Linear-Fresnel-Zone-Plate-Based Two-State Alignment Method for Sub-0.25 µm X-Ray Lithography System

Abstract

In this paper, we present a linear-fresnel-zone-plate-based two-state alignment method developed at the Center for X-ray Lithography. The alignment system uses a linear Fresnel zone plate (LFZP) as a mask alignment mark and an array of dots as a wafer alignment mark. The alignment error signal extraction is based on a two-state modulation of the incident light. The optical system is arranged outside of the exposure X-ray path and alignment can be performed during X-ray exposures. In the experiment, we obtained an alignment signal depth of focus larger than 4 µm and the gap change between the mask and wafer did not affect the alignment position. The X-ray double-exposure experiment on the system demonstrated an alignment accuracy better than 0.035 µm (3σ) on both Al and silicon nitride marks.