Quantitative determination of higher harmonic content in the soft x-ray spectra of toroidal grating monochromator using a reflection multilayer

Study of interface reaction in a B4C/Cr mirror at elevated temperature using soft X-ray reflectivity

Boron carbide is a prominent material for high-brilliance synchrotron optics as it remains stable up to very high temperatures. The present study shows a significant change taking place at 550°C in the buried interface region formed between the Cr adhesive layer and the native oxide layer present on the silicon substrate. An in situ annealing study is carried out at the Indus-1 Reflectivity beamline from room temperature to 550°C (100°C steps). The studied sample is a mirror-like boron carbide thin film of 400 Å thickness deposited with an adhesive layer of 20 Å Cr on a silicon substrate. The corresponding changes in the film structure are recorded using angle-dependent soft X-ray reflectivity measurements carried out in the region of the boron K-edge after each annealing temperature. Analyses performed using the Parratt recursive formalism reveal that the top boron carbide layer remains intact but interface reactions take place in the buried Cr-SiO₂ region. After 300°C the Cr layer diffuses towards the substrate. At higher temperatures of 500°C and 550°C the Cr reacts with the native oxide layer and tends to form a low-density compound of chromium oxysilicide (CrSiO_x). Depth profiling of Si and Cr distributions obtained from secondary ion mass spectroscopy measurements corroborate the layer model obtained from the soft X-ray reflectivity analyses. Details of the interface reaction taking place near the substrate region of boron carbide/Cr sample are discussed.

Study of Higher Diffraction Order Contribution in a Flat Field Grating Spectrograph Using a High-Order Harmonic Source

An experimental study has been carried out to characterize and investigate the performance of a flat-field grating spectrograph (FFGS) for higher diffraction order in the wavelength range of 80-170 Å. An aberration-corrected, mechanically ruled spherical grating with variable line spacing was used as a dispersive element in the spectrograph. The study was carried out using high-order harmonic radiation generated by an interaction of Ti:sapphire laser pulses with inert gas-filled cells. It was observed that the fraction of photons diffracted in the second diffraction order to that of the first order is as high as ∼ 65% at ∼ 80 Å wavelength, which reduces to ∼ 15% at ∼ 150 Å, whereas for the third diffraction order, the fraction was substantially lower, ∼ 15% at ∼ 100 Å. The observed results match well with reflectivity calculated using REFLEC software and also with the measurement carried out using INDUS-1 reflectivity beamline at RRCAT, Indore. The use of a high-order harmonic source for the study is advantageous, as the higher diffraction order contribution can be estimated from a single spectral image, which can be recorded in a single laser shot. The study will be useful for estimation of conversion efficiency-photon flux of a source using a variable line spaced (VLS) grating based spectrograph/monochromator.

Optical constants of e-beam-deposited zirconium dioxide measured in the 55-150 Å wavelength region using the reflectivity technique

In the present study, optical constants of e-beam-deposited zirconium dioxide (ZrO₂) thin film are determined in the 55-150 Å soft x-ray wavelength region using the angle-dependent reflectivity technique. Soft x-ray reflectivity measurements are carried out using the reflectivity beamline at the Indus-1 synchrotron radiation source. Derived optical constants (δ and β) are compared with the tabulated values of Henke et al. [http://henke.lbl.gov/optical_constants/asf.html]. It is found that the measured δ values are consistently lower than the tabulated bulk values in the 70-150 Å wavelength region. In this region, the delta values are lower by 19%-24% from the tabulated data. Below the Zr M₄ edge (66.3 Å), a deviation in delta values is found as ∼2%-21%. These changes are attributed to growth-related defects (oxygen and voids) and variation in film stoichiometry. To the best of our knowledge, the present study gives the first reported experimental values of optical constants for ZrO₂ in the 55-150 Å wavelength region.

Analysis of higher harmonic contamination with a modified approach using a grating analyser

Soft x-ray spectra of the toroidal grating monochromator (TGM) at the reflectivity beamline of Indus-1 synchrotron source are analyzed for higher harmonic contribution. A diffraction grating of central line spacing 1200 l/mm is used to disperse the monochromatic beam received from TGM to quantify the harmonic contents in the 50-360 Å wavelength range. In order to calculate the harmonic contamination, conventionally the intensity of higher order peak is divided by first order peak intensity of the desired wavelength. This approach is found to give wrong estimate as first order peak itself is overlapped by higher order peaks. In the present study, a modified approach has been proposed to calculate harmonic contamination where the intensity contributions of overlapping orders have been removed from the first order diffraction peak of the desired wavelength. It is found that the order contamination in the TGM spectra is less than 15% in the wavelength range of 90-180 Å. The total harmonic contribution increases from 6%-60% in the wavelength range of 150-260 Å. The critical wavelength of Indus-1 is 61 Å hence the harmonic contamination below 90 Å is significantly low. The results obtained with modified approach match well with those obtained by quantitative analysis of multilayer reflectivity data. The obtained higher harmonics data are used to fit the transmission of aluminum edge filter in the 120-360 Å wavelength range.

Restoration of absolute diffraction efficiency and blaze angle of carbon contaminated gratings by ultraviolet cleaning

An UV cleaning technique has been used for the removal of carbon contamination from grating surface, using oxygen free-radicals generated by the absorption of UV radiation (172 nm) in atmospheric oxygen. The extent of restoration of absolute diffraction efficiency (ADE) and grating blaze angle of a carbon-contaminated variable line-spaced concave grating (average line spacing of 1200 lines/mm, blaze angle 3.2 deg, and blaze wavelength 10 nm) has been studied over a 5-70 nm wavelength range. The contamination (due to prolonged use in extreme-ultraviolet spectrograph) resulted in a drastic reduction in the diffraction efficiency of grating, along with a change in the blaze angle. The UV cleaning led to the restoration of blaze angle as well as to an increase in the ADE by more than an order of magnitude for the first two diffraction orders (i.e., from ~0.2 to ~7.2%) for the first diffraction order and 0.1 to ~5% for the second diffraction order at the blaze wavelength. The study is useful for the restoration of carbon-contaminated costly optics.