Effect of alignment mark depth on alignment signal behavior in advanced lithography

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 waf er. Hence, the processing steps can possibly affect the properties ofalignment mark. The alignment mark depth (trench depth) can be varied due to the nature ofprocessing. According optics, a light optical path variation may lead to a destructive interference, which is not good.