Overlay measurement device and method for controlling focus movement and program storage medium therefor

A method for controlling a focus movement of an overlay measurement device, comprising:

• obtaining a first focus graph of a first layer and a second focus graph of a second layer from one or more of images of each focus;
• identifying a reference focus in relation to the obtained first focus graph and the obtained second focus graph and moving the objective lens;
• identifying a first contrast index and a second contrast index for each of the first layer and the second layer of the obtained measurement image,
• identifying a focus corresponding to each of the identified first contrast index and the identified second contrast index,
• deducting a difference between the identified reference focus and the identified focus from the reference focus, and
• moving the objective lens to a focus corresponding to the measurement image by the deducted difference.

A non-transitory computer-readable storage medium comprising a program that comprises instructions for controlling a focus movement of an overlay measurement device,

• the program comprising:
• instructions for obtaining a first focus graph of a first layer and a second focus graph of a second layer from one or more of images of each focus;
• instructions for identifying a reference focus in relation to the obtained first focus graph and the obtained second focus graph and moving an objective lens;
• instructions for identifying a first contrast index and a second contrast index for each of the first layer and the second layer of the obtained measurement image;
• instructions for identifying a focus corresponding to each of the identified first contrast index and the identified second contrast index;
• instructions for deducting a difference between the identified reference focus and the identified focus from the reference focus;
• and
• instructions for moving the objective lens to a focus corresponding to the measurement image by the deducted difference.