Methods to control euv exposure dose and euv lithographic methods and apparatus using such methods

A device manufacturing method comprising:

• controlling EUV exposure dose of a lithographic apparatus having an EUV radiation source configured to generate pulses of EUV radiation by excitation of expanded, heated portions of fuel material by corresponding pulses of excitation laser radiation by controlling, from pulse to pulse, a conversion efficiency with which said laser radiation is converted to said EUV radiation by setting a target conversion efficiency that is lower than a maximum achievable conversion efficiency but sufficient to achieve a target EUV exposure dose, such that both positive and negative dose corrections can be applied between pulses by varying the conversion efficiency above and below said target conversion efficiency;
• patterning said EUV radiation to form a patterned beam of radiation;
• and
• projecting the patterned beam of radiation onto a substrate.

A lithographic apparatus comprising:

• a source of EUV radiation;
• an illumination system configured to condition a radiation beam received from said EUV radiation source;
• a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;
• a substrate table constructed to hold a substrate;
• a projection system configured to project the patterned radiation beam onto a target portion of the substrate;
• and
• a controller configured to control an exposure dose generated by said source of EUV radiation by controlling, from pulse to pulse, a conversion efficiency with which excitation laser radiation is converted to said EUV radiation by excitation of expanded, heated portions of fuel material by setting a target conversion efficiency that is lower than a maximum achievable conversion efficiency but sufficient to achieve a target EUV exposure dose, such that both positive and negative dose corrections can be applied between pulses by varying the conversion efficiency above and below said target conversion efficiency.