Overlay target measurement and shift compensation in charged particle systems and related methods

A system for compensating for response times of different types of sensors of a charged particle system, the system comprising:

• a memory unit configured to store a first secondary electron image of an overlay target, a first backscattered electron image of the overlay target, a second secondary electron image of the overlay target and a second backscattered electron image of the overlay target, wherein the first secondary electron image is indicative of secondary electrons emitted from the overlay target due to a scanning the overlay target with an illuminating electron beam in accordance with a first scan pattern, the first backscattered electron image is indicative of backscattered electrons emitted from the overlay target due to the scanning the overlay target with the illuminating electron beam in accordance with the first scan pattern, the second secondary electron image is indicative of secondary electrons emitted from the overlay target due to a scanning the overlay target with the illuminating electron beam in accordance with a second scan pattern that is opposite to the first scan pattern, and the second backscattered electron image is indicative of backscattered electrons emitted from the overlay target due to the scanning the overlay target with the illuminating electron beam in accordance with the second scan pattern;
• and
• a processing circuit that is configured to:
• determine a secondary electron shift based on a registration between the first secondary electron image and a rotated second secondary electron image;
• determine a backscattered electron shift based on a registration between the first backscattered electron image and a rotated second backscattered electron image;
• and determine a compensation shift applicable to future acquired backscattered electron images based on the backscattered electron shift and the secondary electron shift, wherein the images are acquired using a pixel grab period that is lower than a response period of the backscattered electron sensor used to generate the first backscattered electron image.

A non-transitory computer readable medium for compensating for differences in response times of different types of sensors of a charged particle system, the non-transitory computer readable medium having instructions stored thereon that, when executed by a processor, cause a system to:

• obtain a first secondary electron image of an overlay target and a first backscattered electron image of the overlay target, the first secondary electron image is indicative of secondary electrons emitted from the overlay target due to a scanning the overlay target with an illuminating electron beam in accordance with a first scan pattern, the first backscattered electron image is indicative of backscattered electrons emitted from the overlay target due to the scanning the overlay target with the illuminating electron beam in accordance with the first scan pattern;
• obtain a second secondary electron image of the overlay target and a second backscattered electron image of the overlay target, the second secondary electron image is indicative of secondary electrons emitted from the overlay target due to a scanning the overlay target with the illuminating electron beam in accordance with a second scan pattern that is opposite to the first scan pattern, the second backscattered electron image is indicative of backscattered electrons emitted from the overlay target due to the scanning the overlay target with the illuminating electron beam in accordance with the second scan pattern;
• determine a secondary electron shift based on a registration between the first secondary electron image and a rotated second secondary electron image;
• determine a backscattered electron shift based on a registration between the first backscattered electron image and a rotated second backscattered electron image;
• and
• determine a compensation shift for compensating for the differences in the response times of different types of sensors based on the backscattered electron shift and the secondary electron shift, and wherein the first secondary electron image, the second secondary electron image, the first backscattered electron image and the second backscattered electron image are acquired using a pixel grab period that is lower than a response period of a backscattered electron sensor used to generate the first backscattered electron image.