SYSTEM AND METHOD FOR COMPENSATING FOR THERMAL EFFECTS IN AN EUV LIGHT SOURCE

摘要

A method and apparatus for compensating for thermal effects on the focal spot of a lens used to focus a laser beam on a target material at an irradiation site in a laser produced plasma (LPP) extreme ultraviolet (EUV) light system is disclosed. The EUV energy output of the light system is measured at sample intervals as a proxy for the laser power. The thermal load on the focusing lens is estimated from the measured EUV power, the expected change in the focal length of the lens for the thermal load is calculated, and the lens position is adjusted to compensate for the calculated focal length change. The actual position of the lens may be determined and compared to its desired position, and adjusted to insure that it remains in the desired position.