Finding a robust alignment strategy is one of the key evaluations in defining
photolithography process. Alignment is a process to determine how the current pattern is
placed on the wafer. Alignment is done by an optical system, which means that it is
dependable on the quality of the alignment signal to determine the correct orientation.
Alignment signal is generated by alignment mark, a diffraction grating structure (trench
and line structure) printed on wafer. Hence, the processing steps can possibly affect the
properties of alignment mark. The alignment mark depth (trench depth) can be varied due
to the nature of processing. According optics, a light optical path variation may lead to a
destructive interference, which is not good.
