a phase shifted coherent beam at imaging optics 218 using phase shifted coherent beams allows using two 0 degree dark field masks with one beam shifted by 180 degrees in place of a single dark field mask with both 0 degree and 180 degree phase shifts another example where phase shifting in the optical device 212 can be useful is in a four phase phase shifting mask more specifically a single mask having four phases can be decomposed into two masks each having only two phases to manufacture optical device 212 can shift the phase in one path by 90 degrees relative to the other path optical device 212 can also modify the optical path such that the illuminations of mask 208 and mask 214 are incoherent with respect to each other this provides the ability to expose target 220 through the separate masks without unwanted interference between the two beams for example incoherent illumination is useful for dual exposure aapsm one binary mask one phase shifting mask and contact arrays two separate masks so that contacts are partially isolated and can be exposed using attenuated phase shifting masks and low u03c3 illumination in an alternate embodiment of the invention optical device 212 can include a set of optical elements e.