MODELING OF NOVOLAK-BASED POSITIVE PHOTORESIST EXPOSED TO KRF EXCIMER LASER UV-RADIATION AT 248 NM

A model is presented for calculating the local exposure and the development rate for a novolak-resin naphtoquinone-diaide sensitized photoresist exposed to a KrF excimer laser UV radiation at 248 nm. The measured transmission of the pulsed UV radiation through the resist is presented and compared to the simulated one using the model. Classical bleaching characteristics (i.e., the resist transmittance increases with the dose) are observed at low dose exposure with low energy per pulse. When the dose is increased, the photoresist transmittance reaches a maximum and starts to decrease. This behavior is assumed to be due to UV radiation effect on the resin. The proposed model describes a general photoresist with absorbing components, each with two distinctive initial and postexposure states. The model is applied for the specific case of the novolak based photoresist where the two components are the photoactive compound (PAC) and the resin. Both components affect the light distribution within the resist and the dissolution rate in a developer. An analytical solution for the absorption equation and the rate equations of the two absorbing components' concentration is given as a function of depth and time. The system is also solved numerically by a finite difference method and the photoresist UV transmittance versus the illumination energy is calculated and compared to the experiment. An approximation which assumes exponential light intensity decay in the resist is also presented. The simplified solution is used for fast calculation and for the model parameters extraction. Another nonphysical approximation is proposed here for the transmission versus dose curves. This approximation assumes an exponential decay of each initial state towards its final state and is used for zero order parameter estimation. The development rate of the photoresist exposed to 248-nm radiation is also characterized versus illumination dose. At low dose the dissolution rate increases with illumination and conventional photoresist development models are found to fit the data. However, at higher doses (above about 500-800 mJ/cm2) the dissolution rate decreases with increasing dose. The effect is assumed to be due to resist cross-linking and an empirical model is suggested to describe the development rate at higher doses. This model can be integrated with conventional models to describe the dissolution rate in a wide range of doses. © 1990 IEEE