Analysis of diffraction-based wafer alignment rejection for thick aluminum process

A diffraction-based alignment method has been widely used during lithography processes. The alignment position is obtained by analyzing the light intensity changes of the moiré fringes. In order to ensure the reliability of the alignment, the alignment system sets a preset threshold value during measurement and then rejects the measurement data that do not meet the requirements directly. In the diffraction-based alignment system, the photolithographic technology encounters the situation of alignment rejection frequently when processing the thick aluminum film alignment process and most of the reasons for rejection are wafer quality and delta shift. In this paper, we proposed a fast and adaptable analysis method for asymmetric structures and have carried out a systematic study. The calculation results of our model are verified with a rigorous physical model with high accuracy. The influences of the thickness fluctuation of the deposited aluminum and the sidewall angle of the alignment mark on wafer quality are studied. The reasons and degrees of delta shift induced by the process are also explored and studied. The results show that for the same alignment mark, the thickness fluctuation has a greater impact on wafer quality than the sidewall angle. For different alignment strategies, such as to align to a metal layer or to a contact layer, different alignment rejection is experimentally obtained and analyzed. Our model gave a conclusion as to why aligning to a contact layer mark is more process-robust than to a metal layer. The delta shift is mostly generated by the grating inconsistent of the moiré alignment grating. Process induced wafer distortion of the alignment mark will increase inconsistency and then make the delta shift out of threshold.