Method for optical inspection of nanoscale objects based upon analysis of their defocused images and features of its practical implementation.

A microscopic method to inspect isolated sub 100 nm scale structures made of silicon is presented. This method is based upon an analysis of light intensity distributions at defocused images obtained along the optical axis normal to the sample plane. Experimental measurements of calibrated lines (height 50 nm, length 100 μm, and widths of 40-150 nm in 10 nm steps) on top of a monocrystalline silicon substrate are presented. Library of defocused images of calibrated lines is obtained experimentally and numerically with accordance to experimental setup parameters and measurements conditions. Processing of the measured defocused images and comparison with simulated ones from library allow one to distinguish between objects with a 10 nm change in width. It is shown that influence of optical system aberrations must be taken into account in order to achieve coincidence between simulation and measured results and increase accuracy of line width inspection accuracy. The limits of accuracy for object width measurements using this optical method are discussed.