Improvement of alignment and overlay accuracy on amorphous carbon layers

ArF lithography has shrunk photo resist patterns down to 60nm from 80nm with the help of various RETs (resolution enhancement technologies). Photo resist thickness also has been thinner than ever to increase image contrast and DoF margin and to avoid pattern collapse due to high aspect ratio. Etching process became more difficult and marginal by using thin resist patterning so that new BARC materials having high etching selectivity are required. Since amorphous carbon (a-C) and SiON have good etch selectivity between them, they can be used as hard mask materials for thin resist process. Lithographic alignment system usually uses the light of 400 to 700nm. In general a-C has certain level of light absorption in this wavelength range and the absorption coefficient increases with deposition temperature of a-C. Because a-C film is not suitably transparent to the alignment light, overlay control might get worsen as the thickness of a-Carbon film increased. In this paper, we will present the effect of the thickness of a-Carbon film on alignment signal strength, alignment accuracy and overlay control of various layers. Simulation of alignment signal is conducted and compared with experiment results. It is also studied whether the overlay control can be improved by changing the spectrum of alignment light or structural design of alignment marks. Improvements on alignment accuracy and overlay control are examined by lowering the extinction coefficient, k of a-Carbon film. In conclusion, because photo resist only is not sufficient for a mask during etch step as the thickness decreased further, adoption of new hard mask is inevitable. It is the alignment trouble for a-Carbon that should be cleared before being named as a main stream of new hard mask.