Mask and wafer are located in a helium ambient and exposed to x-rays.
Fringes result from interference between mask gratings and wafer checkerboards of similar periods.
The fringes are imaged onto a CCD camera and analyzed in the frequency domain. The relative mask-wafer position is controlled by piezos with integral capacitive sensors. It was discovered that the spatial phase of fringes from marks on the mask alone could be made sensitive to gap. This gives rise to a fringe pattern, as shown. Also shown are the secondary beams, which traverse the gap a second time at an inclined angle.