Quantitative linewidth measurement down to 100 nm by means of optical dark-field microscopy and rigorous model-based evaluation

The dark-field microscopy method with alternating grazing incidence UV illumination (UV-AGID) developed at the PTB now offers the possibility of measuring lateral structures down to 100 nm. The advantage of this new dark-field method is, in comparison to bright-field microscopy, the better edge localization, since only light that is scattered or diffracted by object details reaches the image plane. Also, in the case of AGID microscopy, the proximity effects are much more reduced. This method can be used for dimensional measurements of micro- and nanostructures, e.g. on wafers, masks or etched Si structures. Determining the linewidth quantitatively from the microscope image is, however, only possible by comparing the measured data with adapted rigorous simulations. To model the intensity distribution in the image, we make use of two different rigorous grating diffraction theories. On the one hand we use the rigorous coupled wave analysis (RCWA) method (for calculations of the diffracted electrical and magnetic fields), on the other hand, the finite elements method (FEM). In particular, it will be discussed what influence the polarization of the illumination and the edge angles of the trapezoidal-shaped lines and spaces have on the microscope image. Also, different quantitative measurements of chromium lines on a COG mask (COG: chromium structures on glass) will be presented. The results will be compared with electron optical measurements.