LITHOGRAPHIC EVALUATION OF A DUV CARBON ATTENUATED PHASE-SHIFT MASK

A DW (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 transmitta nce was similar to 6.4% with a phase angle of similar to 180+/-3 degre es. 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 incr ease significantly with decreasing partial coherence (sigma=0.3) for i solated contact vias. The top down SEM measurements show an approximat ely 30% improvement in DOF over the full range when the APSM is used i nstead of the binary mask. For smaller contacts around 200 nm, DOF pro cess window improvement can be as high as 100%. The smallest vias prin ted were similar to 150 nm in diameter. At higher exposure dose. sidel obes printing was clearly visible showing an asymmetric ''3-leaf clove r'' pattern behaviour. This non-ideal behaviour is attributed to lens aberrations of the stepper. For dense contact via patterns, a signific antly greater process window was found at a higher partial coherence ( sigma=0.6) when compared to a lower sigma. This occurs because in a de nse pattern the sidelobes, which tend to print more at a lower partial coherence, overlap between the vias enhancing their printing in the D UV resist.