Characterizing profile tilt of nanoscale deep-etched gratings via x-ray diffraction

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The authors report the development of fast, nondestructive, and high accuracy metrology for the characterization of profile tilt relative to the surface normal in nanoscale gratings using x-ray diffraction. Gratings were illuminated with a collimated x-ray beam (Cu Kα), similar to variable-angle small-angle x-ray scattering, to record changes of diffraction efficiency (DE) as a function of incidence angle. Simulations using scalar diffraction theory and rigorous coupled wave analysis predict extrema (0th order DE minimized, ±1st order DE maximized) when local grating bars are parallel to the incident x-ray beam. The surface normal was measured independently by reflecting a laser beam from the grating surface. The independent measurements using x rays and laser beams were referenced to each other via a slit reference plane to characterize the bar tilt angle relative to the surface normal. The fast x-ray measurement can be repeated at arbitrary points to study the spatial variation of the bar tilt angle across large gratings. Two test gratings etched with different deep reactive-ion etch chambers were prepared to investigate the performance of the proposed method. The authors report a repeatability of <0.01° and an accuracy of ∼0.08° with a fast scan speed (total integration time of 108 s to scan a line across ∼55 mm large grating samples at an interval of ∼2 mm). High spatial resolution (<50 μm) can be easily achieved at the expense of speed by limiting the incident x-ray spot size. This process is applicable to any periodic nanostructure as long as x-ray diffraction is well modeled.