# CD-SAXS

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 $\scriptstyle (q_x, q_y)$ images can be combined to reconstruct the 3D reciprocal-space, in particular probing the $\scriptstyle (q_y, q_z)$ 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).