Application of two-wavelength optical heterodyne alignment system in XS-1

This article presents the alignment performance of the two-wavelength optical heterodyne alignment system in the x-ray stepper XS-1. The alignment accuracy (mean+3 sigma) obtained by the double-exposure method with a single mask and a Si trench wafer was better than 20 mn. The dependence of the alignment accuracy on Si trench depth indicated that the two wavelengths compliment each other and ensure a 3 sigma of less than 20 mn. The alignment capabilities for other processed test wafers were also investigated by mix-and-match exposure. For etched SiO2, and poly-Si film on a Si trench, an accuracy below 20 nm (3 sigma) was obtained. For AlSiCu film sputtered on etched SiO2, there appeared systematic alignment offsets (i.e., die shift and rotation errors) depended on die position, which are thought to be due to a wafer-induced shift. The systematic offset errors were eliminated by the use of a send-ahead wafer and corrections for individual offsets on each die, and thus the alignment accuracy was improved to 20-40 nm (3 sigma) for each alignment axis. The two-wavelength heterodyne alignment system of the XS-1 has sufficient potential for 130-nm lithography and below.