Patent ID: 8189172

Claim:
A method of measuring aberration in a lithographic apparatus, the method comprising: modulating a radiation beam using an array of individually controllable elements; projecting the modulated radiation beam using a projection system; detecting the projected radiation using a moveable sensor; and measuring aberration in the detected radiation beam, wherein a pattern is provided on the array of individually controllable elements to modulate the radiation beam, wherein the pattern has a repeating structure and is formed from a plurality of features, wherein the plurality of features are dimensioned to generate first order diffraction of the radiation beam that substantially fills the pupil of the projection system, and wherein the moveable sensor steps through and captures an image of an interference pattern in the radiation projected by the projection system, captures an intensity of the image as a function of a stepped distance per step, determines a phase of a first harmonic of the intensity as a function of stepped distance and determines aberrations in the lithographic apparatus based on the phase of the first harmonic.