Computer-aided phase shift mask design with reduced complexity

We propose a new approach to systematic phase shift mask design. In doing so, we constrain the complexity of the mask at a pre-specified level by limiting the number of "features" on the mask. We then optimize the location, size and phase of the features so as to achieve a desired intensity pattern on the wafer. The main advantage of this object-based approach over our previous pixel-based solution is that it results in substantially larger assisting phase shift features, and is therefore easier to fabricate. Our approach can also be used to design masks with proper bias and/or extension of the depth of focus. We show examples of contact hole, bright line and chromeless line-space mask designs. Finally, we show experimental results using the new approach.