Effect of the partial coherence on reflective notching

Abstract

This paper explores through numerical simulation and experiment the role of partial coherent illumination on reflective notching in optical projection printing over a nonplanar substrate. The intensity in the photoresist when patterning a gate line over a LOCOS generated active area well covered in polysilicon was simulated with the rigorous electromagnetic simulation program TEMPEST-PCD which includes partial coherence through a decomposition method. The simulation results are compared to experimental SEM pictures of developed photoresist lines for patterning a gate over a trench with varying moat widths when using a DUV 248 nm stepper with a NA of 0.5 for a (sigma) of 0.3 and 0.6. Variations in linewidth of both plus and minus 20% are observed due to the interactions with the underlying topography. The simulations give physical insight into trends seen in the experimental critical dimension (CD) measurements. Simulations indicate that most of the effects observed are due to lateral specular reflection from the underlying structure aiding in the dissolution of the resist near the feature. These trends seen in simulation and in experiment indicate that the topography has a similar impact on the CD regardless of the coherence.