Critical-Dimension Small-Angle X-ray Scattering (CD-SAXS) is an x-ray scattering technique that can be used to reconstruct the in-plane and out-of-plane structure of nanostructured thin-films. The technique consists of collecting a series of transmission SAXS images, at a variety of sample rotation angles. These images can be combined to reconstruct the 3D reciprocal-space, in particular probing the slice that contains both in-plane and out-of-plane (film normal direction) information.
The technique derives its name from CD-SEM, a realspace microscopy used measure the 'critical dimensions' of a structure. CD-SAXS can also be called rotational-SAXS (RSAXS); indeed the neutron variant is typically called RSANS. It is closely related to a variety of other scattering/diffraction techniques that involve rotating the sample in order to reconstruct reciprocal-space (c.f. pole figures).
CD-SEM is frequently used in the lithography and nanofabrication industry as a metrology for the quality of fabrication process. Similarly, CD-SAXS is ideally suited to quantifying the average structure of well-defined entities such as lithographic line-gratings. Indeed, CD-SAXS can reliably probe a grating's repeat period, height, and sidewall angle (or, more generally, the grating's cross-sectional profile). In principle, this technique can quantify aspects of defects and disorder (e.g. line-edge roughness, LER).
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- GISAXS can directly measure the plane in a single image, but introduces a refraction distortion and beam projection.
- GTSAXS can measure the plane in a single image without distortion, but imposes constraints on sample geometry.
- NIST Dimensional Metrology for Nanoscale Patterns
- NIST tutorial video