The analysis of diffracted light from periodic structures is shown to be a versatile metrology technique applicable to inline metrology for periodic nanostructures. We show that 10 nm changes in periodic structures can be traced optically by means of sub-wavelength diffraction. Polymer gratings were fabricated by electron beam lithography. The gratings have a common periodicity of 6 μm, but different line width, ranging from 370 to 550 nm in 10 nm steps. A comparison between the resulting diffraction patterns shows marked differences in intensity which are used to sense nanometre scale deviations in periodic structures.
Mass production of nanostructured surfaces relies on the periodic repetition of micrometre scale patterns. A unit cell with
nanometre features in the micrometre size range is repeated thousands of times. The ensemble can used as a diffraction
grating for visible light. The relative intensity distribution of the diffraction orders is characteristic for the grating and
sensitive to nanometre scale changes. A newly designed subwavelength diffraction setup allows the measurement in real
time of the diffraction pattern of an illuminated polymer grating with only one detector image. The setup records
diffraction patterns of, for example, polymer gratings with intentionally low scattering contrast and line features ranging
from 610 to 80 nm. Thus, sub-100 nm features can be traced. The comparison of the measured diffraction patterns with
simulated patterns allows to sense nanometre scale deviations from fabrication goals.