A new interferometric alignment technique has been developed that is suitable for aligning x-ray lithography masks or photomasks relative to a substrate. A superposition precision of 200 Å has been demonstrated and in principle ∼100 Å should be possible. The superposition of matching grating type alignment marks on facing plates is determined from the intensities of beams multiply diffracted from the matching alignment marks. The principle of the interferometric alignment technique is described.