DFM Dictionary - Public View/DFMC User Edit
Lithography requires tight phase control since in-phase waves add constructively to expose the resist and out-of-phase waves add destructively, cancelling each other, preventing exposure. In conventional illumination, the zero maxima projects at the midpoint of the two slits with Θ equal to zero. The first maxima occurs at a point where two rays of light travel lengths L and L + λ and add in-phase to construct a maxima.
The second maxima occurs when the two rays have path lengths L and L + 2 λ and so on. Where the maximas of light occurs there is high contrast between the light are dark areas on the silicon to precisely expose the resist. In the areas of low contrast, where minima occur, the constrast between the light and dark areas is less decreasing the uniformity of the exposure. The areas of Low Contrast are typically the areas that limit the latitude of the exposure range in the Process Window.