APPARATUS FOR AND METHOD OF TEMPERATURE COMPENSATION IN HIGH POWER FOCUSING SYSTEM FOR EUV LPP SOURCE

Apparatus for and method of temperature compensating a focusing system in which the focusing system has at least one transmissive optical element having a thermal lens. A reflective optical element is added to the system that has a thermal lens that is complementary to the thermal lens of the transmissive optical element so that the optical characteristics of the two optical elements combined are substantially temperature independent. The respective thermal lenses of the two optical elements are balanced by selecting materials for the reflective optical element that have the correct optical absorption based on the absorption of the transmissive optical element and the relative strengths of the thermal lenses. Provision can also be made for a change in the absorption of the transmissive optical element over time by selecting a value for the absorption of the reflective optical element that exceeds a contemporaneous value for the absorption of the transmissive optical element and then cooling the reflective optical element to reduce the strength of its thermal lens, with provision for increasing the temperature of the reflective optical component over time. The focusing system may also include a pulse combiner for combing pulses from multiple sources. The focusing system is especially applicable to systems for generating EUV light for use in semiconductor photolithography.