Fig. 1. (a) The microscopic picture of modern CPU chip^[18] is on the left, and a close section of the two-layer structure is on the right. (b) Signal formation in DBO schematic diagram^[19,20]; α is proportional to OVL Δl; β is the phase shift due to the optical path difference between the two diffracted beams, and its value is twice the distance difference between the bottom and top gratings.

Fig. 2. Cross-sectional diagram of OVL mark (the bottom grating profile is parabolic).

Fig. 3. Cross-sectional diagram of OVL mark (the bottom grating profile is linearly deformed).

Fig. 4. Variation of error signal with slope w under different orders of diffracted light.

Fig. 5. Cross-sectional diagram of OVL mark (both top and bottom grating profiles are linearly deformed).

Fig. 6. Variation of error signal with amplitude ratio η under different orders of diffracted light. (a) The bottom and top gratings are tilted in the opposite direction (the absolute value of γ is greater than the absolute value of ε), ε = 0.005, γ = −0.01. (b) The bottom and top gratings are tilted in the same direction, ε = 0.005, γ = 0.01.

Fig. 7. Variation of the absolute of function F (ε, γ) with the slope of the top grating ε and bottom grating γ. (a) n = 1; (b) n = 2.

Fig. 8. Variation of 32η*F (ε, γ) (which is defined as F1) with respect to the amplitude ratio η for different orders of diffracted light (the bottom and top gratings are tilted in the same direction, ε = 0.005, γ = 0.01).