Evaluation of double focal plane exposure technique for 248 nm and 193 nm lithography for semi-dense trenches and contacts

Double focal plane exposure technique has the property to increase greatly the depth of focus of a lithographic process and appears to be a solution to fulfil the requirements of the most aggressive lithographic targets. The purpose of this work is to investigate the performances of this technique and to understand its mechanisms, to be able to find the best conditions of use for a given process.A simple model based on aerial images considerations has been developed to determine the behaviours of the main lithographic parameters (DoFmax, Elmax, central dose, shape of the Bossung curves) for various values of die distance between the two focal planes. Comparisons with four experiments have been realized with different conditions (type of pattern, dimensions, wavelength, N.A. and coherence sigma). The possibility to predict the best experimental conditions (trade-off between DoF, El, resolution and LER) has been verified.