Overlay Focus Control for Clearer Wafer Alignment Measurement

The patent implements dynamic focus adjustment by measuring images at multiple different focus positions and selecting the optimal focus based on contrast analysis. Instead of using a fixed focus position, the system adaptively determines the best focus for each measurement location by evaluating contrast values across multiple focus settings, thereby maintaining image clarity despite variations in layer thickness or reflection surface.

The system changes the focus parameter by measuring at multiple predetermined focus positions (first through fourth focuses) and selects the optimal focus position based on contrast index calculations. This parameter variation approach allows the system to adapt to different layer conditions and maintain measurement accuracy.

The patent performs preliminary focus optimization by pre-measuring images at multiple focus positions and calculating contrast indices before the actual overlay measurement. This preliminary action identifies the optimal focus position in advance, preventing the need for repeated focus adjustments during subsequent measurements and reducing overall measurement time.

The system uses feedback from contrast index calculations to automatically determine the optimal focus position. By continuously evaluating contrast values from images captured at different focus positions and selecting the maximum contrast point as the optimal focus, the system achieves rapid and accurate focus determination without requiring manual intervention or extensive trial-and-error.

The system implements self-service focus optimization by automatically measuring images at multiple focus positions, calculating contrast indices, and selecting the optimal focus position without external intervention. The device autonomously determines the best focus for each measurement location based on the contrast maximum, eliminating the need for manual focus adjustment or complex external control systems.

The patent divides the focus optimization process into discrete segments by measuring at four specific focus positions (first, second, third, and fourth focuses) rather than continuously adjusting focus. This segmentation approach simplifies the control system by using predetermined focus positions and straightforward contrast comparison, reducing the complexity of focus control while maintaining measurement accuracy.