Deep subwavelength nanometric image reconstruction using Fourier domain optical normalization

Quantitative optical measurements of deep subwavelength, three-dimensional (3D), nanometric structures with sensitivity to sub-nanometer details address a ubiquitous measurement challenge. A Fourier domain normalization approach is used in the Fourier optical imaging code to simulate the full 3D scattered light field of nominally 15 nm-sized structures, accurately replicating the light field as a function of the focus position. Using the full 3D light field, nanometer scale details such as a 2 nm thin conformal oxide and nanometer topography are rigorously fitted for features less than one-thirtieth of the wavelength in size. The densely packed structures are positioned nearly an order of magnitude closer than the conventional Rayleigh resolution limit and can be measured with sub-nanometer parametric uncertainties. This approach enables a practical measurement sensitivity to size variations of only a few atoms in size using a high-throughput optical configuration with broad application in measuring nanometric structures and nanoelectronic devices. Optical scatterfield microscopy combined with Fourier domain analysis permits image reconstruction of features on a deep-subwavelength scale. Scientists at the National Institute of Standards and Technology in the USA report that their imaging scheme can map features on a scale of just 15 nm when using a blue light-emitting diode, which corresponds to one-thirtieth of the illumination wavelength (450 nm). The approach involves focusing polarized blue light onto a target object and imaging at multiple focus positions using a charge-coupled device camera to capture the scattered field. Fourier domain analysis and a model based on nonlinear regression are used to fit the experimental data to a library of simulations. The researchers verified their approach using 30- and 100-line targets, which had been fabricated by electron-beam lithography with linewidths of 14, 16 and 18 nm.