Reducing algorithmic inaccuracy in scatterometry overlay metrology

A scatterometry overlay (SCOL) target, comprising:

• two or more cells on a sample comprising:
• at least one first segmented target structure on a first layer of the sample segmented with a coarse pitch and a first fine pitch;
• and
• at least one second segmented target structure on a second layer of the sample segmented with the coarse pitch and a second fine pitch, wherein the two or more cells include two or more predetermined offsets between the corresponding at least one first segmented target structure and the at least one second segmented target structure, wherein an overlay signal associated with measured diffraction modes from the two or more cells is indicative of an overlay offset between the first layer and the second layer of the sample, wherein the overlay signal is a periodic function of the overlay offset and is represented by a sum of oscillation frequencies of different orders, wherein the values of the coarse pitch, the first fine pitch, and the second fine pitch are selected to limit, within a specified inaccuracy requirement, an algorithmic error in determining the overlay offset by limiting oscillation frequencies of the overlay signal having an order higher than a predetermined cutoff order.

A method of reducing algorithmic inaccuracy in scatterometry overlay metrology, comprising the steps of:

• designing a scatterometry overlay target on a sample, the scatterometry overlay target comprising:
• at least one first segmented target structure on a first layer of the sample segmented with a coarse pitch and a first fine pitch;
• and
• at least one second segmented target structure on a second layer of the sample segmented with the coarse pitch and a second fine pitch, wherein the two or more cells include two or more predetermined offsets between the corresponding at least one first segmented target structure and the at least one second segmented target structure, wherein an overlay signal associated with measured diffraction modes from the two or more cells is indicative of an overlay offset between the first layer and the second layer of the sample, wherein the overlay signal is a periodic function of the overlay offset and is represented by a sum of oscillatory frequencies of different orders;
• and
• selecting the values of the coarse pitch, the first fine pitch, and the second fine pitch to limit, within a specified inaccuracy requirement, an algorithmic error in determining the overlay offset by limiting oscillation frequencies having an order higher than a predetermined cutoff order;
• providing the designed scatterometry overly target on the sample;
• measuring the overlay signal from the scatterometry overlay target on the sample using a detector to determine an overlay offset between the first sample layer and the second sample layer with the limited algorithmic error;
• and
• generating correction terms to correct for the algorithmic error associated with oscillation frequencies of the overlay signal for one or more additional scatterometry overlay targets.