Fast line-by-line metrology of X-ray gratings

We describe an optical metrology tool for the measurement of X-ray gratings, demonstrated for pattern densities up to 600 lines/mm. Reflection gratings illuminated at glancing angles of incidence are the central dispersive elements used in many short-wavelength monochromators, spanning the energy spectrum from the vacuum ultraviolet to tender X-ray. In modern designs, gratings can be hundreds of mm long, and for high performance, their line-placement accuracy requirements are below 50 nm, a small fraction of the grating spatial period. Gratings with uniform and variable line spacing are both important in X-ray optics. We describe line-by-line pattern measurement using a focused 405 nm laser beam and a distance-measuring interferometer, achieving measurement precision as low as 11 nm rms and speeds up to 0.07 mm/s. Periodic variations in speckle intensity reveal the motion of the grating pattern.