Indus-1 Beamline for Reflectometry

K J S Sawhney, G S Lodha, A K Sinha, M H Modi, A Verma, V K Raghuvanshi, M Nayak  and R V Nandedkar

 Synchrotron Utilization Division

Introduction

The reflectivity beamline is amongst the first beamlines that became operational on Indus-1 in November 2000. This beamline gives monochromatic radiation in the 40–1000 Å wavelength region with a moderate resolution (l/Dl ~ 500) and high intensity.  The beamline is based on a grazing incidence Toroidal Grating Monochromator (TGM) and employs toroidal mirrors for pre- and post- focussing optics. The entire beamline of more than 12 m length is maintained in ultra high vacuum (UHV) and comprises of 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 reflectivity measurements. In addition to reflectivity measurements the beamline is designed for multipurpose applications such as, study of materials (metals, semiconductors, thin films, multilayers, etc.) in VUV and soft x-ray regimes. The details of the beamline along with the initial commissioning and characterisation results are presented here.

Beamline Description

The schematic of the optical configuration of the beamline 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 x 0.1 mm (horizontal x vertical). The beamline acceptance is 10 mrad x 5 mrad and the beamline is designed to cover the 40–1000Å photon wavelength range. The toroidal pre-mirror M₁ images the source at 2:1 demagnification on to the entrance slit S₁ of the monochromator. The monochromator employed 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 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 monochromator has an entrance arm of 1000mm, exit arm of 1414mm, constant deviation of 162º and it is used in first positive order. The monochromatic image of the source at the exit slit (S₂) is imaged onto the sample position by a vertical deflecting toroidal mirror (M₂) with 1:1 demagnification to get a SR beam spot of ~ 1mm x 1mm. All the optical elements are gold coated and the beamline optical scheme is so chosen that the reflected beam from the post-mirror come out in horizontal direction.

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 and thus the beamline has to be maintained in UHV conditions with pressure < 5 x 10^-9 mbar. The beamline hardware consists of four major sections namely, the frontend, the pre-mirror, the monochromator, the post-mirror and the experimental station. The beamline hardware is built in a modular fashion with each section of the beamline separated from the other by a UHV gate valve.

 Figure 1: Schematic of the optical layout of Reflectivity beamline on Indus-1.

Storage ring vacuum is protected from any vacuum failure in the experimental station by a combination of a fast shutter (closing time ~10 msec) and a UHV gate valve placed just downstream of the bending magnet output port. For in-situ fine adjustment of the toroidal mirrors in all the six degrees of freedom viz. three linear and three rotational, UHV compatible precision mirror movement mechanism has been developed. The TGM used in the beamline is a commercial monochromator (M/s Jobin Yvon, France). It has three holographically made gold coated gratings having 200, 600 and 1800 lines /mm. Wavelength scan in the TGM is done using a sine drive mechanism so that the wavelength is proportional to the perpendicular displacement of the sine bar from 0^th -order position which, in turn, is measured using a commercial linear encoder. The grating drive allows the TGM to be run with or without a computer. 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 1.8mm with a resolution of 1 mm in the dispersive (vertical) direction. In the non-dispersive direction, four discrete slit sizes varying from 0.3 mm to 3 mm are available.

Figure 2:  Schematic of the reflectometer station.

To suppress the higher diffraction orders, which are generally quite high in a grazing incidence monochromator, a filter wheel mechanism has been incorporated in the beamline just after the exit slit in which transmission filters in the form of ~ 1000 to 1500 Å thick mesh-supported metal foils (Al, Si, Sn, B, C and In) are mounted. Any one of these filters can be introduced in the SR beam path.

A detector station is installed between S₂ and M₂ in which two soft x-ray detectors (IRD AXUV 100 Si pin detector and windowless Hamamatsu GaAsP Schottky photodiode) are mounted on an UHV-compatible linear translation stage. Any one of the two detectors can be brought into the beam path and the photodiode current measured using a Keithley picoammeter. These detectors are very useful in determining photon flux at various wavelengths and to periodically check, and if need be, maximize the photon flux reaching the sample station.

The experimental station on this beamline is a multipurpose reflectometer. The reflectometer (Figure 2) operates at a vacuum of 1x10^-7 mbar and hence 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 is equipped with commercial two-axes high-vacuum compatible goniometer. Independent as well as coupled rotation of sample and detector is possible with angular resolution of 2.5 mdeg and it is possible to set the reflectometer in either s or p polarization geometry. The axes of the two stages have been pre-aligned (<50 microns) on a coordinate measuring 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. Samples of size up to 80 mm length, 50 mm width, 5 mm height and maximum weight of ~1 Kg can be accommodated. 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. For precise alignment of the optical axis of the SR beam to 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°. 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 milliamperes. Various size pinholes can be inserted just before the sample. Incident beam intensity can be monitored continuously by inserting a gold wire mesh in the incident beam and monitoring the photoelectron current from this mesh. 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 experiments. Sufficient number of additional vacuum 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 form the window of the gate valve to position and align the sample keeping the reflectometer at the atmospheric pressure.

Various sections of the beamline are provided with pumping ports. Pre-pumping in each section is done using a 270 lt./sec turbo molecular pump and the ultimate vacuum (< 5 x 10^-9 mbar) is achieved with the help of 270 lt./sec sputter-ion pumps. To monitor any contamination to the beamline optical components, residual gas analyzers are mounted in the beamline as well as in the reflectometer station.

Beamline Commissioning

The entire beamline was initially setup in air without creating vacuum, which gives much flexibility for fine adjustment of various components of the beamline. Many alignment and diagnostics tools like a digital level, a theodolyte, a He-Ne laser, a CCD camera, a visible light detector, and a soft x-ray detector etc. were used. After the preliminary alignment, vacuum was created in the whole beamline and precise alignment was finally done using the in-situ alignment provisions.

After commissioning the beamline, detailed measurements 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 1].

Table 1: Measured spectral resolution

Measurements 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 3]. Photon flux of the order of 10¹¹–10¹² photons/sec/100mA is available over most of the wavelength range.

Figure 3: Measured photon flux

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

Figure 4: Reflectivity of Platinum- Carbon multilayer (2d= 95.2 Å, number of layer pairs N-30)
measured below and above the carbon edge .(44Å)

VUV/soft x-ray region. A representative reflectivity measurement of Pt/C (2d=90A, N=20 layer pairs) x-ray multilayer reflector measured above and below the carbon K edge (44 Å) is shown in Figure 4. The effect of carbon edge is clearly seen with low reflectivity below the carbon K-edge energy and high Bragg peak reflectivity above the carbon K-edge. Another example of measurement from this beamline is shown in Figure 5 wherein reflectance of C thin film measured at various wavelengths is shown. The extension of critical angle with increase in wavelength is clearly visible.

Figure 5: Reflectivity of Carbon film on a float glass substrate measured at 55, 70, 80 and 100 Å.

Summary:

The TGM based reflectivity beamline and the associated reflectometer station is installed on Indus-1 synchrotron radiation source. The beamline is characterised with respect to photon flux, spectral resolution, and reflectivity data, etc. The beamline is now being used routinely for VUV/soft x-ray reflectivity measurements. This beamline, well equipped with sample manipulation and detector facilities, is a national facility and is available to scientists working in national laboratories, academic institutions or industry for carrying out research.

Acknowledgements

R K Gupta, M.N.Singh, B.Gowrishankar, P.K.Shrivatava, S P L Srivastava and Suraj Das took active part in construction and commissioning of the beamline. Their contribution is gratefully acknowledged. Thanks are also due to the Indus-1 operation staff and health physicists for their support and cooperation.

References

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4.  K.J.S. Sawhney and R.V. Nandedkar, Nucl. Instrum. Meth. A359 (1995) 146

5.  A. Verma, K.J.S. Sawhney, S.K. Chatterjee and R.V. Nandedkar, Vacuum 55 (1999) 95

6.  G.S. Lodha, V.K. Raghuvanshi, M.H. Modi, P.Tripathi, A. Verma and R.V. Nandedkar ,Vacuum 60 (2001) 385