Overlay key of the method of forming the overlay key subject method and overlay accuracy of measurement

Embodiments of the present invention discloses a key of the overlay, comprising: at least two overlay marks, each mark having a first overlay mark and the second sub-sub-symmetrical to each other center mark, marking the first sub- and each of the second marking includes two sub-stripe pattern perpendicular to each other and having a common terminal. The overlay mark having at least two different layers. Also disclosed is a method for overlay key subject methods and measuring overlay accuracy is formed.