Apparatus and method for generating extreme ultraviolet radiation

A method of controlling a feedback system of an extreme ultraviolet (EUV) radiation source, the method comprising:

• measuring a time delay between a pre-pulse and a main pulse from an excitation laser;
• performing an energy measurement by an energy detector of a variation in RN energy generated when target droplets are converted to plasma;
• determining whether a variation in energy measurement is within an acceptable range;
• and
• in response to a variation in energy measurement that is not within the acceptable range of variation in energy measurement, automatically adjusting a configurable parameter of the excitation laser to set the variation in energy measurement within the acceptable range,
• wherein the feedback system is configured to adjust a spatial separation between the pre-pulse and the main pulse caused by at least one of a mechanical and an electrical drift of the excitation laser.

An apparatus for generating extreme ultraviolet (EUV) radiation comprising:

• a droplet generator configured to generate target droplets;
• a final focus module comprising:
• a radiation source configured to adjust a time delay between a pre-pulse and a main pulse at a predetermined position, and
• a steerable mirror;
• an energy detector configured to measure a variation in EUV energy generated when the target droplets are converted to plasma by an excitation laser;
• and
• a controller coupled to the energy detector and programmed to:
• determine whether the variation in energy measurement of radiation detected at the energy detector is within an acceptable range, and
• automatically adjust, in response to a determination that the detected variation in energy measurement is not within an acceptable range, a configurable parameter of the excitation laser,
• wherein the controller is configured to adjust a spatial separation between the pre-pulse and the main pulse caused by at least one of a mechanical and an electrical drift of the excitation laser.