Extending the limits of i-line lithography for via layers and minimization of dense-iso bias

This paper describes the result of patterning contact holes on a TEOS substrate with 365 nm lithography using both binary and phase shift mask techniques. The target CD on the wafer was 0.34 micrometers and the minimum pitch was 0.63 micrometers . Results show significant improvement in depth-of-focus using the phase-shift mask. With the binary mask we obtained 0.6- micrometers DOF for both isolated and dense contact holes while with the phase-shift mask we obtained 1-micrometers DOF. Using the phase-shift mask we can also improve the linearity slightly for dense contacts and significantly for isolated contacts. The effect of pitch on contact size was also studied, showing that the intermediate pitch contacts print larger than the isolated or minimum pitch contacts. Using a mask- to-wafer bias of 60-80nm the largest bias between contacts of various sizes was 25 nm.