Du Juyou, Dai Fengzhao, Wang Xiangzhao
Appl Opt. 2019 Jan 1;58(1):9-14. doi: 10.1364/AO.58.000009.
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.
相位衍射光栅在晶圆对准系统中被广泛用作对准标记。通过使用对准标记的更高衍射级可以获得更高的对准精度。同时，对准精度也受到对准信号的信噪比（SNR）的影响。在实际光刻过程中，由于标记表面不透明膜堆叠尤其是金属层的强烈吸收，对准标记的衍射效率显著降低。衍射效率的降低导致对准信号的信噪比恶化。通常使用等分段相位光栅来提高更高奇数级的衍射效率，从而提高相应对准信号的信噪比。然而，这种光栅的零级和偶数级仍具有相对较高的衍射效率。它们会影响通常用于测量标记位置的奇数衍射级对准信号的信噪比。在本文中，我们提出了一种通过优化对准标记的结构来提高分段相位光栅奇数级衍射效率的方法。此外，在标记位置测量过程中不使用的零级和偶数级的衍射效率被充分降低。仿真结果表明，所提出的方法明显提高了对准信号的信噪比，这对提高对准精度非常有帮助。
