Accuracy of diffraction-based overlay metrology using a single array target

Abstract

We focus on the capability and theoretical limits of a model-based scatterometry method to determine overlay using a single two-dimensional array target. We use our modeling capability to design an optimized test target for scatterometer-based overlay measurements in a range of semiconductor films. We propose a methodology to measure the overlay using a single two-dimensional array target designed with intentional offsets, x and y, between the top and bottom grid arrays along the X and Y directions. This method allows extraction of the two-dimensional overlays from first diffraction order measurements through bi-azimuth angle analysis (0 and 90 deg with respect to the incidence plane), and includes a simple linear response algorithm. Two critical issues are taken into account: correlation of x and y and lithography process errors. We have simulated the diffraction signatures of a two-dimensional target with a pitch of 400 nm and linewidth of 100 nm, and optimized the overlay target design to maximize the measurement sensitivity and minimize the correlation of two axial measurements. We also investigate the influence of parameter variations on overlay measurement error