1. A method of determining an overlay error between two layers of a multiple layer sample, the method comprising: for a plurality of periodic targets that each have a first structure formed from a first layer and a second structure formed from a second layer of the sample, employing an interferometer to modulate substantially a plurality of wavelengths of a broadband source and then acquiring one or more images of the periodic targets, wherein there are predefined offsets between the first and second structures; and determining an overlay error between the first and second structures by analyzing the one or more acquired images from the periodic targets using a scatterometry overlay technique based on the predefined offsets.