Patent ID: 8368041

Claim:
A method of controlling the focal point of the focusing lens in an EUV light source having a laser source emitting a laser beam, a plasma chamber in which droplets of an EUV light emitting material are irradiated by the laser beam at an irradiation site, and a focusing lens for focusing the laser beam to a focal spot within the plasma chamber, comprising: determining an amount of thermal load created on the lens by the laser beam at each point in time of a series of points in time; determining, for each point in time of the series of points in time, an expected change in the focal spot of the lens compared to a nominal focal spot of the lens due to the amount of thermal load; and adjusting, for each point in time of the series of points in time, the position of the lens to compensate for the expected change in the focal spot of the lens compared to the nominal focal spot of the lens due to the amount of thermal load, such that the focal spot remains substantially at the irradiation site over the series of points in time.