Patent ID: 9535341
Date: 2017-01-03
CPC Classifications: G03F

Claim:
1. A method of obtaining information at least indicative of a focal property of a lithographic apparatus, the method comprising: illuminating a patterning device pattern, provided by a patterning device, with a radiation beam having a symmetric illumination mode, the patterning device pattern comprising a first pattern feature that substantially diffracts radiation constituting at least a portion of the radiation beam, and a second pattern feature that does not substantially diffract radiation constituting at least a portion of the radiation beam; illuminating a phase modulation element with radiation emanating from the patterning device, and introducing an asymmetry, relative to an optical axis, substantially only in the substantially diffracted radiation using the phase modulation element; illuminating a radiation beam receiving element with radiation emanating from the phase modulation element to form a receiving element pattern on the receiving element that is related to the patterning device pattern, the receiving element pattern having a first feature that is related to the first pattern feature of the patterning device pattern, and a second feature that is related to the second pattern feature of the patterning device pattern; obtaining positional information regarding the relative positions of the first feature of the receiving element pattern and of the second feature of the receiving element pattern; and determining information at least indicative of the focal property of the lithographic apparatus from the obtained positional information, wherein using the phase modulation element to introduce an asymmetry relative to an optical axis in the substantially diffracted radiation comprises: using the phase modulation element to induce a phase shift between first radiation constituting at least a portion of a diffraction order of the substantially diffracted radiation and second radiation constituting a different portion of that same diffraction order such that the first radiation and second radiation combine with radiation constituting at least a portion of another diffraction order to result in a substantially constant level of radiation when illuminating the radiation beam receiving element.