
116 Semiconductors: Integrated Circuit Design for Manufacturability
For imaging layers with small contact holes, the production of arrays is
executed using masks with attenuated phase-shifting layers in which the
opaque mask material (typically chrome), replaced with a slightly trans-
parent material that also causes a uniform phase shift of 180°, passes only
enough light to cause a dark interference fringe to form at the boundaries of
the features. This enhances image contrast and image integrity but typically
does not allow enough light to pass through the mask to actually expose the
photoresist in the dark areas. Low light lev ...