Larger depth of focus for increased yield

Large depths of focus can be obtained by modifying the pupil function of a projector lens. An image recorder with a threshhold greatly increases the number of phase functions that can be used. Simulations are given that show that it is feasible to increase the depth of focus by a factor of two or more by making modification of the phase in the aperture stop of the projector lens. Examples show new techniques that are used to design pupil-plane phase functions to produce images for contact holes with very little change over at least twice the normal focal depth. The penalty is that the side lobes are higher than for a system without modification in the pupil plane. A larger focal depth gives considerable potential cost savings. For example, the down time for focus correction of a lithographic system and re-work of wafers can be reduced if the focal depth of the projector is increased. Using the smallest numbers that were quoted, the down time for refocus (2% on 20% of the jobs) costs $4M/yr./machine. The 30% of re-work that is caused by focus problems costs $15M/yr./machine. Expensive flatter wafers need not be used. The cost of flatter wafers, when needed (10% of the time) with current machines, is more than $17M/yr./machine. Using the lowest numbers quoted, the totals for down time, re-work, and flatter wafers alone is $36M/yr./machine. In addition, masks can be cheaper because complexity for extending the depth of focus is not needed. We show the phase modifications that were used, how the phase modifications were found, and highlight the differences between the design techniques that we used and the previous work on extending the depth of focus.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.