Patent ID: 7916284
Filing Date: 2011-03-29
Classification: G03F

Abstract:
1. A method of measuring a parameter of a lithographic process by which a target pattern has been printed on a substrate, the method comprising: projecting an image of a reference pattern onto a radiation-sensitive layer of a calibration substrate a plurality of times to form a plurality of calibration patterns, wherein different values of a calibration pattern parameter are used to form different ones of the calibration patterns and the reference pattern comprises first and second parts having different sensitivities to variation in the value of the calibration pattern parameter, the first and second parts of the reference pattern are selected such that, for a given value of the calibration pattern parameter, each of the first and second parts of the reference pattern form a substantially identical pattern with respect to one another in the radiation-sensitive layer; directing an inspection beam of radiation onto the calibration patterns and measuring the radiation reflected or scattered therefrom to obtain a measurement result for each part of each calibration pattern; subtracting the measurement result for a first part of each calibration pattern from the measurement result for a second part of the respective calibration pattern to obtain a plurality of differential measurement results; decomposing each of the differential measurement results into a set of basis functions and associated coefficients and obtaining a relationship between values of the associated coefficients and values of the calibration pattern parameter; projecting an image of the reference pattern onto a radiation-sensitive layer of a substrate to form a target pattern, wherein a value of a target pattern parameter used to form the target pattern is unknown; directing an inspection beam of radiation onto the target pattern and measuring radiation reflected or scattered therefrom to obtain a target measurement result for each part of the target pattern; subtracting the target measurement result for a first part of the target pattern from the target measurement result for a second part of the target pattern to obtain a target differential measurement result; and decomposing the target differential measurement result into a set of coefficients, multiplying a plurality of basis functions, and using a relationship between values of the associated coefficients and values of the calibration pattern parameter to determine the value of the target pattern parameter used to form the target pattern.