Method and apparatus for correcting an optical element with inhomogeneous material properties

A method for correcting an error of a mask using a plurality of laser pulses on the substrate of the mask, the method comprising:

• receiving at least one laser parameter of the plurality of laser pulses,
• receiving a refractive index map of the substrate of the mask,
• determining, for a plurality of positions on the substrate of the mask, a local optical path difference, OPD, based on the at least one laser parameter, wherein the local OPD is caused by deviations in the refractive index map from an average refractive index of the substrate of the mask.

An apparatus for correcting an error of a mask, comprising a laser source for directing a plurality of laser pulses on a plurality of corresponding positions on a substrate of the mask;

• wherein the apparatus is configured to receive at least one first individual laser parameter for each laser pulse of the plurality of laser pulses to be applied at a plurality of corresponding positions on the substrate of the mask;
• the apparatus further comprising:
• a pulse shaping unit adapted for adapting each laser pulse of the plurality of laser pulses according to the at least one first individual laser parameter for the respective laser pulse.