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

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 Ka), 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 mu m) 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. Published by the AVS.