Alternating grazing incidence dark field scanning optical microscopy for dimensional measurements

We present a novel method of dark field optical microscopy for linewidth measurements on microstructures on photo masks and wafers. This method is based on alternating grazing incidence illumination of the specimen, where the angle of incidence of the illumination is perpendicular to the edges or grooves of the specimen. The main advantage of this method is the improved resolving power due to the suppression of the proximity effect and to the high pass characteristic of the optical imaging. Linewidth measurements on different structures are compared with results obtained with conventional dark field and bright field optical microscopy. The experimental results are in good agreement with theoretical simulations based on rigorous diffraction theory. The suppression of the proximity effect is strongly depending on the polarisation of the illuminating light. The quality of the edge localisation is affected by the optical constants of the material, the structure (e.g. single line or periodical structure, corner rounding,...) and on the illumination wavelength. The best results are obtained for single lines and metallic structures.