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Grazing-incidence X-ray analysis techniques which are commonly used for the nondestructive characterization of surfaces and thin films are reviewed. The X-ray reflectivity technicue is used to study surface uniformity and oxidation, layer thickness and density, interface roughness and diffusion, etc. The grazing-incidence in-plane diffraction technique is used to determine in-plane crystallography of epitaxial films. The grazing-incidence asymmetric-Bragg diffraction is used for surface phase identification and structural depth profiling determination of polycrystalline films. Typical examples to illustrate the types of information that can be obtained by the techniques are presented.
The operation of a new polycrystalline phase identification method using the IBM Series/1 minicomputer is described. Data of the unknown can be entered by automatic transfer of previous runs, stored data sets and manually. Full screen menu selections are provided to facilitate operations and correct entries. Typical S/M time for a multi-phase inorganic mixture containing 43 reflections using a 0.3° window averaged 11 sec per 100 standards and with simple chemical prescreening less than 4 sec including program initialization and calculations of comprehensive figures of merit. Interactive options provide graphics terminal comparison of the unknown pattern with selected standards which appear as diffractometer patterns, subtraction of identified standards from the unknown and others. Utility programs permit storing data sets for later analysis, user created files and a program to display any file standard as a diffractometer pattern.
X-ray polycrystalline diffraction was used to track progress toward improving the structural properties of SrS:(Eu,Sm) thin films. These thin films are used as the active layer of the ETOM (Electron Trapping Optical Memory) media. In this study conventional x-ray diffraction and x-ray reflectivity were used to evaluate the effect of two deposition parameters on film structures. Line broadening analysis performed using the Warren-Averbach technique showed the beneficial effects of a hydrogen sulfide reactive atmosphere and the RF magnetron sputtering technique on crystallite size and microstrain. A factor of five improvement in crystallite size and a factor of two reduction in microstrain was observed. Film thickness, density, and interfacial and surface roughnesses were determined for two SrS thin films. The sin2Ψ technique was used to determine the in-plane biaxial stress for two films prepared by different deposition techniques. These films exhibit inhomogeneous stress states with an average stress of less than IMPa.
A method for computer simulation of X-ray powder diffraction patterns which are identical to those obtained experimentally is described. The calculated pattern is generated directly from the d's (or 2θs) and intensities of the phase(s) and is based on a profile fitting algorithm which uses the instrument function to form the profile shapes at all reflection angles. Examples of simulated patterns of mixtures, line broadening, linear and amorphous backgrounds, and counting noise are given.
Results on least-squares refinement of X-ray reflectivity data obtained with a conventional powder diffractometer are reported. A model containing an oxygen contaminated surface on Pt was used to refine experimental data for a “500-Å” Pt film on Si. Values of layer thickness, density, and roughness determined by least-squares refinement agree with those obtained from highresolution reflectivity data. The results were found to be insensitive to the film-surface alignment. An agreement of ±2.3 Å for Pt thickness, 8% for density, and 2.5 Å for roughness was obtained when the surface was aligned to within the divergence of the incident X-ray beam. The least-squares refinement method was also used to analyze two sputtered “300-Å” Pt films deposited at 4 and 20 × 10-6 Torr Ar pressure. Results showed a significant increase in Pt thickness and a decrease in density for the 20 × 10-6 film probably caused by a large amount of Ar trapped in the film.
The integrated and peak intensities of a series of silicon powder samples of various crystallite sizes were measured with a computer automated diffractometer and a profile fitting method (PFM). The accuracy of the PFM was better than 0.003% in computing the integrated intensities. The PFM gave more precise values than would be expected from counting statistics of the peak intensity. The average difference between each measurement and the average intensity was 0.5% with little dependence oo the absolute intensity. Crystallite sizes have a large effect and it is essential to rotate the specimen around the diffraction vector. The best results were obtained with <10 μm particles. Larger sizes decrease the absolute intensities and change the relative intensities. Structure refinement using the POWLS (powder least squares refinement) program showed the presence of (111) preferred orientation even in the <10 μm specimens. R(Bragg) decreased from 4.3% to 0.7% by including the preferred orientation correction in the refinement.
This paper outlines the use of an IBM Series/1 small computer for instrument automation and data reduction for X-ray polycrystalline diffractometry and wavelength dispersive X-ray fluorescence spectrometry. The profile fitting method is used to determine 2θ, d and relative peak and integrated intensities in diffraction, and the fundamental parameters method (LAMA program) is used for quantitative analysis of bulk and thin film samples. The methods are precise and rapid.
The analysis of mixtures of phases which produce complicated composite x-ray powder patterns is greatly facilitated by use of our profile fitting method and the technique of applying it is illustrated with a five-compound mixture. Profile fitting gave higher precision in the determination of the reflection angles and Intensities and resolved overlaps in a much shorter time than with other methods. If the reference standards are obtained with the same precision, a smaller error window width can b e used in the search/match procedure.
A vertical scanning single crystal diffractometer with graphite monochromator and narrow divergent beam controlled by an IBM Series/1 computer was used to study crystal damage in the ion-implanted garnet films including strain effects induced by multi-implantation processes as a function of the types of ions, their energies and doses, growth and annealing temperatures.
The combined derivative method (accompanying paper) was tested with a large number of experimental patterns to illustrate its use in various difficult problems commonly arising in peak search analysis of X-ray diffraction data. Patterns obtained with various step sizes, resolution, counting statistical noise, and profile widths were used. The precision in 2θ determination and overlap resolution are in good agreement with those previously obtained from calculated profiles, raise identification of noise as diffraction peaks was eliminated by using a convolution range proportional to the full width at half maximum. Peak search results (both 2θ and intensity) were also compared to those obtained by profile fitting to illustrate the different characteristics of these two methods.
A precise and practical method for the determination of d-values and lattice parameters from digital diffraction data is described. Systematic errors are corrected mathematically during a d-spacing / lattice-parameter least-Squares refincment process making it unnecessary to use internal standards. X-ray and synchrotron diffraction data of an ICDD alumina plate obtained with a wide variety of experimental conditions and analysis parameters were used to study the precision in the derivation of d-values and the accuracy in the determination of lattice parameters. Results showed that the precision in determining d-values was high with |Δd/d|avg ranging from 2x105 to 4x10-5. Using the results obtained from the high precision XRD analysis as a reference standard, the accuracy in the lattice parameter determinations from the synchrotron diffraction data reached the l-2x10-6] range. Lattice parameters, with an accuracy in the high 10-5 range, were also obtained using parameters commonly used in a routine XRD analysis such as a wide RS (0.11°) for high intensity, peaks only in the front reflection region, no Kα2 stripping, and a Single 2θo parameter for systematic error corrections.
A tcchniquc for high-precision measurement of carbon thin-film thickness using X-ray fluorescence (XRF) is described. A quadratic calibration procedure is used for carbon thin films on silicon. Measurement of carbon-film thickness in a double-layer structure of carbon and CoCrX alloy is complicated by interference effects from the underlying layer. The dependence of the relative precision in measuring thickness (σT/T) on the counting time has been derived. It shows that a precision of 2% for a 25-nm carbon coating can be obtained using a W/C crystal and counting time of 4 minutes. Intensity and resolution advantages provided by the recently developed Ni/C and V/C multilayer synthetic crystals are also described.
This paper presents a comprehensive study of various applications of x-ray fluorescence and diffraction techniques for the characterization of thin films. With the proper use of x-ray instruments and techniques, a fairly complete understanding of the chemical and physical structure of thin films was obtained. The x-ray fluorescence (XRF) method was used for the determination of composition, mass-thickness, and density. The x-ray diffraction (XRD) method was used for structural characterization, including: local atomic arrangements of amorphous materials; phase identification, preferred orientation, crystallite size, stacking faults, microstrain, the annealing behaviors of polycrystalline films; and lattice mismatch between the epitaxial film and its single-crystal substrate.
In order to obtain a clear picture of the capabilities of the x-ray method and the properties of thin films, a series of carefully selected specimens representing a wide range of compositions and thicknesses was used. A number of practical x-ray techniques, which are valuable for this type of analysis, are also introduced.
The structural characterization of thin films is important for research development and manufacturing of electronic, magnetic, optical, and other high-tech materials. The grazing incidence X-ray diffraction technique has bean used successfully for the determination of crystalline phases, structural-depth profiles, crystallite size, and strain, etc. of thin films with thickness's down to a few tens of Å, If the crystal structure, e.g. the distribution of atoms in the unit cell, or the crystallinity and texture (or preferred orientation) of a film is of interest, the conventional Bragg-Brentano diffractometer technique with the θ-2θ scanning geometry has been found to be appropriate.
A comprehensive study of derivative methods for the peak search analysis of X-ray diffraction data was made to determine the relative merits of the methods. The peak positions were best determined by the cubic first derivative method which had an intrinsic error ≤ 0.001°, and random error ∼ ± 0.003 ° to 0.02 ° depending on the counting statistical noise. The quadratic/cubic second derivative method had the highest resolution with a separation limit ≥ 1/2w (w = full width at half maximum). An effective algorithm combining the cubic first derivative and the quadratic/cubic second derivative methods was developed for high precision and resolution. The method uses a full screen menu for parameter selection, and the entire peak search analysis including peak identification and position determination, and graphic and numeric display of results at the color terminal is completed in a few seconds using a time sharing mode on an IBM 3083 central processing unit. The combined derivative method should be also applicable to other spectra such as gamma-rays, X-ray fluorescence, optical, infrared, ESCA, Mossbauer, etc.
An effective and practical computer algorithm has been developed for rapid and precise phase identification of polycrystalline materials by X-ray diffraction methods using the JCPDS database and/or user created standard files. The entire JCPDS file was reorganized for efficient search. Identifications are facilitated by a number of options: automatic correction of systematic errors using internal standard reflections, selectable window widths for file searching, elemental restrictions (chemical prescreening), handling preferred orientation, match without using intensities, match with 3 reflections, and others. A comprehensive algorithm for calculating a figure-of-merit (FOM) is used so that the “correct” phases can easily be identified with highest FOMs. This method has been tested extensively on a wide variety of analyses and is applicable to either a host or a minicomputer.
A method for resolving overlapping X-ray fluorescence spectra by curve fitting is described. The profile shape of an experimental fluorescence line obtained by wavelength dispersive method is represented by a simple pseudo-Voigt function, i.e. a sum of an asymmetric Gaussian and Lorentzian, each of equal width. Results showed that the pseudo-Voigt function matched the experimental profiles with high reliability. The relative Gaussian and Lorentzian contents and the asymmetry of the profiles depended upon the analyzing crystal, coliimating system and the 2θ peak position. For fixed crystal and collimator the smaller the 2θ, the larger the Gaussian content and the lower the asymmetry. The original Gaussian and Loretzian components of the exact Voigt function calculated from the parameters of the fitted pseudo-Voigt function explain the broadening effects of the X-ray emission lines and the instrumental aberrations on observed spectra. Curve fitting method with the psuedo- Voigt function has been used successfully to analyze overlapping fluorescence spectra. Examples and applications include a thin film sample where the Kα and the Kβ lines of adjacent transition elements overlap, and a strontium zirconium oxide specimen where the Zr Kα and the Sr Kβ lines strongly interfere. Concentrations obtained from the resolved individual peak intensities of Zr and Sr Kα lines are within ±1% of the true values.
The instrumentation developed for poly crystalline diffractometry using the storage ring at the Stanford Synchrotron Radiation Laboratory is described. A pair of automated vertical scan diffractometers was used for a Si (111) channel monochromator and the powder specimens. The parallel beam powder diffraction was defined by horizontal parallel slits which had several times higher intensity than a receiving slit at the same resolution. The patterns were obtained with 2:1 scanning with’ a selected monochromatic beam, and an energy dispersive diffraction method in which the monochromator is step-scanned, and the specimen and scintillation counter are fixed. Both methods use the same instrumentation.
X-ray diffraction techniques have been used for the structure characterization of Y-Ba-Cu-O and Tl-Ca-Ba-Cu-O thin films. A powder diffraction analysis of Y-Ba-Cu-O films showed that the films deposited at 650°C on Si are polycrystalline and have an orthorhambic structure similar to that of the YBa2Cu3O7 bulk superconductors. In addition to the conventional powder diffraction technique, both the rocking curve and the grazing incidence diffraction methods were used to characterize a YBa2Cu3O7 film on (110) SrTiO3 substrate. Results showed that the film was epitaxially grown and aligned with its substrate in a true epitaxy. Phase identification and line broadening analyses of Tl-Ca-Ba-Cu-O films showed that the films are comprised of one or more superconducting phases and probably contain stacking faults.