Adjustment of bilayer optical properties and the effect on imaging and etching performance

Thin imaging systems have the potential for excellent lithographic performance and good etch properties. In such systems, the optical absorption of the undercoat and of the imaging layer can be adjusted through formulation and chemistry modifications. As the substrate underneath the resist undercoat changes, the optimal k for the undercoat will change. The reflectivity of the underlayer resist interface will be roughly proportional to the square of the k of the underlayer. As k gets bigger the standing wave in the resist gets stronger, but the effects of varying substrate layer thickness underneath the underlayer are suppressed. It is found that even for very reflective substrate stacks, both types of reflectivity effects are minimal with an undercoat k of about 0.20 to 0.25. The optimal underlayer k depends on how thicken an underlay er is used. Such a system gives better control of reflection and topographic effects than does a single layer plus BARC system. Experiments with different underlayers having different k's show that k can be varied chemically while retaining good etch performance.