å. A Varied Line Spacing Plane Grating Monochromator (VLS-PGM) based soft x-ray beam line has been designed on bending magnet source to cater the above experimental needs in the energy range 100-1500 eV. The beamline is set up at the bending magnet port BL-03 and a reflectometer is installed as an experimental station. This beamline covers the K edges of light elements like C, N and O and the L and M edges of transition elements.

BEAMLINE OPTICAL SCHEME

The soft x-ray reflectivity beamline uses a constant deviation angle variable line spacing plane grating monochromator with Hettrick type optics. This configuration has been chosen because of simplicity of its mechanism and less number of optical elements. The optical layout of the beamline is shown in the Figure 1. This beamline is installed on a 5º port of bending magnet source. The first optical element of the beamline is a horizontally deflecting and vertically mounted toroidal mirror, TM1, which accepts 2 mrad (H) and 3 mrad (V) of the emitted bending magnet radiation. TM1 focuses the light vertically on to the entrance slit S1, and horizontally on to the exit slit S2. The second mirror is a spherical mirror SM, which is vertically deflecting and forms a convergent beam on the plane grating. After SM, the white light is diffracted by the plane grating and desired wavelength is focused on the slit S2. Three interchangeable gratings G1, G2 and G3 of line densities 1200, 400 and 150 lines/mm are used to efficiently cover the whole energy region of 100-1500 eV.

TABLE 1. Major parameters of the soft X-ray reflectivity beamline

Beamline parameters
Energy range 100-1500 eV
Flux 109-1011 ph/sec
Resolution 1000 – 6000
Beam Size 0.5 mm(H) X 0.3 mm (V)
Monochromator VLS-PGM 3 gratings
400-1500 ev Gr1: 1200 l/mm
150-600 eV Gr2: 400 l/mm
100-225 eV Gr3: 150 l/mm

The beamline provides moderate spectral resolution (E/ΔE ≈ 1000-6000) and high photon flux (~109 - 1011 ph/sec) with the use of three gratings that are interchangeable in-situ without breaking the vacuum. The monochromatized light is focused on to the sample by horizontally deflecting and vertically mounted toroidal mirror TM2. The whole beamline operates in ultra high vacuum environment of pressure < 3 x 10-9 mbar. The major parameters of the beamline is given in the Table 1.

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FIGURE 2. (a) Inside view of the reflectometer chamber (b) schematic of sample scanning stages as the sample can be scanned in x-y plane using two lateral translation stages and Z-vertical stage can be used to move the sample in and out of the beam and also to adjust samples of different thicknesses.


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FIGURE 3. (a) Schematic of three mirror assembly for higher order suppressor (top view). Each mirrors are coated with stripes of four different materials as shown in (b). During the operation the three mirrors will set such that the stripes of same materials will remain in the beam path.

EXPERIMENTAL STATION

The reflectometer consists of a two -axes high -vacuum compatible goniometer with x-y-z sample manipulation stages. The scattering geometry is in the vertical plane which is suitable for s-polarized reflectivity measurements as synchrotron light is plane polarized in the horizontal plane. The sample and the detector are mounted on the theta and 2-theta 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 stage. The existing sample holder can accommodate a sample of size up to 300 mm length, 100 mm width and 50 mm height. The maximum weight of the sample in existing configuration can be about 5 Kg. Detector distance from the axis of rotation is 200 mm. Inside view of goniometer and sample mounting stage are shown in Figure 2a. The schematic of sample translation stages is shown in Figure 2b. The reflectometer has a capability of positioning the sample to within 2 microns and the angular position of the sample can be set within 0.001°. The reflectometer can be used as a sample manipulator for undertaking a variety of other experiments too besides the reflectivity. Sufficient number of additional ports has been provided for this purpose. 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 reflectometer at the atmospheric pressure. Incident beam intensity can be monitored continuously by inserting a Nickel wire mesh in the incident beam and monitoring the photoelectron current from this mesh. Detector arm is

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FIGURE 4. (a) Measured (scatter) and fitted (solid line) reflectivity curves of NbC/Si multilayer (b). Reflectivity data of a 300 mm long toroidal mirror measured before and after removal of surface carbon contamination layer from its top surface.


designed to mount multi detectors and therefore different detectors can be mounted at a time. Presently, two silicon photodiodes with different coatings are mounted. Using these detectors, reflectance can be measured over five order of dynamic ranges. The used silicon photodiode detector from International Radiation Detector Inc, USA) has 100 % internal quantum efficiency. The detector signal is measured in terms of current using a Keithley electrometer (6514). The sample stage is electrically isolated from the goniometer body and hence the total electron yield (TEY) signal can be directly measured by connecting a wire on a sample surface. In order to suppress higher harmonics coming from the monochromator different edge filters are installed in the beamline. The high vacuum reflectometer is isolated from the UHV environment of the beamline using a custom designed differential pumping setup which can maintain a pressure difference of 3 order. Thus, the reflectometer can be operated at pressure as high as 1×10-6 mbar with the beamline at 1×10-9 mbar.

To further improve spectral purity of the monochromatic light coming from the PGM, a three mirror based higher order suppressor (HOS) is designed and will be installed before the exit slit of the monochromator (see Figure 1). The schematic of the HOS optical elements is shown in Figure 3 where the mirror M1 and M3 operates at a grazing angle θ/2. Mirror M2 is set at an angle θ and move in transverse direction such that the beam exit will remain fixed during the angular movement of the mirrors M1 and M3. In order to efficiently suppress the higher harmonics in 100-800 eV range different coating materials are used for different energy regime. The three mirrors will have a stripe coating of four different materials i.e. Carbon, Chromium, Silicon and Nickel. During the operation of the HOS the three mirrors can be set in the beam path with similar coatings on respective mirrors. The HOS setup will improve the spectral purity and reduce the harmonic components below 0.1%.

TEST RESULTS

The beamline is being used to carry out different experiments for thin film and multilayer characterization. A representative graph of NbC/Si multilayer (2d=140Å, N=20 layer pairs) measured near 1000eV photon energy is shown in Figure 4. The Bragg peaks upto 4th order is recorded with distinguishable Keissig fringes. Low reflectivity of second order Bragg peak is due to almost equal thicknesses of NbC and Si layer in the multilayer stack. Keissig fringes corresponding to reflection from multilayer total thickness are visible between 2nd and 3rd Bragg peak region. The measurements are performed upto 22 degree incidence angle using silicon photodiode where the background current of 5-10 pA was limiting the dynamic range of reflectivity measurements to 5 orders on logarithmic scale. A best fit obtained using Parratt formalism applying Gaussian roughness distribution at the interfaces is also shown. The similar measurements are carried out on various other multilayers and thin film samples.

Further, the reflectivity system is used to test actual optics also. A carbon contaminated toroidal mirror taken from some other beamline for carbon cleaning experiment was measured for reflectivity performance before and after the removal of surface carbon contamination layer. The measured and fitted reflectivity curves of that mirror (using 1200 eV photon energy) are shown in Figure 4b. It is evident that due to presence of carbon contamination layer on the test mirror the reflectivity curve shows a dip near the critical angle of carbon and after plasma cleaning experiments the dip disappears indicating the cleaning of carbon layer. The fit parameters suggest that the carbon contamination layer has 400Å thickness and a roughness of 60 Å with density of ~75% of graphitic carbon.

The beamline is used for variety of other experiments that includes transmission and absorption measurements in direct and indirect mode. The direct transmission measurement of ultrathin free standing Ni Filter of 0.4 micron thickness is shown in Figure 5a. The indirect absorption measurements of thin film and bulk samples are carried out in total electron yield (TEY) mode by measuring the electron current emitted from sample surface. The measured TEY data of a ZrO2 thin film recorded near O K-edge at different grazing incidence angle is shown in Figure 5b. The TEY at different grazing angle allows one to tune the location of node and antinode of standing wave field inside the sample depth. Beside that the TEY spectra recorded near the absorption edges represent NEXAFS features and give vital information of chemical and physical state of a surface layer. In the present case the TEY spectra measured near oxygen edge shows clear feature of crystal field splitting in the ZrO2 system. The electron transition from O1s state to O 2p state which is hybridized with Zr 4d state give rise to appearance of eg and t2g states separated by 2.61eV.

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FIGURE 5. (a) Transmission curve of Nickel foil measured using 1200 l/mm grating (b) TEY spectra of ZrO2 thin film measured near the O K edge region at different incidence angles.


Radiation hutch

The hutch is comprised of three zones namely the optics hutch, the monochromator hutch and the experimental area. The pre mirror of the beamline is installed in the optics hutch where it receive the full wavelength spectrum of Indus-2 bending magnet emission. For radiation safety point of view this area is shielded and have full safety interlocking scheme. The optics hutch walls are comprised of lead sheets sandwiched between MS plates of recommended thickness.

The first optical elements of the beamline is designed to work at 2 degree incidence angle. The first optics i.e. pre mirror reflects only the soft x-ray part of the bending magnet radiation of Indus-2. Consequently, the monochromator hutch receives the low energy radiation mainly the soft x-rays. The low energy soft x-ray cannot pass through the walls of the vacuum vessels and hence the monochromator hutch has very low radiation background. The monochromator hutch can have an access during the beamline operation.

The experimental hutch is just an enclosure made of glass and composite aluminum panels. The photograph and scheme of the radiation shielding hutch is shown in the Figure
Layout of the soft x-ray beamline hutch.


VLS-PGM Monochromator:

The monochromator system of the beamline has been placed on its design location along with entrance and exit slit assembly. The 8-meter long monochromator section has been leak tested and optical alignment has been carried out. Actual optics of the monochromator has been installed and aligned using the laser light. Inside view of the three gratings installed inside PGM chamber is shown in figure.

Mirror positioning system

The pre and post focusing mirror positioning systems are installed on its design location. Alignment and UHV testing has been carried out using dummy optics. Later, the dummy optics has been replaced with actual toroidal mirrors. Photograph of TM1 system during installation phase is shown in figure.

Contact Number : 244 2503

Contact Person :

Name Email
Dr. M. H. Modi modimh (at) rrcat.gov.in

Last Modification on: January 2019