Method of measuring a property of a substrate, inspection apparatus, lithographic system and device manufacturing method

A device manufacturing method comprising:

• forming a first device layer on a substrate, the first device layer including a plurality of targets;
• measuring N targets of the plurality of targets using an optical measurement system, where N is an integer greater than 2 and each of the N targets is measured Wt times, where Wt is an integer greater than 2 so as to obtain N*Wt measurement values;
• and
• determining R property values using Q equations and the N*Wt measurement values, where R<Q≤N*Wt;
• making a decision about another process step;
• wherein the optical measurement system has at least one changeable setting and, for each of the N targets, measurement values are obtained using different setting values of at least one changeable setting;
• and
• making a decision is based on the R property values.

A non-transitory computer readable medium comprising processor readable instructions to cause an optical measurement system to perform a method of measuring a property of a substrate, the substrate having a plurality of targets formed thereon, the method comprising:

• performing Wt measurements of N targets of the plurality of targets using an optical measurement system that has at least one changeable setting, wherein obtaining the Wt measurements includes:
• setting the at least one changeable settings to a first setting;
• measuring a first value of a first one of the N targets at the first setting of the at least one changeable settings;
• measuring a second value of a second one of the N targets at the first setting of the at least one changeable settings;
• changing the at least one changeable settings to a second setting;
• measuring a third value of the first one of the N targets at the second setting of the at least one changeable settings;
• measuring a fourth value of the second one of the N targets at the second setting of the at least one changeable settings;
• determining R property values of the N targets using plural Q equations and the total of N*Wt measurement values taken, where R<Q≤N*Wt.