Comparison of dark field microscopy with alternating grazing incidence illumination and bright field microscopy

At the PTB, there was developed a new dark field microscopy method in the last years. This procedure makes possible measurements of linewidths below the optical wavelength. It is based on an alternating grazing incidence illumination, whereby the proximity effect of neighbouring edges can be suppressed efficiently. It can be applied for dimensional measurement of micro- and nanostructures, for example CD-measurement of wafers or photomasks. A new prototype is presented, which was developed within a research project. As lightsources two diode lasers with 375 nm are used. Shifting the sample towards the optical axis, a focus series with a UV camera is taken and by means of focus criteria the focal image is determined. This focal image is compared with 2 different adapted rigorous simulation models (Rigorous Coupled Wave Analysis respectively Finite Elements). The results of the linewidth measurement using this special dark field microscopy method are compared with the measurements, which were accomplished using a conventional bright field linewidth measuring system at a wavelength by 365 nm. Pro and cons of both systems will be discussed.