Apparatus and methods for detecting overlay errors using scatterometry

A method for determining an overlay error between at least two layers in a multiple layer sample, the method comprising:

• providing a sample having a plurality of periodic targets that each have a first structure in a first layer and a second structure in a second layer, wherein there are predefined offsets between the first and second structures;
• using a scatterometry overlay metrology, determining and storing scatterometry overlay data for a first set of the periodic targets based on one or more measured optical signals from the first target set on the sample;
• using an imaging overlay metrology, determining and storing imaging overlay data for a second set of the periodic targets based on one or more image(s) from the second target set on the sample;
• and
• selecting the first target set from which the scatterometry overlay data is determined and selecting the second target set from which the imaging overlay data is determined, wherein the selections are based on a criteria selected from a relative performance or a relative speed of the scatterometry overlay metrology or the imaging overlay metrology.