Nanoresolution interface studies in thin films by synchrotron x‐ray diffraction and by using x‐ray waveguide structure

In the last years we performed several measurements with synchrotron radiation of several facilities to reveal interesting interface phenomena on the nanoscale. We used both x‐ray diffraction [1] , [2] (XRD) and spectrometry techniques. In this paper, we briefly summarize the results obtained from diffraction measurements, which lead us to our recent grazing incidence x‐ray fluorescence analysis (GIXRF) and extended x‐ray absorption fine structure (EXAFS) experiments. We show how a combination of experimental methods of GIXRF analysis and EXAFS spectroscopy in fluorescence detection with x‐ray standing waves (XSW) technique was applied for the depth profiling of a‐Si/Co/a‐Si layers with nanometer resolution to monitor the growth of CoSi intermetallic phase. The investigated layers were placed into the waveguide structure formed by two Ta films to increase sensitivity and accuracy of the measurements. Copyright © 2009 John Wiley & Sons, Ltd.     

Portable x‐ray fluorescence spectrometer with a pyroelectric x‐ray generator

A portable x‐ray fluorescence spectrometer was assembled with an x‐ray generator that was driven by a 9 V dry electric battery. Several possible optimum geometries of the x‐ray generator and detector were evaluated, and the results showed that the intensity of fluorescent x‐rays was strong enough when the angle between the x‐ray generator and detector was as small as 30°. The geometrically optimized x‐ray spectrometer was applied to the analysis of paints, plastics and aluminum foils. Pigments in paint and toxic elements in plastic could be easily detected with on‐site analysis. Fe in aluminum foil was quantitatively determined down to the sub‐% level. The detection limit of Fe was 180 ppm for 100 s of measurement. Copyright © 2005 John Wiley & Sons, Ltd.     

Determination of the composition of Ultra‐thin Ni‐Si films on Si: constrained modeling of electron probe microanalysis and x‐ray reflectivity data

The homogeneous bulk assumption used in traditional electron probe microanalysis (EPMA) can be applied for thin‐layered systems with individual layers as thick as 50 nm provided the penetration depth of the lowest accelerating voltage exceeds the total film thickness. Analysis of an NIST Ni‐Cr thin film standard on Si using the homogeneous model yielded certified compositions and application of the same model to ultra‐thin Ni‐Si layers on GaAs yielded their expected compositions. In cases where the same element is present in multiple layers or in the substrate as well as the film, the homogeneous assumption in EPMA alone is not sufficient to determine composition. By combining x‐ray reflectivity (XRR) thickness and critical angle data and using an iterative approach, quantitative compositional data in EPMA can be achieved. This technique was utilized to determine the composition of Ni‐Si ultra‐thin films grown on silicon. The Ni‐Si composition determined using this multi‐instrumental technique matched that of Ni‐Si films simultaneously deposited on GaAs. Copyright © 2008 John Wiley & Sons, Ltd.     

Calculation of attenuation and x‐ray fluorescence intensities for non‐parallel x‐ray beams

With the nowadays widespreaded use of x‐ray optics in x‐ray fluorescence analysis, large convergence or divergence angles can occur. This experimental situation violates a basic assumption of the usual fundamental parameter quantification procedure. In order to take beam divergences in micro x‐ray fluorescence analysis into account, a way of calculating fluorescence intensities numerically by Monte Carlo integration is described. For three examples of typical micro‐XRF set‐ups the fluorescence intensities and their deviation from the parallel beam geometry are calculated. Furthermore, we propose a new approach with ‘equivalent angles’ which correct for the beam divergences in fundamental parameter methods. Copyright © 2003 John Wiley & Sons, Ltd.     

Performance of polycapillary x‐ray lens for x‐ray sources with various spot sizes

The performance of a polycapillary x‐ray lens (PCXRL) is related to the spot size of the x‐ray source. The transmission efficiency of the PCXRL decreases generally with the increase of the x‐ray source spot size. Both the output focal spot size of a focusing PCXRL and the divergence of a parallel PCXRL increase with increasing source spot size. A large source spot size results in more ‘escape halo’, which affects the measured transmission efficiency and the measured output focal spot size. Copyright © 2007 John Wiley & Sons, Ltd.     

Short soft x‐ray sources

In this work we report on the characterization of pulsed soft x‐rays emitted from laser‐produced plasma. The plasma was generated by a pulsed KrF excimer laser focused on pure Si, Cu and Ta targets by 40, 80 and 120 mJ laser energies. The utilized detector was a very sensitive Faraday cup which opportunely biased was able to record time‐resolved signals of x‐rays and to estimate their energy. The found x‐rays energy values were compared with the ion temperature of the plasma obtained by fitting the time‐resolved ion current signals with a shifted Maxwell‐Boltzmann velocity distribution. The results showed that the laser‐produced Ta plasma induced bunches of x‐rays having in average the highest energy values and it was also characterized by ion temperature higher than the ones of the laser‐produced Si and Cu plasmas. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization of homemade x‐ray polycapillaries

The possibilities of performing non‐destructive elemental microanalysis with synchrotron radiation by means of homemade polycapillary optics are described. Using fiber glass technology, monolithic polycapillary half‐lenses were made by drawing packages of glass capillaries at high temperature in a heating furnace. The performance of polycapillary half‐lenses with different geometries was evaluated. A gold knife‐edge scanned at the output of the lenses was used to determine the spot size at different distances. Measurements of photon flux impinging on the entrance end of the lenses and photon flux leaving the lenses were used to obtain the transmission efficiency. The polycapillary half‐lens made in our laboratory was used in a series of XRF microanalysis tests using white beam. The overall efficiency of the system was studied by means of establishing detection limits for various elements in an organic sample. Copyright © 2008 John Wiley & Sons, Ltd.     

Active region of x‐ray emission

The active region of x‐ray emission was studied. The x‐ray emission rates for K and L x‐rays were calculated as a function of the maximum radial distance in integration of matrix elements for radiative transitions and the radius of the active region of x‐ray emission was determined. This radius depends on the multipolarity of the transition and also the sum of numbers of nodes of the wavefunctions concerned. The approximate expression for the radius of the active region is obtained in terms of the mean radius of the wavefunction for the initial vacancy state. Applications of the present results to multiple ionization and to chemical effects are discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

The capabilities of an electrically cooled energy‐dispersive x‐ray detector for quantitative x‐ray fluorescence analysis

The capabilities of the Si PIN diode x‐ray detector were determined and compared with those of a standard Si(Li) detector. The x‐ray fluorescence (XRF) analysis systems assembled with these two detectors included annular radioisotope excitation sources of Cd‐109 and Fe‐55. The systems were calibrated for sensitivity and quantification was performed with fundamental parameters software. Based on the analysis of the standard reference material NIST 2710 (Montana soil), the elemental sensitivities and the limits of detection of both systems were obtained. The elemental sensitivities of the Si PIN detector for fluorescence x‐rays in the energy range up to 10 keV were comparable to those of the Si(Li) detector. At higher fluorescence x‐ray energies the sensitivity of the Si PIN detector gradually decreased and was smaller by a factor of ～4 at 20 keV. The reason was mainly the small thickness of the sensitive volume of the Si PIN diode (0.2 mm) and therefore the smaller relative efficiency of this detector. The assessed limits of detection (LODs) were comparable for the two detectors, which was mainly due to the lower spectral background of the Si PIN detector in excitation with the Cd‐109 source as a result of its smaller sensitive thickness. The accuracy of elemental determinations for the two detectors was comparable and within the limits of the assessed uncertainties, which were calculated considering all the steps of the analysis, i.e. spectrum measurement and analysis, sensitivity calibration and quantification. Copyright © 2004 John Wiley & Sons, Ltd.     

HOPG as powerful x‐ray optics

A new method of producing strongly bent highly oriented pyrolytic graphite (HOPG) is presented. The method allows one to make a crystal of almost arbitrary shape, but has some thickness limits. A review of the results obtained on this type of crystal by different laboratories during the last 10 years is given. An x‐ray intensity gain from a synchrotron radiation source and Mo Kα tube beams of one order of magnitude could be achieved when using a crystal of optimized shape. The application of the crystals for secondary monochromatization in EDXRF improves by one order of magnitude the detection limits for the elements of interest. Copyright © 2003 John Wiley & Sons, Ltd.     

X‐ray fluorescence analysis with a pyroelectric x‐ray generator

Several types of handy x‐ray fluorescence spectrometers are presented. The results obtained with a Niton spectrometer are presented as a goal to develop a laboratory‐made spectrometer using an Amptek Cool‐X pyroelectric x‐ray generator. A small and cheap charge‐up x‐ray emitting device and its spectrum are also presented. Handy x‐ray spectrometers are now progressing rapidly and the detection limits are in the range of a few ppm for certain elements. Copyright © 2005 John Wiley & Sons, Ltd.     

Thin‐film characterization with x‐ray microanalysis. Extending and improving invariant embedding results

Invariant embedding theoretical results for the recorded x‐ray intensities in electron probe microanalysis were used to develop a procedure for thin films. The method presented here gives the possibility of achieving thin‐film characterization directly from characteristic x‐rays intensities measurements, without the necessity to make an explicit calculation of the ?(ρz) function. Theoretical results are found to follow the general trend of experimental data. Copyright © 2003 John Wiley & Sons, Ltd.     

High‐resolution x‐ray analysis with multilayer gratings

Periodic multilayers are nowadays widely used to perform x‐ray analysis in the soft x‐ray range (photon energy lower than 1 keV). However, they do not permit to obtain high‐resolution spectra such as natural or synthetic crystals. Thus, multilayers cannot resolve interferences between close x‐ray lines. It has been shown and demonstrated experimentally that patterning a grating profile within a multilayer structure leads to a diffractive optics with improved resolving power. We illustrate the use of an Mo/B₄C multilayer grating in the Fe L and C K spectral ranges, around 700 and 280 eV, respectively. First, in the Fe L range, the improved spectral resolution enables us to distinguish the Fe Lα and Lβ emissions (separated by 13 eV). In addition, using a sample made of a mix of LiF and an iron ore, we show that it is possible to easily resolve the F K and Fe L emissions. These examples demonstrate that an improved x‐ray analysis can be obtained with multilayer gratings when there is the need to study samples having elements giving rise to close emission lines. Second, in the C K range, by comparing C Kα spectra from B₄C and cellulose, we show that the shape of the emission band is sensitive to the chemical state of the carbon atom. Copyright © 2009 John Wiley & Sons, Ltd.     

Calculation of the transmission efficiency of a monolithic polycapillary x‐ray lens for an x‐ray source of any shape

A method for calculating the transmission efficiency of a monolithic polycapillary x‐ray lens is presented. Using this method, the transmission efficiency of an x‐ray source of any shape can be calculated by using the transmission efficiency measured with a microfocus x‐ray source. Copyright © 2005 John Wiley & Sons, Ltd.     

Spectrum function of polarizer‐scattered x‐ray tube radiation

An equation was derived to calculate the x‐ray tube spectrum from a massive anode and scattered from a homogeneous flat target. The spectrum function of molybdenum anode radiation scattered from a boron carbide polarizer was calculated. Copyright © 2004 John Wiley & Sons, Ltd.     

Planar waveguide‐resonator: a new device for x‐ray optics

Systematic investigations of the width dependence on the x‐ray beam propagation mechanism for a narrow extended slit formed by two plane dielectric plates are presented. It is shown that the mechanism of a multiple consecutive total reflection for Cu Kα radiation dominates in a slit width range s ≥ 3 µm. At the same time the manner of Cu Kα radiation propagation for super‐narrow slits s ≤ 0.1 µm is very different from the multiple total reflection mechanism. The x‐ray beam intensity proves to be constant for all this range of magnitude. This gives grounds to expect that the super‐narrow slit area is characterized by a specific type of mechanism of x‐ray beam propagation: waveguide‐resonance. A simple model for the waveguide‐resonance propagation mechanism based on the formation of a uniform x‐ray standing wave interference field in the total space of a narrow extended slit was developed. The design of a new x‐ray optical device, namely a planar x‐ray waveguide‐resonator, is proposed based on the waveguide‐resonance mechanism. Some properties of the composite planar x‐ray waveguide‐resonator are discussed. It is shown that under specific conditions the composite waveguide can demonstrate a partial tunneling effect of the x‐ray beam. The main applications of the new technique are discussed. Copyright © 2004 John Wiley & Sons, Ltd.     

Chemometrics and its applications to x‐ray spectrometry

Recent developments in curve fitting, multivariate calibration, and pattern recognition in chemometrics, and their application to x‐ray spectrometry, are reviewed. Relatively innovated algorithms, namely genetic algorithms, neural networks and support vector machines, are discussed. Together with the three algorithms, the performances of different algorithms are compared briefly, which mainly includes principal component analysis, partial least‐squares regression, factor analysis, cluster analysis, nearest neighbor methods, linear discriminant analysis, linear learning machine, and soft independent modeling of class analogy. In general, the chemometrics methods are superior to the conventional methods, such as Fourier transform and Marquardt–Levenberg algorithms, to a certain extent. Copyright © 2006 John Wiley & Sons, Ltd.     

Semiconductor Peltier‐cooled detectors for x‐ray fluorescence analysis

We discuss recent results obtained in the development of Si(Li), Si p–i–n, CdTe p–i–n and CdZnTe x‐ray detectors with Peltier coolers for fabrication of laboratory and portable XRF analyzers. The characteristics of Si(Li) Peltier‐cooled detectors are close to those of detectors cooled with the liquid nitrogen and remain the most preferred type of detectors for the tasks of x‐ray fluorescence analysis. Considerable success was obtained in the improvement of the characteristics of CdTe p–i–n detectors and CdZnTe detectors with a metal–semiconductor–metal structure, effective in the energy range up to 100 keV. The spectra of all detectors are presented. Copyright © 2004 John Wiley & Sons, Ltd.     

X‐ray reflectivity from curved liquid interfaces

X‐ray reflectivity studies of the structure of liquid–vapour and liquid–liquid interfaces at modern sources, such as free‐electron lasers, are currently impeded by the lack of dedicated liquid surface diffractometers. It is shown that this obstacle can be overcome by an alternative experimental approach that uses the natural curvature of a liquid drop for variation of the angle of incidence. Two modes of operation are shown: (i) sequential reflectivity measurements by a nanometre beam and (ii) parallel acquisition of large ranges of a reflectivity curve by micrometre beams. The feasibility of the two methods is demonstrated by studies of the Hg/vapour, H₂O/vapour and Hg/0.1 M NaF interface. The obtained reflectivity curves match the data obtained by conventional techniques up to 5α_c in micro‐beam mode and up to 35α_c in nano‐beam mode, allowing observation of the Hg layering peak.