Alignment mark optimization for improving signal-to-noise ratio of wafer alignment signal.

Abstract

Phase diffraction gratings are widely used as alignment marks in wafer alignment systems. A higher alignment accuracy can be obtained by using higher diffraction orders of the alignment mark. Meanwhile, the alignment accuracy is also affected by the signal-to-noise ratio (SNR) of the alignment signal. The diffraction efficiency of the alignment mark is significantly reduced in the practical lithography process because of the strong absorption of the opaque film stacks on the mark surface, especially the metal layer. The reduction of the diffraction efficiency leads to a deterioration of the SNR of the alignment signal. An equal subsegmented phase grating is usually used to improve the diffraction efficiency of higher odd orders, so as to improve the SNR of the corresponding alignment signal. However, there is still a relatively high diffraction efficiency of the zeroth and even orders of such a grating. They will affect the SNR of the alignment signal of odd diffraction orders, which are usually used to measure mark position. In this paper, we propose a method to improve the diffraction efficiency of odd orders for a subsegmented phase grating through optimizing the structure of the alignment mark. Moreover, the diffraction efficiency of the zeroth and even orders, which are not used in the measurement process of mark position, are sufficiently reduced. Simulation results indicate that the SNR of the alignment signal was obviously improved by the proposed method, which is very helpful for improving alignment accuracy.