Improved optical linewidth measurement by means of alternating dark field illumination and model-based evaluation

The dark field microscopy method with alternating grazing incidence illumination (AGID), developed at the PTB, offers the possibility to measure structures with lateral dimensions below the classical resolution limit. Moreover, this method offers the advantage of better dimensional measurements of phase objects, since higher contrasts compared to conventional bright field microscopy are obtained. As an example, the AGID-method can be used for the determination of linewidths on photomasks or wafers. A newly developed prototype measuring system based on the alternating dark field illumination is be presented. This system uses two diode lasers with the wavelength 374 nm as lightsources. This new method is be compared with conventional bright field microscopy. The distribution of intensity of an image of a given structure depends on the illumination and the geometrical parameters as well on the optical parameters. In particular the dependence on the polarization and the angle of incidence of the illumination is discussed. For modeling of the intensity distribution in the image we are using two different rigorous diffraction theories. On the one hand we use the rigorous coupled wave analysis (RCWA) method for the calculation of the diffracted electric and magnetic fields and on the other hand the finite elements (FEM) method. These two models are used for determining the linewidth from measurement data of the new prototype system. It will be shown that the newly developed system is a suitable tool for optical linewidth measurements which represents a meaningful addition to existing optical linewidth measurement systems.