Clear-field photo-masks offer significant advantages over dark-field photo-masks for some important classes of target patterns, including small isolated features and dense arrays of contacts. This work compares lithographic performance of clear-field and dark-field images when mask patterns are optimized for respective mask tones. Since the purpose is to study optical behavior, computed images without resist models were compared. In order to explore performance limits, optimized masks were not constrained to limit their complexity.
Calculated images were compared for clear-field and dark-field masks, with either opaque or 6% transmission, 180-degree phase-shifted absorbers. In each case, mask patterns were independently optimized to print the targets, which were a set of square and rectangular arrays of contact holes with various dimensions and pitches. The range of the target patterns extended to the limits of ArF resolution with water immersion. Because the intent was to compare inherent optical performance of positive and negative-tone imaging, the study did not use resist models that would combine materials properties or behaviors into the results, but simply applied a constant threshold to calculated intensities to obtain images. Contrast, MEEF, and deviation of images with defocus were the basis of optimizing the mask patterns, and were compared for the four combinations of mask tones and absorbers. Best contrast and MEEF were obtained with bright-field masks that had attenuated, phase-shifting absorbers. The amount of improvement depended on the size of the mask patterns relative to that of their corresponding targets, set here by varying the intensity threshold for the images during mask optimization. Differences in how the images of the four types of masks changed with defocus were statistically insignificant.