METHODS TO CONTROL EUV EXPOSURE DOSE AND EUV LITHOGRAPHIC METHODS AND APPARATUS USING SUCH METHODS.

摘要

EUV exposure dose in a lithographic apparatus is controlled pulse to pulse by varying a conversion efficiency with which a pulse of EUV radiation is generated from an excitation of a fuel material by a corresponding pulse of excitation laser radiation. Conversion efficiency can be varied in several different ways, by varying the proportion of a fuel material that intersects a laser beam, and/or by varying a quality of the interaction. Mechanisms to vary the conversion efficiency can be based on variation of a laser pulse timing, variation of pre-pulse energy, and/or variable displacement of a main laser beam in one or more directions. Steps to maintain symmetry of the generated EUV radiation can be included.