Partial coherence optimization for printing chain contacts using an attenuated phase-shifting mask

Attenuated phase-shifting masks (APSM) are being used to push the limits of optical lithography. When compared to conventional chrome-on-glass (COG) masks, improvements in both minimum resolution and total process window are observed. However, these improvements are not always realized because a phenomenon called 'side lobes' can generate undesirable printed defects, thereby reducing the total process window. An attenuated phase-shifting mask was used to print isolated and chain contacts on an I-line stepper. The side lobe-limited process window for this process was maximized by optimizing the partial coherence and dose required to print both features.