Lithographic evaluation of a DUV carbon attenuated phase shift mask

Abstract

A DUV (248 nm) attenuated phase shift mask (APSM) using a hydrogenated amorphous carbon film was fabricated and its lithographic performance was evaluated against a binary Cr mask. The attenuated mask transmittance was ∼6.4% with a phase angle of ∼180±3° . Exposures on DUV photoresist were performed using an excimer laser stepper on contact vias arranged in both isolated and dense patterns. The depth-of-focus (DOF) process window of the APSM was found to increase significantly with decreasing partial coherence (σ=0.3) for isolated contact vias. The top down SEM measurements show an approximately 30% improvement in DOF over the full range when the APSM is used instead of the binary mask. For smaller contacts around 200 nm, DOF process window improvement can be as high as 100%. The smallest vias printed were ∼150 nm in diameter. At higher exposure dose, sidelobes printing was clearly visible showing an asymmetric "3-leaf clover" pattern behaviour. This non-ideal behaviour is attributed to lens aberrations of the stepper. For dense contact via patterns, a significantly greater process window was found at a higher partial coherence (σ=0.6) when compared to a lower σ. This occurs because in a dense pattern the sidelobes, which tend to print more at a lower partial coherence, overlap between the vias enhancing their printing in the DUV resist.