Evaluation of Overlay Accuracy for 100-nm Ground Rule in Proximity X-Ray Lithography

Abstract

An evaluation was made of the overall overlay accuracy of proximity X-ray lithography (PXL), which includes mask placement error, the alignment error of the stepper, and deformation arising from wafer processing. The relative placement accuracy of X-ray masks between first and second layers was below 22 nm (3σ) for both the x- and y-directions. Exposure was carried out using an optical heterodyne alignment system and magnification correction, and an overlay accuracy below 30 nm (mean ±3σ) was obtained. The overlay error was broken down into various components to estimate the error budget. The analysis revealed the alignment error of the X-ray stepper to be about 25 nm, and the common in-plane deformation to be about 11 nm. The common in-plane deformation was almost the same as the relative mask placement accuracy without scale error. The accuracy obtained in this study clearly demonstrates the suitability of PXL for the fabrication of 100-nm ground rule devices. Moreover, the analysis indicates that it is also applicable to the 70-nm technology node.