Method of detecting repeating defects in lithography masks on the basis of test substrates exposed under varying conditions

A method, comprising:

• exposing a resist layer provided above a substrate by a lithography process using a specific photolithography mask and a first setting of exposure parameter values to generate a first plurality of images, at least one parameter value of said first setting being above an allowable range applied when using said specific photolithography mask during exposure of product substrates;
• generating a second plurality of images using said specific photolithography mask and a second setting of exposure parameters, said at least one parameter value in said second setting being below said allowable range;
• and
• comparing said first and second pluralities of images to identify defects that repeatedly occur in said first plurality of images.

A method of estimating printability of mask defects, the method comprising:

• forming for each of a plurality of different process threads first images using a specific photolithography mask and a first setting of exposure parameters above an allowable range used for processing product substrates in said plurality of different process threads and second images using said specific photolithography mask and a second setting of exposure parameters below said allowable range;
• identifying defects repeatedly occurring in said first and second images associated with each of said different process threads;
• and
• determining printability of said repeatedly occurring defects of each of said different process threads on the basis of said first and second images.