Patent ID: 8872144

Claim:
A system for laser beam focus control in a laser produced plasma extreme ultraviolet light source, comprising: a focusing element configured to receive light reflected from a target when the target is illuminated by a laser source, the focusing element focusing the received reflected light along an optical path, a beam splitter located on the optical path configured to receive the focused reflected light and to split the focused reflected light into a first split beam and a second split beam, a sensing device configured to receive the first split beam from the beam splitter at a first sensing location on the sensing device, a reflecting element configured to receive the second split beam from the beam splitter and to direct the second split beam to a second sensing location on the sensing device, and a controller coupled to the sensing device, the controller configured to: determine an image size of the first split beam at the first sensing location on the sensing device, determine an image size of the second split beam at the second sensing location on the sensing device, generate a focus correction signal indicating a front focus condition when the image size of the first split beam at the first sensing location on the sensing device is smaller than the image size of the second split beam at the second sensing location on the sensing device, generate a focus correction signal indicating a back focus condition when the image size of the first split beam at the first sensing location on the sensing device is larger than the image size of the second split beam at the second sensing location on the sensing device, and provide the generated focus correction signal to a laser focus controller.