Small-Angle X-ray Scattering
X-ray reflectometry is a technique for investigating the near-surface structure of many materials. It probes the electron density with a depth resolution of less than one nm for depths of up to several hundred nm. The method involves measuring the reflected X-ray intensity as a function of X-ray incidence angle (typically small angles are used). The method is used for studies of thin films and multilayers of metals, semiconductors and polymers. It can accurately determine films thickness, density, average roughness, and the roughness correlation function.
Grazing Incidence X-ray Scattering and Diffraction on Thin Films
Grazing incidence X-ray scattering or diffraction (GIXS) refers to a method where the incident X-ray beam makes a small (typically about 1 degree) angle to the sample surface. This has an advantage, which is particularly important for thin films, of limiting the penetration depth of the X-rays into the sample with consequently low background scattering from the substrate. Often, the exit angle is also small and, thus, the scattering vector, Q, is in the plane of the sample; one measures diffraction from planes perpendicular to the sample surface. This geometry is shown in Figure 3. By varying the incidence angle, one can change the penetration depth of the X-rays from several nm up to typically several 100 nm (determined by the absorption length). In some cases, it is useful to have a small incidence angle, but a large exit angle. This preserves the low background from the grazing incidence geometry, but allows measurements with Q tilted with respect to the sample surface (i.e., the diffraction planes are tilted).
Surface diffraction refers to diffraction applied to two dimensional, adsorbed layers, surface reconstruction and relaxation, and buried interfaces; this area includes a number of methods, including GIXS and reflectivity. Surface diffraction is frequently used for surface crystallography - the determination of atomic structure of adsorbed layers and at surfaces and buried interfaces. This involves intensity measurements of Bragg diffraction rods (from two dimensional layers) and crystal truncation rods (from surfaces and interfaces). Surface diffraction is also used in studies of surface and adsorbed-layer phase transitions.
Small Angle X-ray Scattering for Materials Science
Small-angle X-ray scattering (SAXS) is a well-established characterization method for microstructure investigations in various materials. It can probe structural inhomogeneities (really electron density differences) from the near atomic scale (1nm) to the micron scale (1000nm). The method involves measuring the scattered X-ray intensity as a function (typically small) scattering angle angles and is generally performed in transmission. SAXS is used to characterize the size scale of inhomogeneities (e.g, pores, inclusions, second phase regions) in polymer blends, microemulsions, geological materials, bones, cements and ceramics.