Automated X-Ray Scattering

February 1, 2007
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Designed for the structural analysis of advanced materials and thin films, a new X-ray diffraction system combines flexible instrument geometry with a comprehensive knowledge-based software platform.

Above: The SmartLabTM X-ray diffraction system.


X-ray scattering is a powerful tool in the determination of the structure of materials. It is possible to calculate the positions of scattering centers within a material by looking at the pattern of X-rays scattered by the sample. Since X-rays are scattered by electrons, these scattering centers are associated with regions of electron density. Atoms, molecules, nanoparticles, pores, etc. can all give rise to characteristic X-ray scattering patterns that can be modeled to determine structural features in advanced materials.

In the ceramic industry, X-ray scattering is used routinely to examine the phase composition of bulk materials, thickness and perfection of thin films, and residual stress and texture of composites. Applications exist across a range of ceramic disciplines, from basic research and process development to QC/QA analyses for materials production. Examples include the study of ferroelectrics for optical devices, hydroxyl apatites in dentistry, and high Tc superconductors for power transmission.

An X-ray diffractometer is an instrument that measures the intensity of scattered X-rays as a function of position relative to the sample. The instrument consists of a source of X-rays, a goniometer to move the sample into different orientations, and a moveable detector to detect X-rays at different positions. For the non-specialist, these measurements can be complicated to make, especially when many different types of measurements are required for the complete characterization of a sample. Traditional, multi-purpose diffractometers require the reconfiguration of a number of critical optical components in order to be optimized for a particular measurement type. These reconfigurations can be time consuming and lead to imprecise results when done incorrectly.

Figure 1. The system's patented Cross Beam Optics.

An Award-Winning Alternative

Recently, a new high-resolution diffractometer has been developed with horizontal sample mounting theta-theta geometry.* Designed for the structural analysis of advanced materials and thin films, the system combines flexible instrument geometry with a comprehensive knowledge-based software platform. The unit enables users with little or no expertise in diffraction methods to quickly and easily make advanced structural measurements.

The new system offers a complete range of X-ray diffraction measurements in one fully automated tool suitable for use by the non-specialist. It combines a high-resolution, high-powered horizontal sample mount X-ray diffractometer with an automated, knowledge-based control system. Offering simplified automated operation, the system can address a full range of samples, including bulk solids, liquids, powders and thin films. In addition, it can analyze all crystalline forms, such as perfect, textured, polycrystalline, disordered and amorphous materials, and provide a complete range of structural X-ray measurements, including powder X-ray diffraction (PXRD), high-resolution X-ray diffraction (HRXRD), grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), and small angle X-ray scattering (SAXS).

PXRD methods use signature diffraction patterns from powdered crystalline materials as fingerprints in the identification of crystalline composites. In contrast, HRXRD and XRR measurements are used to gauge the structure and perfection of thin films and multilayer interfaces. SAXS analysis provides structural information on materials that may not be crystalline or that may have very long periodicities that are difficult to measure with traditional diffraction techniques.

The unit uses Cross Beam Optics (CBO) as the foundation of a fully automated flexible optical system (see Figure 1). CBO allows the system to operate in both parallel beam and focusing geometries without reconfiguring the diffractometer system. Both geometries are permanently mounted, simultaneously aligned and user selectable.

*SmartLabTM, developed by Rigaku Group, Tokyo, Japan, and winner of the 2006 R&D 100 Award.

Figure 2. The software's main status screen shows the system's current configuration and available optics.

The system offers completely automated control and sensing of all downstream monochromators and slit optics. Non-expert users benefit by the time saved and ease of use afforded by a dual geometry system, as well as by the ability of the software** to monitor and recommend optics configurations. The software is used to automate all processes, from the setting and aligning of the optics to sample measurements.

The software is pre-programmed at the factory with various optical alignment, sample alignment, and data analysis applications packages called modular guidance packages (GPAKS). The software's main status screen is shown in Figure 2. GPAKS contain the techniques and know-how to make advanced measurements, such as film thickness, texture and pore/particle size analyses. GPAKS suggest the best optical configuration for each application, check the hardware settings and run automatic alignment sequences.

The unit contains a horizontal sample mount and a theta/theta goniometer with high-resolution scanning in both the plane of the sample surface and perpendicular to the sample surface. This geometry provides users with a simple, uniform, stress-free mount for all sample types. The high-resolution scanning in both parallel and orthogonal directions eliminates the requirement for users to change and realign the instrument configuration when making in-plane and out-of-plane measurements. Also available is a 9 kW rotating anode X-ray source. This high-power source is optimal for many applications, including the measurement of ultra-thin films.

**SmartLab GuidanceTM software, developed by Rigaku Group.

Figure 3. X-ray reflectivity of BaSrTiO3 films drops sharply with the increase in film thickness due to growing surface roughness.

Application Example

Consider the study of a typical ferroelectric ceramic material like BaSrTiO3. Thin films of single crystal BaSrTiO3 have potential applications in piezoelectric actuators, electro-optical devices and non-volatile memories. A crucial aspect of all of these applications is the structural quality of the films in nanometer scale. In this example, X-ray reflectivity, X-ray rocking curve measurements and traditional wide angle XRD were used to analyze the composition and surface roughness of laser deposited BaSrTiO3 thin films of 10, 50 and 150 nm thick on Nb doped SrTiO3(001) substrates.

As shown in Figure 3, the X-ray reflectivity of these films drops sharply with an increase in film thickness due to growing surface roughness. Theoretical modeling of the reflectivity curves, assuming uniform composition throughout the entire thickness, estimates the surface roughness of the 10, 50 and 150 nm films to be about 0.4, 1.0 and 4.4 nm, respectively. Moreover, the 150 nm film shows a smaller critical angle for total external reflection when compared to the 10 and 50 nm films, suggesting that the average density of this film is lower than that of the thinner films. Since BaTiO3 has a higher density than SrTiO3, this means that the 150 nm film has a higher Sr/Ba ratio than the other two films.

Figure 4. X-ray theta/2theta scans in the vicinity of SrTiO3 (002) reflection.

This dependence of average composition on thickness is also evident in the diffraction patterns of these films. From the position of the film peaks, the average out-of-plane lattice constants of the 10, 50 and 150 nm films are about 0.3984 nm, 0.3984 nm and 0.3946 nm, respectively. Because cubic SrTiO3 has a smaller lattice constant (0.391 nm) than the tetragonal BaTiO3 (a=b=0.399 m, c=0.404 nm), this indicates that the 150 nm film contains more Sr than Ba, a conclusion that is consistent with the results of X-ray reflectivity (see Figure 4).

Figure 5. High-resolution X-ray rocking curves of three samples.

Figure 5 shows high-resolution rocking curves of the same three samples. The rocking curves of the 10 and 50 nm samples are narrow and indicate no strain relaxation. The 150 nm film shows a broadened rocking curve, which is a typical feature of relaxed films with dislocations.

Comprehensive Results

As evidenced in the preceding example, the new system can combine three very different measurements to provide detailed information on a series of ferroelectric samples. Automatic alignment, CBO and advanced software combine to create a flexible, intelligence-based X-ray diffraction instrument.

The system's software is used to gather information about the samples, suggest measurement configurations, set up the diffractometer, and execute measurements with the help of an interactive graphical user interface. The components of the overall system design combine to make X-ray diffraction measurements easy and accessible.

For additional information regarding X-ray diffraction instrumentation, contact Rigaku Americas Corp. at 9009 New Trails Dr., The Woodlands, TX 77381; (281) 362-2300; fax (281) 364-3628; e-mail info@Rigaku.com; or visit www.Rigaku.com.

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