Patent ID: 8178263

Claim:
A method of increasing a depth of focus of at least a part of a pattern feature imaged by a lithographic apparatus, the method comprising: illuminating a patterning device pattern, provided by a patterning device, with a radiation beam, the patterning device pattern comprising a pattern feature that diffracts the radiation beam to form a plurality of diffracted beams of radiation; illuminating a phase modulation element with the diffracted beams of radiation emanating from the patterning device, and using the phase modulation element to control the phase of at least a portion of radiation constituting each of the diffracted beams of radiation to form a first phase-wavefront having a first focal plane for the lithographic apparatus, and form a second phase-wavefront having a second focal plane for the lithographic apparatus, the first and second focal planes being offset relative to one another gong an optical axis of the lithographic apparatus, wherein controlling the phase of the radiation constituting at least a portion of the diffracted beams of radiation comprises: controlling the phase of a first portion of a first diffracted beam of radiation and a corresponding first portion of a second diffracted beam of radiation so that the first portions of the first and second diffracted beams of radiation are subjected to a first phase change which results in an at least partial formation of the first phase-wavefront, and controlling the phase of a second portion of the first diffracted beam of radiation and a corresponding second portion of the second diffracted beam of radiation so that the second portions of the first and second diffracted beams of radiation are subjected to a second phase change which results in an at least partial formation of the second phase-wavefront.