Overlay mark and overlay method of semiconductor structure

An overlay measurement method of a semiconductor structure, comprising:

• forming four sub-overlay marks on a scribe line of a substrate, wherein each sub-overlay mark comprises an inner region and an outer region, and a plurality of first mandrel structures located on the substrate of the inner region, and a plurality of second mandrel structures located on the substrate of the outer region;
• forming a mask layer to cover the first mandrel structure and the second mandrel structure;
• forming a first patterned photoresist layer on the mask layer;
• performing a first overlay measurement step on the first patterned photoresist layer, the first mandrel structure and the second mandrel structure;
• forming a second patterned photoresist layer on the mask layer;
• and
• performing a second overlay measurement step on the second patterned photoresist layer, the first mandrel structure and the second mandrel structure.