METHOD AND SYSTEM FOR GENERATING A SPHERICAL ABERRATION SIGNAL ERROR

摘要

The invention relates to a method and system comprising: - a detector for obtaining first focus error signal (FESn,1) and second focus error signal (FESn,2) dependent on radiation in first and second zones in a cross­-section of a radiation beam, respectively, - processing means for subtracting said second focus error signal (FESn,2) to said first focus error signal (FESn,1) so as to generate a spherical aberration signal (SAESn), - processing means for generating said second focus error signal (FESn,2) according to the formula (I) wherein α is a parameter, and A2, B2, C2 and D2 comprise output signals from a plurality of radiation detection sectors located in said respective second zone.