Overlay Measurement Error Research Articles

Calibration method of overlay measurement error caused by asymmetric mark.

With the process nodes extending to sub-10-nm in advanced semiconductor manufacturing, the overlay requirements keep progressively scaling down, which makes it very important to measure overlay precisely for monitoring on-product performance. The overlay mark being asymmetrical when generated via the lithography process, this asymmetry will be slightly variated even in the same process or same lot, and it will bring overlay measurement error. In general, the wafer alignment data are used for correcting this overlay measurement error, utilizing its wavelengths and polarizations dependence. However, there is a residual error that cannot be removed because the structures of the wafer alignment mark and overlay mark are different and are affected by the process differently. In this paper, a new method is proposed for calibrating the overlay measurement error introduced by the asymmetric mark, which is based on the relationship between measurement data of the overlay mark and the single layer mark. The validity is verified by simulation with different types of asymmetric mark. It is very useful for improving overlay measurement accuracy and for understanding how the overlay measurement error is affected by the asymmetric mark.

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

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