Patent ID: 8677288

Claim:
A method of fabricating a photomask for use in lithography, comprising: providing an OPC model; calibrating the OPC model by: quantifying several different optical functions of a first mask having first mask patterns, quantifying the same optical functions of a second mask having mask patterns that correspond to the first mask patterns, respectively, but differ from the first mask patterns by a predetermined bias, and calculating the differences between the optical functions, respectively, of the first and second masks, wherein each of the optical functions is a respective type of result of a photolithography process using the mask and dependent on the geometry of the mask patterns of the mask, and the quantifying of the optical functions of the first mask, the quantifying of the same optical functions of the second mask, and the calculating of the differences between the optical functions are preformed for each pair of corresponding patterns of the first and second masks, generating a sensitivity matrix that includes data of values of the calculated differences in optical function for each of the optical functions, whereby the sensitivity matrix also contains data of variations among the values of the calculated differences for each of the optical functions, analyzing the sensitivity matrix to select one or more optical functions from among said several different optical functions, pulling data from the sensitivity matrix pertaining to the selected optical functions and clustering the data pulled from the sensitivity matrix, and selecting respective ones of the first patterns in consideration of how the data clusters, and designating the selected first patterns as test patterns, whereby the first mask patterns are sampled; and altering a layout of mask patterns of a photomask according to the calibrated OPC model, and fabricating the mask patterns according to the altered layout.