Automatic Alignment Technique For X-Ray Lithography Using Moire Signals In Reflection

Abstract

This paper deals with an automatic and precise alignment technique between a wafer and a mask in X-ray lithography, using two pairs of moire gratings, one on the wafer and another on the mask in each pair. The moire signals are detected in zeroth order beams in reflection. These two moire optical signals are converted to digital signals by A/D converters, and then the digital signals input to a microcomputer. The microcomputer controls the alignment of X stage through a stepping motor, using the difference between these two digitalized moire signals. The precision of the present system is resulted from the pitch of the gratings as an error signal, the relative displacement between the gratings per step of the stepping motor, the resolution of the A/D converters and S/N of the system. The response time of alignment is determined by the response time of the feed back loop. Experiments were carried out by using a grating of 25 μm pitch. The minimum displacement corresponding to one step of the stepping motor was 14 nm. The resolution of the A/D converters was 12 bits and the response time was 1.2 s. With these parameters a control reproducibility better than 50 nm was obtained. Improvement in the control precision by an order of magnitude is expected by using a pair of quadruple gratings, a combined unit of 4 matched photodiode and PZT controllers. The design considerations of a suitable X-ray lithographic mask alignment system are given.