Methods and apparatus for predicting performance of a measurement method, measurement method and apparatus

A method of inspecting structures that have been formed on a substrate by a lithographic process, the method comprising:

• illuminating a first target with first radiation;
• substantially simultaneously illuminating a second target with second radiation, the first radiation and the second radiation including wavelengths less than 100 nm;
• detecting, at one or more first detection locations, portions of the first radiation that have been diffracted in a first direction by features of the first target;
• and
• detecting, at one or more second detection locations, portions of the second radiation that have been diffracted in a second direction by features of the second target.

A method of manufacturing devices comprising:

• a lithographic process step, wherein, before or after performing the lithographic process step, measurements are obtained of first and second targets on a substrate by a method comprising,
• illuminating a first target with first radiation,
• substantially simultaneously illuminating a second target with second radiation, the first radiation and the second radiation including wavelengths less than 100 nm,
• detecting, at one or more first detection locations, portions of the first radiation that have been diffracted in a first direction by features of the first target, and
• detecting, at one or more second detection locations, portions of the second radiation that have been diffracted in a second direction by features of the second target;
• and
• using the obtained measurements to adjust parameters of the lithographic process step for the processing of the substrate and/or further substrates.

A non-transitory computer program product comprising machine-readable instructions for causing a processor to implement the processing arrangement of an inspection apparatus to perform operations comprising:

• illuminating a first target with first radiation;
• substantially simultaneously illuminating a second target with second radiation, the first radiation and the second radiation including wavelengths less than 100 nm;
• detecting, at one or more first detection locations, portions of the first radiation that have been diffracted in a first direction by features of the first target;
• and
• detecting, at one or more second detection locations, portions of the second radiation that have been diffracted in a second direction by features of the second target.