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, which is used to define the 'critical dimension' 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).

References

Gratings

• Wen-li Wu, Eric K. Lin, Qinghuang Lin and Marie Angelopolous [Small angle neutron scattering measurements of nanoscale lithographic features] Journal of Applied Physics 2000, 88, 7298 doi: 10.1063/1.1324688
• Ronald L. Jones, Tengjiao Hu, Eric K. Lin, Wen-Li Wu, Rainer Kolb, Diego M. Casa, Patrick J. Bolton and George G. Barclay Small angle x-ray scattering for sub-100 nm pattern characterization Applied Physics Letters 2003, 83, 4059 doi: 10.1063/1.1622793
• Tengjiao Hu, Ronald L. Jones, Wen-li Wu, Eric K. Lin, Qinghuang Lin, Denis Keane, Steve Weigand and John Quintana Small angle x-ray scattering metrology for sidewall angle and cross section of nanometer scale line gratings Journal of Applied Physics 2004, 96, 1983 doi: 10.1063/1.1773376
• Chengqing Wang, Ronald L. Jones, Eric K. Lin, Wen-Li Wu, Bryan J. Rice, Kwang-Woo Choi, George Thompson, Steven J. Weigand and Denis T. Keane Characterization of correlated line edge roughness of nanoscale line gratings using small angle x-ray scattering Journal of Applied Physics 2007, 102, 024901 doi: 10.1063/1.2753588

Lithographic structures

Charles M. Settens; Aaron Cordes; Benjamin D. Bunday; Abner F. Bello; Vimal K. Kamineni; Abhijeet Paul; Jody Fronheiser; Richard J. Matyi Assessment of critical dimension small-angle x-ray scattering measurement approaches for FinFET fabrication process monitoring Journal of Micro/Nanolithography, MEMS, and MOEMS 2014, 13 (4), 041408 doi: 10.1117/1.JMM.13.4.041408

See Also

• GTSAXS can measure the $\scriptstyle (q_{y},q_{z})$ plane in a single image (but constrains sample geometry).