If the opaque regions on a COG mask become partially transmitting with a 180Â° phase relative to the bright regions, as shown in Fig. 6.1, an attenuated PSM results [15, 16]. As opposed to an alternating PSM where all phase regions image onto the wafer, the partially transmitting background of an attenuated PSMâusually with an intensity transmission T of about 7%â does not print. Rather, destructive interference between the clear area and the partially transmitting background enhances the image contrast of the bright region. To illustrate this behavior, consider coherent images of an edge shown in Fig. 6.2. The attenuated PSM image has a sharper transition at the mask edge, resulting in a more robust image and greater exposure latitude. Depth of focus increases as well because the improved image quality is maintained over a larger focus range. Process latitude enhancement is demonstrated in Fig. 6.3, where the DOF-EL curves of an isolated k 1 =0.5 contact hole for COG and attenuated PSM are shown. Exposure latitude improves from 19% to 26%; depth of focus more than doubles.
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