Evaluation of alignment accuracy in processed wafers and SiC masks on a scattered-light alignment system for x-ray aligners

The alignment performances of the video-based scattered-light alignment (SLA) system for 0.1 μm lithography are described in this article. The SLA system has high sensitivity to the silicon carbide (SiC) mask without an antireflection coating (ARC). This article especially focuses on the alignment accuracy in processed wafers and the dependency of the alignment accuracy on the SiC membrane thickness. A series of alignment tests was done on a lab-based vertical wafer stage using the SiC masks. In order to evaluate the alignment accuracy in processed wafers, we prepared four processed wafer types: nitride, oxide, poly-Si, and aluminum. The high position sensing repeatability in the range of 4.8–6.4 nm (3σ) was obtained using the combination of the four processed wafers and a 2-μm-thick SiC membrane without the ARC. We also obtained the alignment accuracy using the wafer alignment marks only, resulting in an alignment accuracy of 10.0, 8.8, 9.1, and 35.7 nm (3σ) for the nitride, oxide, poly-si, and aluminum wafers, respectively. In addition, the dependency of the alignment accuracy on the SiC membrane thickness was obtained using the membranes with thicknesses of 2, 3, 4, and 5 μm. An improvement of the alignment accuracy in the aluminum is discussed.