Reticle defect printability and photoresist modeling of contact structures

Over the past few years there has been a growing interest in using advanced image formation techniques to enhance optical lithography resolution. Techniques such as Optical Proximity Correction (OPC) and phase shifting involve changes in reticle manufacturing which increase the printability risk of small reticle defects and therefore impact wafer yields. There have been several experimental and simulation studies on the printability of sub-half micron defects using both reduction and 1X photolithography equipment. In general these studies have focused on the printability effects of line and space features. However, OPC is frequently implemented to control the size and shape of contact structures. This study was performed to gain a better understanding of the behavior of contact hole defects in a 1X lithography system using both a moderate and a high contrast photoresist. A test reticle was created with 0.72 micrometer contact holes containing edge, corner and isolated central defects in programmed sizes from 0.15 to 0.4 micrometer, and exposed on a submicron 1X stepper. Printability was determined by measurement of the normalized area of the contact (NCA). Reticle defect printability of the contact structures was modeled for each photoresist using a three-dimensional (3D) optical lithography simulation tool. The experimental NCA data was compared to modeled results to validate the simulator. Cross sectional contact simulations were then prepared to show the relative impact on the placement of the defect in the contact structure. Both the simulation and the experimental results show the relative sensitivity of the two photoresists to the printability of defects in the contact hole structure. This analysis enhances the understanding of the criticality of defect sizes in contact arrays and allows users to predict defect printability issues for new photoresists.