(a) Scattering images of a silicon grating at an incident angle of = 0.76°. The direct beam position (qz = 0) and the surface horizon position () are denoted by the black dotted line and the blue solid line, respectively. (b) A cross-sectional SEM view of the grating. It is modeled by a rectangular shape with a trapezoidal bottom. (c) The intensity profile along qz of first- (blue) and fourth-order (red) BRs [taken from the rectangular areas marked in panel (a)], fitted with the corresponding form factor (solid lines). (d) A three-dimensional representation of the form factor of the modeled shape shown in panel (b).