An alignment technique using diffracted moiré signals

Abstract

The authors have proposed an automatic and precise alignment technique for proximity printing in x-ray lithography, using two pairs of moiré gratings, with moiré signals from each pair being 180° out of phase with each other. A control reproducibility better than 50 nm was obtained with the mask and the wafer alignment system. This technique has also been applied to projection photolithography, as part of a program to mix and match optical projection lithography with proximity x-ray lithography. An alignment accuracy of ±0.12 μm(3σ) was obtained by an actual step and repeat photolithographic system. This paper discusses experimentally determined characteristics of diffracted moiré signals appropriate for automatic alignment. The moiré alignment technique was determined to be suitably precise for mix and match lithography once difficulties with interference between the parallel mask and wafer in the proximity system and chromatic effects in the projection system were overcome.