An illumination system for a microlithographic stepper has a light source
that emits light of selected wavelength(s) along an optical path toward a
photomask. An aperture mask is positioned in the path of the illumination
light and between the light source and the photomask. The aperture mask
has a dithered pattern of pixels. The intensity of the pattern controls
the illumination of the photomask. The masking aperture pattern defines
one or more zones of illumination. Each zone has elements that are
patterned in accordance with a selected wavelength of incident light to
diffract the incident light into an illumination pattern for illuminating
a photomask. Each of the elements is constructed with a matrix of pixels.
In the preferred embodiment the array of pixels is 8.times.8. The number
of elements is generally greater than 3.times.3.