Patent ID: 8663881

Claim:
A method of controlling a radiation source apparatus in a lithographic system, the method comprising: providing a radiation source for emitting electromagnetic radiation at an EUV wavelength at a desired location; providing a radiation collector for receiving the emitted radiation and forming a beam of EUV radiation focused at a virtual source point; providing an illumination system for receiving the EUV radiation from said virtual source point and conditioning the beam to provide a desired distribution of EUV radiation over a target area; supporting a patterning device at said target area so as to generate from the conditioned beam a patterned beam of EUV radiation and using said patterned beam of radiation to apply a pattern to a substrate; detecting a residual asymmetry of the EUV radiation in said conditioned beam at said patterning device; and adjusting a parameter of said radiation source in response to the detected residual asymmetry so as to reduce the asymmetry by feedback control.